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The design of hole-transport materials to stabilize the performance of perovskite solar cells Chiykowski, Valerie Anne 2019

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  THE DESIGN OF HOLE-TRANSPORT MATERIALS TO STABILIZE THE PERFORMANCE OF PEROVSKITE SOLAR CELLS   by   Valerie Anne Chiykowski  B.Sc., Queen’s University, 2014    A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF  THE REQUIREMENTS FOR THE DEGREE OF   DOCTOR OF PHILOSOPHY    in   THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES   (CHEMISTRY)   THE UNIVERSITY OF BRITISH COLUMBIA  (VANCOUVER)  June 2019    © Valerie Anne Chiykowski, 2019     ii  The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the dissertation entitled: The Design of Hole-Transport Materials to Stabilize the Performance of Perovskite Solar Cells   submitted by  Valerie A. Chiykowski in partial fulfillment of the requirements for the degree of  Doctor of Philosophy in Chemistry   Examining Committee: Curtis Berlinguette Supervisor  Michael Wolf Supervisory Committee Member Laurel Schafer Supervisory Committee Member Mark MacLachlan University Examiner Peyman Servati University Examiner Additional Supervisory Committee Members: Dan Bizzotto Supervisory Committee Member  iii Abstract Over six decades of photovoltaic research have led to the emergence of a highly efficient technology called perovskite solar cells (PSCs). Despite the recent sharp rise in power conversion efficiencies (PCEs) of this technology, PSCs have not yet been deployed at scale owing in part to the unsatisfactory stability of devices. The stability issues are due to the light absorbing layers being susceptible to dissolution and the hole-transport material (HTM) layers undergoing morphological changes under real life conditions. This dissertation seeks to suppress mechanisms of PSC degradation through the design of HTMs. I designed a series of five structurally similar HTMs to study the effect of triphenylamine (TPA) location and number on the thermal stability (i.e., glass transition temperature, Tg) of the HTM layer. My studies demonstrate that where the TPA units are positioned about a spiro-carbon core can shift the Tg upwards of 30 °C.   I designed HTMs that can be electrochemically and thermally polymerized to yield an encapsulation layer for the PSC. I demonstrated that the polymerized HTM layer decreases film wettability and can be incorporated into a PSC device. I interrogated a series of three structurally analogous donor-acceptor (D-A) architectures (i.e., monopodal, bipodal and tripodal architectures) to determine the role of molecular structure on the hole mobility of HTMs. From these experiments, I learned that “monopodal” D-A architectures yielded the highest hole mobilities because of the low computed reorganization energy, small polaron stabilization energy and hole extraction potential associated with this HTM. Overall, I demonstrated three mechanisms to suppress either the degradation of the photoactive perovskite layer, the morphological changes to the HTM layer or the instability caused  iv by additives in HTM films. I suggest future design principles to yield stable PSC devices towards the commercialization of this technology.               v Lay Summary Solar energy is an important renewable energy source. Covering merely 0.16% of the earth’s surface could provide enough electricity to meet the projected global demand of 2040, without any contribution from non-renewable sources such as oil and gas (though issues surrounding energy storage and distribution would have to be investigated in parallel to solar cell development). Solar panels must offer warranties of 20 years to be considered commercially viable for installation and application. The research in this dissertation strives to improve the stability of a particular solar cell technology, called perovskite solar cells, which exhibit high efficiencies and low costs of fabrication. Specifically, the experiments in this dissertation were designed to create new materials that do not: dissolve in water; change shape in heat; and rely on unstable dopants in order to conduct charge effectively.                         vi Preface Chapter 3 is adapted from the paper “Design Rules for High Mobility Xanthene-Based Hole-Transport Materials”, submitted for publication. The work was supervised by Prof. Aspuru-Guzik at the University of Toronto and Prof. Berlinguette. The project was designed and developed by myself. The computational screening was conducted by Dr. Daniel Tabor in the Aspuru-Guzik group. The synthesis, structural characterization, photophysical and electrochemical analysis of seven compounds were carried out by myself and Dr. Yang Cao in the Berlinguette group. The conductivity studies were performed by myself and David Dvorak in the Berlinguette Group. The mobility model was developed and carried out by Dr. Pascal Freiderich in the Aspuru-Guzik group.  The perovskite solar cell studies were carried out by Dr. Hairen Tan in the Sargent group. The manuscript was written by myself and Dr. Daniel Tabor.  Chapter 4 is adapted from the paper “Precise Control of Thermal and Redox Properties of Organic Hole-Transport Materials”, Angew. Chem. Int. Ed., 2018, 57, 15529-15533. The work was supervised by Prof. Aspuru-Guzik at the University of Toronto, Prof. Sargent at the University of Toronto and Prof. Berlinguette. The project was designed and developed by myself and Dr. Yang Cao in the Berlinguette group. The computational screening and calculations  were conducted by Dr. Daniel Tabor in the Aspuru-Guzik group. The synthesis, structural characterization, photophysical and electrochemical analysis of seven compounds were carried out by myself and Dr. Yang Cao in the Berlinguette group. Hole mobility and perovskite solar cell studies were carried out by Dr. Hairen Tan in the Sargent group. The manuscript was written by myself, Dr. Daniel Tabor and Dr. Hairen Tan with contributions from Prof. Aspuru-Guzik, Prof. Sargent and Prof. Curtis Berlinguette.  vii  Chapter 5 research was supervised by Prof. Sargent at the University of Toronto and Prof. Berlinguette. The project was designed and developed by myself. The synthesis, structural characterization, photophysical and electrochemical analysis of two compounds was carried out by myself. The conductivity studies were performed by myself and David Dvorak in the Berlinguette Group. Perovskite solar cells were fabricated and tested by Dr. Hairen Tan in the Sargent group.  Chapter 6 was supervised by Prof. Berlinguette. The project was designed and developed by myself. The computational calculations were carried out by Dr. Carolyn Virca in the Berlinguette group. The synthesis, structural characterization, photophysical and electrochemical analysis of three compounds was carried out by myself. The conductivity and hole mobility studies were performed by myself and David Dvorak in the Berlinguette Group.                        viii Table of Contents  Abstract ................................................................................................................................... iii Lay Summary ............................................................................................................................ v Preface ...................................................................................................................................... vi Table of Contents .................................................................................................................. viii List of Tables .......................................................................................................................... xiv List of Figures ......................................................................................................................... xv List of Schemes ....................................................................................................................... xix List of Abbreviations .............................................................................................................. xx Acknowledgments ............................................................................................................... xxvii Dedication........................................................................................................................... xxviii Chapter 1: Motivation and Outline .............................................................................................. 1 1.1 Motivation for research .......................................................................................................... 1 1.2 Outline of dissertation............................................................................................................. 4 Chapter 2: Literature Review ....................................................................................................... 7 2.1 Perovskite solar cells .............................................................................................................. 7 2.1.1 Components and materials ............................................................................................. 7 2.1.2 Key Photovoltaic Metrics ............................................................................................... 9 2.2 Perovskite layer .....................................................................................................................10 2.2.1 Structure and role of the perovskite layer ....................................................................10  ix 2.3 Hole transport material layer ................................................................................................12 2.3.1 Roles and properties of hole transport materials .........................................................12 2.3.2 Instability of hole transport material layer ..................................................................19 2.4 Solutions to perovskite solar cell instability using hole transport materials .....................21 2.4.1 Polymer hole transport materials for improved moisture stability of perovskite layer .................................................................................................................................................21 2.4.2 High glass transition temperature hole transport materials for improved thermal stability of perovskite solar cells ...........................................................................................23 2.4.3 Donor-acceptor hole transport materials for dopant-free perovskite solar cells .......23 Chapter 3 : Tuning the Electrochemical and Electronic Properties of Spiro-Centred Hole Transport Materials through Substituent Identity .................................................................26 3.1 Introduction ...........................................................................................................................26 3.2 Results and discussion ..........................................................................................................28 3.2.1 Computational screening ..............................................................................................28 3.2.2 Synthesis ........................................................................................................................29 3.2.3 Electrochemical and photophysical characterization ..................................................29 3.2.4 Bulk thermal properties ................................................................................................32 3.2.5 Electrical properties ......................................................................................................33 3.2.6 Bulk Simulation ............................................................................................................34 3.2.7 Perovskite solar cell characterization ..........................................................................35  x 3.3 Conclusions ...........................................................................................................................36 3.4 Experimental .........................................................................................................................37 3.4.1 Computational screening ..............................................................................................37 3.4.2 Synthesis ........................................................................................................................37 3.4.3 Electrochemical and photophysical properties ............................................................43 3.4.4 Bulk thermal properties ................................................................................................43 3.4.5 Electrical properties ......................................................................................................44 3.4.6 Bulk simulation .............................................................................................................44 3.4.7 Perovskite solar cell characterization ..........................................................................45 Chapter 4 : Controlling the Thermal Stability and Electrochemistry of Hole-Transport Material Films ...............................................................................................................................47 4.1 Introduction ...........................................................................................................................47 4.2 Results and discussion ..........................................................................................................49 4.2.1 Computational screening ..............................................................................................49 4.2.2 Synthesis ........................................................................................................................53 4.2.3 Electrochemical and photophysical characterization ..................................................53 4.2.4 Bulk thermal properties ................................................................................................58 4.2.5 Electrical properties ......................................................................................................58 4.2.6 Perovskite solar cell characterization ..........................................................................61 4.3 Conclusion .............................................................................................................................62  xi 4.4 Experimental .........................................................................................................................63 4.4.1 Computational screening ..............................................................................................63 4.4.2 Synthesis ........................................................................................................................64 4.4.3 Electrochemical and photophysical properties ............................................................68 4.4.4 Bulk thermal properties ................................................................................................68 4.4.5 Electrical properties ......................................................................................................69 4.4.6 Perovskite solar cell characterization ..........................................................................69 Chapter 5 : Polymerizable Hole-Transport Materials for Improved Moisture Stability of Perovskite Solar Cells ...................................................................................................................71 5.1 Introduction ...........................................................................................................................71 5.2 Results and discussion ..........................................................................................................73 5.2.1 Synthesis ........................................................................................................................73 5.2.2 Electrochemical and photophysical properties ............................................................76 5.2.3 Bulk thermal properties ................................................................................................78 5.2.4 Contact angle measurements ........................................................................................79 5.2.5 Electrical properties ......................................................................................................80 5.2.6 Perovskite solar cell characterization ..........................................................................81 5.3 Conclusion .............................................................................................................................82 5.4 Experimental .........................................................................................................................83 5.4.1 Synthesis ........................................................................................................................83  xii 5.4.2 Electrochemical and photophysical properties ............................................................86 5.4.3 Bulk thermal properties ................................................................................................86 5.4.4 Contact angle measurements ........................................................................................86 5.4.5 Electrical properties ......................................................................................................87 5.4.6 Perovskite solar cell characterization ..........................................................................87 Chapter 6 : Effect of Donor-Acceptor Architecture on Hole-Transport Material Hole Mobility ...........................................................................................................................................89 6.1 Introduction ...........................................................................................................................89 6.2 Results and discussion ..........................................................................................................92 6.2.1 Synthesis ........................................................................................................................92 6.2.2 Electrical properties ......................................................................................................94 6.2.3 Bulk thermal properties ................................................................................................95 6.2.4 Electrochemical and photophysical properties ............................................................97 6.2.5 Computed properties .................................................................................................. 100 6.3 Conclusions ........................................................................................................................ 101 6.4 Experimental ...................................................................................................................... 102 6.4.1 Synthesis ..................................................................................................................... 102 6.4.2 Electrical properties ................................................................................................... 106 6.4.3 Bulk thermal properties ............................................................................................. 107 6.4.4 Electrochemical and photophysical properties ......................................................... 107  xiii 6.4.5 Computed properties .................................................................................................. 108 Chapter 7: Conclusions and Future Directions ..................................................................... 110 7.1 Conclusions ........................................................................................................................ 110 7.2 Future directions ................................................................................................................ 111 References ............................................................................................................................. 115 Appendices ............................................................................................................................ 126 Appendix 1: Chapter 3 ........................................................................................................ 126 Appendix 2: Chapter 4 ........................................................................................................ 137 Appendix 3: Chapter 5 ........................................................................................................ 148 Appendix 4: Chapter 6 ........................................................................................................ 156                     xiv List of Tables  Table 2-1 Electrochemical and electron transport properties, and glass transition temperatures of organic HTMs ........................................................................................................................... 15	Table 3-1 Summary of computed reduction potentials and HOMO-LUMO energy gaps for the spiro-R series ........................................................................................................................... 28	Table 3-2 Summary of photophysical, electrochemical and bulk thermal properties for spiro-R series......................................................................................................................................... 31	Table 3-3 Conductivity data for spiro-OMeTAD and spiro-R series ....................................... 34	Table 3-4 Perovskite solar cell data for spiro-OMeTAD and spiro-R series ............................ 36	Table 4-1 Calculated electronic frontier molecular orbital properties and contributions to reorganization energies (B3LYP/6-311+G(d,p) evaluated at B3LYP/6-31G(d) optimized geometries) of the HTM series and spiro-OMeTAD ................................................................ 52	Table 4-2 Electrochemical, bulk thermal and electrical properties of HTM series .................... 54	Table 4-3 Summary of DPV oxidation peaks for HTM series................................................... 57	Table 4-4 Summary of spectroscopic and electrochemical properties for HTM series. ............. 58	Table 4-5 Device characteristics for perovskite solar cells containing HTM series. .................. 62	Table 5-1 Electrochemical, photophysical, and glass transition temperature data for HTMs. ..... 79	Table 5-2 Statistical performance data of HTMs in perovskite solar cell devices. ..................... 82	Table 6-1 Electrical, electrochemical, photophysical, and bulk thermal properties of TPA-1, TPA-2, and TPA-3 ................................................................................................................... 96	Table 6-2 Computed factors affecting hole mobility for TPA-1, TPA-2, and TPA-3 .............. 101	  xv List of Figures  Figure 1-1 Power conversion efficiencies (PCEs) of select first, second, and third generation solar cell technologies with current certified record PCEs. Figure adapted from NREL Best Research-Cell Efficiencies Graph.1.............................................................................................. 2	Figure 1-2 (a) Schematic illustration of a conventional PSC (b) electron (e-) and hole (h+) flow through PSC components, and (c) molecular structure for organic HTM X60.............................. 3	Figure 1-3 Outline of organic HTM molecular design for dissertation chapters .......................... 4	Figure 2-1 Energy levels (e.g., valence band (VB) and conduction band (CB) of the perovskite and TiO2-Cl, HOMO and LUMO levels of the HTM) relevant to various device performance parameters (e.g., Voc and ΔG) ...................................................................................................... 8	Figure 2-2  J-V and power density curves highlighting key performance metrics for a PSC ...... 10	Figure 2-3 General perovskite (AMX3) structure with examples of A (e.g., CH3NH3+, CH(NH2)2+ and Cs+), M (e.g., Pb2+ and Sn2+) and X (i.e., Cl-, Br-, I-) identities.......................... 11	Figure 2-4 Examples of linear, starburst and spiro organic HTMs ............................................ 14	Figure 2-5 MilliporeSigma pricing per gram of HTM ............................................................... 15	Figure 2-6 Morphological changes to the HTM layer above the Tg ........................................... 20	Figure 2-7 Chemical structures of p-dopants (LiTFSI, Zn(TFSI)2, FK209 and F4-TCNQ). ....... 21	Figure 2-8 Examples of polymer HTMs employed for improved PSC moisture stability .......... 22	Figure 2-9  Common donor, pi and acceptor units employed for D-A HTMs ............................ 24	Figure 3-1  Molecular representations of the spiro-R series under investigation. ...................... 26	Figure 3-2 Cyclic voltammograms for spiro-R series recorded in 0.1 M n-NBu4PF6 DCM solutions at room temperature. Data were collected at a scan rate of 50 mV/s. Dashed lines indicate EHOMO values. ............................................................................................................... 30	 xvi Figure 3-3  (a) UV-Vis absorption spectra and (b) emission spectra for spiro-R series dissolved in DCM ..................................................................................................................................... 32	Figure 3-4 (a) Simulated (grey) and experimental (red) hole mobilities of spiro-OMeTAD and spiro-R series, (b) computed hole mobilities of the spiro-R series with the contributions by electronic coupling, reorganization energy, conformational disorder and electrostatic disorder. . 35	Figure 4-1 Molecular representations of spiro-OMeTAD68 and the HTM series under investigation. ............................................................................................................................ 49	Figure 4-2 HOMO and LUMO orbitals of each molecule in the HTM series, calculated at the BP86/def2-SVP level of theory. Isosurfaces are at 0.03 and -0.03 values for the wavefunction. . 50	Figure 4-3 Spin density difference maps for the HTM cations evaluated at the B3LYP/6-311+G(d,p) level of theory at the B3LYP/6-31G(d) cation adiabatic minima. ........................... 52	Figure 4-4 Cyclic voltammograms of HTM series in 0.1 M n-NBu4PF6 DCM solutions, with a polished Pt working electrode, Pt wire counter electrode and Ag/AgCl reference electrode. ...... 54	Figure 4-5 Differential pulse voltammograms (DPVs) for HTM series normalized in amplitude to their first oxidation event. The number of electrons that contribute to each redox process was determined by integrating the area beneath the peak fitting. (a) The first deconvoluted oxidation events are shown in shaded colors to highlight the progressively anodic shift of EHOMO for HTM-F (red), HTM-X (blue) and HTM-X′ (green), respectively. Traces for (b) X60 (black) and (c) HTM-FX′ (orange) are deconvoluted to highlight that the EHOMO is linked to the TPA unit positioned on the fluorene unit. ................................................................................................. 56	Figure 4-6  (a) UV-Vis absorption spectra for HTM solutions (2.0 × 10-5 M) recorded in DCM. (b) Emission spectra for HTM series recorded in DCM excited at lowest energy absorption band recorded in Table 4-4. ............................................................................................................... 57	 xvii Figure 4-7 Performance of perovskite solar cells with HTMs: (a) J-V curves of hole-only devices with HTM series; (b) J-V curves of solar cells with HTM series measured under solar simulator (100 mW cm-2); (c) Histogram of PCEs for 44 devices with HTM-FX′; (d) J-V curves of champion device with HTM-FX′ scanned in reverse and forward directions (scanning rate of 20 mV/s). ....................................................................................................................................... 60	Figure 4-8 (a) Steady-state PL and (b) time-resolved PL decay of perovskite films with spiro-OMeTAD, X60 and HTM-FX' and (c) time-resolved PL decay of perovskite films with HTM-F, HTM-X and HTM-X'. ......................................................................................................... 61	Figure 5-1 Molecular structures of VNPB,27 HTM-FX′26,119  and spiro-VNPB. ....................... 72	Figure 5-2 Cyclic voltammograms (CVs) over 20 scans for spiro-NPB between 0.5-1.6 V vs NHE (grey), spiro-VNPB between 0.5-1.2 V vs NHE (teal) and spiro-VNPB between 0.5-1.6 V vs NHE (purple) recorded in 0.1 M n-NBu4,PF6 DCM solutions at room temperature. Vertical arrows indicate increasing current response with subsequent scans suggestive of electropolymerization. Horizontal arrows indicate open-circuit potential. .................................. 77	Figure 5-3 Differential pulse voltammograms (DPV) for spiro-NPB (black) and spiro-VNPB (teal) ......................................................................................................................................... 77	Figure 5-4 Absorption profiles for spiro-NPB (black) and spiro-VNPB (teal) ......................... 78	Figure 5-5 Contact angle data of spiro-OMeTAD, spiro-NPB, spiro-VNPB and thermally polymerized spiro-VNPB over 30 minutes. .............................................................................. 80	Figure 5-6 Sheet resistance data for spiro-OMeTAD, spiro-NPB, spiro-VNPB and thermally polymerized spiro-VNPB ......................................................................................................... 81	Figure 6-1 Molecular representations of TPA-1, TPA-2, and TPA-3 (collectively referred to as TPA-X series) which vary in D-A architectures. ....................................................................... 91	 xviii Figure 6-2 a) Schematic of hole-only device for hole mobility measurements and b) hypothetical J-V curve of a hole-only device highlighting the ohmic and SCLC regimes for HTM hole mobility measurements. ............................................................................................................ 95	Figure 6-3 Hole mobilities determined from SCLC region of hole-only devices containing spiro-OMeTAD, TPA-1, TPA-2, and TPA-3. ................................................................................... 95	Figure 6-4 Grazing incidence XRD data for TPA-1, TPA-2, and TPA-3 on glass .................... 97	Figure 6-5 Cyclic voltammograms (CVs) of TPA-1, TPA-2, and TPA-3 recorded in 0.1 M n-NBu4PF6 DCM solutions at room temperature. Data were collected at a scan rate of 50 mV/s. .. 98	Figure 6-6 Visual representation of computed energies relevant to reorganization energy components (λ1 and λ2), hole extraction potential (HEP), and ionization potentials (IP) of neutral and charge HTM molecules....................................................................................................... 98	Figure 6-7  (a) Experimental UV-Vis absorption, (b) experimental emission, and (c) theoretical absorption spectra for TPA-1, TPA-2, and TPA-3. ................................................................... 99	Figure 6-8 Calculated frontier orbitals of optimized structures for  TPA-1, TPA-2, and TPA-3 plotted at an absolute isovalue of 0.02. .................................................................................... 100	Figure 7-1 Proposed architecture of future directions for PSCs where there the interface between the perovskite and HTM layers is manipulated by surface functionalization of the AMX3 perovskite structure and the incorporation of polymerizable HTM. ......................................... 112	Figure 7-2 Proposed molecular structure of monopodal D-A HTM with spiro-core to increase Tg and opportunity to vary donor and acceptor identities and EHOMO. ........................................... 113	      xix List of Schemes  Scheme 2-1 Synthetic schemes for (a) spiro-OMeTAD and (b) X60 ........................................ 18	Scheme 3-1 Synthetic scheme for spiro-R series ...................................................................... 29	Scheme 4-1 Synthetic scheme for HTM series. ........................................................................ 53	Scheme 5-1 Synthetic scheme of spiro-VNPB and spiro-NPB ................................................. 75	Scheme 6-1 Synthesis schemes for TPA-1, TPA-2, and TPA-3 ............................................... 93	                             xx List of Abbreviations A amp Ag(TFSI)2 silver bis(trifluoromethanesulfonyl)amide  AMX3 general perovskite structure AMX3 ᐧ H2O general monohydrate perovskite sheet A2MX6 ᐧ 2H2O general dihydrate perovskite octahedra BPA biphenylamine B3LYP Becke’s three-parameter exchange functional (B3) and the Lee-Yang-Parr correlation functional (LYP) 13C carbon-13 CB conduction band CH3NH3+ methylammonium CH3NH3PbBr3 methylammonium lead bromide CH3NH3PbI3 methylammonium lead bromide Champion PCE maximum power conversion efficiency achieved with a single device cm centimeter CuBr copper bromide CuI copper iodide CuSCN copper thiocyanate CV cyclic voltammogram d doublet d inter-electrode spacing dir direct D-A donor-acceptor DCM dichloromethane DFT density functional theory  xxi DMSO dimethylsulfoxide DPV differential pulse voltammetry DSC differential scanning calorimetry DSSC dye-sensitized solar cell EI electron impact EHOMO highest occupied molecular orbital energy level ELUMO lowest unoccupied molecular orbital energy level EHOMO-LUMO energy gap between HOMO and LUMO energy levels EF1 first oxidation of fluorene TPAs EF2 second oxidation of fluorene TPAs EX1 first oxidation of xanthene TPAs EX2 second oxidation of xanthene TPAs EFermi Fermi level Ereorg reorganization energy E*+ energy of neutral molecule at cation minimum geometry E+ energy of cation at cation minimum geometry E* energy of cation at neutral minimum geometry ESI electron-spray ionization ETL electron transport layer EtOAc ethylacetate eV electron volts F4-TCNQ 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane FA formamidinium FF fill factor FK209 tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III) tri[hexafluorophosphate]  xxii FTO fluorine-doped tin oxide G Gibb’s free energy or driving force 1H proton HOMO highest occupied molecular orbital HPLC high pressure liquid chromatography HTM hole-transport material inv inverse ITO indium-doped tin oxide J current density Jsc short-circuit current density J-V current density-voltage l length LiTFSI lithium(I) bis (trifluoromethylsulfonyl)imide LUMO lowest unoccupied molecular orbital M molar  MA methylammonium MeO-TPD N,N,N′,N′-tetrakis(4-methoxyphenyl)benzidine MeOTPA-TPA N4,N4-bis(4'-(bis(4-methoxyphenyl)amino)-[1,1'-biphenyl]-4-yl)-N4',N4'-bis(4-methoxyphenyl)-[1,1'-biphenyl]-4,4'-diamine MeO-TPTE N4,N4'-([1,1'-biphenyl]-4,4'-diyl)bis(N4,N4',N4'-tris(4-methoxyphenyl)-[1,1'-biphenyl]-4,4'-diamine  MeO-TPTR N4-(4′-(bis(4-methoxyphenyl)amino)-[1,1′-biphenyl]-4-yl)-N4,N4′,N4′-tris-(4-methoxy phenyl)-[1,1′-biphenyl]-4,4′-diamine MHz megahertz min minute  mmol millimole mol Mole  xxiii mV millivolt m/z mass-to-charge ratio m- meta substituted  NiOx nickel oxide nm nanometer n-Bu4NPF6  tetrabutylammonium hexafluorophosphate NHE normal hydrogen electrode NMR nuclear magnetic resonance OLED organic light-emitting diode P3HT poly(3-hexylthiophene-2,5-diyl)  -o ortho substituted  -p para substituted PEDOT poly(3,4-ethylenedioxythiophene) PEPPSI-iPr [1,3-Bis(2,6-Diisopropylphenyl)imidazol-2-ylidene](3-chloropyridyl)palladium(II) dichloride catalyst PCE power conversion efficiency PL photoluminescence PMMA poly(methyl methacrylate) ppm parts per million PSC perovskite solar cell PSS polystyrene sulfonate PTFE polytetrafluoroethylene PTAA poly(triaryl amine) Pin power input Pmax power maximum Pout power output  xxiv R resistance rpm rotations per min s second S Siemens SCLC space-charge limited current SiO2 silicon dioxide spiro-F N2,N2,N2',N2',N7,N7,N7',N7'-octakis(4-fluorophenyl)spiro[fluorene-9,9'-xanthene]-2,2',7,7'-tetraamine spiro-FOMe N2,N2',N7',N7-tetrakis(4-fluorophenyl)-N2,N2',N7',N7-tetrakis(4-methoxyphenyl)spiro[fluorene-9,9'-xanthene]-2,2',7',7-tetraamine spiro-H N2,N2,N2',N2',N7,N7,N7',N7'-octaphenylspiro[fluorene-9,9'-xanthene]-2,2',7,7'-tetraamine spiro-Me N2,N2,N2',N2',N7,N7,N7',N7'-octa-p-tolylspiro[fluorene-9,9'-xanthene]-2,2',7,7'-tetraamine spiro-NPB N2,N2,N7,N7-tetrakis(4-methoxyphenyl)-N3',N6'-di(naphthalen-1-yl)-N3',N6'-diphenylspiro[fluorene-9,9'-xanthene]-2,3',6',7-tetraamine spiro-OMeTAD N2,N2,N2′,N2′,N7,N7,N7′,N7′-octakis(4-methoxyphenyl)-9,9′-spirobi[9H-fluorene]-2,2′,7,7′-tetramine spiro-p,o-OMe N2,N2',N7,N7'-tetrakis(2-methoxyphenyl)-N2,N2',N7,N7'-tetrakis(4-methoxyphenyl)spiro[fluorene-9,9'-xanthene]-2,2',7,7'-tetraamine Spiro-Me N2,N2,N2',N2',N7,N7,N7',N7'-octa-p-thiomethylspiro[fluorene-9,9'-xanthene]-2,2',7,7'-tetraamine spiro-TPD N2,N2',N7,N7'-tetraphenyl-N2,N2',N7,N7'-tetra-p-tolyl-9,9'-spirobi[fluorene]-2,2',7,7'-tetraamine  Spiro-VNPB N2,N2,N7,N7-tetrakis(4-methoxyphenyl)-N3',N6'-di(naphthalen-1-yl)-N3',N6'-bis(4-vinylphenyl)spiro[fluorene-9,9'-xanthene]-2,3',6',7-tetraamine ssDSSC solid-state dye-sensitized solar cell t thickness Tc crystallization temperature  xxv Tg glass transition temperature Tm melting temperature tBP 4-tert-butylpyridine TD time dependent TiO2 titanium dioxide TiO2-Cl chlorine-doped compact titanium dioxide TFSI (trifluoromethane)sulfonimide TLC thin-layer chromatography TPA triphenylamine TPD N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine TPTE N4,N4'-([1,1'-biphenyl]-4,4'-diyl)bis(N4,N4'-diphenyl-N4'-(p-tolyl)-[1,1'-biphenyl]-4,4'-diamine)  UV ultraviolet UV-Vis ultraviolet-visible  V voltage V volt vac vacuum VB valence band VNPB N2,N2,N7,N7-tetrakis(4-methoxyphenyl)-N3',N6'-di(naphthalen-1-yl)-N3',N6'-bis(4-vinylphenyl)spiro[fluorene-9,9'-xanthene]-2,3',6',7-tetraamine Voc open-circuit voltage W watt X60 N2,N2,N2',N2',N7,N7,N7',N7'-octakis(4-methoxyphenyl)spiro[fluorene-9,9'-xanthene]-2,2',7,7'-tetraamine Zn(TFSI)2 zinc di[bis(trifluoromethylsulfonyl)imide] μh hole mobility  xxvi " conductivity ΔE1/2 electronic coupling ΔG Gibbs free energy change ε0 vacuum permittivity εr dielectric constant εHOMO computed vertical oxidation potential εLUMO computed vertical ionization energy λmax absorbance wavelength maximum λen emission wavelength maximum μm micrometer δ chemical shift (ppm) °C degree Celsius 3D three dimensional 2D two dimensional                  xxvii Acknowledgments I would like to thank my research supervisor, Dr. Curtis P. Berlinguette. Thank you for allowing me to pursue research avenues and collaborations that I was impassioned about and I appreciate the opportunities provided to me by the way of conferences and meetings. I would like to acknowledge NSERC Create Sustainable Synthesis and NSERC PGSD for partial funding. I would like to thank the excellent scientists I collaborated with: Dr. Daniel Tabor at Harvard University; Dr. Pascal Friedreich, Dr. Hairen Tan, Andrew Proppe, Dr. Edward Sargent and Dr. Alán Aspuru-Guzik at the University of Toronto; and Dr. Yang Cao, Dr. Carolyn Virca and David Dvorak at the University of British Columbia.  I would like to thank the entire Berlinguette group for support at group meetings and forming a community in and out of the lab. Thank you Angelica, Aiko, Rebecka, Damon, Mitch, Perry, Karen, Chris Brown and Kyle for the coffees and company throughout the rollercoaster that is graduate school. Thank you Ryan, Chris Voth and Ben Mowbray for the moving therapy on morning runs and helping me find an escape from my studies. Thank you to Kyle for countless lunches, coffees, strolls and runs and for being a patient sounding-board for all aspects of life. Thank you Carrie and Terri for providing support and love in the most motivating way. Thank you Rebecca, for being so supportive both in and out of the lab. I cannot imagine my graduate school experience without your intelligence, modesty and friendship, and nor would I ever dare to.  Thank you Alex, for being by my side through the best and worst times and building a nice life for ourselves, warm in our home.  Finally, thank you to my family for believing in me when I was unable to believe in myself. Your support means everything.  ********  xxviii Dedication To my mom, for showing me what a strong woman looks like.  1 Chapter 1: Motivation and Outline 1.1 Motivation for research Power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) have increased from 13.0% to 22.7% since 2013 (Figure 1-1).1 Despite this sharp rise in PCE, PSCs have not yet been deployed at scale.2,3 The primary reasons PSCs have not been commercialized are because of materials costs (e.g., gold electrodes are needed for state-of-the-art-devices),4,5 toxicity (e.g., lead is used in the light absorbing layer)6–8 and unsatisfactory stability (e.g., devices are notoriously unstable to moisture and air).6,9–11 The stability issues are due to the light absorbing layers being susceptible to dissolution12,13 and the hole transport material (HTM) layers undergoing morphological changes under real life conditions.14,15 With the goal of improved PSC stability in mind, the key outcomes of this dissertation are: encapsulation of the perovskite layer to prevent solvation upon moisture exposure; determination of a structure-property relationship for morphological stabilities (i.e., glass transition temperature, Tg) of HTM layers at elevated temperatures; and design of conductive HTMs without the requirement of additives.  2  Figure 1-1 Power conversion efficiencies (PCEs) of select first, second, and third generation solar cell technologies with current certified record PCEs. Figure adapted from NREL Best Research-Cell Efficiencies Graph.1  PSCs contain five layers: a perovskite layer; an electron transport layer (ETL); a conductive glass substrate; a hole transport material layer; and a metal counter-electrode (Figure 1-2a).16,17 The perovskite layer, typically a >500-nm film of inorganic-organic hybrid crystals (e.g., CH3NH3PbBr3),18,19 absorbs the visible portion of the solar spectrum to generate electron-hole pairs (Figure 1-2b). The ETL (e.g., mesoscopic or compact titanium dioxide, TiO2) extracts the photo-excited electrons from the perovskite and transports the electrons to the conductive glass substrate (e.g., indium-doped tin oxide, ITO or fluorine-doped tin oxide, FTO on glass).5,20 The HTM layer extracts holes from the perovskite layer and transports the holes to the counter-electrode, which is usually gold or silver.18,21 HTM layers are usually 100-200 nm films of redox-active organic compounds (e.g., X60 in Figure 1-2c)22,23 or extended metal oxide solids (e.g., NiOx).24  The  3 objectives of this dissertation (Figure 1-3), and the corresponding literature review in Chapter 2, are confined to organic HTMs.    Figure 1-2 (a) Schematic illustration of a conventional PSC (b) electron (e-) and hole (h+) flow through PSC components, and (c) molecular structure for organic HTM X60        4 1.2 Outline of dissertation  Figure 1-3 Outline of organic HTM molecular design for dissertation chapters  Chapter 3 describes how electrochemical and charge transport properties of HTMs can be modulated through substituent modification on the HTM molecule. How terminal substituents of HTMs affect the hole mobilities of HTMs is, quite surprisingly, not well understood. I therefore designed a series of HTMs with an X60 core and seven different substitution patterns to test how  5 steric bulk and electronic factors impact the hole mobilities. This work yielded empirical evidence showing sterically bulky, non-polar substituents increased the hole mobilities of HTMs. This chapter also includes collaborative work outlining a computational model implicating the role of low conformational and electrostatic disorder exhibited by large, non-polar substituents for high hole mobility. Chapter 4 describes how the location of the triphenylamine (TPA) groups about the X60 core impacts the thermal stabilities of HTMs.  There were few known design principles that teach how to modify spiro-based organic HTMs to modulate glass transition temperatures (Tgs) above 90 °C (this property is needed because the cell heats up during operation).25 I discovered that the Tg of an HTM is linked to the location, and not number, of TPA units appended to the spiro-carbon core.  My studies showed that where the TPA units were positioned about the spiro-carbon core could shift the Tg upwards of 30 °C.  This design principle was published in Angewandte Chemie International Edition to help the field make more thermally stable PSC devices.26 Chapter 5 describes my goal of using novel polymerizable HTMs that serve to encapsulate PSCs and protect them from moisture. Perovskite layers dissolve when exposed to moisture causing irreversible degradation of PSCs.6,12 Polymer HTMs can encapsulate the perovskite layer of PSCs, preventing water ingress into the cell.27 I installed vinyl functionalities to the spiro-carbon core that are capable of undergoing thermal and electrochemical polymerization to form a polymeric HTM layer.  This approach successfully lowered film wettability and illuminated a unique approach for making encapsulation layers.  Chapter 6 outlines my on-going exploration of designing “donor-acceptor” (D-A) HTMs with the goal of bypassing the need for dopants in the HTM films. While dopants are typically needed to get to appropriate hole conductivities, they are a source of device instability because  6 they are (i) hygroscopic and (ii) susceptible to migration during operation.28–30  I therefore designed, synthesized, and tested a series of D-A architectures in pursuit of HTMs with high hole mobilities.  From these experiments, I learned that “monopodal” D-A architectures yielded the highest hole mobilities because of the low reorganization energy, small polaron stabilization energy and hole extraction potential calculated for this HTM.  This achievement notwithstanding, PSCs exhibiting high PCEs with these candidate materials have not yet been realized.  A summary of this work and how it fits into the broader PSC community is provided in Chapter 7.  I also discuss how the field can build on these results to make stable, high efficiency PSCs.                    7 Chapter 2: Literature Review 2.1 Perovskite solar cells Perovskite solar cells (PSCs) have been certified to produce a power conversion efficiency (PCE) approaching 23% (Figure 1-1).  This value is striking in that it is approaching the performance of commercial silicon solar cells, which have been certified to exhibit a PCE of 25.3%.  PSCs, however, can be fabricated at lower temperatures (i.e., <150 °C)2,16,31 than the temperatures required for fabricating commercial silicon solar cells (i.e., >1800 °C).32,33  The challenge is that while commercial silicon solar panels can last decades, the most stable PSCs do not last longer than six weeks without encapsulation.28 Fundamentally new approaches are needed to make PSCs durable for commercially relevant time periods.  2.1.1 Components and materials  PSCs contain five layers: a perovskite layer; an electron transport layer (ETL); a conductive glass substrate; a hole transport material (HTM) layer; and a metal counter-electrode (Figure 2-1).16,17 The perovskite layer, typically a >500-nm film of inorganic-organic hybrid crystals with the general structure AMX3 (e.g., CH3NH3PbI3),18,19 absorbs the visible portion of the solar spectrum to generate electron-hole pairs. The ETL (i.e., chlorine-doped compact titanium dioxide, TiO2-Cl herein) extracts the photo-excited electrons from the perovskite and transports the electrons to the conductive glass substrate (i.e., indium-doped tin oxide, ITO in this work).5,20 The HTM layer extracts holes from the perovskite layer and transports the holes to the counter-electrode, which is a sputter-deposited gold layer in this dissertation.18,21 HTM layers are usually 100-200 nm films of redox-active organic compounds (e.g., X60 in Figure 1-2c)22,23 or extended metal oxide solids (e.g., NiOx).24  I focus on altering the molecular structure of organic HTMs related to X60 while holding  8 the perovskite material, ETL, conductive glass substrate and counter-electrode material of the PSC constant.   Figure 2-1 Energy levels (e.g., valence band (VB) and conduction band (CB) of the perovskite and TiO2-Cl, HOMO and LUMO levels of the HTM) relevant to various device performance parameters (e.g., Voc and ΔG)   During solar cell operation, photoexcitation of electrons from the valence band (VB) to conduction band (CB) of the perovskite generates electron-hole pairs (Figure 2-1).18  Electrons are then extracted from the CB of the perovskite and transported through the TiO2-Cl layer to the anode.20  Electrons generate current by traveling through the TiO2-Cl, ITO and circuit before returning back to the PSC at the gold counter-electrode. Holes extracted from the VB of the perovskite by the HTM layer are transported to the gold cathode.  The relative positions of the band edges must be positioned appropriately for these processes to occur, and without significant energy losses (see Section 2.1.2).34,35    9  2.1.2 Key Photovoltaic Metrics  The PCE is the ratio of maximum electrical power, Pmax , that can be extracted from the power of the incident solar radiation, Pin (Equation 2-1). #$% =	 ()*+(,- 	× 	100%	 = /01	×	231	×	44(,- 	× 	100%                           (Equation 2-1) Pmax is proportional to the short-circuit current (Jsc), open-circuit potential (Voc) and fill factor (FF). Jsc is the maximum current measured when no voltage bias is applied against the cell (i.e., at short circuit), and corresponds to the y-intercept of the current-voltage (J-V) curve (Figure 2-2).36  The Voc is the voltage measured when no current is passed through the device (i.e., at open circuit) and corresponds to the x-intercept of the J-V curve.37  Voc corresponds approximately to the potential energy difference between the Fermi level (EFermi) of the TiO2-Cl and the highest-occupied molecular orbital (HOMO) level of the HTM (Figure 2-1).38,39   FF is the ratio between Pmax and  Jsc × Voc (Equation 2-2) and reflects the quality of the solar cell fabrication and J-V curve. Deviation from the ideal J-V curve (FF < 1.0) is caused by shunt and series resistance in the PSC. Series resistance is the loss of energy along the favoured charge flow pathway (e.g., high hole transfer resistance to the HTM, low mobility of holes through the HTM, low conductance of electrons  through the ETL). Shunt resistance is the energy loss through parallel charge flow pathways (e.g., deleterious electron-hole pair recombination or excited-state electron transfer to the HTM).                                     55 = 	 2)*+	×	6)*+/01	×	231	 = ()*+/01	×	231	                                (Equation 2-2)  10   Figure 2-2  J-V and power density curves highlighting key performance metrics for a PSC   2.2 Perovskite layer 2.2.1 Structure and role of the perovskite layer The light absorbing perovskite layer has the structure of AMX3 where A is a large cation (e.g., Cs+, CH3NH3+ or HC(NH2)2+),19,40,41 M is a metal cation (e.g., Pb2+ or Sn2+),42,43 and X is a halide (i.e., Cl-, Br- or I-)44,45 (Figure 2-3).  AMX3 adopts an octahedral geometry repeating in all three dimensions. The perovskite formulation used in this work (i.e., Cs0.05HC(NH2)20.81CH3NH30.14PbI2.55Br0.45) was selected because these films exhibit molar extinction coefficients greater than 105 cm-1 throughout the visible spectrum (it is characterized by an abrupt onset of absorption at 700 nm).46,47   11  Figure 2-3 General perovskite (AMX3) structure with examples of A (e.g., CH3NH3+, CH(NH2)2+ and Cs+), M (e.g., Pb2+ and Sn2+) and X (i.e., Cl-, Br-, I-) identities    The perovskite structure (i.e., AMX3) can react readily with water (acting as a Lewis base) to produce a Lewis acid (e.g., CH3NH3I) and a metal halide salt (e.g., PbI2) (Equation 2-3).9 The Lewis acid can react to yield HI and CH3NH2 which is driven by dissolution in water (Equation 2-4).  The final product, CH3NH2, is volatile (i.e., boiling point of -6 °C) and susceptible to evaporation, even prior to operation.11  CH3NH3PbI3 (s) ⇄ PbI2 (s) + CH3NH3I (aq)                   (Equation 2-3)    CH3NH3I (aq) ⇄ CH3NH2 (aq) + HI (aq)                      (Equation 2-4) An alternative degradation pathway arises from the susceptibility of the 3D perovskite structure to monohydrate and dihydrate formation in the presence of water: The perovskite (e.g., CH3NH3PbI3) can react with one equivalent of water to produce a monohydrate species (e.g., CH3NH3PbI3 ᐧ H2O) or two equivalents of water to produce a dihydrate species (e.g., (CH3NH3)4PbI6 ᐧ 2H2O) (Equations 2-5 to 2-6).6 The first hydration results in the breakdown of 3D  12 perovskite into 2D perovskite sheets, while the second hydration results in the breakdown of 2D perovskite sheets into individual octahedra. The relative ratio of monohydrate to dihydrate species depends on the relative humidity of storage and the duration of moisture exposure. The degradation mechanisms of perovskites with water are irreversible and decrease the PCEs of PSCs over time.6,48          4 CH3NH3PbI3 + 4 H2O ⇄ 4 [CH3NH3PbI3 ᐧ H2O]                           (Equation 2-5)   4 CH3NH3PbI3 ᐧ H2O ⇄ [(CH3NH3)4PbI6 ᐧ 2H2O] + 3 PbI2 + 2 H2O           (Equation 2-6) 2.3 Hole transport material layer 2.3.1 Roles and properties of hole transport materials The HTM serves two key functions: hole extraction from the perovskite layer and hole transport to the gold cathode.  The HTM must therefore be designed with appropriate energy levels, high conductivity, and high hole mobility, while being physically robust against elevated temperatures of a solar cell.  The ideal HTM should therefore satisfy the following criteria: ○ HOMO energy level positioned 70 mV more positive than the perovskite valance band;34,35 ○ Conductivity greater than 10-5 S/cm;49,50 ○ Hole mobility greater than 10-4 cm2/Vs;50 ○ Glass transition temperature, Tg, greater than 90 °C.25  It is difficult to modulate any one of these properties through structural modifications without impacting the others. Addressing this dynamic situation through rational molecular design is an overarching theme of this entire dissertation.  While HTMs can be broken down into inorganic (e.g., CuSCN, CuBr and NiOx)24,51–54 and organic materials, this dissertation will be limited to the design and study of exclusively organic  13 HTMs.  The primary reason for the focus on organic HTMs is that the structures can be more easily modulated through molecular design, enabling direct tuning of the four key properties listed above. Organic HTMs can be further divided into (i) linear, (ii) starburst or (iii) spiro architectures50,55 (Figure 2-4), with each class defined by different HOMO energy levels, hole mobilities, conductivities and glass transition temperatures.   Linear HTMs were originally developed for organic light emitting diodes (OLEDs) decades before their use in PSCs.14,56–59  Linear HTMs are molecules of repeating monomer units, ranging in size from small molecules (e.g., one unit) to polymers (e.g., >20 units).  The HOMO levels and hole mobilities of linear HTMs are dependent on oligomer substituents and length, respectively.  Triphenylamine (TPA) units are common monomers in linear HTMs because of reversible, facile redox chemistry and established synthetic chemistry (e.g., synthesized using Ullmann coupling or Buchwald-Hartwig coupling).  The redox chemistry of these HTMs is exquisitely sensitive to substitution at the TPA unit.  For example, the destabilized EHOMO of MeO-TPD (i.e., -4.3 eV) relative to the EHOMO of  TPD (i.e., -5.3 eV) is due to the electron-donating strength of methoxy substituents stabilizing the oxidized form of the compound.60 The higher EHOMO of MeO-TPD results in a greater Gibbs free energy for hole extraction but a lower Voc and, hence, PCE (Equation 2-1).  The hole mobilities increase with increasing number of TPA units, which was demonstrated clearly by Xu et al. (e.g., 6.19 × 10-5, 9.82 × 10-5 and 1.47 × 10-4,cm2/Vs for MeO-TPD, MeO-TPTR, and MeO-TPTE, respectively, in Figure 2-4.61  Finally, the cost to purchase commercially available HTMs also increases with linear HTM length (e.g., TPA, TPD, TPTE and PTAA which range from 3 to 2640 $/g; see Figure 2-5).  In other words, high hole mobility comes at a financial cost.62   14   Figure 2-4 Examples of linear, starburst and spiro organic HTMs              15 Table 2-1 Electrochemical and electron transport properties, and glass transition temperatures of organic HTMs  Compound EHOMO hole mobility conductivity Tg eV cm2 V-1 s-1 S cm-1 °C TPD -5.3 - - 60 MeO-TPD -4.3 6.19 × 10-5 4.95 × 10-7 a 71 MeO-TPTR -4.33 9.82 × 10-5 2.75 × 10-7 a 110 MeO-TPTE -4.33 1.47 × 10-5 6.98 × 10-7 a 137 MeOTPA-TPA -5.13 1.08 × 10-4 - - spiro-TPD -5.3 - - 115 spiro-OMeTAD -5.11 2.00 × 10-4  4.61 × 10-6 b 128 X60 -5.18 1.9 × 10-4 1.1 × 10-4 c 107 Measured with a  3% FK209 b 20% LiTFSI c 20% LiTFSI and 3% FK209    Figure 2-5 MilliporeSigma pricing per gram of HTM   16  Starburst structures, described as non-linear oligomers, typically have higher hole mobilities than analogous isomers with linear architectures. Ko et al. demonstrated this trend by designing starburst MeOTPA-TPA with a hole mobility of 1.08 × 10-4 cm2/Vs, while linear isomer MeO-TPTE exhibits a hole mobility of 1.54 × 10-5 cm2/Vs.63 When comparing these linear and starburst HTMs, it is also evident that the EHOMO is stabilized by 0.8 eV in the starburst configuration, which is favourable for increasing the Voc and PCE when employed in a PSC device. Sterically bulky spiro-centres increase the Tg of organic HTMs relative to homologous linear HTMs.64,65  Spiro HTMs are characterized by two 8-systems juxtaposed across a tetrahedral sp3-hybridized atom (e.g., carbon, silicon).  Naito and Miura demonstrated that steric bulk across a strained linkage led to more thermally stable films. Thermal stability tracks the Tg of the material.66,67 The Tg of linear TPD vs spiro-TPD is 60 °C vs 115 °C, respectively, and the Tg of linear MeO-TPD vs spiro-OMeTAD is 71 °C and 128 °C, respectively.56 Solar cells are known to reach temperatures approaching 90 °C  during operation, caused by solar exposure and thermal losses in the cell. The spiro-carbon core in both cases raises the Tg of the material above the 90 °C threshold necessary to avoid morphological changes during the operation of a PSC. Spiro-centres also increase the conductivity of organic HTMs relative to homologous linear HTMs. The conductivity of spiro-OMeTAD is approximately an order of magnitude higher than linear MeO-TPD (4.61 × 10-6 S/cm vs 4.95 × 10-7 S/cm for spiro-OMeTAD and MeO-TPD, respectively).60,61 Spiro-OMeTAD is a state-of-the-art HTM containing four redox active TPA groups substituting a spirobifluorene core.68 Spiro-OMeTAD is difficult to synthesize because it requires: an air-sensitive Suzuki coupling; a moisture-sensitive lithiation reaction or Grignard generation; a bromination reaction; an air-sensitive Buchwald-Hartwig coupling; and chromatographic purification at most steps (Scheme 2-1a).69–71  X60, however, can be synthesized in fewer steps and  17 was reported in 2016 by Xu et al. and Maciejczyk et al. to generate similar PCEs to spiro-OMeTAD in a PSC.22,23  X60 is an HTM that can be synthesized by a condensation reaction with >95% yield without a column, followed by a Buchwald-Hartwig coupling.  X60 rivals spiro-OMeTAD in performance (i.e., both >20% PCE in a PSC) but can be synthesized at a fifth of the cost.22 Moreover, the more accessible synthetic route for X60 affords the opportunity to synthesize series of compounds to study structure-property relationships of HTMs without undertaking laborious syntheses.  X60 motivated the design of many HTMs studied in this dissertation (Figure 1-3).           18  Scheme 2-1 Synthetic schemes for (a) spiro-OMeTAD and (b) X60   19 2.3.2 Instability of hole transport material layer In the HTM field, Tg is the temperature at which materials undergo a transition from the glassy, amorphous state, in which they exist following spin-coating at room temperature, to another morphology.59,72  Glass transition is an endothermic process (the absorption of heat by the material from the surroundings) that can be measured by a differential scanning calorimeter (DSC) as the point at which more heat flow is required to increase the temperature of bulk material at a sustained rate. The crystallization temperature, Tc, is the temperature at which an HTM film transitions from an amorphous phase to a crystalline phase. This phase change is exothermic and can be measured by a DSC as a decrease in the heat flow required to increase the temperature of a bulk material at a sustained rate.73 Not all materials display a crystallization temperature if the regular, ordered stacking of molecules is not energetically favourable.  Morphological changes, pinholes and phase segregation of the HTM layer in a PSC can result from HTMs with low Tg values (Figure 2-6).  Morphological changes to HTM films from the glassy, amorphous state occur above a material’s Tg and can lead to small polycrystallite formation, decreased mechanical strength and phase segregation from dopants and additives. Grain boundaries between crystallites can trap charge carriers and lower the charge-transport properties of an HTM film.74 Films may exhibit decreased mechanical strength which may result in defects in the form of pinholes at elevated temperatures.75  Morphology changes can also result in phase segregation of the HTM from the added dopants which causes inconsistencies in film composition and conductivity.76  20  Figure 2-6 Morphological changes to the HTM layer above the Tg   The conductivity (!) of a p-type semiconductor (e.g., an HTM film in a PSC) is proportional to the density of mobile holes (nh), mobility of holes ("h) and elementary charge (e):     !h = nhe"h            (Equation 2-7)  Most HTM films are not intrinsically conductive, and hence, ancillary oxidants (i.e., dopants in Figure 2-7) are frequently added to induce chemical oxidation, increase the density of mobile holes, and maximize conductivity.77,78  Dopants either react directly with the HTM in a one or two electron redox reaction (this includes FK209 or Ag(TFSI)2, respectively) or indirectly with oxygen (in the case of LiTFSI) to yield oxidized equivalents of the HTM material (e.g., HTM+).79,80  21   Figure 2-7 Chemical structures of p-dopants (LiTFSI, Zn(TFSI)2, FK209 and F4-TCNQ).  LiTFSI is the most commonly used dopant in the PSC field (Figure 2-7). A major challenge arises when employing LiTFSI because Li+ is hygroscopic and can result in the ingress of water into the cell via the HTM layer.30  Li+ is also a small cation and can migrate toward the anode during cell operation.81  This ion migration results in inconsistencies in HTM film doping and conductivity which are diagnosed by the hysteresis of forward and backward J-V scans of the PSC.82,83  Alternative dopants to LiTFSI include other TFSI salts (e.g., FK209 and Zn(TFSI)2 mentioned above) and organic molecular dopants (e.g., F4-TCNQ in Figure 2-7).28,29,84,85  Seo and coworkers demonstrated that PSCs doped with exclusively Zn(TFSI)2 maintained PCEs above 20% following 600 h exposure at 50 °C and 40% relative humidity.28  2.4 Solutions to perovskite solar cell instability using hole transport materials 2.4.1 Polymer hole transport materials for improved moisture stability of perovskite layer  There is little data available on modifying the HTM layer to improve PSC stability.  In 2017, Kelly et al. investigated the use of P3HT nanowires in PMMA matrices as the HTM layers of a PSC.12 The P3HT nanowires performed the role of an HTM and the PMMA matrix prevented the entry of water into the PSC. Liquid-phase water stability tests revealed that devices with 80:20 NS S CF3F3COOOOTFSI-Li+ TFSI-CoNNNNNNNNNCNNCNC CNFFFFF4-TCNQZn2+ 2 TFSI-3+3 TFSI-FK209 22 PMMA/P3HT compositions completely retained performance metrics (i.e., PCE, Jsc, Voc and FF) following 60 s of exposure to water droplets on the surfaces of the devices.  Vapour-phase water stability tests, however, revealed that the PCE dropped to 0% over seven days in a 99% humid environment regardless of the PMMA concentration. This decline in PCE implicates the importance of vapour-phase stability studies, in addition to studying the liquid-phase stability, to understand the durability of PSCs during moisture exposure.  Figure 2-8 Examples of polymer HTMs employed for improved PSC moisture stability  Sargent and co-workers hypothesized that VNPB polymers could protect the perovskite layer of a PSC from the entry of water and suppress subsequent moisture degradation.27 VNPB was thermally polymerized on the perovskite layer of a PSC to yield devices with 16.5% PCE. X-ray diffraction (XRD) was used to quantify degradation of the perovskite by measuring the amount of PbI2 byproduct formed relative to the intact CH3NH3PbI3 perovskite (Equation 2-3). XRD revealed that devices covered by VNPB degraded less than devices without the crosslinked HTM layer. This study demonstrates the potential for polymerizing molecular HTMs in situ to improve the stability of the perovskite layer against water ingress into PSCs. The VNPB structure inspired the novel spiro-centred HTMs developed in Chapter 5.  N NOOnSnPMMA P3HT VNPB 23 2.4.2 High glass transition temperature hole transport materials for improved thermal stability of perovskite solar cells  Increasing Tg through steric bulk (e.g., substituent changes and oligomer length) improves morphological stability and this incorporation of steric bulk is the inspiration for research outlined in Chapter 4. The use of bulky phenyl groups to increase Tg was demonstrated in 1996 by Sun et al. for a series of oligomeric HTMs: MeO-TPD; MeO-TPTR; and MeO-TPTE (Figure 2-4 and Table 2-1).86,87 The phenyl moieties of a TPA adopt pinwheel orientations that inhibit regular stacking of HTMs and increase the stabilities of amorphous films. Smaller substituents (e.g., halogens or alkyl groups) also increase the Tg of HTMs relative to otherwise homologous materials.64 Naito demonstrated that increasing the size of halogen substituents results in steric disruptions to regular packing and increases the Tg of an HTM. Incorporation of electron-withdrawing halogens, however, significantly lowers the HOMO level for the HTM and decreases the free energy change necessary for hole extraction from the perovskite.  Chapter 4 reveals design principles for improving the Tg values of spiro-centred HTMs more than 30 °C with judicious placement of bulky TPA units, without impacting the HOMO level of HTMs.  2.4.3 Donor-acceptor hole transport materials for dopant-free perovskite solar cells It has been recognized that HTMs can have high conductivities without the use of dopants,88 offering an opportunity to overcome the hygroscopic nature and diffusion of LiTFSI.89  D-A materials have demonstrated >15% PCE in PSCs.88-90 Dopant-free HTMs typically are designed with large planar moieties to induce stacking,90 and/or integration of donor-acceptor (D-A) structures.91  Large planar moieties improve stacking between HTM molecules and decrease the distance required for holes to be transported.88,92  Bi et al. has argued that the strong light  24 absorption properties characteristic to D-A HTMs could be beneficial in creating dual-light absorbing systems within the PSC.93 This claim is in disagreement with convention where the HTM is designed to not absorb light.   Figure 2-9  Common donor, pi and acceptor units employed for D-A HTMs    The molecular structure of D-A HTMs in the literature varies significantly in architecture (e.g., the number and identity of donor and acceptor groups) and organic class (e.g., linear or starburst). Yang et al. examined the effect of donor group identity between two monopodal D-A HTMs and determined that HTMs with benzo[1,2-b:4,5-b′]dithiophene donor units yielded higher mobility films than a fluorinated alternative (i.e., 5,6-difluoro-2,1,3-benzothiadiazole). 94 Steck and coworkers compared thiophene and EDOT π-bridges in bipodal (A-π-D-π-A) HTMs and found that thiophene linkages yielded HTMs with minimal visible light absorption, often considered favourable for use in PSCs.95 The effect of D-A architectures on the conductivities and hole mobilities of HTM films have not been previously investigated. In Chapter 6, three architectures  25 of D-A HTMs (i.e., monopodal, bipodal and tripodal structures) are investigated using computational and experimental techniques to design stable, dopant-free HTMs.   The HTM layer can be designed to overcome the unsatisfactory stability of PSCs in moisture and heat. HTMs can be designed to: encapsulate the perovskite layer to prevent solvation upon moisture exposure; suppress morphological changes at elevated cell temperatures; and eliminate hygroscopic and migrating additives in the device. Preventing these three degradation pathways for PSCs could be vital for guaranteeing stable PCEs for PSC commercialization.                         26 Chapter 3 : Tuning the Electrochemical and Electronic Properties of Spiro-Centred Hole Transport Materials through Substituent Identity 3.1 Introduction Spiro-OMeTAD has prevailed as a state-of-the-art hole transport material (HTM) since its discovery by Bach et al. in 1998.68  The rigidity imparted by the spiro-carbon core causes an increase in glass transition temperature (Tg) relative to homologous linear HTMs.64,65  The spiro-carbon core therefore prevents morphological changes to the HTM during perovskite solar cell (PSC) operation. The four triphenylamine (TPA) units appended to the spiro-carbon core are incorporated to achieve reversible redox chemistry.96  X60 was recently reported to be a viable alternative to spiro-OMeTAD (Figure 3-1).22,23 X60 also contains four TPA units appending a strained spiro-carbon core but the asymmetric spiro[fluorene-9,9′-xanthene] core of X60 can be synthesized in two steps - and at a fifth of the cost of the symmetric spirobifluorene core of spiro-OMeTAD (Scheme 2-1).  X60 reliably yields power conversion efficiencies (PCEs) greater than 20% in PSCs, values that are commensurate to those with spiro-OMeTAD.26  Figure 3-1  Molecular representations of the spiro-R series under investigation.  27  The effect of substituents on the charge transport properties of spiro-based HTMs remains poorly understood.  Spiro-OMeTAD contains methoxy substituents, which were originally incorporated by Bach et al. to ensure an appropriate “matching of energy levels” between the valence band of the light absorbing layer (originally a ruthenium dye) to the highest occupied molecular orbital (HOMO) of the HTM.68 Polander et al. demonstrated that the HOMO of the HTM must be at least 70 mV less positive than the valence band of a PSC for efficient hole extraction.34,35  The HOMO level (EHOMO) of an HTM can be modulated through substituent identity and location, but how these substituents affect the hole mobilities of HTMs is not clearly defined. There is a pressing need to develop design rules for high mobility HTMs, while also retaining control over the EHOMO. The series of seven HTMs (spiro-R, Figure 3-1) was selected to contain modifications to the peripheral substituents appending the spiro[fluorene-9,9′-xanthene] core of X60 to determine the role of substituent identity on hole mobility. We selected the series of seven analogous compounds from a computational screen of more than 25 compounds on the basis of appropriate HOMO level for use in a PSC.  We systematically investigated the electrochemical, photophysical and charge transport properties of the series and compared HTM characteristics to those of spiro-OMeTAD. In parallel, we conducted a state-of-the-art multiscale simulation of the spiro-R hole mobilities and elucidated the role of substituents on hole mobility. Our computational model implicated the importance of low conformational and electrostatic disorder, common to large, non-polar substituents, for high hole mobility.    28 3.2 Results and discussion 3.2.1 Computational screening Computational screening by Dr. Daniel Tabor of the Aspuru-Guzik group at the University of Toronto was used to select HTM substitution patterns on the basis of appropriate HOMO energy levels for hole extraction from the perovskite layer.  A series of 25 molecules containing the spiro[fluorene-9,9′-xanthene] core was modelled by generating up to 20 conformers of each compound using B3LYP/6-31+G level of theory.  The reduction potentials (#HOMO in Table 3-1) were calculated as an approximation of the HOMO energy levels (EHOMO) of the HTMs.  The valence band of the perovskite Cs0.05FA0.81MA0.14PbI2.55Br0.45 used in our PSC studies is determined to be -5.82 eV in the solid state,20 and minimum free energy change of 0.07 eV is necessary for hole extraction from the valence band of the perovskite to the HOMO of the HTM.34,35   Known compound X60 deviated from experimental values by approximately 0.6 eV which is estimated to be a correction necessary going from a gaseous computation to experimental results. On this basis, HTMs with an εHOMO lower than -5.15 eV were anticipated to exhibited negligible hole extraction. Seven candidate materials were identified on this basis of εHOMO; they are shown in Figure 3-1.  Table 3-1 Summary of computed reduction potentials and HOMO-LUMO energy gaps for the spiro-R series compound εHOMO (eV) εHOMO-LUMO (eV) X60 -4.49 3.66 spiro-p,o-OMe -4.82 3.77 spiro-Me -4.88 3.79 spiro-SMe -5.09 3.60 spiro-FOMe -4.96 3.72 spiro-H -5.03 3.82 spiro-F -5.37 3.78  29 3.2.2 Synthesis The seven compounds were synthesized by coupling a one-pot condensation product with four equivalents of a secondary amine. The synthesis of the seven candidate HTM compounds followed a modified literature procedure where 2,7-dibromofluorenone was reacted with 4-bromophenol to yield a tetra-brominated product (4Br). 4Br was isolated in >95% yield by a precipitation in methanol before undergoing four one-pot Buchwald-Hartwig couplings with four equivalents of secondary amine (BPA-R) (Scheme 3-1). BPA-R compounds were either purchased (in the case of BPA-OMe, -Me and -H) or synthesized (in the case of BPA-o,p-OMe, -SMe, -FOMe and -F).  All intermediate and final compounds were characterized by 1H nuclear magnetic resonance (NMR) spectroscopy, 13C NMR, low and high resolution mass spectrometry.  Scheme 3-1 Synthetic scheme for spiro-R series   3.2.3 Electrochemical and photophysical characterization Cyclic voltammograms (CVs) were recorded for all HTMs in 0.1 M n-NBu4PF6 DCM solutions to experimentally determine the EHOMO  values for the series (Figure 3-2 and Table 3-2). CVs showed four peaks in current representing four electron transfer events for each compound, which matches the four TPA units present on each molecule.  The EHOMO of these materials occurred between 0.67 to 0.96 V vs NHE, with a clear trend that EHOMO  increases with increasing  30 electron-withdrawing character of substituents. The trend observed for the experimentally determined EHOMO is fully supported by the εHOMO determined from density functional theory (DFT) calculations. The methoxy substituents on X60 and spiro-p,o-OMe contributed to the lowest  EHOMO  values measured for the series (0.67 and 0.69 vs NHE, respectively) because these substituents donate electron density toward the amine nitrogens and stabilize oxidized HTM molecules.  Fluorine substituents were the most electron-withdrawing of the series owing to the high electronegativity of fluorine and, hence, spiro-F exhibited the highest EHOMO measured at 0.96 V vs NHE.   Figure 3-2 Cyclic voltammograms for spiro-R series recorded in 0.1 M n-NBu4PF6 DCM solutions at room temperature. Data were collected at a scan rate of 50 mV/s. Dashed lines indicate EHOMO values.      31 Table 3-2 Summary of photophysical, electrochemical and bulk thermal properties for spiro-R series compound $maxa (nm) $emb (nm) EHOMO-LUMOc (eV) EHOMOd (V vs NHE) ELUMOe (V vs NHE) Tgf (°C) Tmf (°C) X60 306, 365 429 2.97 0.67 -2.30 107 >300 spiro-p,o-OMe 313, 354 413 3.07 0.69 -2.38 101 >300 spiro-Me 307, 388 414 3.02 0.77 -2.25 127 >300 spiro-SMe 327, 395 427 2.97 0.84 -2.13 125 270 spiro-FOMe 303, 381 416 3.02 0.84 -2.17 96 >300 spiro-H 306, 381 405 3.08 0.90 -2.18 98 >300 spiro-F 300, 376 405 3.06 0.96 -2.10 114 >300 a Absorption maximum for HTM in DCM. b Excitation at higher energy $max. c Determined from intersection of absorption and emission spectra. d The half-wave potential from the first oxidation of cyclic voltammograms of HTMs in DCM referenced to NHE with the addition of Fc/Fc+. e  The LUMO determined from the addition of EHOMO-LUMO to EHOMO.  f Determined by differential scanning calorimetry (DSC).    All seven HTMs exhibited similar absorption profiles with maximum absorption peaks (%max) between 300-327 nm and notable lower energy shoulders at 354-395 nm (Figure 3-3a and Table 3-2).  The absorption bands in the UV region make these HTMs good candidates for either normal or inverted PSC devices as the HTMs would not compete with the photoactive layer for visible light absorption from solar radiation, regardless of the side of the device being illuminated.  The emission data (Figure 3-3b) were collected to determine the bandgap (EHOMO-LUMO) of each spiro-R compound from the intersection of the absorption and emission spectra.  ELUMO  can be determined by adding the spectroscopically derived bandgap EHOMO-LUMO and the electrochemically derived EHOMO. An ELUMO  smaller in magnitude than -2.00 V vs NHE is necessary to ensure that there is no deleterious flow of photoexcited electrons from the conduction band of the perovskite to the LUMO of the HTM and instead, only productive electron flow from the conduction band of the perovskite to the conduction band of the TiO2-Cl.97 All experimental ELUMO values were smaller  32 in magnitude than -2.10 V vs NHE and, therefore, electron transfer to the HTM will be suppressed (Table 3-2).  Figure 3-3  (a) UV-Vis absorption spectra and (b) emission spectra for spiro-R series dissolved in DCM  3.2.4 Bulk thermal properties The seven compounds exhibited glass transition and melting temperatures above the maximum operating temperature of a PSC (i.e., 90 °C).50  The glass transition temperature (Tg) and melting temperature (Tm) were determined using differential scanning calorimetry (DSC) for each material in the series (Table 3-2).  The Tg values for all seven HTMs were measured between 96-127 °C. The Tm values were all above 270 °C for the spiro-R series. An HTM should exhibit a Tg and Tm greater than 90 °C to avoid morphological and phase changes to the glassy, amorphous HTM film as the temperature of a PSC increases during operation due to solar irradiation and thermal losses in the cell. All seven HTMs displayed suitable bulk thermal properties for PSC application and are not expected to undergo morphological or phase changes during operation.    33 3.2.5 Electrical properties Conductivity measurements were performed for each of the spiro-R films with 20 mol% LiTFSI dopant and 250 mol% tBP additive.  LiTFSI is the most commonly used dopant in the PSC field and is employed to increase the number of charge carriers and conductivity of an HTM film.79,80  tBP was added to all films to increase film uniformities by suppressing phase segregation between dopant molecules and HTMs during spin coating.76  Four parallel gold electrodes were deposited uniformly on each of the spin-coated films and a current was applied from 2 × 10-9 to 1 × 10-7 A through the two outer contacts.  The responding voltage was measured between the two inner contacts.  The conductivity values tabulated in Table 3-3 were calculated from the slope of the current vs. voltage plots, and the film thicknesses were determined by optical profilometry.  The three highest conductivities of 5.8 × 10-5, 6.9 × 10-5 and 2.5 × 10-4 S/cm were measured for X60, spiro-SMe and -Me, respectively which are the HTMs containing the largest substituents of the series.  The asymmetrically substituted spiro-FOMe exhibited the lowest conductivity (1.9 × 10-6 S/cm). Spiro-F, containing polar fluorine substituents, exhibited the second lowest conductivity of the series (i.e., 1.6 × 10-5 S/cm).                   34 Table 3-3 Conductivity data for spiro-OMeTAD and spiro-R series  compound conductivity (S/cm) spiro-OMeTAD 1.2 ± 0.2 × 10-4 X60 5.8 ± 0.1 × 10-5 spiro-p,o-OMe 3.7 ± 0.7 × 10-5 spiro-Me 2.5 ± 0.6 × 10-4 spiro-SMe 6.9 × 10-5 spiro-FOMe 1.9 × 10-6 spiro-H 2.8 × 10-5 ± 2.8 × 10-6 spiro-F 1.6 × 10-5 Conductivity measured for films composed of HTM, 20 mol% LiTFSI and 250 mol% tBP  3.2.6 Bulk Simulation The hole mobilities of the spiro-R series were calculated from experimentally determined conductivities by assuming a constant charge carrier density for all HTMs. The agreement between experimentally determined hole mobilities and computationally derived values (Figure 3-4a) lends credence to us gleaning design principles from our mobility model.  The hole mobilities were computed as a product of the electronic coupling, reorganization energy, conformational disorder and electrostatic disorder. Conformational disorder is due to geometrical differences between the molecules in the amorphous structure. Electrostatic disorder is due to the electrostatic interaction between molecules with their amorphous environment. The contributions by conformational disorder and electrostatic disorder appear to vary most significantly between compounds in the series and impact the mobilities the greatest of the four factors.   Conformational disorder was minimized by increasing the steric bulk of substituents which implicated the importance of large substituents (e.g., methyl, methoxy and thiomethyl groups) on HTM hole mobility. The  35 electrostatic disorder was impacted by the polarities of the substituents where dense, polar and asymmetric substituents negatively impacted the computed electrostatic disorder and hole mobility values. In particular, symmetric substitution of large, non-polar moieties (e.g., methyl groups in the case of spiro-Me) can effectively lower both conformational disorder and electrostatic disorder to yield higher hole mobilities than smaller, polar substituents.     Figure 3-4 (a) Simulated (grey) and experimental (red) hole mobilities of spiro-OMeTAD and spiro-R series, (b) computed hole mobilities of the spiro-R series with the contributions by electronic coupling, reorganization energy, conformational disorder and electrostatic disorder.    3.2.7 Perovskite solar cell characterization PSC studies were carried out by Dr. Hairen Tan of the Sargent group at the University of Toronto. Each of the HTMs was then tested in a PSC with the device architecture: glass/ITO/TiO2-Cl/perovskite/HTM/Au. All HTMs were doped with 20 mol% LiTFSI to increase the number of charge carriers and the conductivities of the films. Table 3-4 summarizes the performance of PSCs fabricated with identical methods for each HTM. X60 demonstrated a PCE of 20.9% in a PSC which was commensurate with state-of-the-art HTM spiro-OMeTAD (PCE of 21.2%).  The six other HTMs yielded PCEs of 3.5% or below. We had anticipated that the Voc values of PSCs would  36 increase with increasing electron-withdrawing strength of substituents and stabilizing of HTM EHOMO but this trend was not observed. We hypothesized that the poor conductivities and hole mobilities of the majority of the materials (e.g., spiro-FOMe and -F) resulted in the poor overall device metrics and prevented Voc values from reflecting the energy difference between the EFermi of the TiO2-Cl and the EHOMO of the HTM.  Table 3-4 Perovskite solar cell data for spiro-OMeTAD and spiro-R series  compound Voc (V) Jsc (mA/cm2) FF PCE (%) Spiro-OMeTAD 1.12 23.4 0.80 21.2 X60 1.12 23.4 0.79 20.9 spiro-p,o-OMe 1.03 13.5 0.75 3.5 spiro-Me 0.68 0.16 0.34 0.04 spiro-SMe 0.92 1.25 0.34 0.39 spiro-FOMe 0.77 0.04 0.20 0.01 spiro-H 0.75 0.3 0.14 0.04  PSCs prepared with Cs0.05FA0.81MA0.14PbI2.55Br0.45 perovskite mixture and 20 mol% LiTFSI dopant/250 mol% tBP additive in HTM layer   3.3 Conclusions Relatively bulky, non-polar substituents (e.g. methyl substituents) yield high hole mobility materials. We were able to draw this conclusion by screening a library of 25 HTM compounds which differed in both substituent identity and position about the core.  We selected seven candidate materials on the basis of computed εHOMO values.  We studied the effect of both substituent identity and location on the electrochemical, photophysical and charge transport properties of seven candidate HTMs. Owing to the agreement between experimental and simulated hole mobility data, we were able to extract design principles for high hole mobility HTMs from  37 our model.  Sterically bulky, non-polar substituents are beneficial for decreasing the conformational and electrostatic disorder to yield high hole mobility materials.   3.4 Experimental 3.4.1 Computational screening Cores were assembled from spiro[fluorene-9,9′-xanthene] moieties. To each of these cores, there were various modifications in the positions and the functional groups attached to the triphenylamine (TPA) units and the TPA substitution positions themselves. Each molecule was then subject to the same computational pipeline. Conformers were generated using the RDKit package. These conformers were optimized at the B3LYP/def2-SV(P) level of theory and the frontier molecular orbital (i.e., HOMO and LUMO) energies obtain were obtained. The Gaussian0930 package was used for these electronic structure calculations. 3.4.2 Synthesis  All reagents were obtained from MilliporeSigma, Alfa Aesar or Fisher Scientific and used as received. All reactions were performed using toluene that was passed through a solvent purification system prior to use. Standard inert atmosphere and Schlenk techniques were carried out under nitrogen.  All reactions and purifications were carried out in the dark. Purification by column chromatography was performed using silica (Silicycle: Ultrapure Flash Silica).  Analytical thin-layer chromatography (TLC) was performed on aluminum-backed sheets pre-coated with silica 60 F-254 adsorbent (250 μm thick; Silicycle, QC, Canada) and visualized under UV light.  Routine 1H and 13C NMR spectra were collected on a Bruker AV300 or Bruker AV400 inv/dir instrument at ambient temperatures, operating at 400 MHz and 100 MHz, respectively.   Chemical  38 shifts (δ) are reported in parts per million (ppm) using the residual signals δ 7.26 and 77.0 for CDCl3, δ 2.05 and 29.8 for acetone-d6 and δ 2.50 and 39.4 for DMSO-d6 as internal references for 1H and 13C, respectively.  4Br (2,2',7,7'-tetrabromospiro[fluorene-9,9'-xanthene]):  2,7-dibromo-9-flourenone (1.08 g, 3.20 mmol), 4-bromophenol (5.53 g, 32.0 mmol) and methane sulfonic acid (0.82 mL, 12.8 mmol) were mixed at 150 °C for 18 h under N2. The reaction mixture was cooled to room temperature and product was precipitated with the addition of MeOH. Separation by filtration and rinsing with MeOH yielded 2.09 g (96.5%) of product as white powder. 1H NMR (300 MHz, Acetone-d6) δ = 8.01 (d, J = 8.1 Hz, 1 H), 7.69 (dd, J = 8.2, 1.8 Hz, 1H), 7.49 (dd, J = 8.8, 2.4 Hz, 1H), 7.44 (d, J = 1.7 Hz, 1 H), 7.31 (d, J = 8.8 Hz, 1H), 6.53 (d, J = 2.4 Hz, 1H). 13C NMR (100 MHz, Acetone-d6): δ = 155.5, 150.0, 137.6, 132.2, 130.4, 129.0, 124.8, 122.9, 122.0, 119.2, 116.2 ppm. HRMS (EI) m/z: 643.76141 [(M+)] calcd for C25H12O79Br4 m/z: 643.76216. BPA-p,o-OMe (2-Methoxy-N-(4-methoxyphenyl)aniline): 2-iodoanisole (1.00 g, 4.27 mmol), p-anisidine (0.53 g, 4.27 mmol), palladium acetate (0.061 g, 0.30 mmol), tri-tert-butylphosphine (0.110 g, 0.491 mmol) and potassium tert-butoxide (1.03 g, 10.7 mmol) were added to toluene (15 ml) under N2. The reaction mixture was stirred at reflux for 16 h. The reaction mixture was cooled to room temperature. Purification by silica column chromatography (SiO2: hexanes/EtOAc, 8:1) yielded 0.807 g (81%) of pale yellow solid product. 1H NMR (400 MHz, CDCl3) δ = 7.12, ( (d, J = 8.8 Hz, 1 H), 7.04 (dd, J = 7.7, 1.7 Hz, 1H), 7.05-6.77 (m, 6H), 5.97 (s, 1H). 13C NMR (100 MHz, Acetone-d6): δ = 155.8, 147.8, 135.9, 135.5, 123.2, 121.4, 119.0, 115.1, 113.1, 110.7, 56.0 ppm. HRMS (EI) m/z: 230.1190 [(M+)] calcd for C14H16NO2  m/z: 230.1181. BPA-SMe (Bis(4-(thiomethyl)phenyl)amine): (4-bromophenyl)methylsulfane (2.03 g, 10.0 mmol), 4-(methylthioaniline) (1.39 g, 10.0 mmol), palladium acetate (0.112 g, 0.50 mmol), tri- 39 tert-butylphosphine (0.100 g, 0.5 mmol) and potassium tert-butoxide (1.68 g, 15.0 mmol) were added to toluene (15 mL) under N2. The reaction mixture was stirred at reflux for 16 h. The reaction mixture was cooled to room temperature. Purification by silica column chromatography (SiO2: hexanes/EtOAc, 10:1) yielded 1.55 g (44%) of yellow/brown solid product. 1H NMR (400 MHz, CDCl3) δ = 7.20 (d, J = 8.6 Hz, 4H), 6.94 (d, J = 8.6 Hz, 4H), 5.62 (br, 1H), 2.42 (s, 6H). 13C NMR (101 MHz, CDCl3) δ = 141.3, 129.9, 129.3, 118.6, 77.5, 77.2, 76.8, 17.9. HRMS (ESI) m/z: 262.0721  [(M+)] calcd for C14H16NS2  m/z: 262.0724. BPA-FOMe (4-Fluoro-N-(4-methoxyphenyl)aniline): 4-bromoanisole (2.81 g, 15.0 mmol), 4-fluoroaniline (1.67 g, 15.0 mmol), palladium acetate (0.168 g, 0.75 mmol), tri-tert-butylphosphine (0.151 g, 0.75 mmol) and potassium tert-butoxide (2.53 g, 22.5 mmol) were added to toluene (15 ml) under N2. The reaction mixture was stirred at reflux for 16 h. The reaction mixture was cooled to room temperature. Purification by silica column chromatography (SiO2: hexanes/EtOAc, 10:1) yielded 1.60 g (49.1%) of pale yellow waxy solid product. BPA-F (bis(4-fluorophenyl)amine): 4-bromo-fluorobenzene (3.15 g, 18 mmol), 4-fluoroaniline (2.00 g, 18 mmol), palladium acetate (0.29 g, 1.4 mmol),  tri-tert-butylphosphine (0.32 g, 1.4 mmol) and potassium tert-butoxide (2.00 g, 90 mmol) were added to toluene (15 ml) under N2. The reaction mixture was stirred at reflux for 16 h. The reaction mixture was cooled to room temperature. Purification by silica column chromatography (SiO2: hexanes/EtOAc, 5:1) yielded 2.29 g (62%) of dark brown oil product.  1H NMR (400 MHz, CDCl3) δ = 6.97, ( (d, J = 1.1 Hz, 8 H), 5.46 (s, 1H). 13C NMR (100 MHz, Acetone-d6): δ = 159.1, 156.7, 140.0, 119.6, 119.5, 116.2, 116.0 ppm. HRMS (EI) m/z: 205.07029 [(M+)] calcd for C12H9NF2  m/z: 205.07031. X60 (N2,N2,N2',N2',N7,N7,N7',N7'-octakis(4-methoxyphenyl)spiro[fluorene-9,9'-xanthene]-2,2',7,7'-tetraamine): bis(4-methoxyphenyl)amine (1.14 g, 5.00 mmol), 4Br (0.678 g, 1.00 mmol),  40 palladium acetate (0.018 g, 0.08 mmol),  tri-tert-butylphosphine (0.018 g, 0.08 mmol) and potassium tert-butoxide (0.45 g, 5.00 mmol) were added to toluene (15 ml) under N2. The reaction mixture was stirred at reflux for 16 h. The reaction mixture was cooled to room temperature. Purification by silica column chromatography (SiO2: hexanes/EtOAc, 4:1) yielded 0.731 g (59%) of product as pale green solid. 1H NMR (400 MHz, CDCl3) δ = matched literature. 13C NMR (100 MHz, CDCl3): δ = 155.7, 155.4, 154.9, 147.8, 146.8, 143.3, 141.7, 141.4, 133.1, 125.6, 125.5, 124.4, 124.2, 122.8, 121.9, 119.8, 118.7, 117.4, 114.6, 114.5. 55.6, 54.4. HRMS (ESI) m/z: 1241.5085 [(M+H)] calcd for C81H69N4O9  m/z: 1241.5065. spiro-p,o-OMe (N2,N2',N7,N7'-tetrakis(2-methoxyphenyl)-N2,N2',N7,N7'-tetrakis(4-methoxyphenyl)spiro[fluorene-9,9'-xanthene]-2,2',7,7'-tetraamine):  2-methoxy-N-(4-methoxyphenyl)aniline (1.27 g, 5.50 mmol), 4Br (0.800 g, 1.23 mmol), palladium acetate (0.020 g, 0.1 mmol),  tri-tert-butylphosphine (0.022 g, 0.1 mmol) and potassium tert-butoxide (0.50 g, 5.50 mmol) were added to toluene (15 mL) under N2. The reaction mixture was stirred at reflux for 16 h. The reaction mixture was cooled to room temperature. Purification by silica column chromatography (SiO2: hexanes/EtOAc, 4:1) yielded 0.686 g (45%) of product as pale green solid. 1H NMR (400 MHz, CD2Cl2) δ = 7.24 (d, J = 9.0 Hz, 2H), 7.12 (m, 4H), 8.97 (m, 4H), 6.85 (m, 14H), 6.71 (m, 18H), 6.16 (d, J = 2.7 Hz, 2H), 3.73 (d, J = 15.4 Hz, 12H), 3.46 (d, J = 12.9 Hz, 12H)  . 13C NMR (100 MHz, CD2Cl2): δ = 155.7, 155.6, 155.5, 155.0, 154.7, 147.7, 146.5, 143.2, 141.5, 141.0, 135.9, 135.8, 132.4, 129.3, 129.0, 126.2, 126.1, 125.9, 124.8, 122.8, 121.5, 121.4, 121.3, 119.7, 119.3, 119.1, 116.6, 116.3, 114.3, 114.2, 113.3, 113.1, 55.7, 55.6, 55.5, 55.5, 54.8. HRMS (ESI) m/z: 1240.4989  [(M+H)] calcd for C81H68N4O9  m/z: 1240.4986. spiro-Me (N2,N2,N2',N2',N7,N7,N7',N7'-octa-p-tolylspiro[fluorene-9,9'-xanthene]-2,2',7,7'-tetraamine): di-p-tolylamine (1.46 g, 7.4 mmol), 4Br (1.00 g, 1.5 mmol), palladium acetate (0.024  41 g, 0.12 mmol),  tri-tert-butylphosphine (0.027 g, 0.12 mmol) and potassium tert-butoxide (0.71  g, 5.00 mmol) were added to toluene (15 mL) under N2. The reaction mixture was stirred at reflux for 16 h. The reaction mixture was cooled to room temperature. Purification by silica column chromatography (SiO2: hexanes/EtOAc, 15:1) yielded 0.910 g (22%) of product as light brown solid. 1H NMR (400 MHz, Acetone-d8) δ = 7.46 (d, J = 8.3 Hz, 2H), 7.09-6.99 (m, 20H), 6.86 (d, J = 8.4 Hz, 8H), 6.82 (dd, J = 8.3, 2.3 Hz, 4H), 6.76 (d, J = 8.4 Hz, 8H), 6.39 (d, J = 2.6 Hz, 2H), 4.72 (s, 12H), 5.10 (s, 12H).  13C NMR (100 MHz, CDCl3): δ = 155.7, 155.4, 154.9, 147.8, 146.8, 143.3, 141.7, 141.4, 133.1, 125.6, 125.5, 124.4, 124.2, 122.8, 121.9, 119.8, 118.7, 117.4, 114.6, 114.5, 77.5, 77.2, 76.8, 55.6, 54.4. HRMS (ESI) m/z: 1113.5469  [(M+H)] calcd for C81H69N4O  m/z: 1113.5471. spiro-SMe: (N2,N2,N2',N2',N7,N7,N7',N7'-octa-p-thiomethylspiro[fluorene-9,9'-xanthene]-2,2',7,7'-tetraamine): bis(4-(methylthio)phenylamine (0.915 g, 3.5 mmol), 4Br  (0.454 g, 0.70 mmol), palladium acetate (0.040 g, 0.18 mmol),  tri-tert-butylphosphine (0.036 g, 0.18 mmol) and potassium tert-butoxide (0.561  g, 5.00 mmol) were added to toluene (15 mL) under N2. The reaction mixture was stirred at reflux for 16 h. The reaction mixture was cooled to room temperature. Purification by silica column chromatography (SiO2: hexanes/EtOAc, 12:1) yielded 0.451 g (47%) of product as yellow solid. 1H NMR (400 MHz, CDCl3) δ = 7.38 (d, J = 8.2 Hz, 2H), 7.17 – 7.07 (m, 16H), 7.05 – 6.77 (m, 24H), 6.36 (d, J = 2.6 Hz, 2H), 2.46 (s, 12H), 2.42 (s, 12H).  spiro-FOMe (N2,N2',N7',N7-tetrakis(4-fluorophenyl)-N2,N2',N7',N7-tetrakis(4-methoxyphenyl)spiro[fluorene-9,9'-xanthene]-2,2',7',7-tetraamine): N-(4-fluorophenyl)aniline (0.760 g, 3.5 mmol), 4Br (0.454 g, 0.70 mmol), palladium acetate (0.18 g, 0.18 mmol),  tri-tert-butylphosphine (0.036 g, 0.18 mmol) and potassium tert-butoxide (0.561  g, 5.0 mmol) were added  42 to toluene (15 mL) under N2. The reaction mixture was stirred at reflux for 16 h. The reaction mixture was cooled to room temperature. Purification by silica column chromatography (SiO2: hexanes/EtOAc, 20:1 to 10:1) yielded 0.620 g (74.2%) of product as pale brown solid. 1H NMR (400 MHz, CDCl3) δ = 7.33 (d, 2H, J = 8.0 Hz), 6.98 (d, 2H, J = 8.0 Hz), 6.93- 6.76 (m, 38H), 6.33 (d, 2H, J = 2.7 Hz), 3.80 (s, 6H), 3.76 (s, 6H). 13C NMR (101 MHz, CDCl3): δ 159.6, 159.3 (d, J = 60.6 Hz), 159.0, 156.9 (d, J = 59.3 Hz), 156.0, 155.7, 155.6, 147.6, 147.2, 144.6 (d, J = 2.5 Hz), 144.2 (d, J = 2.7 Hz), 143.1, 141.0, 140.8, 133.5, 126.2, 125.6, 125.5, 124.8, 124.7 (d, J = 7.8 Hz), 123.2, 123.0 (d, J = 7.7 Hz), 122.6, 120.1, 119.2, 117.8, 115.9 (d, J = 22.5 Hz), 115.7 (d, J = 22.4 Hz), 114.8, 114.7, 55.6, 55.6, 54.4. spiro-H (N2,N2,N2',N2',N7,N7,N7',N7'-octaphenylspiro[fluorene-9,9'-xanthene]-2,2',7,7'-tetraamine): diphenylamine (1.96 g, 11.6 mmol), 4Br (1.50 g, 2.3 mmol), palladium acetate (0.037 g, 0.18 mmol),  tri-tert-butylphosphine (0.045 g, 0.18 mmol) and potassium tert-butoxide (1.11  g, 11.6 mmol) were added to toluene (30 mL) under N2. The reaction mixture was stirred at reflux for 16 h. The reaction mixture was cooled to room temperature. Purification by silica column chromatography (SiO2: hexanes/EtOAc, 12:1) yielded 1.35 g (59%) of product as light brown solid. 1H NMR (400 MHz, Acetone-d6) δ = 7.52 (d, 2H, J = 8.0 Hz), 7.32-7.18 (m, 16H), 7.10-7.00 (m, 8H), 6.97 (dd, 8H, J = 7.5, 1.2 Hz), 6.92-6.85 (m, 10H), 6.41 (d, 2H, J = 2.6 Hz). HRMS (ESI) m/z: 1001.4203  [(M+Na)] calcd for C71H54N4ONa  m/z: 1001.4195. spiro-F (N2,N2,N2',N2',N7,N7,N7',N7'-octakis(4-fluorophenyl)spiro[fluorene-9,9'-xanthene]-2,2',7,7'-tetraamine): bis(4-fluorophenyl)amine (0.414 g, 2.0 mmol), 4Br (0.300 g, 0.45 mmol), palladium acetate (0.012 g, 0.072 mmol),  tri-tert-butylphosphine (0.018 g, 0.072 mmol) and potassium tert-butoxide (0.100  g, 0.87 mmol) were added to toluene (15 mL) under N2. The reaction mixture was stirred at reflux for 16 h. The reaction mixture was cooled to room temperature. Purification by  43 silica column chromatography (SiO2: hexanes/EtOAc, 15:1) yielded 0.204 g (39%) of product as light brown solid.  1H NMR (400 MHz, CDCl3) δ = 7.36 (d, J = 8.8 Hz, 2H), 7.01 (d, J = 8.8 Hz, 2H, 6.89-6.77 (m, 38H), 6.26 (d, J = 2.6 Hz, 2H). 13C NMR (101 MHz, Chloroform-d) δ 160.0, 159.6, 157.6, 157.2, 155.5, 147.5, 147.4, 144.0, 144.0, 143.8, 142.9, 133.8, 129.2, 128.4, 125.6, 125.5, 125.4, 125.3, 124.4, 124.3, 123.5, 123.2, 120.4, 119.7, 118.1, 116.3, 116.1, 115.9, 54.5. HRMS (ESI) m/z: 1145.3438  [(M+Na)] calcd for C71H46N4OF8Na  m/z: 1145.3442. 3.4.3 Electrochemical and photophysical properties   Solution-phase electrochemical data were recorded with a CHI660D potentiostat using a platinum wire counter electrode, Ag/AgCl reference electrode, and a platinum working electrode. A 0.1 M n-NBu4PF6 DCM solution at ambient temperature was used for all HTMs.  Ag/AgCl in saturated KCl was used as the reference electrode and was calibrated versus the normal hydrogen electrode (NHE) by addition of Fc/Fc+ as an internal reference.  CVs were acquired for 0.5 mM solutions of each HTM at a scan rate of 50 mV s-1.  Solution UV-vis absorption spectra were recorded using a Cary 5000 UV-vis spectrophotometer. Photoluminescence (PL) spectra were recorded with a Cary Eclipse. All samples were measured in a 1 cm cell at room temperature with DCM as the solvent. Concentration of 2 × 10-5 M and 1 × 10-5 M were used for solution absorption and emission measurements, respectively.   3.4.4 Bulk thermal properties DSC curves of compounds were collected using a Netzsch DSC 214 Polyma instrument under a nitrogen protective atmosphere. For each measurement, two aluminum crucibles were used, one as empty reference and the other for sample measurement. A customized heating program was  44 developed to include a 5 min hold at the initial temperature of 50 °C and four consecutive heating-cooling cycles (Tmin = 50 °C, Tmax = 300 °C, 10 K/min). Data from the last 3 heating segments were used to identify the Tm and extrapolate the Tg of the material. 3.4.5 Electrical properties  Four parallel Au electrodes with a spacing of 0.75 mm and length of 23 mm were used to measure the film conductivities in the dark and under ambient conditions. A Keithley 2400 sourcemeter was used to force current through the outer electrodes while sensing the voltage across the inner two electrodes. A linear fit of each measurement was used to determine the resistance, R, and the geometry of samples was then used to calculate the conductivity, &, according to the equation & = ()*+    (Equation 3-1) where l is the electrode length, d is the inter-electrode spacing and t is the film thickness. The film thickness was measured using a Bruker DektakXT profilometer. 3.4.6 Bulk simulation  For simulation of thin film properties such as HOMO/LUMO energy distributions and hole mobility, we used the multiscale modeling approach described by Friederich et al.98,99 This approach includes the parameterization of molecule-specific force fields, the generation of atomically-resolved morphologies using a Monte Carlo simulated annealing protocol,100 the analysis of the electronic structure of the molecules in their amorphous environment,101 the calculation of electronic couplings, reorganization energies, energy disorder and, finally, the calculation of the hole mobility using an effective medium model.102 We generated and analyzed three morphologies (each with approximately 1000 molecules) for all seven materials to have  45 sufficient statistics for the calculation of the hole mobility which sensitively depends on the energy disorder or the width of the distribution of HOMO energies. 3.4.7 Perovskite solar cell characterization The planar perovskite solar cells were fabricated according to our previous work.20 Briefly, pre-patterned ITO coated glass substrates were sequentially cleaned using detergent, acetone, and isopropanol. The TiO2-Cl electron transport layers were spin-coated on ITO substrates from the colloidal nanocrystal solutions, and annealed on a hot plate at 150 °C for 30 min in ambient air. After the substrates had cooled, we transferred the substrates immediately to a nitrogen-filled glovebox for the deposition of perovskite films. The Cs0.05FA0.81MA0.14PbI2.55Br0.45 precursor solution (1.4 M) was prepared in a mixed solvent of DMF and DMSO with a volume ratio of 4:1. The molar ratios of PbI2/PbBr2 and FAI/MABr were both fixed at 0.85:0.15, molar ratio of CsI/(FAI+MABr) is 0.05:0.95, and the molar ratio of (FAI+MABr+CsI)/(PbI2+PbBr2) was fixed at 1:1. The perovskite films were deposited onto the TiO2-Cl substrates with two-step spin coating procedures. The first step was 2000 rpm for 10 s with an acceleration of 200 rpm/s. The second step was 6000 rpm for 20 s with a ramp-up of 2000 rpm/s. Chlorobenzene (100 µL) was dropped on the spinning substrate during the second spin-coating step at 10 s before the end of the procedure. The substrate was then immediately transferred on a hotplate and heated at 100 °C for 20 min. After cooling down to room temperature, the hole-transport layer was subsequently deposited on top of the perovskite film by spin coating at 4000 rpm for 30 s using a chlorobenzene solution which contained 72.3 mg/mL of HTMs and 28.8 µL/mL of tBP, as well as 520 mg/mL of LiTFSI in acetonitrile). Finally, 100 nm Au contact was deposited on top of HTMs by electron-beam evaporation in an Angstrom Engineering deposition system.   46 Current density-voltage (J-V) characteristics were measured using a Keithley 2400 source meter under illumination of a solar simulator (Newport, Class A) at the light intensity of 100 mW cm−2 as verified using a certified reference solar cell (Newport). A spectral mismatch factor of 1 was used for all J-V measurements. Unless otherwise stated, the instantaneous J-V curves were measured with a scanning rate of 50 mV s-1 from 1.2 V to -0.1 V. The active area was determined using an aperture shade mask (0.049 cm2) placed in front of the solar cell to avoid overestimation of the photocurrent density.                47 Chapter 4 : Controlling the Thermal Stability and Electrochemistry of Hole-Transport Material Films 4.1 Introduction  Hole-transport materials (HTMs) are a key component of organic light-emitting diodes (OLEDs)59,103,104 and solid-state solar cell devices.105,106 High performance HTMs for perovskite solar cells (PSCs) should exhibit high glass transition temperatures (Tg) to suppress the formation of grain boundaries in glassy films which can impede effective charge transfer.49,107 It is imperative that the HTM contains an appropriately positioned and reversible redox couple to mediate charge extraction from the photo-oxidized perovskite material, and be capable of conducting charge through the charge transport layer.35,108 These materials should also have high temporal stability under elevated temperatures and humidity.27,109,110 Organic HTMs are amenable to low-temperature solution processing while also being capable of mediating high power conversion efficiencies (PCEs).50,111 Spiro-OMeTAD68 (Figure 4-1) is a widely-used organic HTM and is capable of reaching PCEs greater than 20% when integrated in PSCs.20,112 The spiro-OMeTAD molecule teaches how a spiro-carbon center can impart a rigid 3D structure that leads to a high Tg (125 °C).113 The redox-active TPA substituents functionalized with methoxy groups also yield reversible redox couples at potentials appropriate for use in a PSC.114 This combination of structural and electrochemical properties contributes to the high hole mobilities (2 × 10-4 to 5 × 10-5 cm2 V-1s-1) that have proven so effective for spiro-OMeTAD.115 As described in Chapter 2, X60 is an HTM disclosed in early 201622,23 which contains an asymmetric fluorene-9,9′-xanthene core functionalized with TPA groups, analogous to that of spiro-OMeTAD, to generate hole-transport properties that produce high PCEs in the PSCs.22,23,114 The simple reaction chemistry associated with spiro-based X60 inspired us to elucidate how each  48 of the TPA units appending the unsymmetric spiro-core affects the HTM properties and the corresponding performance in a PSC. I discovered that the TPA units positioned about the highly conjugated fluorene fragment are the electrochemically relevant units (i.e., associated with the redox couple that represents the HOMO energy level, EHOMO) while those positioned on the xanthene moiety affect the structural and bulk thermal properties.26 I was able to resolve this structure-property relationship by placing TPA groups exclusively on the fluorene (HTM-F) or the xanthene (HTM-X) units of the core. These results pointed to the xanthene substituents governing the bulk thermal properties, prompting me to test how para- and meta-substitution of TPAs at the xanthene moiety (i.e., HTM-X and HTM-X′, respectively) affects the Tgs of these films. This decoupling of the EHOMO levels and bulk thermal properties (e.g., Tg) yielded HTM-FX′ which is characterized by a strikingly high Tg of 137 °C. This HTM was found to be capable of generating a PCE of 20.8% in a PSC, a value that compares favorably with those measured for spiro-OMeTAD and X60 under the same experimental conditions. Moreover, devices containing HTM-FX′ could be prepared with a notably high level of reproducibility. This identification of how the TPAs impact the redox activity, thermal properties and device characteristics will guide the design of new high-performance HTMs.   49  Figure 4-1 Molecular representations of spiro-OMeTAD68 and the HTM series under investigation.  4.2 Results and discussion 4.2.1 Computational screening Computational screening was carried out by Dr. Daniel Tabor of the Aspuru-Guzik group at the University of Toronto. Replacing one of the two fluorene units in spiro-OMeTAD with a xanthene unit to form X6022,23 results in a  break in molecular symmetry and enables analysis of which specific TPA groups affect the redox properties and thermal properties of HTMs. Computational methods were used to screen the five HTMs in Figure 4-1 containing this unsymmetrical core for appropriate frontier orbital energy levels, low reorganization energies and photophysical properties. The frontier orbitals were modeled (Figure 4-2) and the computed reduction potentials corresponding to the first oxidation event (εHOMO) were predicted to all exhibit suitable reduction potentials for hole extraction from the perovskite layer.   50   Figure 4-2 HOMO and LUMO orbitals of each molecule in the HTM series, calculated at the BP86/def2-SVP level of theory. Isosurfaces are at 0.03 and -0.03 values for the wavefunction.  51  The reorganization energies (Ereorg) for the entire HTM series (Table 4-1) were calculated using density functional theory (DFT) and evaluated using Nelsen’s four point method.23,116,117 HTMs containing TPA moieties at the Rx or Rx′ positions of the HTM scaffold were predicted to exhibit smaller Ereorg values that are favourable for efficient hole transport according to Marcus theory.118 When considering each component term of the Ereorg calculation independently (Table 4-1), both the vertical ionization and the adiabatic ionization energies are substantially higher for compounds functionalized at exclusively the fluorene or xanthene units relative to X60 and HTM-FX′ (substituted at both the fluorene and xanthene units) due to less diffuse hole delocalization (Figure 4-3). The net relative effect of these two terms largely cancel out in a reorganization energy calculation thereby yielding similar Ereorg values across the series. The vertical electron affinity of the cation at the adiabatic cation minimum appears to be the more dominant factor that affects Ereorg, and HTM-F was found to contain the largest relative contribution from this term.  Modeling the spin-density differences of the adiabatic minima for the singly oxidized molecules of the series (Figure 4-3) provides a visualization of the hole on an oxidized HTM and reveals differences in hole delocalization within the series. Fluorene was confirmed to be a highly conjugated unit and the positions of TPA groups on the xanthene moieties affect the electronic structure of the oxygen bridge. All computed Ereorg values were relatively low, between 0.14-0.20 eV and close to that of spiro-OMeTAD (0.14 eV).  52   Figure 4-3 Spin density difference maps for the HTM cations evaluated at the B3LYP/6-311+G(d,p) level of theory at the B3LYP/6-31G(d) cation adiabatic minima.   Table 4-1 Calculated electronic frontier molecular orbital properties and contributions to reorganization energies (B3LYP/6-311+G(d,p) evaluated at B3LYP/6-31G(d) optimized geometries) of the HTM series and spiro-OMeTAD Compound εHOMO  (eV) εLUMO  (eV) εHOMO-LUMO (eV) E*+ (eV) E+  (eV) E*  (eV) Ereorg  (eV) X60 -4.54 -1.06 3.48 0.076 5.321 5.400 0.154 HTM-F -4.60 -1.12 3.48 0.108 5.429 5.525 0.205 HTM-X -4.65 -1.20 3.46 0.081 5.523 5.598 0.152 HTM-FX′ -4.52 -1.03 3.48 0.072 5.273 5.359 0.158 HTM-X′ -4.90 -1.14 3.76 0.071 5.739 5.812 0.144 Spiro-OMeTAD -4.52 -1.06 3.48 0.059 5.235 5.319 0.143  53 4.2.2 Synthesis The preparation of X60 followed a modified literature procedure where 2,7-dibromofluorenone is reacted with 4-bromophenol to yield a tetra-brominated product that, in turn, undergoes a Pd-catalyzed Buchwald-Hartwig coupling with excess 4,4′-dimethoxyphenylamine to yield the product.23 This procedure was adapted to access the rest of the HTM series (Scheme 4-1) using either non-brominated fluorenone for HTM-X and HTM-X′ and/or 3-bromophenol for HTM-FX′ and HTM-X′. All compounds in the HTM series were prepared in merely two reaction steps under relatively benign reaction conditions and a single column purification step. By contrast, the spiro-OMeTAD core alone requires a multi-step synthesis involving air and moisture-sensitive reagents, hazardous lithiation reactions and several time-consuming column chromatographic purification steps.68  Scheme 4-1 Synthetic scheme for HTM series.  4.2.3 Electrochemical and photophysical characterization Cyclic voltammograms (CVs) for each HTM were recorded in 0.1 M n-NBu4PF6 DCM solutions to determine the EHOMO for the series (Table 4-2 and Figure 4-4). The EHOMO values for  54 HTMs bearing TPAs on the fluorene (X60, HTM-F and HTM-FX′) were all found to be ~0.66 V vs NHE, which is slightly more positive than spiro-OMeTAD (0.63 V vs NHE) and therefore appropriate for energy alignment with the perovskite layer.   Figure 4-4 Cyclic voltammograms of HTM series in 0.1 M n-NBu4PF6 DCM solutions, with a polished Pt working electrode, Pt wire counter electrode and Ag/AgCl reference electrode.  Table 4-2 Electrochemical, bulk thermal and electrical properties of HTM series  Compound EHOMO a (V vs NHE) εHOMOb (eV) Tgc (°C) Hole Mobility   (× 10-4 cm2 V-1s-1)d Spiro-OMeTAD 0.63 −5.13 125 6.9 X60 0.67 −5.17 107 1.4 HTM-F 0.67 −5.17 104 0.4 HTM-X 0.76 −5.26 94 0.5 HTM-FX′ 0.66 −5.16 137 4.8 HTM-X′ 0.97 −5.47 130 0.3 a The half-wave potential corresponding to the first oxidation process in the cyclic voltammogram. Measured in DCM and referenced to NHE by addition of ferrocene. b εHOMO = − (EHOMO + 4.50 eV). c Determined by differential scanning calorimetry (DSC). All Tm values determined to be >300 °C and above the detection limits of the instrument. d  Determined by space-charge-limit current (SCLC)  method from doped HTMs.  55 Differential pulse voltammetry (DPV) was used to deconvolute and quantify each of the successive oxidation events for the series (Figure 4-5 and Table 4-3). The number of oxidation events tracked the number of TPA units on the HTM, and we were able to assigned the redox activity to specific TPA units by taking a number of different observations into account. For example, the EHOMO levels for the HTMs with only para- or meta-substituted xanthene TPAs (HTM-X and HTM-X′, respectively) are higher than that of the HTM with only fluorene TPAs (HTM-F) and reflect a lower degree of electronic coupling based on successive oxidation of their two TPA groups (Figure 4-5a). This conjecture is fully supported by the trend in computed εHOMO values (Table 4-3).  The first reduction potentials (EHOMO) for X60 and HTM-FX′ can therefore be linked to the first oxidations of the fluorene TPAs (EF1), in that the first oxidation event of X60 and HTM-FX′ matches that measured for HTM-F (0.67 V vs NHE). The second oxidation event for X60 at 0.87 V vs NHE is attributed to the first oxidation of the xanthene TPAs (EX1) by benchmarking against HTM-X (0.78 V vs NHE). The third and fourth oxidation events of X60 are assigned using the second Faradaic events for HTM-F (EF2) and HTM-X (EX2) according to their electronic coupling (ΔE1/2 in Figure 4-5) with the first two oxidations. Similarly, all four oxidation events for HTM-FX′ have been resolved even though the second, third and fourth oxidation features overlapped significantly and appeared to be a three-electron process (0.8 to 1.1 V vs NHE). My assignments of the redox activity for each TPA unit, which were corroborated by modeling the degree of hole delocalization of the cationic HTM species (Figure 4-3), clearly show that the redox activity relevant to the HOMO level is confined to the TPA groups appended to the fluorene units for X60 and HTM-FX′. This finding indicates that the xanthene units can be modified without compromising the EHOMO levels corresponding to the fluorene TPA units.    56  Figure 4-5 Differential pulse voltammograms (DPVs) for HTM series normalized in amplitude to their first oxidation event. The number of electrons that contribute to each redox process was determined by integrating the area beneath the peak fitting. (a) The first deconvoluted oxidation events are shown in shaded colors to highlight the progressively anodic shift of EHOMO for HTM-F (red), HTM-X (blue) and HTM-X′ (green), respectively. Traces for (b) X60 (black) and (c) HTM-FX′ (orange) are deconvoluted to highlight that the EHOMO is linked to the TPA unit positioned on the fluorene unit.        57 Table 4-3 Summary of DPV oxidation peaks for HTM series Compound EF1 EF2 ΔE1/2 EX1 EX2 ΔE1/2 (V vs NHE) (mV) (V vs NHE) (mV) X60 0.685 0.983 297 0.869 0.991 122 HTM-F 0.685 0.982 296 - - - HTM-X - - - 0.777 0.915 138 HTM-FX′ 0.672 0.989 317 0.927 1.007 80 HTM-X′ - - - 0.905 0.987 82   The similar UV-visible absorption and emission maxima (Figure 4-6 and Table 4-4) of X60, HTM-F,  and HTM-FX′ confirm the similar electronic structures of the compounds’ frontier orbitals. All HTMs under study exhibit limited light absorption in the visible region which may be beneficial in maximizing light absorption by the perovskite layer (Figure 4-6a).   Figure 4-6  (a) UV-Vis absorption spectra for HTM solutions (2.0 × 10-5 M) recorded in DCM. (b) Emission spectra for HTM series recorded in DCM excited at lowest energy absorption band recorded in Table 4-4.      58 Table 4-4 Summary of spectroscopic and electrochemical properties for HTM series. Compound λmax  a (nm) λem  b (nm) EHOMO-LUMO c (eV) X60 392, 305 428 2.97 HTM-F 391, 307 428 3.07 HTM-X 308 413 2.97 HTM-FX′ 398, 304 428 3.02 HTM-X′ 304 396 3.02 a Absorption maximum for HTM in DCM. b Excitation at higher energy λmax. c Determined from the intersection point of absorption and emission spectra. 4.2.4 Bulk thermal properties The melting point (Tm) and Tg of each HTM measured by differential scanning calorimetry (DSC) are tabulated in Table 4-2. The Tm  and Tg values for every member of the series were measured to be >300 °C and >93 °C, respectively. A Tg value higher than 90°C is required to avoid morphology changes during solar cell operation.49 There is no apparent trend between number of TPA units and glass transition temperature (e.g., Tg (X60) = 107 °C c.f. Tg (HTM-F) = 104 °C). However, substitution at the meta-position of the xanthene moiety was found to increase Tg by over 30 °C (e.g., Tg (HTM-X′) = 130 °C c.f. Tg (HTM-X) = 94 °C). This striking increase in Tg through meta-substitution of the xanthene was also observed in the case of HTM-FX′ (Tg = 137 °C c.f. 107 °C for X60). 4.2.5 Electrical properties  The current-voltage (J-V) characteristics of “hole-only” devices with various HTMs are shown in Figure 4-7a and are used to determine the hole mobilities of the HTMs under study. The HTMs are doped with  tBP and LiTFSI. The hole mobilities tabulated in Table 4-2 were determined  59 using the space-charge-limited current (SCLC) method. The mobilities of spiro-OMeTAD, X60 and HTM-FX′ obtained here are comparable to the previous reported values.23,115,119 The mobility of HTM-FX′ is close to that of spiro-OMeTAD and is much higher than that of all other HTM compounds in the series under study. While the EHOMO level of HTM-FX′ is similar to X60, the higher mobility is ascribed to the meta-substitution affecting the chemical structure or electronic delocalization. The low hole mobilities of HTM-X and HTM-X′ may be due to the EHOMO levels being too stabilized for efficient doping by LiTFSI. Given that the extent of hole doping for HTM-F and X60 by LiTFSI is expected to be similar based on the similarities in EHOMO, the low mobility of HTM-F is attributed to the higher Ereorg value (0.21 eV) relative to the other HTMs (<0.16 eV).     60  Figure 4-7 Performance of perovskite solar cells with HTMs: (a) J-V curves of hole-only devices with HTM series; (b) J-V curves of solar cells with HTM series measured under solar simulator (100 mW cm-2); (c) Histogram of PCEs for 44 devices with HTM-FX′; (d) J-V curves of champion device with HTM-FX′ scanned in reverse and forward directions (scanning rate of 20 mV/s).  The HTMs exhibited similar charge extraction kinetics and effective photoluminescence quenching for all HTMs as revealed by time-resolved and steady-state photoluminescence (PL) measurements (Figure 4-7). Hole extraction from the perovskite layer to each of the HTMs in this study was not found to be a limiting factor, except for HTM-X′ which showed incomplete PL quenching (Figure 4-7a and c). The HTMs exhibit similar charge extraction kinetics between spiro-OMeTAD, X60, and HTM-FX′ (Figure 4-7b) while the initial PL decay rates in HTMs with only two TPA moieties follow the trend of HTM-F > HTM-X > HTM-X′.  61  Figure 4-8 (a) Steady-state PL and (b) time-resolved PL decay of perovskite films with spiro-OMeTAD, X60 and HTM-FX' and (c) time-resolved PL decay of perovskite films with HTM-F, HTM-X and HTM-X'.  4.2.6 Perovskite solar cell characterization Each of the HTMs were then tested in planar PSCs with the device architecture: glass/ITO/TiO2-Cl/perovskite/HTM/Au by Dr. Hairen Tan of the Sargent group at the University of Toronto. Table 4-5 summarizes the statistical performance of devices fabricated with otherwise-identical device processing for each HTM, and the J-V curves of champion devices for each HTM are shown in Figure 4-7b. Champion PCE values indicate the maximum power conversion efficiency achieved with a single device. PSCs containing HTM-F, HTM-X, and HTM-X′ all generated low PCE values. This observation is attributed to the low hole mobilities (Figure 4-7a) precluding effective transport of carriers through the HTM layers. X60 results in PCEs that are lower than spiro-OMeTAD and the best device yielded a PCE of 19.5% which is comparable to a previous report.23 HTM-FX′ exhibited slightly higher PCEs than X60 (20.8% vs 19.5%, respectively) and the small performance enhancement can be attributed to the higher hole mobility imparted by the change of TPA substitution pattern on the xanthene unit (4.8 vs 1.4 × 10-4 cm2V-1s-1, respectively). HTM-FX′ produced a device performance that rivals that of spiro-OMeTAD. PSCs containing HTM-FX′ exhibit excellent reproducibility, as indicated by the  62 narrow PCE distribution over 44 devices (Figure 4-7c). Moreover, PSCs containing HTM-FX′ also show negligible hysteresis in the J-V measurements (Figure 4-7d).  These results highlight that meta-substitution of the xanthene groups to be an important handle for achieving high-efficiency PSCs.     Table 4-5 Device characteristics for perovskite solar cells containing HTM series.  Compound Voc (V) Jsc (mA cm-2) FF (%) PCE (%) Champion PCE (%) Spiro-OMeTAD 1.16 ± 0.02 21.9 ± 0.3 77.7 ± 1.7 19.7 ± 0.5 20.4 X60 1.16 ± 0.01 21.2 ± 0.3 74.0 ± 2.3 18.2 ± 0.7 19.5 HTM-FX′ 1.17 ± 0.01 21.7 ± 0.3 77.5 ± 2.0 19.7 ± 0.6 20.8 HTM-F 1.08 ± 0.02 14.7 ± 0.9 40.6 ± 1.9 6.5 ± 0.5 7.0 HTM-X 0.88 ± 0.11 3.8 ± 0.6 31.6 ± 2.2 1.1 ± 0.2 1.3 HTM-X′ 0.77 ± 0.06 1.4 ± 0.1 26.1 ± 1.3 0.2 ± 0.1 0.3  Statistical performance of 41 spiro-OMeTAD devices, 24 X60 devices, 44 HTM-FX′ devices, 8 HTM-F devices, and 4 devices for each HTM-X and HTM-X′, is shown here.   4.3 Conclusion Breaking the symmetric core of spiro-OMeTAD by replacing one of the two fluorene units with a xanthene unit to form X6023 enabled us to experimentally resolve how the positions of TPA groups about the core affect the redox properties and glass transition temperatures of HTMs relevant to high efficiency PSCs. TPA groups positioned on the highly conjugated fluorene moiety  63 renders the HTM easier to oxidize, which, in turn, increases the free energy change for hole-extraction from the perovskite layer. The glass transition temperatures, however, are governed by the TPAs about the xanthene unit: substitution at the meta-position raises the Tg by 30 °C over those substituted at the para-position. The synergistic effects of a positively shifted EHOMO and high glass transition temperature in HTM-FX′ yielded a device power conversion efficiency of 20.8%, which is commensurate with current state-of-the-art PSCs. Electrochemical, photophysical and structural findings were fully supported by computational analysis of the frontier orbitals and reorganization energies of the neutral HTMs and spin density difference maps of cationic species, thereby highlighting the role predictive tools can play in screening future state-of-the-art HTMs. This set of design principles that enables selective control of redox and mesoscopic properties will be elaborated on in future chapters in pursuit of robust higher performance PSCs and can be used to inform the rational design and accelerated discovery of high efficiency materials with robust performance during cell operation. 4.4 Experimental 4.4.1 Computational screening Up to 20 conformers were generated for each molecule using MMFF94 force field120 with the RDKit package.121 Each conformer was then used as the starting point for a set of DFT calculations. Geometry optimizations were performed at both the B3LYP/6-31G(d) and BP86/def2-SVP levels of theory. The HOMO and LUMO levels were calculated at the B3LYP/6-311+G(d,p) level of theory at the B3LYP/6-31G(d) minima. These gaps are shown in Table 4-1. Vertical excitation energies were obtained using time-dependent (TD) DFT at the B3LYP/def2-TZVP, CAM-B3LYP/def2-TZVP, ωB97X-D/def2-TZVP and  levels of theory, with the first 15  64 states obtained. The resulting spectra are plotted in Figure A4-11. Reorganization energy (Ereorg) for hole transfer were calculated using the Nelsen four point method23,116,117 at both the B3LYP/6-31G(d) and ωB97X-D/6-31G(d) levels of theory: Ereorg = [M+(0) - M(0)] + [M+(+) - M(+)]   (Equation 4-1) Where M(0) and M(+) are the neutral and oxidized HTMs with a neutral geometry and M+(0) and M+(+) are the neutral and oxidized HTMs with the oxidized geometry.  In addition, the HOMO and LUMO orbitals were obtained for the neutral molecules and spin density differences were determined for the radical cations at both the neutral and the cation minimum geometry. The ground state electronic structure energy calculations were performed with the QChem 4.2 suite.122 The excited-state calculations for the modeled UV-Vis spectra, the orbital plots and the spin density difference maps were generated with Gaussian09, revision D.01123 with VMD employed for visualization.124 4.4.2 Synthesis  All reagents were obtained from MilliporeSigma, Alfa Aesar or Fisher Scientific and used as received. All reactions were performed using toluene from a solvent purification system. Standard inert atmosphere and Schlenk techniques were carried out under nitrogen. All reactions and purification were carried out in the dark. Purification by column chromatography was performed using silica (Silicycle: Ultrapure Flash Silica). Analytical thin-layer chromatography (TLC) was performed on aluminum-backed sheets pre-coated with silica 60 F-254 adsorbent (250 μm thick; Silicycle, QC, Canada) and visualized under UV light. Routine 1H and 13C NMR spectra were collected on a Bruker AV400 inv/dir instrument at ambient temperatures, operating at 300 MHz and 100 MHz, for 1H and 13C nuclei, respectively. Chemical shifts (δ) are reported in parts per million (ppm) using the residual signals δ 7.26 and 77.0 for CDCl3, δ 2.05 and 29.8 for acetone- 65 d6 and δ 2.50 and 39.4 for DMSO-d6 as internal references for 1H and 13C, respectively. Standard abbreviations indicating multiplicity are used as follows: s = singlet; d = doublet; t = triplet; m = multiplet. Br4-FX (2,2',7,7'-tetrabromospiro[fluorene-9,9'-xanthene]): 2,7-dibromo-9-flourenone (1.08 g, 3.20 mmol), 4-bromophenol (5.53 g, 32.0 mmol) and methane sulfonic acid (0.82 mL, 12.8 mmol) were mixed at 140 °C for 18 h under N2. The reaction mixture was cooled to room temperature and product was precipitated with the addition of MeOH. Separation by filtration and rinsing with MeOH yielded 2.09 g (96.5%) of product as white powder. 1H NMR (400 MHz, CDCl3): δ = 8.01 (d, 2H,  J = 8.1 Hz), 7.69 (dd, 2H, J = 8.2, 1.8 Hz), 7.49 (dd, 2H,  J = 8.8, 2.4 Hz), 7.44 (d, 2H,  J = 1.7 Hz), 7.31 (d, 2H,  J = 8.8 Hz), 6.53 (d, 2H,  J = 2.4 Hz). 13C NMR (100 MHz, CDCl3): δ = 155.5, 150.0, 137.6, 132.2, 130.4, 129.0, 124.8, 122.9, 122.0, 119.2, 116.2. HRMS (EI): m/z = 643.76141 [M+] (calcd for [C25H12OBr4]+: m/z = 643.76216). Br4-FX′ (2,3',6',7-tetrabromospiro[fluorene-9,9'-xanthene]): Compound was prepared following a modified literature procedure for Br4-X. 1H NMR (400 MHz, CD2Cl2): δ = 7.73 (d, 2H,  J = 8.2 Hz), 7.60 (dd, 2H, J = 8.2, 1.8 Hz), 7.38 (dd, 2H, J = 8.8, 2.4 Hz), 7.28 (d, 2H,  J = 1.8 Hz), 7.17 (d, 2H, J = 8.8 Hz), 6.49 (d, 2H, J = 2.4 Hz). 13C NMR (100 MHz, CD2Cl2): δ = 156.1, 150.5, 138.2, 132.6, 132.6, 130.8, 129.4, 125.2, 123.2, 122.6, 119.6, 116.4. HRMS (EI): m/z = 643.76192 [M+] (calcd for [C25H12OBr4]+: m/z = 643.76216). Br2-F (2,7-dibromospiro[fluorene-9,9'-xanthene]): Compound was prepared following a modified literature procedure for Br4-X. 1H NMR (400 MHz, CDCl3): δ = 7.63 (d, 2H, J = 8.1 Hz), 7.50 (dd, 2H, J = 8.1, 1.8 Hz), 7.31 – 7.13 (m, 6H), 6.92 – 6.78 (m, 2H), 6.38 (dt, 2H, J = 7.8, 1.1 Hz). Characterization data has been previously reported.125  66 Br2-X (2',7'-dibromospiro[fluorene-9,9'-xanthene]): Compound was prepared following a modified literature procedure for Br4-X. 1H NMR (400 MHz, CDCl3): δ = 7.93 – 7.79 (m, 2H), 7.44 (tt, 2H, J = 7.5, 1.2 Hz), 7.35 – 7.24 (m, 6H), 7.15 (dd, 2H, J = 14.5, 1.0 Hz), 6.49 (dd, 2H, J = 2.4, 1.0 Hz). HRMS (EI): m/z = 487.94118 [M+] (calcd for [C25H14O79Br2]+: m/z = 487.94114). Br2-X′ (3',6'-dibromospiro[fluorene-9,9'-xanthene]): Compound was prepared following a modified literature procedure for Br4-X. 1H NMR (400 MHz, CDCl3): δ = 7.80 (dt, 2H, J = 7.6, 0.9 Hz), 7.49 – 7.34 (m, 4H), 7.23 (td, 2H, J = 7.5, 1.1 Hz), 7.12 (dt, 2H,  J = 7.6, 0.9 Hz), 6.91 (dd, 2H, J = 8.4, 2.0 Hz), 6.26 (d, 2H, J = 8.4 Hz). 13C NMR (101 MHz, CDCl3): δ = 154.4, 151.7, 139.7, 129.5, 128.8, 128.4, 127.0, 125.7, 123.9, 121.2, 120.3, 120.1, 77.5, 77.2, 76.8, 53.7. HRMS (EI): m/z = 487.94119 [M+] (calcd for [C25H14O79Br2]+: m/z = 487.94114). X60 (N2,N2,N2',N2',N7,N7,N7',N7'-octakis(4-methoxyphenyl)spiro[fluorene-9,9'-xanthene]-2,2',7,7'-tetraamine): bis(4-methoxyphenyl)amine (1.14 g, 5.00 mmol), Br4-FX (0.678 g, 1.00 mmol), palladium acetate (0.018 g, 0.080 mmol),  tri-tert-butylphosphine (0.018 g, 0.080 mmol) and potassium tert-butoxide (0.45 g, 5.0 mmol) were added to toluene (15 mL) under N2. The reaction mixture was stirred at reflux for 16 h. The reaction mixture was cooled to room temperature. Purification by silica column chromatography (SiO2: hexanes/EtOAc, 4:1; Rf = 0.24) yielded 0.731 g (59.2%) of product as pale green solid. 1H NMR (400 MHz, CDCl3): δ = matched literature. 13C NMR (100 MHz, CDCl3): δ = 155.7, 155.4, 154.9, 147.8, 146.8, 143.3, 141.7, 141.4, 133.1, 125.6, 125.5, 124.4, 124.2, 122.8, 121.9, 119.8, 118.7, 117.4, 114.6, 114.5. 55.6, 54.4. HRMS (ESI): m/z = 1241.5085 [M+H]+ (calcd for [C81H69N4O9]+:  m/z = 1241.5065). HTM-FX′ (N2,N2,N3',N3',N6',N6',N7,N7-octakis(4-methoxyphenyl)spiro[fluorene-9,9'-xanthene]-2,3',6',7-tetraamine): Compound was prepared following a modified literature procedure for X60.23,119 Purification by silica column chromatography (SiO2: hexanes/EtOAc, 5:1;  67 Rf = 0.21) yielded product as pale green solid in 78.2% yield. 1H NMR (400 MHz, acetone-d6): δ = 7.50 (d, 2H, J = 9.0 Hz), 7.05 – 6.96 (m, 8H), 6.95 – 6.84 (m, 16H), 6.78 (dd, 12H, J = 7.9, 4.4 Hz), 6.41 (dd, 2H,  J = 8.5, 2.4 Hz), 6.38 – 6.31 (m, 4H), 3.75 (d, 24H, J = 3.4 Hz). 13C NMR (100 MHz, acetone-d6): δ = 157.3, 156.8, 156.7, 152.6, 149.8, 148.9, 141.8, 141.3, 133.6, 128.7, 127.7, 126.8\, 121.3, 120.6, 118.8, 118.0, 116.0, 115.6, 115.4, 107.7, 55.7, 55.7, 54.2. HRMS (ESI): m/z = 1240.4988 [M]+ (calcd for [C81H68N4O9]+:  m/z = 1240.4986). HTM-F (N2,N2,N7,N7-tetrakis(4-methoxyphenyl)spiro[fluorene-9,9'-xanthene]-2,7-diamine): Compound was prepared following a modified literature procedure for X60.23 1H NMR (400 MHz, CDCl3): δ = 7.46 – 7.40 (m, 2H), 7.15 (ddd, 2H, J = 8.5, 7.1, 1.6 Hz), 7.06 (dd, 2H, J = 8.2, 1.3 Hz), 6.94 – 6.80 (m, 14H), 6.75 – 6.63 (m, 8H), 6.60 (dd, 2H, J = 7.8, 1.6 Hz), 3.75 (s, 12H).  HTM-X (N2',N2',N7',N7'-tetrakis(4-methoxyphenyl)spiro[fluorene-9,9'-xanthene]-2',7'-diamine): Compound was prepared following a modified literature procedure for X60.23 1H NMR (400 MHz, CDCl3): δ = 7.55 (d, 2H, J = 7.5 Hz), 7.26 – 7.14 (m, 8H), 7.02 (d, 2H, J = 8.9 Hz), 6.79 – 6.67 (m, 8H), 6.60 (d, 6H, J = 9.0 Hz), 6.06 (d, 2H, J = 2.8 Hz), 3.72 (s, 12H). 13C NMR (100 MHz, CDCl3): δ = 155.1, 155.0, 146.6, 143.5, 141.2, 139.7, 128.0, 127.6, 125.4, 125.0, 122.3, 121.4, 120.1, 117.2, 114.4, 55.6, 54.7. HRMS (ESI): m/z = 786.3083 [M+H]+ (calcd for [C53H42N2O5]+:  m/z = 786.3094). HTM-X′ (N3',N3',N6',N6'-tetrakis(4-methoxyphenyl)spiro[fluorene-9,9'-xanthene]-3',6'-diamine): Compound was prepared following a modified literature procedure for X60.23 1H NMR (400 MHz, CDCl3): δ = 7.75 (dt, 2H, J = 7.5, 1.0 Hz), 7.34 (m, 2H), 7.24 – 7.23 (m, 4H), 7.03 (d, 8H, J = 8.9 Hz), 6.79 (d, 8H, J = 8.9 Hz), 6.59 (d, 2H, J = 2.4 Hz), 6.31 (dd, 2H,  J = 8.6, 2.4 Hz), 6.14 (d, 2H, J = 8.6 Hz), 3.77 (s, 12H). 13C NMR (100 MHz, CDCl3): δ = 156.2, 155.5, 152.1, 148.7, 140.6, 139.8, 128.3, 128.1, 127.6, 127.2, 125.8, 119.9, 116.4, 115.1, 114.8, 106.9, 77.5,  68 77.2, 76.8, 55.6).  HRMS (ESI): m/z = 786.3089 [M+H]+ (calcd for [C53H42N2O5]+:  m/z = 786.3094). 4.4.3 Electrochemical and photophysical properties      Solution-phase UV-Vis absorption spectra were collected on a Cary 5000 spectrophotometer. Photoluminescence spectra were recorded with a Cary Eclipse fluorimeter. All samples were measured in a 1-cm quartz cell at room temperature in HPLC grade DCM. The concentrations of the DCM solutions of analytes for UV-Vis and photoluminescence measurements were approximately 2 × 10-5 mol∙L-1 and 1 × 10-5 mol∙L-1, respectively.  Solution-phase electrochemical data were recorded with a CHI660D potentiostat at room temperature using a platinum wire counter electrode and a platinum working electrode. Ag/AgCl in saturated KCl was used as the reference electrode and was calibrated versus the normal hydrogen electrode (NHE) by the addition of Fc+/Fc.  A 0.1 M n-NBu4PF6 electrolyte solution in DCM was used for all HTMs. CV data were acquired for 0.5 mM solutions of each HTM at a scan rate of 50 mV s-1. DPV data were collected with the same instrument, electrodes and electrolyte conditions with an amplitude of 0.05 V, a pulse width of 0.05 s, sampling period of 0.5 s and pulse period of 0.5 s.   4.4.4 Bulk thermal properties DSC curves of compounds were collected using a Netzsch DSC 214 Polyma instrument under a nitrogen protective atmosphere. For each measurement, two aluminum crucibles were used, one for material measurement and the other as empty reference. A customized heating program was developed to include a 5 min hold at the initial temperature of 50 °C, and four continuous cycles between to 50 °C and 300 °C with a heating/cooling rate of 10 K/min and 5 min holds at 50 °C  69 and 300 °C. Data from the heating segment of last 3 cycles were used to identify the Tm and extrapolate the Tg of the material. 4.4.5 Electrical properties The hole-only devices were fabricated with a structure of glass/ITO/PEDOT:PSS/HTMs/Au according to previous literature.20 The HTMs were doped with tBP and LiTFSI. The dopant concentration was kept constant for all HTMs and the same as used in PSCs. Hole mobility was investigated by the space-charge-limit current (SCLC) method, which can be described as the following equation: µ = (8d3J)/(9ε0εrV2), where µ is the hole mobility, J is the current density, ε0 is the vacuum permittivity, εr is the dielectric constant of the material (normally taken to approach 3 for organic semiconductors), V is the applied bias, and d is the film thickness.115 The applied bias was measured from 0 to 5 V.  Current density-voltage (J-V) characteristics were measured using a Keithley 2400 source meter under illumination of a solar simulator (Newport, Class A) at the light intensity of 100 mW cm−2 as verified using a certified reference solar cell (Newport). A spectral mismatch factor of 1 was used for all J-V measurements. Unless otherwise stated, the instantaneous J-V curves were measured with a scanning rate of 50 mV s-1 (voltage step of 10 mV and delay time of 200 ms) from 1.2 V to -0.1 V. The active area was determined using an aperture shade mask (0.049 cm2) placed in front of the solar cell to avoid overestimation of the photocurrent density. No excess encapsulation or preconditioning procedure was used. 4.4.6 Perovskite solar cell characterization The planar perovskite solar cells were fabricated according to previous work.20 Briefly, pre-patterned indium tin oxide (ITO) coated glass substrates were sequentially cleaned using detergent,  70 acetone, and isopropanol. The TiO2-Cl electron transport layers were spin-coated on ITO substrates from the colloidal nanocrystal solutions, and annealed on a hot plate at 150 °C for 30 min in ambient air. After the substrates had cooled, substrates were transferred immediately to a nitrogen-filled glovebox for the deposition of perovskite films. The Cs0.05FA0.81MA0.14PbI2.55Br0.45 precursor solution (1.4 M) was prepared in a mixed solvent of DMF and DMSO with a volume ratio of 4:1. The molar ratios of PbI2/PbBr2 and FAI/MABr were both fixed at 0.85:0.15, molar ratio of CsI/(FAI+MABr) is 0.05:0.95, and the molar ratio of (FAI+MABr+CsI)/(PbI2+PbBr2) was fixed at 1:1. The perovskite films were deposited onto the TiO2-Cl substrates with two-step spin coating procedures. The first step was 2000 rpm for 10 s with an acceleration of 200 rpm/s. The second step was 6000 rpm for 20 s with a ramp-up of 2000 rpm/s. Chlorobenzene (100 µL) was dropped on the spinning substrate during the second spin-coating step at 10 s before the end of the procedure. The substrate was then immediately transferred on a hotplate and heated at 100 °C for 20 min. After cooling down to room temperature, the hole-transport layer was subsequently deposited on top of the perovskite film by spin coating at 4000 rpm for 30 s using a chlorobenzene solution which contained 72.3 mg/mL of HTMs and 28.8 µL/mL of tBP, as well as 17.5 µL/mL of LiTFSI (520 mg/mL in acetonitrile). Finally, 100 nm Au contact was deposited on top of the HTM film by electron-beam evaporation in an Angstrom Engineering deposition system.         71 Chapter 5 : Polymerizable Hole-Transport Materials for Improved Moisture Stability of Perovskite Solar Cells 5.1 Introduction There are many challenges that remain for perovskite solar cells (PSCs) to be a commercially-deployed technology.2,3 One key challenge is the sensitivity of the photo-active perovskite layer to water. Water drives the conversion of the 3-dimensional AMX3 structure into 2-dimensional monohydrate sheets (i.e., AMX3ᐧ H2O) and eventually into isolated dihydrate octahedra formulated as A2MX6 ᐧ 2H2O (Equations 2-3 and 2-4).6 One solution to this problem is to encapsulate the PSC to simply prevent water from entering the device. Hwang and coworkers achieved negligible decreases in the PCEs of PSCs over 30 days in ambient environment by coating the entire solar cell with hydrophobic Teflon (polytetrafluoroethylene, PTFE) 126 while Dong et al. demonstrated the use of silica (SiO2) encapsulation of PSCs for stabilized PCEs in 65% humid environments.127 The use of carbon films,128 epoxy resins,129 and HTM design for PSC encapsulation have also been investigated as engineering approaches to improving PSC stability. All these approaches add extra components and weight to the solar cell. One potentially powerful approach is to use polymeric HTM films to preclude water from reaching the perovskite layer. Kelly et al. investigated the use of hole-transporting P3HT nanowires in hydrophobic PMMA matrices as the HTM layer of a PSC and saw improved stability of devices in liquid phase stability studies.12 Others have employed PEDOT:PSS mixtures as the hole-transporting layer. 43,130 Solution deposition of polymers can be challenging, however, because the low solubilities of these high molecular weight materials creating processing challenges.  To overcome this issue, the photo-driven, electrochemical and thermal polymerization of solution-deposited monomers in situ provides an alternative means of accessing polymer HTM  72 layers.  UV-active cinnamate-based moieties have been appended to traditional triphenylamine (TPA)-based HTMs to obtain photo-crosslinkable HTMs.131 Zhu et al. electrochemically polymerized carbazole-containing monomers on indium-doped tin oxide (ITO) conductive glass for HTM layers in inverted PSC architectures.110 Sargent and coworkers thermally crosslinked monomers of VNPB (Figure 5-1), which were capable of reaching PCEs of 16.5% in a PSC.27 Only a handful of in situ polymerization approaches and functionalities have been explored, providing the opportunity to design HTMs with the desirable properties of state-of-the-art HTMs and new crosslinkable functionalities.   Figure 5-1 Molecular structures of VNPB,27 HTM-FX′26,119  and spiro-VNPB.   The challenges of designing HTMs with crosslinkable functionality are to maintain: appropriate redox potentials for charge extraction; high glass transition temperatures; and high conductivities for use in PSCs. We recently demonstrated the importance of methoxy-substituted  73 TPA units flanking fluorene moieties to maintain a highest occupied molecular orbital (HOMO) energy level (EHOMO) at 0.69 V vs NHE and ensure sufficient (i.e., >70 mV) free energy change for hole extraction from the perovskite valence band.26 We also determined, in work reported in Chapter 4, that meta-substitution on the xanthene portion of the spiro[fluorene-9,9'-xanthene] core of HTM-FX′ improved the thermal stability of the material (characterized by glass transition temperature, Tg) relative to para-substitution.  We set out to develop a polymerizable HTM that exhibits similar electrochemistry and thermal stability as state-of-the-art HTMs, as well as low wettability as a film intended for PSC encapsulation. spiro-VNPB was designed to contain: the redox-active TPA units of HTM-FX′; the thermal stability (i.e., Tg > 90 °C) of HTM-FX′; and the crosslinkable functionality of VNPB. Hence, styrene-containing aryl amines were installed in the meta-position on the xanthene to create polymerizable spiro-VNPB with high Tg values (Tg = 151 °C). We hypothesized that spiro-VNPB could be polymerized by electrochemical and thermal means to produce polymerized spiro-VNPB with lower film wettability than control compound spiro-NPB as determined by time-resolved contact angle measurements. We also aimed to demonstrate the use of polymerized spiro-VNPB in an operating PSC.  5.2 Results and discussion 5.2.1 Synthesis  The spiro[fluorene-9,9'-xanthene] core was synthesized with bromide and chloride substituents on the different halves for subsequent selective coupling at the fluorene and xanthene portions of the molecule (Scheme 5-1). Syntheses of spiro-VNPB and spiro-NPB proceeded similar to that of HTM-FX′ with the spiro[fluorene-9,9'-xanthene] core being synthesized in >95%  74 yield in a condensation reaction. Selective Buchwald-Hartwig coupling at the aryl bromide was achieved with different conditions (i.e., 100 °C with palladium acetate and tri-tert-butylphosphine ligands) than at the aryl chloride (i.e., 110 °C with PEPPSI-iPr catalyst). As such, methoxy-substituted TPA units were installed on the fluorene moiety for similar electrochemistry as HTM-FX′ and vinyl-functionalized TPA units were installed at the meta-position of the xanthene moiety for greater thermal stability and the opportunity to polymerize HTM monomers.   75  Scheme 5-1 Synthetic scheme for spiro-VNPB and spiro-NPB  76 5.2.2 Electrochemical and photophysical properties The electrochemical properties of spiro-NPB and spiro-VNPB in 0.1 M n-NBu4PF6 DCM solutions were tested by cyclic voltammetry (Figure 5-2). The cyclic voltammograms (CVs) recorded on spiro-NPB revealed four Faradaic events, which we attribute to the oxidation of the four aryl amine groups appended to the spiro core.  The CVs were reproducible over 20 scans.  In the case of spiro-VNPB, an oxidative scan produced two reversible oxidation events at 0.71 and 0.95 V vs NHE followed by an irreversible oxidation at higher potentials. Successive CV cycling between 0.5 V and 1.6 V range led to a progressively increasing current response, indicative of conductive polymer deposition on the working electrode at higher potentials.  We observed a purple film deposited on the working electrode following the 20 scans between 0.5 V and 1.6 V vs NHE or when collecting differential pulse voltammograms of spiro-VNPB above 1.2 V vs NHE (Figure 5-3). No polymerization was observed when spiro-VNPB was subjected to cycling over a narrower range of 0.5-1.2 V vs NHE (Figure 5-2). These results point to the electropolymerization and deposition of spiro-VNPB occurring when the vinyl-containing aryl amine units are oxidized at potentials above 1.2 V vs NHE.     77   Figure 5-2 Cyclic voltammograms (CVs) over 20 scans for spiro-NPB between 0.5-1.6 V vs NHE (grey), spiro-VNPB between 0.5-1.2 V vs NHE (teal) and spiro-VNPB between 0.5-1.6 V vs NHE (purple) recorded in 0.1 M n-NBu4,PF6 DCM solutions at room temperature. Vertical arrows indicate increasing current response with subsequent scans suggestive of electropolymerization. Horizontal arrows indicate open-circuit potential.  Figure 5-3 Differential pulse voltammograms (DPV) for spiro-NPB (black) and spiro-VNPB (teal)  78  The EHOMO for spiro-NPB and spiro-VNPB were measured to be 0.71 and 0.72 V, respectively.  These EHOMO values indicated that the free energy change for charge extraction from the perovskite layer is similar to HTM-FX′ (EHOMO =  0.67 V vs NHE). The UV-visible absorption and emission maxima (Figure 5-4 and Table 5-1) of spiro-VNPB and spiro-NPB confirm that all materials under study absorb minimally in the visible region which may be beneficial in maximizing the solar absorption by the perovskite layer in a PSC. Absorption and emission onsets are the same for both materials and, hence, the bandgap (EHOMO-LUMO) determined from the intersection of these two spectra are identical for the two materials.  5.2.3 Bulk thermal properties The Tg values of 147 °C and 151 °C for spiro-NPB and spiro-VNPB, respectively, were determined by differential scanning calorimetry (DSC; Table 5-1), and satisfy the key requirement that the Tg must be greater than 90 °C to remain in the amorphous phase during device operation.  Both of these values are also higher than that measured for commercially available X60 (Tg = 107 °C) owing to the bulky aryl amine groups being positioned at the meta-position of the xanthene moiety.26  Figure 5-4 Absorption profiles for spiro-NPB (black) and spiro-VNPB (teal)  79   Table 5-1 Electrochemical, photophysical, and glass transition temperature data for spiro-NPB and spiro-VNPB Compound EHOMO a (V vs NHE) λmax  b (nm) λem  c (nm) Tgd (°C) spiro-NPB 0.71 362, 306 428 147 spiro-VNPB 0.72 316, 339, 392 428 151 a The half-wave potential corresponding to the first and second oxidation processes in the cyclic voltammogram. Measured in DCM and referenced to NHE by addition of ferrocene. b Absorption maxima for HTM in DCM. c Excitation at higher energy λmax in DCM d Determined by DSC. Both Tm values determined to be >300 °C and above the detection limits of our instrument.   5.2.4 Contact angle measurements  The contact angle of a water droplet on the surface of a film reports the wettability of a material.132,133 The retention of a water droplet on the surface of a film indicates low wettability and the ability to prevent liquid water ingress below the encapsulation layer. Contact angle measurements of water droplets on spin-coated spiro-OMeTAD, spiro-NPB, non-polymerized spiro-VNPB and polymerized spiro-VNPB films on glass substrates were carried to determine the wettability of the various HTM films. Photographic images of the droplets (backlit for contrast) were captured and contact angles were measured using inhouse software. Spiro-OMeTAD, spiro-NPB and thermally polymerized spiro-VNPB held water droplets with similar contact angles of 70, 73 and 72°, respectively; however, the droplets collapsed within seconds to wet the glass substrates below and delaminate the films from the surface. A contact angle of 79 ° was observed for thermally polymerized spiro-VNPB films which retained water droplets (>60 °) throughout the 30-min experiment.  The overall decrease in size of the water droplet implicates evaporation as the cause for the contact angle decrease (Figure 5-5).   80  Figure 5-5 Contact angle data of spiro-OMeTAD, spiro-NPB, spiro-VNPB and thermally polymerized spiro-VNPB over 30 minutes.   5.2.5 Electrical properties We measured the sheet resistivity of ~100 nm thick films of spiro-OMeTAD, spiro-NPB, non-polymerized spiro-VNPB and thermally polymerized spiro-VNPB (5 min at 150 °C) spin-coated on glass substrates with 20 mol% LiTFSI dopant and 250 mol% tBP additive, during 180 min of air exposure (Figure 5-6) to test the feasibility of these films as charge transport layers in PSCs. Spiro-OMeTAD was significantly less resistive than spiro-NPB, spiro-VNPB and thermally polymerized spiro-VNPB. The high resistivity of spiro-NPB, spiro-VNPB and thermally polymerized spiro-VNPB indicated that the naphthyl substituents appended to the xanthene moiety are more insulating than the phenyl substituents common to both spiro-OMeTAD and X60. The high resistivity of these novel materials may limit their use as the sole HTM layer in PSCs. Thermally polymerized spiro-VNPB exhibited lower sheet resistivity than molecular spiro-VNPB which supports the polymerization of this material for PSC encapsulation.  81  Figure 5-6 Sheet resistance data for spiro-OMeTAD, spiro-NPB, spiro-VNPB and thermally polymerized spiro-VNPB   5.2.6 Perovskite solar cell characterization PSC studies were carried out by Dr. Hairen Tan of the Sargent group at the University of Toronto. Spiro-VNPB was incorporated into a perovskite device by annealing a 10-nm thick layer above the perovskite layer and below a hole-transporting spiro-OMeTAD layer (Table 5-2). PSCs with the spiro-VNPB encapsulating layer yielded PCEs of 6.1% while devices containing exclusively spiro-OMeTAD yielded PCEs of 18.0%. This 12% difference in PCEs was attributed to the higher sheet resistivity of annealed spiro-VNPB relative to spiro-OMeTAD. The deleterious impact of high sheet resistivity was confirmed by constructing PSCs with exclusively spiro-VNPB which yielded PCEs of merely 1.2%. The high resistivity and low PCE yielded by spiro-VNPB prompts research into polymerizable HTMs with higher conductivity imparted by stable dopants (e.g., F4-TCNQ) or optimized encapsulation layer thickness (e.g., <10 nm).   82  Table 5-2 Statistical performance data of spiro-OMeTAD and spiro-VNPB in perovskite solar cell devices. material Voc (V) Jsc (mA/cm2) FF  PCE (%) spiro-OMeTAD 1.10 21.4 0.77 18.0 spiro-VNPB / spiro-OMeTAD  0.93 13.9 0.47 6.1 spiro-VNPB (thermally polymerized) 1.03 3.1 0.39 1.2 Spiro-VNPB was thermally polymerized at 150 °C for 15 minutes. Data collected for Cs0.05FA0.81MA0.14PbI2.55Br0.45 perovskite layer on TiO2-Cl under of 100 mW cm−2 lamp.   5.3 Conclusion  We designed spiro-VNPB to contain: the redox active TPA units and thermal stability (i.e., Tg > 90 °C) of HTM-FX′; and the crosslinking functionality of VNPB. We discovered that spiro-VNPB could be polymerized either electrochemically (at potentials greater than 1.2 V vs NHE), or thermally (at temperatures above 120 °C). Contact angle measurements demonstrated that ther mal polymerization of spiro-VNPB decreased the wettability of films and prevented the penetration of water through the HTM film, relative to films of spiro-OMeTAD, spiro-NPB and non-polymerized spiro-VNPB. The lower wettability of polymerized spiro-VNPB demonstrated that it could be used as an encapsulation layer to potentially improve the moisture stability of PSCs. PSCs fabricated with spiro-VNPB as an encapsulation layer and spiro-OMeTAD as an HTM layer resulted in PCEs of 6.1%. Further research into optimizing the encapsulation process (i.e., conductivity, layer thickness and polymerization method) is ongoing.   83 5.4 Experimental 5.4.1 Synthesis   All reagents were obtained from MilliporeSigma, Alfa Aesar or Fisher Scientific and used as received. All reactions were performed using toluene from a solvent purification system prior to use. Standard inert atmosphere and Schlenk techniques were carried out under nitrogen. All reactions and purification were carried out in the dark. Purification by column chromatography was performed using silica (Silicycle: Ultrapure Flash Silica). Analytical thin-layer chromatography (TLC) was performed on aluminum-backed sheets pre-coated with silica 60 F-254 adsorbent (250 μm thick; Silicycle, QC, Canada) and visualized under UV light. Routine 1H and 13C NMR spectra were collected on a Bruker AV400 inv/dir instrument at ambient temperatures, operating at 300 MHz and 100 MHz, for 1H and 13C nuclei, respectively. Chemical shifts (δ) are reported in parts per million (ppm) using the residual signals δ 7.26 and 77.0 for CDCl3, δ 2.05 and 29.8 for Acetone-d6 and δ 2.50 and 39.4 for DMSO-d6 as internal references for 1H and 13C, respectively. Standard abbreviations indicating multiplicity are used as follows: s = singlet; d = doublet; t = triplet; m = multiplet. Y2Br2Cl (2,7-dibromo-3',6'-dichlorospiro[fluorene-9,9'-xanthene]): 2,7-dibromo-9-flourenone (1.50 g, 4.43 mmol), 3-chlorophenol (5.50 g, 45.0 mmol) and methane sulfonic acid (1.7 mL, 17.5 mmol) were mixed at 140 °C for 18 h under N2. The reaction mixture was cooled to room temperature and product was precipitated with the addition of MeOH. Separation by filtration and rinsing with MeOH yielded 2.36 g (95.4%) of product as white powder. 1H NMR (400 MHz, CDCl3):  δ =  7.63 (d, J = 8.1 Hz, 2H), 7.52 (dd, J = 8.1, 1.8 Hz, 2H), 7.28 – 7.16 (d, 2H), 6.82 (dd, J = 8.5, 2.1 Hz, 2H), 6.30 (d, J = 8.5 Hz, 2H). 13C NMR (100 MHz, CDCl3): δ = 155.9, 151.2,  84 137.5, 134.2, 131.8, 129.0, 128.9, 124.4, 122.6, 121.6, 117.4, 77.3, 77.0, 76.7. HRMS (EI): m/z = 555.86333 [M]+ (calcd for [C25H12Br2Cl2O]+:  m/z = 555.86319). Y592Cl (3',6'-dichloro-N2,N2,N7,N7-tetrakis(4-methoxyphenyl)spiro[fluorene-9,9'-xanthene]-2,7-diamine): Y2Br2Cl (0.80 g, 1.43 mmol), bis(4-methoxyphenyl)amine (0.66 g, 2.87 mmol), palladium acetate (0.023 g, 0.11 mmol),  tri-tert-butylphosphine (0.023 g, 0.11 mmol) and potassium tert-butoxide (0.34 g, 0.64 mmol) were added to toluene (15 mL) under N2. The reaction mixture was stirred at reflux for 16 h. The reaction mixture was cooled to room temperature. Purification by silica column chromatography (SiO2: hexanes/EtOAc, 4:1; Rf = 0.31) yielded 1.11 g (91.0%) of product as yellow powder. 1H NMR (400 MHz, Acetone-d6) δ =  7.61 (d, J = 8.4 Hz, 2H), 7.16 (d, J = 8.4 Hz, 2H), 7.01 (dd, J = 8.4, 2.2 Hz, 2H), 6.95 – 6.74 (m, 18H), 6.68 (d, J = 2.1 Hz, 2H), 6.59 (d, J = 8.4 Hz, 2H), 3.74 (s, 12H). 13C NMR (101 MHz, Acetone-d6) δ =  206.3, 157.2, 157.1, 155.8, 152.3, 149.5, 141.8, 141.7, 134.0, 133.4, 130.2, 127.2, 127.2, 125.3, 125.1, 121.7, 121.5, 121.0, 120.9, 118.2, 117.6, 117.5, 115.6, 115.6, 55.8, 54.4, 30.6, 30.4, 30.2, 30.0, 29.8, 29.6, 29.4, 29.3. HRMS (ESI): m/z = 854.2305 [M+H]+ (calcd for [C53H40N2Cl2O5]+:  m/z = 854.2314). spiro-VNPB (N2,N2,N7,N7-tetrakis(4-methoxyphenyl)-N3',N6'-di(naphthalen-1-yl)-N3',N6'-bis(4-vinylpheny)spiro[fluorene-9,9'-xanthene]-2,3',6',7-tetraamine): Y59-Cl (1.12 g, 1.31 mmol), N-(4-vinylphenyl)naphthalen-1-amine (0.80 g, 3.27 mmol), PEPPSI-iPr (0.089 g, 0.13 mmol),  and potassium tert-butoxide (0.50 g, 5.2 mmol) were added to toluene (15 mL) under N2. The reaction mixture was stirred at reflux for 16 h. The reaction mixture was cooled to room temperature. Purification by silica column chromatography (SiO2: hexanes/EtOAc, 4:1; Rf = 0.22) yielded 0.65 g (40.7 %) of product as pale yellow powder. 1H NMR (400 MHz, Acetone-d6)  85 δ = 7.94 (d, J = 8.1 Hz, 2H), 7.88(dd,  J = 8.4, 2.2 Hz, 2H), 7.58 – 7.40 (m, 4H), 7.44 – 7.09 (m, 12H), 6.99 – 6.83 (m, 12H), 6.83 – 6.68 (m, 12H), 6.64 (d, J=8.2 Hz, 2H),  6.60 (dd, J = 8.1, 1.1 Hz, 2H), 6.50 (s, 2H), 6.44 (d, J = 8.2 Hz, 2H), 5.61 (dd, J = 17.6, 1.1 Hz, 2H), 5.07 (dd, J = 10.9, 1.1 Hz, 2H), 3.73 (s, 12H). 13C NMR (101 MHz, Acetone-d6) δ =  206.3, 157.0, 156.4, 152.8, 149.1, 149.0, 148.6, 143.8, 141.9, 137.4, 136.5, 133.6, 132.6, 132.2, 129.9, 129.6, 129.2, 129.2, 128.4, 128.2, 128.0, 127.7, 127.5, 127.4, 127.2, 126.3, 124.8, 122.6, 121.40, 120.8, 120.0, 118.5, 118.0, 115.6, 112.5, 110.0, 55.9, 30.6, 30.4, 30.2, 30.0, 29.8, 29.6, 29.4. HRMS (ESI): m/z = 1273.5260 [M+H]+ (calcd for [C89H69N4O9]+:  m/z = 1273.5268). spiro-NPB (N2,N2,N7,N7-tetrakis(4-methoxyphenyl)-N3',N6'-di(naphthalen-1-yl)-N3',N6'-diphenylspiro[fluorene-9,9'-xanthene]-2,3',6',7-tetraamine) Y59-Cl (0.55 g, 0.64 mmol), N-phenylnaphthalen-1-amine (0.35 g, 1.61 mmol), PEPPSI-iPr (0.044 g, 0.064 mmol),  and potassium tert-butoxide (0.247 g, 2.6 mmol) were added to toluene (15 mL) under N2. The reaction mixture was stirred at reflux for 16 h. The reaction mixture was cooled to room temperature. Purification by silica column chromatography (SiO2: hexanes/EtOAc, 4:1; Rf = 0.22) yielded 0.32 g (41.5 %) of product as pale brown solid. 1H NMR (400 MHz, Acetone-d6) δ 7.92 (d, J = 8.2 Hz, 2H), 7.88 (d, J = 8.5 Hz, 2H), 7.83 (d, J = 8.3 Hz, 2H), 7.47 (m, 6H), 7.35 (m, 4H), 7.30 (m, 8H), 7.17 (d, J = 8.4 Hz, 4H), 6.96 (d, J = 7.8 Hz, 4H), 6.88 (m, 8H), 6.74 (m, 12H), 6.54 (dd, J = 8.6, 2.2 Hz, 2H), 6.47 – 6.37 (m, 4H), 3.71 (s, 12H). 13C NMR (101 MHz, Acetone-d6) δ 206.3, 157.0, 156.5, 152.8, 149.3, 149.1, 148.9, 144.0, 141.8, 138.6, 136.5, 133.6, 132.3, 130.2, 129.9, 129.6, 129.2, 129.1, 128.4, 127.9, 127.6, 127.5, 127.3\, 127.1, 126.3, 124.8, 123.3, 123.2, 121.4, 120.7, 119.5, 118.5, 117.4, 115.6, 109.3, 55.9, 54.4. HRMS (ESI): m/z = 1221.4987 [M+H]+ (calcd for [C85H65N4O5]+:  m/z = 1221.4955).  86 5.4.2 Electrochemical and photophysical properties     Solution UV-Vis absorption spectra were collected on a Cary 5000 spectrophotometer. Photoluminescence spectra were recorded with a Cary Eclipse fluorimeter. All samples were measured in a 1-cm quartz cell at room temperature in HPLC grade DCM. The concentrations of the DCM solutions of analytes for UV-Vis and photoluminescence measurements were 2 × 10-5 mol L-1 and 1 × 10-5 mol L-1, respectively.  Solution electrochemical data were recorded with a CHI660D potentiostat at room temperature using a platinum wire counter electrode and a platinum working electrode. Ag/AgCl in saturated KCl was used as the reference electrode and was calibrated versus the normal hydrogen electrode (NHE) by the addition of Fc+/Fc.  A 0.1 M n-NBu4PF6 DCM solution was used for all HTMs. CV data were acquired for 0.5 mM solutions of each HTM at a scan rate of 50 mV s-1.  5.4.3 Bulk thermal properties DSC curves of compounds were collected using a Netzsch DSC 214 Polyma instrument under a nitrogen protective atmosphere. For each measurement, two aluminum crucibles were used, one for material measurement and the other as empty reference. A customized heating program was developed to include a 5 min hold at the initial temperature of 50 °C, and four continuous cycles between to 50 °C and 300 °C with a heating/cooling rate of 10 K/min and 5 min holds at 50 °C and 300 °C. Data from the heating segment of last 3 cycles were used to identify the Tm and extrapolate the Tg of the material.  5.4.4 Contact angle measurements  Contact angles were measured using a camera capture of a water droplet on an HTM surface spin-coated (at 3000 rpm for 45 seconds) on glass substrates. Images were taken following  87 deposition of a single 10 µL droplet of deionized water on the HTM surface. When droplets were stable on HTM surface (angle >25°C), the image was further analyzed by inhouse software. Images were captured after 30 minutes to assess droplet stability on the film surface.  5.4.5 Electrical properties Four parallel Au electrodes with a spacing of 0.75 mm and length of 23 mm were used to measure the film conductivities in the dark and under ambient conditions. A Keithley 2400 sourcemeter was used to force current through the outer electrodes while sensing the voltage across the inner two electrodes. A linear fit of each measurement was used to determine the resistance, R. The film thickness was measured using a Bruker DektakXT profilometer.   5.4.6 Perovskite solar cell characterization The planar perovskite solar cells were fabricated according to our previous work.20 Briefly, pre-patterned indium tin oxide (ITO) coated glass substrates were sequentially cleaned using detergent, acetone, and isopropanol. The TiO2-Cl electron transport layers were spin-coated on ITO substrates from the colloidal nanocrystal solutions, and annealed on a hot plate at 150 °C for 30 min in ambient air. After the substrates had cooled, we transferred the substrates immediately to a nitrogen-filled glovebox for the deposition of perovskite films. The Cs0.05FA0.81MA0.14PbI2.55Br0.45 precursor solution (1.4 M) was prepared in a mixed solvent of DMF and DMSO with a volume ratio of 4:1. The molar ratios of PbI2/PbBr2 and FAI/MABr were both fixed at 0.85:0.15, molar ratio of CsI/(FAI+MABr) is 0.05:0.95, and the molar ratio of (FAI+MABr+CsI)/(PbI2+PbBr2) was fixed at 1:1. The perovskite films were deposited onto the TiO2-Cl substrates with two-step spin coating procedures. The first step was 2000 rpm for 10 s  88 with an acceleration of 200 rpm/s. The second step was 6000 rpm for 20 s with a ramp-up of 2000 rpm/s. Chlorobenzene (100 µL) was dropped on the spinning substrate during the second spin-coating step at 10 s before the end of the procedure. The substrate was then immediately transferred on a hotplate and heated at 100 °C for 20 min. After cooling down to room temperature, the hole-transport layer was subsequently deposited on top of the perovskite film by spin coating at 4000 rpm for 30 s using a chlorobenzene solution which contained 72.3 mg/mL of HTMs and 28.8 µL/mL of tBP, as well as 17.5 µL/mL of LiTFSI (520 mg/mL in acetonitrile). Finally, 100 nm Au contact was deposited on top of HTM films by electron-beam evaporation in an Angstrom Engineering deposition system.  Current density-voltage (J-V) characteristics were measured using a Keithley 2400 source meter under illumination of a solar simulator (Newport, Class A) at the light intensity of 100 mW cm−2 as verified using a certified reference solar cell (Newport). A spectral mismatch factor of 1 was used for all J-V measurements. Unless otherwise stated, the instantaneous J-V curves were measured with a scanning rate of 50 mV s-1 from 1.2 V to -0.1 V. The active area was determined using an aperture shade mask (0.049 cm2) placed in front of the solar cell to avoid overestimation of the photocurrent density.         89 Chapter 6 : Effect of Donor-Acceptor Architecture on Hole-Transport Material Hole Mobility 6.1 Introduction Perovskite solar cells (PSCs) rely on hole-transport materials (HTMs) to selectively conduct holes from the photoactive perovskite layer to the cathode. HTM layers are usually 100-200 nm thick films of extended metal oxide solids (e.g., NiOx)24,134,135 or redox-active organic compounds (e.g.,  spiro-OMeTAD).68,136 Organic HTMs are attractive due to their readily modifiable structures that enable acute control of the frontier energy levels, light absorption, and thermal stability.77,137 Organic HTMs typically require doping in order to optimize conductivity and to realize high device power conversion efficiencies (PCEs).79,138 Doping is most frequently achieved through chemical oxidation with an ancillary oxidant. LiTFSI is an ionic dopant added to HTM layers that has been utilized in PSC devices to yield PCEs greater than 20%.136,139  However, lithium ions are hygroscopic and can therefore draw water into the PSC, causing dissolution of the photoactive perovskite layer.12 Lithium ions are also susceptible to migration through an HTM film during PSC operation, leading to inconsistencies in the film morphology and conductivity over time.85,140 Another common ionic dopant is FK209, which can also yield PCEs that are >20%.141,142 Seo et al. recently demonstrated that Zn(TFSI)2 is a stable dopant, but it is currently sold at 10 times the price of LiTFSI from distributors such as MilliporeSigma.28,143 Molecular dopants, such as F4-TCNQ, avoid the hygroscopic nature of lithium ions, but still exhibit a 30% decrease in device PCE over eight days of stability studies.29 This scenario provides the impetus to make “dopant-free” HTMs, which do not require hygroscopic, migratory and/or expensive additives that compromise device stability and deployment.  90 Donor-acceptor (D-A) HTMs are a promising class of molecules because they can yield high conductivities without dopants.93,144 D-A HTMs contain at least one electron-rich donor (e.g., triphenylamine or S,N-heteropentacene) and at least one electron-withdrawing acceptor group (e.g., dicyanoacrylate or 3-(3-ethylrhodanine)).93 D-A molecules typically provide predictable redox chemistry at the donor unit and strong light absorption properties owing to the allowed HOMO-LUMO (i.e., D-A) electronic transitions.91,145 It is currently not clear why these D-A materials exhibit high conductivities without the use of dopants, but I submit that the ability for these molecules to exist in a charge-separated form when exposed to visible light may be integral to yielding a “pseudo-doped” species when photoexcited.  The work reported in this chapter seeks to develop a structure-property relationship between the D-A architecture (e.g., monopodal, bipodal, and tripodal architectures) and hole mobility. Previous research has been limited to only comparing the effect of varying the donor, π-bridge, or acceptor identities between similar D-A architectures. Yang et al. examined the effect of donor group identity between two structurally-similar (monopodal) D-A HTMs and determined that HTMs with benzo[1,2-b:4,5-b′]dithiophene donor units yielded higher mobility films than a fluorinated alternative (i.e., 5,6-difluoro-2,1,3-benzothiadiazole).94  Steck and coworkers compared thiophene and EDOT π-bridges between donor and acceptor moieties in structurally-similar (bipodal) A-D-A HTMs and found that thiophene linkages yielded HTMs with minimal visible light absorption, which likely allowed for greater light absorption by the perovskite layer.95  A variety of other D-A HTMs have been disclosed in literature but to the best of my knowledge, no study has been conducted on the effect of structurally-dissimilar D-A architectures on the hole mobilities of HTMs.  91 In this chapter, I report on-going investigations of monopodal, bipodal, and tripodal D-A architectures to determine which form exhibits the highest hole mobility. I conducted a controlled study of three homologous D-A HTMs bearing monopodal, bipodal, or tripodal architectures (TPA-1, TPA-2, and TPA-3, respectively; Figure 6-1). Triphenylamine (TPA) was selected as the donor unit because it is electron-rich and displays the reversible redox chemistry necessary for hole extraction and hole transport by the HTM.146,147 Thiophene was used as the π-bridge and dicyanoacrylate was used as the acceptor group. Preliminary results reveal that TPA-1 exhibited the highest hole mobility of the series. I provide herein a comprehensive summary of the electrochemical, thermal, and photophysical properties in tandem with computational data (e.g., reorganization energy, polaron stabilization energy, hole extraction potential) in pursuit of understanding why this topology yields the highest hole mobility.  Figure 6-1 Molecular representations of TPA-1, TPA-2, and TPA-3 (collectively referred to as TPA-X series) which vary in D-A architectures.  92 6.2 Results and discussion 6.2.1 Synthesis The number of acceptor groups had a significant impact on the ease of synthesis for the three HTMs (collectively referred to as TPA-X) in the series. Different synthetic approaches were used for each material based on the increasing difficulty of coupling one, two, or three aldehyde-substituted thiophene groups (electron-withdrawing units) by Suzuki coupling (Scheme 6-1). In the case of TPA-3, the Suzuki coupling of three equivalents of (5-formylthiophen-2-yl)boronic acid to a tri-brominated TPA was low yielding (<20% isolated yield for the penultimate product) and, hence, this step was amended to the coupling of 2-thienylboronic acid (74% isolated yield) followed by an acylation reaction (72% isolated yield) for an overall yield of 53% for the penultimate product. The general synthetic pathway for all three HTMs, however, started with a mono-, di- or tri-brominated TPA, followed by the installation of the pi-bridge and aldehyde group in one or two steps, and finished with a Knoevenagel condensation to append the cyanoacrylate acceptor group in place of the aldehyde (Scheme 6-1). For characterization, purified HTMs were dissolved in toluene and spin-coated on glass or indium-doped tin oxide (ITO) on glass substrates at 2000-3000 rpm for ~100 nm thick films, as determined by optical profilometry.  93  Scheme 6-1 Synthesis schemes for TPA-1, TPA-2, and TPA-3  94 6.2.2 Electrical properties  Hole mobility is the intrinsic ability of a material to transport holes through a film and is independent of factors such as doping and oxidation by air.  I prepared “hole-only devices” (i.e., ITO/PEDOT:PSS/HTM/Au; Figure 2a) to measure the hole mobilities of HTM films using the space-charge limited current (SCLC) method (Figure 2b). Films were prepared by spin-coating dissolved solutions of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and then TPA-X series on etched ITO substrates. Gold contacts were deposited by sputtering to complete the hole-only devices. Hole mobility (µh) is calculated from quadratic (SCLC) region of current-voltage (J-V) plots where current is controlled by the charge carriers injected by contacts and is not impacted by the number of charge carriers present in the film from doping or ambient oxidation of the material. The µh can be extracted from the slope of the SCLC region using Equation 6-1:            μh = (8d3J)/(9ε0εrV2)            (Equation 6-1) where d is the film thickness, J is the current density, ε0 is the vacuum permittivity, εr is the dielectric constant of the material (normally assumed to be 3 for organic semiconductors), and V is the applied bias. An HTM with intrinsically high hole mobility will ensure sustained current in a cell and will contribute to higher fill factors (FFs). The measurements on the TPA-X series were found to be in the range of 2.88 × 10-5 - 4.49 × 10-4 cm2/Vs (Figure 6-3).  TPA-1 produced the highest hole mobility (4.49 × 10-4 cm2/Vs) for the series, and is commensurate with the state-of-the-art HTM spiro-OMeTAD without the use of dopants.   95    Figure 6-2 a) Schematic of hole-only device for hole mobility measurements and b) hypothetical J-V curve of a hole-only device highlighting the ohmic and SCLC regimes for HTM hole mobility measurements.    Figure 6-3 Hole mobilities determined from SCLC region of hole-only devices containing spiro-OMeTAD, TPA-1, TPA-2, and TPA-3.  6.2.3 Bulk thermal properties I then set out to resolve why TPA-1 yields the highest hole mobility for this series of amorphous HTM films.  I investigated the glass transition temperature (the temperature at which a bulk material transitions from glassy, amorphous phase to a different phase) by measuring the  96 endothermic phase transition with differential scanning calorimetry (DSC).  The trend shows that increasing the number of acceptor units increases the glass transition temperatures (Tg) of the bulk HTMs: The Tgs of TPA-1, TPA-2, and TPA-3 were 59, 116, and 127 °C, respectively (Table 6-1).  These Tg values, along with grazing incidence X-ray diffraction (XRD) data (Figure 6-4), indicate that all three HTMs are amorphous glasses when deposited by spin-coating at room temperature.  On this basis, we can rule out different phases being responsible for the differences in hole mobilities.  It is also worth noting that TPA-1 has a Tg of 59 °C, which is typically too low for use in devices (values of >90 °C are needed for stable operation). The less conductive TPA-2 and TPA-3 films however, both exhibited higher Tg values (116 and 127 °C, respectively) appropriate for use in a PSC.  Table 6-1 Electrical, electrochemical, photophysical, and bulk thermal properties of TPA-1, TPA-2, and TPA-3 Compound hole mobility (m2/Vs) IPv (eV) εHOMO a (V vs NHE) EHOMO b (V vs NHE) λmax  c (nm) λem  d (nm) Tge (°C) TPA-1 4.49×10-4 6.56 1.14 1.19 517  649 59 TPA-2 2.88×10-5 6.74 1.24 1.24 519 642 116 TPA-3 1.17×10-4 6.92 1.34 1.26 504  641 127 a The computed IP(v) and εHOMO were conducted using B3LYP/6-31G(d,p) level of theory and εHOMO was referenced against experimental data for TPA-2. b The half-wave potential corresponding to the first and second oxidation processes in the cyclic voltammogram measured in DCM and referenced to NHE by addition of ferrocene. c Absorption maximum for HTM in DCM. d Excitation at higher energy λmax in DCM. e Determined by differential scanning calorimetry (DSC). All Tm values determined to be >300 °C and above the detection limits of our instrument.   97  Figure 6-4 Grazing incidence XRD data for TPA-1, TPA-2, and TPA-3 on glass  6.2.4 Electrochemical and photophysical properties Increasing the number of electron-withdrawing acceptor groups increased the highest occupied molecular orbital energy level (EHOMO) of the TPA-X series: 1.19, 1.24, and 1.26 V vs NHE for TPA-1, TPA-2 and TPA-3, respectively (cyclic voltammogram (CV) data in Table 6-1 and Figure 6-5). The computed highest occupied molecular orbital (HOMO) energy levels (ϵHOMO) were 1.14, 1.24 and 1.35 V vs NHE. The low EHOMO of TPA-1 is proportional to the adiabatic ionization potential (IPa), which is the difference in energy between the optimized neutral molecule (i.e., HTM) and oxidized molecule (i.e., HTM+; Figure 6-6).  The IPa is proportional to a number of properties related to hole mobility, including reorganization energy and small polaron stabilization energy. It should be noted that the electron-withdrawing substituents result in more stabilized EHOMO values than typically observed for non-D-A HTMs (i.e., 0.67 V vs NHE for state-of-the-art HTM spiro-OMeTAD). The stabilized EHOMO values may preclude use in PSCs because HOMOs may not be sufficiently high in energy (i.e., >70 mV less positive than perovskite valence  98 band) to participate in a one-electron redox chemistry relevant for hole extraction from the perovskite conduction band.   Figure 6-5 Cyclic voltammograms (CVs) of TPA-1, TPA-2, and TPA-3 recorded in 0.1 M n-NBu4PF6 DCM solutions at room temperature. Data were collected at a scan rate of 50 mV/s.   ` Figure 6-6 Visual representation of computed energies relevant to reorganization energy components (λ1 and λ2), hole extraction potential (HEP), and ionization potentials (IP) of neutral and charge HTM molecules.  99  Increasing the number of acceptor units increased the molar extinction coefficients (ε) of TPA-X HTMs (Figure 6-7a).  The increase in ε is fully supported by time-dependent density functional theory (TD-DFT) calculations, which reported increasing oscillator strength of the HOMO-LUMO (D-A) transition from TPA-1 to TPA-3 (Figure 6-7c).  The agreement between experimental and theoretical spectra confirmed the modelling used for TPA-1, TPA-2, and TPA-3 and the parameters relating to hole mobility extracted from the models (i.e., reorganization energies, small polaron stabilization energies, and hole extraction potentials). The increase in vertical ionization potential (IPv in Table 6-1) from TPA-1 to TPA-3 is proportional to a number of properties related to hole mobility, including reorganization energy and small polaron stabilization energy.  Figure 6-7  (a) Experimental UV-Vis absorption, (b) experimental emission, and (c) theoretical absorption spectra for TPA-1, TPA-2, and TPA-3.    100  Figure 6-8 Calculated frontier orbitals of optimized structures for  TPA-1, TPA-2, and TPA-3 plotted at an absolute isovalue of 0.02.  6.2.5 Computed properties Reorganization energy, small polaron stabilization energy, and hole extraction potential (Figure 6-6) are three factors that affect hole mobility and were computed and tabulated in Table 6-2.  Reorganization energy (Ereorg) is the amount of energy required to modify an HTM from neutral geometry to oxidized geometry. According to Marcus theory, the rate of charge transfer is exponentially proportional to Ereorg and, hence, a low Ereorg will contribute to faster hole transfer and higher hole mobility.  The Ereorg values computed for TPA-1, TPA-2, and TPA-3 were 0.23, 0.40, and 0.43 eV, respectively.  The small polaron stabilization energy (λ1) estimates the self-trapping energy of holes on an HTM molecule and is inversely proportional to hole mobility. The small polaron stabilization energies for TPA-1, TPA-2, and TPA-3 are 0.11, 0.19, and 0.27 eV, respectively. Lastly, the hole extraction potential (HEP) must be small to avoid electron-hole pair  101 recombination and to promote charge dissociation with minimal Coulombic interactions.  The hole extraction potentials for TPA-1, TPA-2, and TPA-3 are 6.33, 6.34, and 6.48 eV, respectively.  The reorganization energies, small polaron stabilization energies and hole extraction potentials all predicted that TPA-1 would demonstrate the highest hole mobility of the series under study, thereby supporting my experimental results.  Table 6-2 Computed factors affecting hole mobility for TPA-1, TPA-2, and TPA-3 Compound Ereorg   (eV) small polaron stabilization energy (eV) hole extraction potential (eV) TPA-1 0.23 0.11 6.33 TPA-2 0.40 0.19 6.34 TPA-3 0.43 0.27 6.48  6.3 Conclusions This study provides the first systematic analysis of hole mobility, photophysical, electrochemical, and bulk thermal properties for a series of D-A HTM analogues. Significant differences in hole mobility were experimentally determined, and TPA-1 exhibited the highest hole mobility of the series (4.49 × 10-4 cm2/Vs). Increasing the number of acceptor units imparted measurable increases in EHOMO values and the ε values of the TPA-X series under study. The high hole mobility of TPA-1 was explained by the low computed reorganization energy, small polaron stabilization energy, and hole extraction potential of this HTM. More HTMs should be designed with the under-investigated monopodal structure of TPA-1, but with a focus on destabilizing the EHOMO through the addition of less electron-withdrawing acceptor groups and increasing Tg with steric bulk to ensure efficient function in a PSC.  102 6.4 Experimental  6.4.1 Synthesis  All reagents were obtained from MilliporeSigma, Alfa Aesar or Fisher Scientific and used as received. All reactions were performed using toluene that was passed through a solvent purification system prior to use. Standard inert atmosphere and Schlenk techniques were carried out under nitrogen.  All reactions and purification were carried out in the dark. Purification by column chromatography was performed using silica (Silicycle: Ultrapure Flash Silica).  Analytical thin-layer chromatography (TLC) was performed on aluminum-backed sheets pre-coated with silica 60 F-254 adsorbent (250 μm thick; Silicycle, QC, Canada) and visualized under UV light.  Routine 1H and 13C NMR spectra were collected on a Bruker AV300 or Bruker AV400 inv/dir instrument at ambient temperatures, operating at 400 MHz and 100 MHz, respectively.   Chemical shifts (δ) are reported in parts per million (ppm) using the residual signals δ 7.26 and 77.0 for CDCl3, δ 2.05 and 29.8 for acetone-d6 and δ 2.50 and 39.4 for DMSO-d6 as internal references for 1H and 13C, respectively. Work on this project is ongoing and full characterization by 1H NMR, 13C NMR and high resolution mass spectrometry is lacking at the time of submission.  TPA-1TA (5-(4-(diphenylamino)phenyl)thiophene-2-carbaldehyde): 4-bromo-N,N-diphenylamine (0.200 g, 0.62 mmol), (5-formylthiophen-2-yl)boronic acid (0.126 g, 0.80 mmol), palladium tetrakis(triphenylphosphine) (0.036 g, 0.030 mmol) and potassium acetate (0.200 g, 2.04 mmol) were added sequentially to 25 mL of sparged THF/H2O (9:1, v/v) under N2. The reaction mixture was stirred and left at reflux for 16 h, then cooled to room temperature. THF was then removed in vacuo. A crude mixture was extracted with 20 mL of distilled water and 3 × 20 mL DCM. The organic phase was collected, dried over MgSO4 and reduced under pressure. Purification by silica column chromatography (SiO2: hexanes/DCM of 9:1, Rf = 0.37) yielded  103 0.145 g (75.4%) of product as an orange solid. 1H NMR (300 MHz, Acetone-d6) δ 9.91 (s, 1H), 7.93 (d, J = 4.0 Hz, 1H), 7.73 – 7.65 (m, 2H), 7.56 (d, J = 4.0 Hz, 1H), 7.41 – 7.31 (m, 4H), 7.20 – 7.12 (m, 6H), 7.08 – 7.00 (m, 2H). TPA-1 (2-((5-(4-(diphenylamino)phenyl)thiophen-2-yl)methylene)malononitrile): TPA-1TA (0.167  g, 0.47 mmol), malononitrile (0.037 g, 0.56 mmol) and piperidine (1 mL) were added to acetonitrile (20 mL) under N2. The mixture was stirred and left at reflux for 16 h, then cooled to room temperature and solvent was removed in vacuo. Purification by silica column chromatography (SiO2: hexanes/EtOAc of 4:1, Rf = 0.31) yielded 0.171 g (86.1%) of product as a metallic red solid. 1H NMR (300 MHz, Acetone-d6) δ 8.37 (s, 1H), 7.95 (d, J = 4.1 Hz, 1H), 7.77 – 7.70 (m, 2H), 7.65 (d, J = 4.2 Hz, 1H), 7.43 – 7.33 (m, 4H), 7.22 – 7.13 (m, 6H), 7.09 – 7.01 (m, 2H). TPA-2Br (4-bromo-N-(4-bromophenyl)-N-phenylaniline): triphenylamine (1.50 g, 6.12 mmol) and N-bromosuccinimide (2.29 g, 12.85 mmol) was added to dichloromethane (20 mL) under N2 at 0 °C. Reaction mixture was brought to room temperature and was stirred for 16 h. The reaction mixture was quenched with distilled water (20 mL) and extracted with 3 × 20 mL DCM. The organic phase was collected, dried over MgSO4 and solvent was removed in vacuo. Purification by silica column chromatography (SiO2: hexanes/DCM of 4:1, Rf = 0.45) yielded 2.33 g (96.1%) of product as a white solid.  TPA-2TA (5,5'-((phenylazanediyl)bis(4,1-phenylene))bis(thiophene-2-carbaldehyde)): TPA-2Br (0.200 g, 0.50 mmol), (5-formylthiophen-2-yl)boronic acid (0.170 g, 1.01 mmol), palladium tetrakis(triphenylphosphine) (0.086 g, 0.070 mmol) and potassium acetate (0.299 g, 3.05 mmol) were added sequentially to 25 mL of sparged THF/H2O (9:1, v/v) under N2. The reaction mixture was stirred and left at reflux for 16 h, then cooled to room temperature. THF was then removed in  104 vacuo. A crude mixture was extracted with 20 mL of distilled water and 3 × 20 mL DCM. The organic phase was collected, dried over MgSO4 and reduced under pressure. Purification by silica column chromatography (SiO2: hexanes/EtOAc of 3:1, Rf = 0.21) yielded 0.133 g (57.1%) of product as a tacky yellow solid.  1H NMR (300 MHz, Acetone-d6) δ 9.93 (s, 2H), 7.95 (d, J = 4.0 Hz, 2H), 7.83 – 7.75 (m, 4H), 7.60 (d, J = 4.0 Hz, 2H), 7.48 (d, J = 4.0 Hz, 2H), 7.27-7.12 (m, 7H) TPA-2 (2,2'-((((phenylazanediyl)bis(4,1-phenylene))bis(thiophene-5,2-diyl))bis(methaneylylidene))dimalononitrile): TPA-2TA (0.130  g, 0.28 mmol), malononitrile (0.044 g, 0.67 mmol) and piperidine (2 mL) were added to acetonitrile (20 mL) under N2. The mixture was stirred and left at reflux for 16 h, then cooled to room temperature and solvent was removed in vacuo. Purification by silica column chromatography (SiO2: hexanes/EtOAc of 2:1, Rf = 0.68) yielded 0.131 g (83.3%) of product as a purple-red solid. 1H NMR (300 MHz, Acetone-d6) δ 8.40 (d, J = 0.6 Hz, 2H), 7.97 (d, J = 4.1 Hz, 2H), 7.84 – 7.79 (m, 4H), 7.71 (d, J = 4.1 Hz, 2H), 7.45 (t, 2H), 7.28 – 7.18 (m, 7H). TPA-3Br (tris(4-bromophenyl)amine): triphenylamine (1.50 g, 6.12 mmol) and N-bromosuccinimide (3.81 g, 21.4 mmol) was added to dichloromethane (20 mL) under N2 at 0 °C. Reaction mixture was brought to room temperature and was stirred for 16 h. The reaction mixture was quenched with distilled water (20 mL) and extracted with 3 × 20 mL DCM. The organic phase was collected, dried over MgSO4 and solvent was removed in vacuo. Evaporation yielded 2.70 g (91.9%) of product as a white solid. TPA-3T (tris(4-(thiophen-2-yl)phenyl)amine): TPA-3Br (0.500 g, 1.05 mmol), thiophen-2-ylboronic acid (0.725 g, 3.45 mmol), palladium tetrakis(triphenylphosphine) (0.362 g, 0.300 mmol) and potassium bicarbonate (0.333 g, 3.14 mmol) were added sequentially to 25 mL of sparged toluene/H2O (10:1, v/v) under N2. The reaction mixture was stirred and left at reflux for  105 16 h, then cooled to room temperature. THF was then removed in vacuo. A crude mixture was extracted with 20 mL of distilled water and 3 × 20 mL DCM. The organic phase was collected, dried over MgSO4 and reduced under pressure. Purification by silica column chromatography (SiO2: hexanes/EtOAc of 7:1, Rf = 0.36) yielded 0.382 g (73.6%) of product as a yellow crystalline solid.  TPA-3TA (5,5',5''-(nitrilotris(benzene-4,1-diyl))tris(thiophene-2-carbaldehyde)): TPA-3T (0.385 g, 0.78 mmol) was dissolved in 1,2-dichlorobenzene (20 mL). Phosphorous oxychloride (0.718 g, 4.7 mmol, 0.44 mL) and DMF (0.513 g, 7.04 mmol, 0.55 mL) were added dropwise by syringe. Mixture was heated to 85 °C for 7 h, then reaction was cooled to room temperature. 0.1 M Na2CO3 (30 mL) was added slowly and allowed to reaction for 30 min. A crude mixture was extracted with 3 × 20 mL DCM. The organic phase was collected, dried over MgSO4 and reduced under pressure. Purification by silica column chromatography (SiO2: DCM, Rf = 0.62) yielded 0.323 g (72.4%) of product as a yellow crystalline solid. 1H NMR (300 MHz, Acetone-d6) δ 9.93 (s, 3H), 7.96 (d, J = 4.0 Hz, 3H), 7.76 (dd, J = 16.3, 8.7 Hz, 6H), 7.62 (d, J = 4.0 Hz, 3H), 7.21 (d, J = 8.7 Hz, 6H), 7.02 (d, J = 8.7 Hz, 3H). TPA-3 (2,2',2''-(((nitrilotris(benzene-4,1-diyl))tris(thiophene-5,2-diyl))tris(methaneylylidene))trimalononitrile): TPA-3TA (0.250  g, 0.40 mmol), malononitrile (0.095 g, 1.44 mmol) and piperidine (2 mL) were added to acetonitrile (20 mL) under N2. The mixture was stirred and left at reflux for 16 h, then cooled to room temperature and solvent was removed in vacuo. Purification by silica column chromatography (SiO2: hexanes/EtOAc of 2:1, Rf = 0.57) yielded 0.159 g (83.1%) of product as a purple-red solid. 1H NMR (300 MHz, Acetone-d6) δ 8.42 (s, 3H), 7.99 (d, J = 4.1 Hz, 3H), 7.87 (d, J = 8.7 Hz, 6H), 7.75 (d, J = 4.1 Hz, 3H), 7.32 (d, J = 8.7 Hz, 6H).  106 6.4.2 Electrical properties For conductivity measurements, four parallel Au electrodes with a spacing of 0.75 mm and length of 23 mm were used to measure the film conductivities in the dark and under ambient conditions. A Keithley 2400 source meter was used to force current through the outer electrodes while sensing the voltage across the inner two electrodes. A linear fit of each measurement was used to determine the resistance, R, and the sample geometry was then used to calculate the conductivity, !, according to the equation ! = #$%&    (Equation 6-2) where l is the electrode length, d is the inter-electrode spacing and t is the film thickness. The film thickness was measured using a Bruker DektakXT profilometer. Light soaking chamber was manufactured in house. The light source is a 1000 W metal halide lamp with 1 sun exposure (wavelength range: ~390-750 nm). A built-in fan is used to keep temperatures below 45ºC as measured by a thermocouple. The hole-only devices were fabricated with a structure of glass/ITO/PEDOT:PSS/HTMs/Au according to previous literature. The HTMs were doped with tBP and LiTFSI. The dopant concentration was kept constant for all HTMs and the same as used in PSCs. Hole mobility was investigated by the space-charge-limit current (SCLC) method, which can be described with the Equation 6-1 where µ is the hole mobility, J is the current density, ε0 is the vacuum permittivity, εr is the dielectric constant of the material (normally taken to approach 3 for organic semiconductors), V is the applied bias, and d is the film thickness. The applied bias was measured from 0 to 5 V.    107 6.4.3 Bulk thermal properties DSC curves of compounds were collected using a Netzsch DSC 214 Polyma instrument under a nitrogen protective atmosphere. For each measurement, two aluminum crucibles were used, one for material measurement and the other as empty reference. A customized heating program was developed to include a 5 min hold at the initial temperature of 50 °C, and four continuous cycles between to 50 °C and 300 °C with a heating/cooling rate of 10 K/min and 5 min holds at 50 °C and 300 °C. Data from the heating segment of last 3 cycles were used to identify the Tm (melting point) and extrapolate the Tg (glass transition temperature) of the material. X-ray diffraction measurements were performed with a Rigaku Smartlab diffractometer in parallel beam geometry using Cu Kα radiation. The data were collected with a 0.04° step size from 2-90° at a rate of 1.6 s/step. A 0.3° grazing incidence angle was used.  6.4.4 Electrochemical and photophysical properties Solution electrochemical data were recorded with a CHI660D potentiostat at room temperature using a platinum wire counter electrode and a platinum working electrode. Ag/AgCl in saturated KCl was used as the reference electrode and was calibrated versus the normal hydrogen electrode (NHE) by the addition of Fc+/Fc.  A 0.1 M n-NBu4PF6 DCM solution was used for all HTMs. CV data were acquired for 0.5 mM solutions of each HTM at a scan rate of 50 mV s-1.      Solution UV-Vis absorption spectra were collected on a Cary 5000 spectrophotometer. Photoluminescence spectra were recorded with a Cary Eclipse fluorimeter. All samples were measured in a 1-cm quartz cell at room temperature in HPLC grade DCM. The concentrations of the DCM solutions of analytes for UV-Vis and photoluminescence measurements were 2 × 10-5 mol L-1 and 1 × 10-5 mol L-1, respectively.    108  6.4.5 Computed properties  DFT calculations were performed for all molecules in the gas phase using B3LYP/6-31G(d,p) in Gaussian 09. Vibrational frequencies were computed to ensure structures were optimized to energy minima and the total energy of each compound was taken from the total thermal and electronic energy. Theoretical UV-vis absorption spectra were obtained from the results of TD-DFT calculations and orbital images were generated using the cubegen utility. To approximate the geometry of the TPA-X molecules within an HTM film, the rotational angle between the phenyl rings was held fixed at approximately 10 °. The geometric restriction was taken as a reasonable estimate of the reduced mobility of the TPA cores in a film due to the good agreement of the computed oxidation potentials and absorption spectra with empirical data. The oxidation potentials were calculated using isodesmic reactions with the empirical oxidation potential of TPA-2 as the reference potential. Systematic errors in computing oxidation potentials are eliminated through the use of isodesmic reactions.Ref1 ΔG  for the isodesmic reactions can be calculated using the change in free energy of the reaction TPA-Xneut + TPA-2+ →  TPA-Xox + TPA-2   (Equation 6-4) ΔG = Gproducts - Greactants     (Equation 6-5) Where TPA-Xneut and TPA-Xox are the neutral and oxidized TPA-X compound of interest and  TPA-2+ and TPA-2 are the oxidized and neutral TPA-2 molecule. ΔG for the isodesmic reaction is then used to calculate the oxidation potential E0 using the equation: E0 = -ΔG/F + E0ref     (Equation 6-6) Where F is Faraday’s constant and E0ref is the oxidation potential of the reference compound (E0ref = 1.24 V vs SHE). The oxidation potential of TPA-2 is accurate by design as a result of using the  109 empirical oxidation potential of TPA-2 as the reference potential. Reorganization energies (Ereorg) for hole transfer were calculated according to:  Ereorg = λ1 + λ2 = [M+(0) - M(0)] + [M+(+) - M(+)]           (Equation 6-7) Where M(0) and M(+) are the neutral and oxidized HTMs with a neutral geometry and M+(0) and M+(+) are the neutral and oxidized HTMs with the oxidized geometry. The small polaron stabilization energy (SPE) was calculated as the energy difference between the adiabatic and vertical ionization potentials (IP(a)-IP(v)).Ref1 The hole extraction potential is the potential difference between the electronic band gap, estimated as the HOMO-LUMO gap, and the first singlet excited state.Ref2 It was calculated using  Eb =  ΔH-L - E1      (Equation 6-8) Where ΔH-L is the HOMO-LUMO energy gap and E1 is the energy of the first singlet excited state.                   110 Chapter 7: Conclusions and Future Directions 7.1 Conclusions The main objectives of this thesis were to suppress degradation of the perovskite layer and the hole transport material (HTM) layer to improve the commercial viability of perovskite solar cells (PSCs).  My experiments were designed to address moisture-induced degradation of the photoactive perovskite layer,148,149 thermal instability of HTM film morphologies,150 and issues associated with the diffusion of dopants, which are added to improve HTM film conductivity.28,151 The experiments in this dissertation outlined how to develop HTMs that can encapsulate photo-active perovskite layers to provide protection against water exposure, and how to design HTMs that circumvent morphological changes and dopant diffusion. Chapter 3 described how the highest occupied molecular orbital (HOMO) energy levels and hole mobilities could be modulated with the substituents appended to a spiro[fluorene-9,9'-xanthene] core.  I designed and synthesized a series of seven spiro-centred HTMs with substituents which vary in steric bulk, electron-withdrawing strength and location about the HTM. Sterically bulky, non-polar substituents increased the hole mobilities of HTMs by decreasing conformational and electrostatic disorder.  These experimental results were supported by a computational model developed from bulk multi-dimensional simulations of approximately 1000 molecules. Chapter 4 defined a previously unreported structure-property relationship whereby the location of triphenylamine (TPA) units appended to a spiro-carbon core significantly impacted the glass transition temperatures (Tgs) of HTMs.  Tgs must be maintained above 90° C to prevent morphological changes to the HTM layer during PSC operation, and I found that the location of TPA groups around a spiro[fluorene-9,9′-xanthene]) core influences both the Tg and HOMO level of the HTM.26  We leveraged this structure-property relationship to improve the morphological  111 stability of films while ensuring sufficient free energy change for hole-extraction from the perovskite. These design principles were applied in subsequent projects to ensure morphologically stable HTMs with appropriate energetics for hole extraction.  Chapter 5 showcased an HTM with polymerizable functionality to form an encapsulation layer in a PSC to prevent the ingress of water into the cell.  I designed a novel HTM that could undergo thermal or electrochemical polymerization, which successfully lowered film wettability. However, the material only exhibited a 6.1% power conversion efficiency (PCE) in PSC proof-of-concept experiments, and hence, further optimization of the encapsulation process and long-term stability studies remain to be conducted. Chapter 6 contributed to a research effort which develops donor-acceptor (D-A) HTMs that do not require dopants to achieve useful conductivities (> 1 × 10-4 S/cm).  A series containing monopodal, bipodal, and tripodal D-A HTMs was developed and investigated using physical and computational methods. The monopodal D-A HTM demonstrated the highest hole mobility because of the low computed reorganization energy, small polaron stabilization energy and hole extraction potential.  These HTMs may be a useful new class of dopant-free PSCs, but more work is required. 7.2 Future directions  Spiro-VNPB in Chapter 5 was designed with styrene groups to undergo thermal polymerization to yield films less resistant to water penetration over time. The low conductivity of spiro-VNPB, however, resulted in PSCs with 1% PCE. I propose the investigation of spiro-VNPB at a styrene-functionalized perovskite surface to create a monolayer of crosslinked HTM at the perovskite/HTM interface. The interface between the perovskite and HTM layer must be optimized to increase the efficiency of hole extraction following light-absorption. Following  112 preparation of the 500-700 nm thick perovskite layer with the desired bulk AMX3 composition for high performance, the organic cation (A) at the surface could be replaced with a styrene-functionalized ammonium (Figure 7-1). The styrene-functionalized ammonium could, in turn, be exploited to crosslink with a thin layer of spiro-VNPB by thermal annealing. A 200-nm layer of a champion HTM (e.g., HTM-FX′ or spiro-OMeTAD) could be deposited on top of the thermally polymerized spiro-VNPB layer. This should improve both the perovskite/HTM interface by providing a covalent linkage between the perovskite and HTM and prevent the penetration of water by incorporating a polymerized network.  Figure 7-1 Proposed architecture of future directions for PSCs where there the interface between the perovskite and HTM layers is manipulated by surface functionalization of the AMX3 perovskite structure and the incorporation of polymerizable HTM.  113 The monopodal donor-acceptor (D-A) architecture of TPA-1 in Chapter 6 exhibited high hole mobility without the use of dopants, the high redox potential (>1.2 V vs NHE) conferred limited free energy change for hole extraction and the low Tg (59 °C) precluded use in a PSC.  Computational screening could be employed to predict the effects of more weakly electron-withdrawing acceptor groups on the redox potential of candidate compounds (Figure 7-2). Laborious synthesis and characterization would only need to be performed on materials that have been predicted to exhibit greater than 70 mV more positive HOMO level than the perovskite conduction band to ensure sufficient free energy change for charge extraction.  Figure 7-2 Proposed molecular structure of monopodal D-A HTM with spiro-core to increase Tg and opportunity to vary donor and acceptor identities and EHOMO.  The high Tg imparted by the spiro-carbon core relative to isomeric linear HTMs and the high hole mobility of monopodal D-A HTMs could potentially be combined to design thermally stable, dopant-free HTMs.64,65 The incorporation of more rigid structures within the D-A  114 architecture should increase the Tg values of the HTMs studied above 90 °C (this property is needed for practical operation).  Greater rigidity can be incorporated in either the donor group (e.g., bridged TPA units) or by creating a D-A architecture across a spiro-carbon core.  Additionally, shortening the D-A molecule by eliminating the pi-bridge could restrict the conformational freedom of the molecule and increase the Tg values of these HTMs.  Over 2000 publications on improving the stability of PSCs have been published, but many challenges remain. 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Nanoscale 2017, 9 (41), 15778–15785.    126 Appendices Appendix 1: Chapter 3   Figure A3-1 1H NMR of spiro-p,o-OMe in CD2Cl2 at ambient temperature.  127  Figure A3-2 13C NMR of spiro-p,o-OMe in CD2Cl2 at ambient temperature.       128  Figure A3-3 1H NMR of spiro-Me in CD2Cl2 at ambient temperature.    129 Figure A3-4 13C NMR of spiro-Me in CD2Cl2 at ambient temperature.    130  Figure A3-5 1H NMR of spiro-SMe in CDCl3 at ambient temperature.   131  Figure A3-6 13C NMR of spiro-SMe in CDCl3 at ambient temperature.    132  Figure A3-7 1H NMR of spiro-FOMe in CDCl3 at ambient temperature.   133  Figure A3-8 13C NMR of spiro-FOMe in CDCl3 at ambient temperature.    134  Figure A3-9 1H NMR of spiro-F in CD2Cl2 at ambient temperature.     135  Figure A3-10 13C NMR of spiro-F in CD2Cl2 at ambient temperature.    Figure A3-11 Resistance data for spiro-OMeTAD and spiro-R series with (a) 20% Li+ and (b) 20% Li+ and 3% Co(III) doping      136 Table A3-1 Conductivity data for spiro-OMeTAD and spiro-R series  compound Conductivity  20% Li+ (S/cm) Conductivity  20% Li+ and 3% Co(III) (S/cm) Spiro-OMeTAD 1.23 × 10-4 1.67 × 10-4 X60 5.81 × 10-5 2.11 × 10-4 spiro-p,o-OMe 3.72 × 10-5 2.20 × 10-5 spiro-Me 2.47 × 10-4  1.17 × 10-5 spiro-SMe 6.92 × 10-5 3.75 × 10-6 spiro-FOMe 1.90 × 10-6 4.32 × 10-5 spiro-H 2.82 × 10-5 7.43 × 10-6 spiro-F 1.57 × 10-5 1.67 × 10-4    Table A3-2 Perovskite solar cell data for spiro-R series doped with LiTFSI and Co(III)TFSI compound Voc (V) Jsc (mA/cm2) FF PCE (%) spiro-Me 0.87 0.21 0.28 0.05 spiro-SMe 0.83 3.35 0.27 0.79 spiro-FOMe 0.93 0.29 0.27 0.07  Perovskite solar cells prepared with Cs0.05FA0.81MA0.14PbI2.55Br0.45 perovskite mixture, 20% LiTFSI dopant, 3% Co(III)TFSI and 250% tert-butylpyridine.             137  Appendix 2: Chapter 4   Figure A4-1 1H NMR of HTM-X in CDCl3 at ambient temperature.   138  Figure A4-2 13C{1H} NMR of HTM-X in CDCl3 at ambient temperature    139  Figure A4-3 1H NMR of HTM-X′ in CDCl3 at ambient temperature.    140  Figure A4-4 13C{1H} NMR of HTM-X′ in CDCl3 at ambient temperature   141  Figure A4-5 1H NMR of HTM-FX′ in acetone-d6 at ambient temperature.     142  Figure A4-6 13C{1H} NMR of HTM-FX′ in acetone-d6  at ambient temperature.     143  Figure A4-7 1H NMR of Br2-X′ in CDCl3 at ambient temperature     144  Figure A4-8 13C{1H} NMR of Br2-X′ in CDCl3 at ambient temperature.    145  Figure A4-9 1H NMR of Br4-FX′ in CD2Cl2 at ambient temperature.    146  Figure A4-10 13C{1H} NMR of Br4-FX′ in CD2Cl2 at ambient temperature.   Figure A4-11 Theoretical UV-Vis spectra of the HTM series, calculated at the TD-CAM-B3LYP/def2-TZVP level of theory, based on the lowest 15 excited states. A linewidth of 0.3 eV is used, before plotting in units of nm.155   147   Figure A4-12 (a) MPP and of champion HTM-FX′ device and (b) EQE curve of solar cell.               148    Appendix 3: Chapter 5   Figure A5-1 1H NMR of Y2Br2Cl  in CDCl3 at ambient temperature.  149  Figure A5-2 13C NMR of Y2Br2Cl  in CDCl3 at ambient temperature.    150  Figure A5-3 1H NMR of Y59-Cl  in CDCl3 at ambient temperature.  151  Figure A5-4 13C NMR of Y59-Cl  in CDCl3 at ambient temperature.  152  Figure A5-5 1H NMR of spiro-VNPB  in CDCl3 at ambient temperature.  153  Figure A5-6 13C NMR of spiro-VNPB in CDCl3 at ambient temperature.  154   Figure A5-7 1H NMR of spiro-NPB  in CDCl3 at ambient temperature.   155  Figure A5-8 13C NMR of spiro-NPB in CDCl3 at ambient temperature.                      156  Appendix 4: Chapter 6   Figure A6-1 1H NMR of TPA-1TA in acetone-d6 at ambient temperature.  157  Figure A6-2 1H NMR of TPA-1 in acetone-d6 at ambient temperature.  158  Figure A6-3 1H NMR of TPA-2TA in acetone-d6 at ambient temperature.  159  Figure A6-4 1H NMR of TPA-2 in acetone-d6 at ambient temperature.   160  Figure A6-5 1H NMR of TPA-3TA in acetone-d6 at ambient temperature.  161  Figure A6-6 1H NMR of TPA-3 in acetone-d6 at ambient temperature.         

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