UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Highly flexible top-emitting phosphorescent organic light emitting diodes (OLEDs) Wang, Yan 2014

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
24-ubc_2014_november_wang_yan.pdf [ 8.16MB ]
Metadata
JSON: 24-1.0165557.json
JSON-LD: 24-1.0165557-ld.json
RDF/XML (Pretty): 24-1.0165557-rdf.xml
RDF/JSON: 24-1.0165557-rdf.json
Turtle: 24-1.0165557-turtle.txt
N-Triples: 24-1.0165557-rdf-ntriples.txt
Original Record: 24-1.0165557-source.json
Full Text
24-1.0165557-fulltext.txt
Citation
24-1.0165557.ris

Full Text

Highly Flexible Top-Emitting Phosphorescent Organic Light Emitting Diodes (OLEDs)   by  Yan Wang  B.Sc., The University of British Columbia, 2011  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Electrical and Computer Engineering)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  October 2014 © Yan Wang, 2014 ii  Abstract  Organic Light Emitting Diodes (OLEDs) have become attractive for flat panel display industry, with applications ranging from mobile phone screens to TVs. They have several advantages over inorganic LEDs such as high contrast ratio, wide viewing angle, faster response time, scalable large area processing and most importantly mechanical flexibility. OLEDs on flexible substrates can endure certain level of mechanical deformation such as bending, rolling or folding without disruption of the performance. The current demonstrated flexibility for OLEDs is up to a few centimeters or millimeters bending radius, depending on the materials, substrates and device structures. More flexible OLEDs with bending radius of curvature on the order of microns will be needed for applications in wearable, roll-up, or foldable displays and bezel-free screens and flexible signage systems.   This thesis presents the design, fabrication and characterization of highly flexible and foldable top-emitting OLEDs made on 50 micron thick polyimide (PI) plastic substrates, which can achieve approximately 200 microns bending radius of curvature (folding) without visible damage or impact on emission brightness and uniformity. To the best of our knowledge these are the most flexible phosphorescent OLEDs and first foldable OLEDs ever reported. We believe such flexibility is the benefit of the mechanical stability and low film thickness of the PI substrate. The surface roughness of PI had been the major limitation of its application as OLED substrates, and in this thesis a special side-angle evaporation method is proposed to improve the step coverage of deposited thin films of materials on PI without the requirement of buffer layers. The same method is also proved to be applicable for fabricating OLEDs on much rougher substrates iii  such as Scotch tapes, and fiberglass and transparency sheets. The OLEDs fabricated on above substrates are also presented and characterized. iv  Preface  I proposed the topic of highly flexible Organic Light Emitting Diodes (OLEDs) and designed the final device structure presented in Chapter 2. I also proposed and optimized the side-angle evaporation method in Chapter 2 to fabricate the highly flexible OLEDs on polyimide substrates and characterized them in Chapter 3. I also fabricated OLEDs on rough substrates such as Scotch tape, fiber glass, lecture transparency sheets and glossy paper presented in Chapter 4, using the same side-angle evaporation method.   Two short manuscripts are being written and planned for publications. First manuscript is “Highly Flexible Top-emitting Phosphorescent OLEDs on Polyimide” based on the methodology in Chapter 2 and results in Chapter 3. The second is on “Top-emitting Phosphorescent OLEDs on Rough Plastic Substrates” based on the methodology in Chapter 2 and results in Chapter 4.  v  Table of Contents  Abstract .......................................................................................................................................... ii Preface ........................................................................................................................................... iv Table of Contents ...........................................................................................................................v List of Figures .............................................................................................................................. vii List of Abbreviations .................................................................................................................. xii Acknowledgements .................................................................................................................... xiii Dedication ................................................................................................................................... xiv Chapter 1: Introduction ................................................................................................................1 1.1 Organic Light Emitting Diodes ....................................................................................... 1 1.2 Background and Challenges ........................................................................................... 1 1.3 Motivation & Thesis Objective ....................................................................................... 3 1.4 Thesis Organization ........................................................................................................ 7 Chapter 2: Experimental ...............................................................................................................9 2.1 Device Structure.............................................................................................................. 9 2.2 Transparent Electrode ................................................................................................... 10 2.3 Substrates ...................................................................................................................... 11 2.4 Structure Design and Materials Selection ..................................................................... 16 2.4.1 Emitting Layer .......................................................................................................... 17 2.4.2 ETL, HTL and HIL ................................................................................................... 18 2.4.3 EIL and Cathode ....................................................................................................... 18 2.5 Fabrication Method ....................................................................................................... 21 vi  2.6 Side-angle Evaporation Method ................................................................................... 31 Chapter 3: OLED on PI ..............................................................................................................38 3.1 Flexibility ...................................................................................................................... 38 3.2 Bending Test ................................................................................................................. 42 3.3 Mechanical Strength ..................................................................................................... 45 3.4 Encapsulation and Luminance ...................................................................................... 47 Chapter 4: OLED on Various Rough Substrates & Future Work .........................................51 4.1 OLED on Scotch Tape .................................................................................................. 51 4.2 OLED on Fiberglass Sheet............................................................................................ 54 4.3 OLED on Transparency Sheets ..................................................................................... 57 4.4 OLED on Glossy Paper ................................................................................................. 59 4.5 Future Work .................................................................................................................. 61 Chapter 5: Conclusion .................................................................................................................62 Bibliography .................................................................................................................................64 Appendices ....................................................................................................................................69     vii  List of Figures  Figure 1.1 Current displays have bezel around the displaying area. The bezel is the reserved space to layout the wiring and inter-connects of all the pixels, as well as sealing and packaging on the edge. ..................................................................................................................................... 5 Figure 1.2 Screens without bezel or with edge displaying can be achieved by bending and hiding the bezel to the back with flexible OLED technology. ................................................................... 5 Figure 1.3 Comparison of OLED displays of small and large bending radius of curvature along the edges for bezel-free display (top graph) and edge display (bottom graph) applications. ......... 6 Figure 2.1 Schematic structure of an OLED device. .................................................................... 10 Figure 2.2 SEM images of commercially available PI films’ surface, showing some of the surface roughness on the surface. The surface has irregularly shaped micron size and nanometer size topographical features. The PI surface is pre-sputtered with 6 nm of gold to provide slightly electrical conductivity for imaging. .............................................................................................. 14 Figure 2.3 AFM images of the PI surface in a scan of 1*1 um area, showing smaller featured roughness on the order of 10 nm................................................................................................... 15 Figure 2.4 100 nm of Al deposited on glass substrates. Left: aluminum seen through the bottom glass. Right: the same aluminum seen from the top. The two photos are taken at the same ambient light condition with a white paper hanging above the samples. ..................................... 19 Figure 2.5 SEM images of the top aluminum surface. Left: the yellow-ish area. Right: the black area. ............................................................................................................................................... 20 Figure 2.6 The device structure of the OLED and the thickness of each layer. ........................... 22 viii  Figure 2.7 Photo of thermal evaporation system. The four copper heating arms are marked with numbers 1-4. Source #1 is mounted between arms “1” and “2” and source #2 is mounted between arms “3” and “4”. ............................................................................................................ 23 Figure 2.8 Schematic diagram of the evaporator chamber. .......................................................... 24 Figure 2.9 Fabrication process, deposition and patterning of layers. a) PI substrate. b) Ag (100 nm) + Ca (20 nm) patterned with shadow mask. c) Alq3 (40 nm) + Ir(ppy)3:CBP (20 nm) + NPB (45 nm) + MoO3 (2 nm). d) Au (15 nm) patterned with shadow mask. ....................................... 26 Figure 2.10 Photos of OLED fabricated on PI with active area showing different color due to difference thickness of the organic layers, and OLED on ITO coated glass substrate. From left to right, the thickness of total organic layers of the PI OLED are: thin, natural and thick, showing pale pink, purple and green colors in the active area respectively. The OLED fabricated with ITO instead of Au as anode (rest of the layers are the same) does not show any colors in the active area. ............................................................................................................................................... 28 Figure 2.11 SEM images of 15 nm Au anode of OLED fabricated on glass (left two images) and PI (right two images) substrates. ................................................................................................... 30 Figure 2.12 Side-angle evaporation method compared to regular perpendicular evaporation. .... 32 Figure 2.13 Schematic diagrams of perpendicular evaporation (top) and side angle evaporation (bottom). In both graphs, materials (green layer) are deposited on a rough surface (pink) from a line of sight direction (green arrows). “A, B, C, D” represent surface topographical areas of spikes, slopes, valleys and flat regions respectively. The blue lines in top graph at A,B,C,D denote the film thickness measured in the line of sight direction. The red lines in in both graphs denote the actual film thickness. ................................................................................................... 34 ix  Figure 2.14 Schematic diagram of the changed configuration of the evaporator to achieve the side-angle evaporation. ................................................................................................................. 35 Figure 2.15 SEM images of the Au anode of the OLED fabricated on glass (left two images) and on PI (right two images). .............................................................................................................. 36 Figure 2.16 From left to right, the photos of the PI OLEDs which are bended in stress, tensile, and around a thin metal edge. ....................................................................................................... 37 Figure 3.1 OLED folded into an equivalent bending radius of curvature ~200 μm with no visible degradation in brightness, color and uniformity before and after the bending. Scale bar is 2.0 mm. ............................................................................................................................................... 38 Figure 3.2 Strain calculation of a bended OLED, assuming OLED films are on the outermost of the substrate and the middle line of the substrate is the neutral axis with zero strain. ................. 39 Figure 3.3 SEM images of the folded area of the OLED in Figure 3.1. In the images the vertical direction corresponds to the line of folding. ................................................................................. 40 Figure 3.4 Photo showing a PI OLED that has been scratched by a tweezers head in the silver areas where the arrows point. After scratching more than 10 times the silver is still in strong adhesion to the PI substrate........................................................................................................... 41 Figure 3.5 Approach to edge-free displaying whose displaying area can occupy 100% of the screen by hiding the electrical wiring and the rest of the OLED to the side or back. .................. 42 Figure 3.6 The 3D drawing of the bending test apparatus. The texts mark the major parts of the setup. ............................................................................................................................................. 43 Figure 3.7 Photo of the bending test apparatus holding two powered on OLED devices. ........... 44 Figure 3.8 In the order from upper-left to bottom-right, photos for bending test of two PI OLEDs after 0, 1000, 2000, 3000, 4000, 5000 cycles. .............................................................................. 45 x  Figure 3.9 Photos of a PI OLED being repeatedly poked in the active area from the back. a), b), c) showing the OLED being punched in the active area from the back at different locations, where a black dot appears (circled), and when the pointy stress is removed the black dot disappears. d) After a series of poking in different regions, the OLED shows not damage or degradation in performance. ......................................................................................................... 46 Figure 3.10 SEM images of the poked areas in the pointy stress test. .......................................... 47 Figure 3.11 OLED encapsulated between two pieces of thermal plastic with aluminum foil as electrical contact. .......................................................................................................................... 49 Figure 3.12 The relative color spectrum of the OLED. ................................................................ 49 Figure 3.13 CIE index of the OLED. ............................................................................................ 50 Figure 4.1 OLED devices fabricated on Scotch tape. Powered at 6 V ......................................... 52 Figure 4.2 SEM micrographs of the Scotch tape surface before (top two images, pre-sputtered with 6 nm of gold) and after (bottom two images) the OLED fabrication. All the SEM images are taken from a 45 degrees tilt. .......................................................................................................... 53 Figure 4.3 OLED fabricated on fiber glass substrate powdered at 4.5 V. .................................... 55 Figure 4.4 SEM images of fiber glass before (top two images, pre-sputtered with 12 nm of gold) and after (bottom two images) fabrication of OLED. ................................................................... 56 Figure 4.5 OLED made on transparency sheet substrate powered at 6 V. ................................... 57 Figure 4.6 SEM images of the transparency films surface before (top two) and after (bottom two) OLED fabricated on top. The films without OLED are sputtered with 6 nm of gold for SEM imaging. ........................................................................................................................................ 58 xi  Figure 4.7 SEM images of glossy paper surface before (top two images) and after (bottom two images) fabricating OLED on top. The glossy paper without OLED are sputtered 6 nm of gold for imaging. ................................................................................................................................... 60  xii  List of Abbreviations  AFM: Atomic Force Microscope Alq3: Tris(8-hydroxyquinolinato)aluminium CBP: 4,4’-Bis(N-carbazolyl)-1,1’-biphenyl EIL: Electron Injection Layer EQE: External Quantum Efficiency ETL: Electron Transporting Layer HIL: Hole Injection Layer HOMO: Highest Occupied Molecular Orbital HTL: Hole Transporting Layer IQE: Internal Quantum Efficiency Ir(ppy)3: Tris[2-phenylpyridinato-C2,N]iridium(III) ITO: Indium tin oxide LUMO: Lowest Unoccupied Molecular Orbital NPB: N,N’-Di(1-naphthyl)-N,N’-diphenyl-(1,1’-biphenyl)-4,4’-diamine OLED: Organic Light Emitting Diode PET: Polyethylene terephthalate PI: Polyimide SEM: Scanning Electron Microscopy TPBi: 1,3,5-tris(N-phenylbenzimiazole-2-yl)benzene xiii  Acknowledgements  I offer my enduring gratitude to my supervisor Dr. Peyman Servati who has supported, helped and guided me from the beginning and never lost faith in me. I owe particular thanks to Dr. Saeid Soltanian for his selfless help and guidance to me throughout the completion of this thesis, as well as his diligence and attitude that have deeply inspired me. My sincere gratitude to Dr. Bobak Gholamkhass who has been enlarging my vision of science and providing coherent answers to my endless questions.  I thank my lab colleges Dr. Nima Mohseni Kiasari, Zenan Jiang, Rowshan Rahmanian, Rubaiya Rahman for their help, support and inspiration in the experimental work, as well as their warmth and kindness as my dearest friends.   Sincere acknowledge to funding of Natural Sciences and Engineering Research Council (NSERC) of Canada, Canada Foundation for Innovation (CFI) and that all work has been carried out at Flexible Electronics and Energy Lab (FEEL) of UBC.  Special thanks are owed to my parents, whose have supported me throughout my years of education, both morally and financially. xiv  Dedication  To my father Shiyou Wang and my mother Lanlan Cui for their unconditional love, selfless support, heart-warming encouragement, and unlimited faith in me.          1  Chapter 1: Introduction 1.1 Organic Light Emitting Diodes A Light Emitting Diode (LED) is a semiconductor device that transforms electric energy into light. It is commonly used as a high efficiency replacement of traditional fluorescent tubes and tungsten lamps for lighting applications, as well as back lighting source in the current display technologies such as TVs, cell phones and computer screens. An Organic Light Emitting Diode (OLED) is an LED made with organic semiconductor materials instead of inorganic compounds that may include expensive or toxic elements such as In, Ga, As, etc. Since the discovery of the first electro luminance in 1965 [1] and the fabrication of the first OLED device in 1990 [2], OLED has become an interesting topic among researchers and will revolutionize the display industry in the 21st century. Compared to traditional inorganic LED lights and LED displays, OLED has superior optical performance such as high luminance efficiency, high contrast ratio, wide viewing angle, and faster response time. In addition, active matrix OLED (AMOLED) displays are self-illuminant and do not require a back light source, resulting in a very thin display devices, which are critical for mobile applications such as cell phones, tablets and laptops. Another unique and incomparable property of OLED is its flexibility, which means the device can endure certain level of mechanical deformation such as bending, rolling or folding while maintaining its performance without any disruption.   1.2 Background and Challenges Despite the many advantages of the OLEDs described above, there are still many challenges yet to be tackled before OLEDs can obtain its full potential for display applications. This section 2  briefly reviews some of the research interests of OLEDs that can be improved for better performances and novel applications.  The external quantum efficiency (EQE), which is the efficiency in conversion of electric energy to light, is the product of two efficiencies, the internal quantum efficiency (IQE) and the light extraction efficiency. The former is the efficiency of the number of electric carriers transforming into number of emitted photons and the latter is the proportion of generated photons that actually escaped the device and can be associated to emission from the device. The maximum IQE as well as the emission spectrum of an OLED mainly depend on the types of the emitting materials, which will be reviewed in more details in the next chapter where the selection of materials and design of device structure are discussed. The light extraction efficiency on the other hand is limited by the total internal reflection when the generated light tries to come out of the OLED (higher refractive index) into the air (lower refractive index). As a result the average light extraction efficiency of an OLED without any light extraction enhancement is only about 20% [3]. Various light extraction techniques have been thoroughly reviewed in literatures such as adding surface micro lenses, surface texturing mesh, scattering layer, spontaneously form buckles, replacing glass substrate by high refractive index substrates, and so on [3][4][5][6][7].   Another important research area is the development of white OLEDs for lighting and display applications. It is known that the human eyes see the combination of red, green and blue colors (RGB) as white color. Therefore, the development of white OLED generally lies in the research of red, green and blue emitting materials. Some red and green emitters have been well established over the years but the development of high efficiency and long lifetime deep blue 3  emitters still remain a challenge due to difficulty in synthesis of stable blue-emitting phosphors with high quantum efficiency, as well as solving the problem that blue photons have a higher energy (2.75eV at 450 nm wavelength) and tend to be easily re-absorbed by surrounding materials (red, green emitters, charge transporting materials) with lower band gap causing materials degradation [8][9].   Another aspect of OLED researchers have been working on is the encapsulation of OLEDs by low cost materials and methods. Since the OLEDs consist of materials sensitive to oxygen or humidity, the device needs to be well-sealed and maintain a low permeation for a long period of time. The recent development of encapsulation methodology is thoroughly reviewed in different literatures [10]. The major technique is atomic layer deposition or molecular layer deposition, which is to coat a layer of low permeability material on top of OLED to seal the transfer of oxygen and water molecules from air to the OLED materials.   1.3 Motivation & Thesis Objective  The development in flexibility of OLED on the other hand has been paid less attention than the aspects reviewed above, because the current applications of OLED stay mostly in flat panel displays such as in cell phones and TVs, which demand more in optical performances such as high efficiency, broad color spectrum (RGB emitters), long lifetime but not necessarily high flexibility or repeatedly bending endurance. For example, in 2014 LG demonstrated the world’s first curved mobile phone LG G Flex with curved OLED screen, while Samsung presented Samsung Galaxy Edge that has OLED screens with extended displaying area across the edge, capable of displaying information on that edge. In addition, several companies have made 4  wearable watches as well as curved TVs, all of which have OLED displays of bending radius at the order of centimeters to meters. The motivation and need for even more flexible OLEDs is to make super portable displays, wearable devices, and particularly rollable and foldable displays that require OLED devices that can operate at millimeters or less than a millimeter radius of curvature. The eventual objective is to make devices or displays that can be rolled up or folded like a piece of paper and carried easily in pockets. Only then we will reach the full potential of flexible OLED technology, since OLEDs are uniquely thin and flexible.   Another more foreseeable motivation for highly flexible OLED is as a potential solution to make bezel-free displays. As shown in Figure 1.1 the current state-of-the-art screens on our cell phones, tablets or laptops all have a bezel around the displaying area, which is the reserved space to layout the wiring and inter-connects of all the pixels, as well as sealing and packaging on the edges. Although current fabrication techniques have managed to make the bezels much thinner than they used to be many years ago, it is still impossible to completely eliminate the presence of the bezel with current technology. Screens with smaller or zero bezel and large display area, and even screens that can display on the side edge are favored and demanded by people. As shown in Figure 1.2, flexible OLED display can potentially fulfill this goal by bending the bezel to the back and achieve bezel-free displays with maximum displaying area, as well as displaying images on the edges.   5   Figure 1.1 Current displays have bezel around the displaying area. The bezel is the reserved space to layout the wiring and inter-connects of all the pixels, as well as sealing and packaging on the edge.    Figure 1.2 Screens without bezel or with edge displaying can be achieved by bending and hiding the bezel to the back with flexible OLED technology.   6   Figure 1.3 Comparison of OLED displays of small and large bending radius of curvature along the edges for bezel-free display (top graph) and edge display (bottom graph) applications.   The visibility of the bezel and whether the screen can be sharply or roundly curved at the edges highly depend on the flexibility and the smallest radius of curvature of the OLED displays. 7  Figure 1.3 shows schematic drawings of bezel-free displays and edge displays, with comparison between R=0.2 mm and R=2 mm bending radius at the edges (drawn up to scale). In the case of bezel-free displays (top graph), a smaller radius of curvature allows the display to bend more sharply at the edges, resulting nearly invisible bezel. In the case of the edge displays (bottom graph), a smaller bending radius enables sharper bending and transition of the display across the edges, showing better appearance and enabling more flat displaying area on the edges.   The objective of this thesis is to fabricate highly flexible OLED, by designing OLED device structures, selecting the appropriate materials and substrates, developing new fabrication methods and optimizing the fabrication parameters and conditions. As demonstrated in the results later, the fabricated OLEDs on PI substrate show excellent flexibility in terms of bending radius, repeated bending, as well as high mechanical stability. The proposed fabrication method is also proved to be effective to fabricate OLEDs on several types of rough substrates such as Scotch tape, fiberglass and transparency sheets.   1.4 Thesis Organization The thesis is constructed with five chapters. Chapter 1 introduced the OLED technology in terms of applications and advantages of OLED, brief review of some of challenges of OLEDs, and finally motivation to make more flexible OLEDs and the objective of the thesis. In Chapter 2 the experimental details are discussed including the design of device structure, material selection, fabrication methods and optimization. Chapter 3 discusses the characterization of OLEDs fabricated on polyimide substrate in terms of flexibility, endurance under repeated bending, mechanical stability and luminance properties. In Chapter 4, OLEDs fabricated on other types of 8  substrates such as Scotch tape, fiberglass, transparency sheets, and glossy paper are presented and discussed. Chapter 5 is the conclusion of the thesis.    9  Chapter 2: Experimental 2.1 Device Structure Figure 2.1 shows a schematic diagram of the general structure of an OLED device, where an emitting material is sandwiched between two electrodes (cathode and anode), with an Electron Transporting Layer (ETL) and a Hole Transporting Layer (HTL) on each side to selectively transport electrons or holes to the emitting layer respectively. In addition, an Electron Injection Layer (EIL) and a Hole Injection Layer (HIL) are required to assist the injection of electrons and holes from the cathode and anode into ETL and HTL, respectively. When an electrical bias is applied between the cathode and anode, the electrons and holes migrate from the electrodes towards the emitting layer, where the negative electrons and positive holes recombine and release the electric energy in the form of emitted photons. The layers in between the two electrodes of the OLED are usually transparent in the visible spectrum. Also at least one of the electrodes needs to be transparent in order for the generated photons to escape. The OLED layers are fabricated on top of a substrate such as glass or plastic, which could also be either transparent or opaque. If the emitted light is designated to escape from the bottom electrode and substrate side, the OLED is called bottom-emitting; if the light escapes through the top electrode side, the OLED is called top-emitting. The advantages and disadvantages of top-emitting and bottom-emitting devices have been reviewed and compared in literature [11]. On another note, customarily the OLEDs having ITO bottom anode and metal top cathode are called regular structure OLEDs. Therefore OLEDs having bottom cathode and top anode, like the structure shown in Figure 2.1, are called inverted structure OLEDs.  10   Figure 2.1 Schematic structure of an OLED device.   2.2 Transparent Electrode One of the most important layers in OLED is the transparent electrode. For a conductive transparent material, a thicker film generally gives a higher conductivity but lower transmittance, and vice versa. A good balance of conductivity and transmittance for the electrode is required in optoelectronic devices such as OLEDs or solar cells by having a low sheet resistance while maintaining a high transmittance of lights. Indium-tin-oxide (ITO) is the most commonly used transparent electrodes in OLEDs and solar cells. Despite its good properties in conductivity and optical transmittance, it has several downsides that make it less favorable for flexible OLEDs. One reason is as reviewed in section 1.2 that in OLED devices although all the layers are either metallic reflective or transparent, about 80% of the generated photons are actually trapped inside 11  the device due to total internal reflection due to the difference in refractive indices of glass substrates (n=1.55), organic materials (n=1.7) and ITO electrode (n=1.8 to 2.2) compared to air (n=1.0) [3]. ITO usually has the largest refractive index among all the OLED layers and therefore need to be replaced by other transparent conductors. Another drawback of the ITO as transparent conductor is its fragility and lack of flexibility. Fabrication of ITO also requires high temperature and vacuumed condition, which limits its deposition onto flexible substrates such as plastics and makes it even less favorable as a flexible electrode. For the above two reasons researchers have been working on various transparent electrode materials to replace ITO in the OLED for better efficiency and flexibility. For example, OLEDs with graphene [12][13] and carbon nanotubes [14][15] electrodes have been reported. Semi-transparent thin films of gold is another popular candidate because of its optical transparency, high electrical conductivity, chemical stability, and ease of processing and patterning [16][17]. A recent study reports fabrication of OLEDs with high external quantum efficiency of 63% using thin film of gold as semi-transparent anode coupled with several light extraction techniques [17]. Compared with the brittleness of ITO, gold is also well known for excellent ductility and malleability that will serve the purpose of fabricating highly flexible OLEDs. Therefore for above reasons as well as the availability of deposition methods and equipment in our lab, thin film of gold was selected as the semi-transparency electrode of our OLED.   2.3 Substrates Substrates is another key element which needs to be appropriately selected for achieving highly flexible OLEDs. In general an OLED device fabricated on a thinner substrate endures less strain during bending and deformation, and therefore can reach smaller bending radius of curvature 12  without affecting the performance of the device. When selecting flexible substrates for OLED some general requirements include: good mechanical strength, low surface roughness, high transparency, high thermal stability, low moisture and oxygen permeability, ease of handling and patterning during fabrication process. Polyethylene terephthalate (PET) is a popular material used as OLED substrates. OLEDs fabricated on PET substrates with ITO [18], carbon nanotube [14],  PEDOT [19][20], and combined multilayers [21]–[23] bottom electrodes have been reported. Despite many good properties of PET, the low thermal stability of PET greatly limits its application in industrial OLED fabrications because high temperature processing such as annealing and thermal evaporation is involved in certain steps of the fabrication process. Flexible OLED displays made on thin metal foil has been reported [24], as well as OLED made on 40 µm transparent cellulose film that reaches 1.5 mm radius of curvature [25].  Compared to the flexible substrates mentioned above and other plastic materials, polyimide (PI) shows superior properties such as exceptionally high thermal stability, low thermal expansion, excellent mechanical robustness, chemical stability, low costs and wide commercial availability [26]. It has a higher glass transition temperature Tg~360 ℃ than PET (Tg~78 ℃) and PEN (Polyethylene naphthalate, Tg~122 ℃), as well as a lower thermal expansion coefficient of ~3.4 ppm/K compared to PET (~10 ppm/K), PEN (~19 ppm/K), metal foil (~17 ppm/K), reaching similar value as non-alkali glass (~3 ppm/K) [27]. Despite such good physical and mechanical properties, PI has been seldom used as an OLED substrate for the following two reasons. First, PI is not optically transparent in the visible spectrum and therefore the OLED fabricated on PI has to be top-emitting, which limits the number of choices for top transparent electrodes. It should be noted that many of the transparent electrodes such as ITO, graphene, carbon nanotubes 13  are difficult to employ as a top electrode. To deal with the low transparency of PI, literature has reported chemical methods to synthesize co-polymer of PI in order to enhance its transmittance [26]. However, such methods greatly increase the time and costs of the OLED fabrication process. The much simpler and easier way is to develop a top-emitting OLED using a feasible transparent conductor as top electrode. As we discussed in the previous section we have selected thin film of gold as anode, which is easier to deposit as a top electrode than ITO, graphene or carbon nanotubes.   Another reason why PI is not suitable as OLED substrate is that commercially available low-cost PI films usually have very rough surface that do not meet the level of smoothness required for OLED fabrication. Figure 2.2 shows the typical SEM images of different positions on the surface of commercially available PI films showing high level of roughness. At the scale of the roughness showed in Figure 2.2, the thin layer of sputtered Au (6 nm) on the surface for SEM imaging does not change or alter the actual surface topography. But to see smaller sized roughness in the flat areas of PI, Atomic Force Microscope (AFM) was employed to examine the PI surface roughness at lower scale without the need of pre-sputtering gold. The AFM images in Figure 2.3 show much smaller features on the surface of PI with roughness in the order of 10 nm.   14   Figure 2.2 SEM images of commercially available PI films’ surface, showing some of the surface roughness on the surface. The surface has irregularly shaped micron size and nanometer size topographical features. The PI surface is pre-sputtered with 6 nm of gold to provide slightly electrical conductivity for imaging.   15   Figure 2.3 AFM images of the PI surface in a scan of 1*1 um area, showing smaller featured roughness on the order of 10 nm.   A thin film such as OLED (total thickness around 200 nm) deposited on the rough surface of PI is likely to be discontinuous or have non-uniform film thickness. The discontinuous metal electrode will not provide enough conductivity. On the other hand discontinuous organic layers would have thin areas that might have higher currents density causing overheat of that local area and burn the device. The common method to deal with rough substrates surface in OLED   fabrication is to coat a buffer layer to reduce the surface roughness. For example, OLED display has fabricated on 10 um thick PI substrate which PI substrate coated with ~1um thick of SiNx and SiO2 as buffer layer [27]. That process is however very costly and time consuming. In this work, we fabricate OLEDs directly on the as purchased and commercially available PI films without using any buffer layer or additional surface treatments.  Kapton PI films with thickness of 25 μm, 50 μm and 100 μm were obtained from DuPont. Later results show that OLEDs fabricated on 25 μm PI get short or burned quite often during the 16  handling and measurements probably because the 25 μm PI lacks enough mechanical strength to withstand the stress during handling of the substrates. The OLED made on 100 μm PI on the other hand shows good mechanical strength and is very robust during the handling, fabrication and characterization. However, it has low flexibility and very difficult to bend and characterize the OLED as a flexible device. The 50 μm thick PI appeared to be with the optimum thickness conditions that provides good flexibility as well as high mechanical strength. Therefore, 50 μm thick PI film was used as the substrates for our OLED. The fabrication and optimization of method will be discussed in more details in section 2.5.  2.4 Structure Design and Materials Selection The emitter of the OLED mostly determines the emission color of the OLED and the maximum internal quantum efficiency (IQE) of the OLED, i.e. the efficiency of converting electrons and holes into photons. When electrons and holes meet in the emitting material, they first combine to form electron-hole pairs called excitons, which make radiative decay and generates photons. Since both electrons and holes have random spins, quantum mechanics states that 25% of the excitons formed are in singlet states and 75% are in triplet states. In traditional florescent emitting materials only the singlet excitons can make radiative decay and becomes emissions because the radiative decay of triplet states are quantum mechanically forbidden. As a result, the maximum efficiency is theoretically limited to 25% [28]. The discovery of phosphorescent small molecules emitters in 1998 [29] takes a step further in improving the IQE of OLED because they can convert both singlet and triplet excitons into photons and can therefore theoretically achieve nearly 100% IQE [30]. However, achieving high IQE usually requires a lot optimization in device structures such as controlling ETL and HTL thickness to balance the number of electrons 17  and holes reaching the emitting layer. Other aspects involve optimizing the organic materials’ HOMO (Highest Occupied Molecular Orbit) and LUMO (Lowest Unoccupied Molecular Orbit) levels alignment between different layers, such as adding more interfacial layers to reduce energy berries between layers, and doping or mixing materials to obtain more effective electrons and holes injection and transportation. Since the focus of this thesis is flexibility of OLED rather than investigating the properties of the organic materials, the selection for emitter, ETL and HTL materials will be the most commonly used ones in reference literatures. Also, the purpose of this thesis is to develop methodology to fabricate flexible OLED with commonly used organic materials and therefore it can be generalized and applied to other types of organic emitters, ETL and HTL materials.   2.4.1 Emitting Layer As mentioned earlier, phosphorescent small molecules have theoretically four times higher efficiency than fluorescent materials. Among various phosphorescent small molecules Ir(ppy)3 is one of the most commonly used green OLED emitters [28]. Phosphorescent molecules are often doped in an organic host material as combined emitting layer rather than used in its pure form, for achieving higher efficiency and longer operating lifetime, as well as for the sake of saving the consumption of the expensive phosphorescent molecules that usually consist of expensive heavy metal elements such as Ir, Rh, Ru and Pt [31][32][33]. A study in 1999 reports that doping 6% weight percentage of Ir(ppy)3 in CPB gives the highest emission efficiency for Ir(ppy)3 mixtures [34]. The same ratio and recipe has been widely used and external quantum efficiency of as high as 63% was reported using Ir(ppy)3 as phosphorescent emitter [17]. Therefore in this thesis the same recipe of Ir(ppy)3 doped 6% weight in CPB was used as the emitting layer of our OLED.  18  2.4.2 ETL, HTL and HIL Alq3 and NPB are commonly used electron and hole transporting small molecules that have been often reported in literatures [13][25][35][36][37][38]. In some cases we also used TPBi, which is another electron transporting small molecule commonly used in literatures [13][17][35][39], as the ETL. MoO3 is selected as the HIL for our OLED as the hole injection effect of MoO3 to different metals has been thoroughly studied, established [40] and has been effectively used as HIL for thin film of gold anode [17] .   2.4.3 EIL and Cathode LiF/Al and Ca/Ag are well established and most commonly used as electron injection layer and cathode materials in most OLED literatures, for example in [13][17][20][36]. The former combination is more suitable and have been more often used in literatures since Al is cheaper than Ag and does not involve Ca, which is easily oxidized once exposed in air [13][17][20][35][39]. But during the experiments it is found that the Al deposited by thermal evaporation on the PI substrate as bottom electrode is dark rather than metallic reflective, i.e. absorbent instead of reflective in the visible light spectrum. We ruled out the possibility of impurity, cross contamination and oxidation because the aluminum source used for deposition is 99.99% pure and the storage, handling and deposition do not introduce any contamination, humidity or oxygen. Figure 2.4 shows 100 nm of Al deposited on transparent glass substrates with a “T” shape pattern, which shows absorbent aluminum black surface. When aluminum is seen through the bottom glass the surface looks shiny and reflective, but when aluminum is seen from the top it shows dark yellow-ish or completely black in some areas.   19   Figure 2.4 100 nm of Al deposited on glass substrates. Left: aluminum seen through the bottom glass. Right: the same aluminum seen from the top. The two photos are taken at the same ambient light condition with a white paper hanging above the samples.    The top aluminum surface (Figure 2.4, right photo) were examined under SEM in the yellowish area and the black area. The corresponding SEM micrographs are shown in Figure 2.5. Quite different surface conditions in the two images can be seen and the blacker area (right image) seems to have smaller grains than the less dark yellowish color area (left image).  20   Figure 2.5 SEM images of the top aluminum surface. Left: the yellow-ish area. Right: the black area.   The thermal evaporation of aluminum generally requires much higher temperature than other metals such as silver, calcium or gold in the chamber. The evaporated aluminum on the substrates experience continuous high temperature heat radiation during the evaporation process. Several literatures have reported surface deformation or transformation of thin aluminum films under annealing or thermal evaporation, causing various types of surface topographical changes and transformations [41][42]. In our case the surface of aluminum facing the glass presumably experience less surface transformation than the exposed surface of the aluminum, own to the restraining by the smooth surface of the glass. No conclusions can be drawn at this point, but the dark color of the aluminum surface could be caused by the heat exposure during the thermal evaporation process  After repeating the deposition of Al on both glass and PI substrates followed by examination under SEM for many times, we get the same consistent results. Therefore instead, Ca/Ag films were used as the electron injection layer and bottom cathode. And 100 nm thick Ag film 21  evaporated on both glass and PI substrates were found to provide a smooth, shiny and metallic reflective surface.   2.5 Fabrication Method The final device structure of the OLEDs fabricated in this study is shown in Figure 2.6. From bottom to top, the layers are: PI substrate (50 μm), Ag (100 nm), Ca (20 nm), Alq3 (40 nm), Ir(ppy)3:CBP (20nm, 6% w.t. doped), NPB (45 nm), MoO3 (2 nm), and Au (15 nm). The thickness of each layer are initially referred to in literatures: Au and MoO3 from the literature [17] while Alq3, Ir(ppy)3:CBP and NPB are referred from the literature [35]. Then the optimized thicknesses are determined by repeatedly fabrication and characterization of devices which will be discussed in later sections. The Ag cathode is opaque and reflective. Top Au anode is semi-transparent and therefore the device is top emitting. The Ag is 100 nm thick since it needs to be thick enough to be highly reflective. Ca (work function=2.9 eV) is 20 nm thick and is to lower the work function of Ag (4.3 eV) to better match with the LUMO level of Alq3 (-3.1 eV) [43] for effective electrons injection.   22   Figure 2.6 The device structure of the OLED and the thickness of each layer.  All the OLED layers in Figure 2.6 including the metals, organic small molecules and inorganic materials are deposited on the PI substrate by thermal evaporation. Compared to other Physical Vapor Deposition (PVD) methods such as e-beam evaporation and sputtering, the thermally evaporated films tend to have better quality and uniformity. In addition, organic small molecules (phosphorescent emitters and host materials, most ETL, HTL materials) can only be thermally evaporated but not e-beam evaporated or sputtered. In this thesis, the materials are thermally evaporated in an EVAP evaporation system inside an MBraun nitrogen glove box. All the evaporation process are conducted at high vacuum conditions with pressure <10-5 mbar, and all 23  the materials are stored and handled in a constantly circulated and filtered nitrogen atmosphere with both oxygen and humidity level <0.1 ppm.   Figure 2.7 Photo of thermal evaporation system. The four copper heating arms are marked with numbers 1-4. Source #1 is mounted between arms “1” and “2” and source #2 is mounted between arms “3” and “4”.  24   Figure 2.8 Schematic diagram of the evaporator chamber.   Figure 2.7 and Figure 2.8 show the digital photograph and the schematic diagram of the thermal evaporation system. The materials to be evaporated are placed in a tungsten or molybdenum boat (metal, inorganic materials), or a quartz crucible (organic materials) sitting in a tungsten heating basket, which are then mounted to the copper heating arms of the evaporator chamber. The substrates covered with shadow mask were placed on a rotary plate at the top of the chamber. The chamber is then closed and vacuumed to pressure <10-5 mbar. When the vacuum is reached to the desire level, the evaporation process starts by passing current through the boat or heating basket to resistively heat up the source above its vaporizing temperature. The source vapor radiates in all radial directions with long mean free path (longer than the size of the chamber) 25  due to the high vacuum level. The vapor arrives at the samples’ surface and condenses into solid phase and gradually forms a thin film. The film thickness and deposition rate are constantly monitored by two quartz sensors to a digit of 0.01 angstrom. The tooling factor, i.e. the ratio of the amount of material reaching the samples to the amount of material reaching the sensors, is calibrated prior to the evaporation process. For the calibration process, a thick layer of film was evaporated on glass substrates with a set thickness (e.g. 100 nm of Ag) and then the actual thickness of the films were measured by thickness measurements equipment such as profilometers. The tooling factor of the system is then calculated by the following equation.  𝑇𝑜𝑜𝑙𝑖𝑛𝑔𝑎𝑐𝑡𝑢𝑟𝑎𝑙 = 𝑇𝑜𝑜𝑙𝑖𝑛𝑔𝑎𝑝𝑝𝑟𝑜𝑥𝑖𝑚𝑎𝑡𝑒 ×𝑇ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠𝑎𝑐𝑡𝑢𝑟𝑎𝑙𝑇ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠𝑠𝑒𝑡 (1)  The films of the OLED are evaporated subsequently on the substrate and the Ag cathode and Au anode films were patterned using shadow masks. Figure 2.9 displays schematics and actual samples during each steps of the fabrication process. The emitting layer, which consist of two chemicals of Ir(ppy)3 doped in CBP with 6% w.t. ratio, cannot be thermally evaporated as two separate layers. To achieve the desired doping ratio we first mixed the Ir(ppy)3 and the CPB powder with a 6% w.t. ratio. The pre-mixed powder was then evaporated as one single material. Another option is to evaporate the two materials simultaneously using two separate material sources with accurately controlled deposition rates to achieve deposited films with the desire doping level. This was found to be extremely difficult to achieve with our evaporation system. It also requires perfect timing that deposition of both films should be started and finished simultaneously. We therefore used the former method, i.e. evaporating a mixture of Ir(ppy)3:CPB.  26  s Figure 2.9 Fabrication process, deposition and patterning of layers. a) PI substrate. b) Ag (100 nm) + Ca (20 nm) patterned with shadow mask. c) Alq3 (40 nm) + Ir(ppy)3:CBP (20 nm) + NPB (45 nm) + MoO3 (2 nm). d) Au (15 nm) patterned with shadow mask.  27   Note that after the evaporation, the active area shows a purple color probably because of the microcavity effect between the Ag cathode and Au anode [16][44]. In case of the Ag and Au micro-cavity, a possible explanation for the color is that the ambient light that entered the OLED layers are reflected between the Ag and Au and get constructive and destructive superposition at different wavelengths. When the light comes out, light of some wavelengths are trapped inside the OLED due to destructive superposition while light of some other wavelengths get constructive superposition and are able to escape, causing different colors depending on the size of the gap between the Ag and Au, i.e. the total thickness of the organic layers (Alq3 + Ir(ppy)3:CBP + NPB). Figure 2.10 shows digital photographs of three OLEDs fabricated with the same thickness of anode and cathode layers, but different thickness of the organic layers (Alq3 + Ir(ppy)3:CBP + NPB). The middle PI OLED is the one fabricated with the thickness reported in Figure 2.6; the left OLED is when the thickness of organics are >30% thinner, showing a pale orange color; the right OLED is when the thickness of organics are >30% thicker, showing a green color in the active area. For comparison, OLED fabricated with ITO anode instead of Au anode does not show any color at all but simple seeing the shiny metal cathode in the active area. 28   Figure 2.10 Photos of OLED fabricated on PI with active area showing different color due to difference thickness of the organic layers, and OLED on ITO coated glass substrate. From left to right, the thickness of total organic layers of the PI OLED are: thin, natural and thick, showing pale pink, purple and green colors in the active area respectively. The OLED fabricated with ITO instead of Au as anode (rest of the layers are the same) does not show any colors in the active area.   It was shown that the micro-cavity of the OLED can be tuned by deposition of a layer of metal oxide such as Ta2O5 to adjust the gap width between the anode and cathode [17]. In this study the width of micro-cavity was tuned by scaling the total thickness of the organic layers. We also varied the thickness of Alq3 (the ETL) and NPB (the HTL) to balance the number of electrons and holes reaching the emitting layer seeking the brightest visual emission. After repeatedly fabrication of OLED devices, the optimized thickness of Alq3, Ir(ppy)3:CBP, and NPB are gradually obtained as shown in Figure 2.6.   A major issue that has not yet been addressed is the roughness of PI substrates. Following the fabrication steps discussed in the previous section, OLED was fabricated on both PI and glass 29  substrates. It turned out that with the same fabrication condition and device structure, the OLEDs made on glass work well, but OLEDs made on PI were open circuit. It was found that the sheet resistance of the thin gold film deposited as top electrode on the OLEDs made on the PI substrate is too high. For the same thickness of Au (15 nm), the sheet resistance of the Au film for OLEDs with the glass substrate was measured to be ~100 Ω/□ with optical transmittance of ~67% at 550 nm wavelength, while the sheet resistance of those with the PI substrate was measured to be ~106 Ω/□ with optical transmittance of ~66% at 550 nm wavelength. Further investigation under SEM revealed that the measured sheet resistance is in good agreement with the morphology and continuity of the films. The corresponding SEM images of the top Au anode in Figure 2.11 show that the Au deposited on glass substrate as OLED top electrode is much more continuous than the Au deposited on PI as top electrode, which forms as discontinuous grains and islands of Au.  30   Figure 2.11 SEM images of 15 nm Au anode of OLED fabricated on glass (left two images) and PI (right two images) substrates.   Using the thermal evaporation, a number of Au films were deposited on the PI substrate at different conditions such as different deposition rate, rotary speed, heating power in order to improve the continuity and therefore the conductivity of the thin Au film. In the optimum condition Au films with the thickness of 12 nm deposited at pressure ~2x10-5 mbar on PI substrate shows sheet resistance of ~20 Ω/□ and transmission of ~55% (at 550 nm). The same fabrication condition was then used to fabricate OLEDs but the OLEDs fabricated at such low vacuum condition, had poor performance with non-uniformity emission and many of the devices 31  burned immediately. The poor performance of those devices can be attributed to the non-uniformity of the deposited organic and injection layers at low vacuum level, where the organic materials was probably poorly evaporated as larger grains. Therefore, lowering vacuum level seems to work in improving gold film quality but not for organic materials. The overall performance of OLED fabricated under this conduction is not the optimum.   2.6 Side-angle Evaporation Method The high roughness of the PI surface is the main reason for observing discontinuity on the thin film of gold caused by the poor step coverage of the rough surfaces by thermal evaporation, which is a line of sight deposition. In the regular thermal evaporation configuration, the source is located directly beneath the target and therefore the vaporized materials are deposited on the substrate from a direction almost perpendicular to the substrate. We propose a new side-angle evaporation method (shown in Figure 2.12) where instead of perpendicular evaporation onto the substrate, the source is deposited to the substrate from a side angle with respect to a rotating substrate.   32   Figure 2.12 Side-angle evaporation method compared to regular perpendicular evaporation.   Compared to the scale of the roughness and size of features on the surface of the PI substrate (micron or nanometer), the evaporating source is located very far away (more than ten centimeters) and therefore the incoming materials to any local point of the PI surface can be considered in parallel rather than radial. Figure 2.13 shows a schematic diagram of regular perpendicular evaporation in comparison with side-angle evaporation. Presumably, in slope regions (e. g. location B), film deposited with normal evaporation tends to have a thinner coating than other areas (e. g. locations A and D) and therefore produce a non-uniform film. On the other 33  hand, the films deposited by the side-angle evaporation is more likely to have a more conformal and uniform thickness, because the materials are deposited from all directions, which would provide higher chance to deposit on all parts of a rough surface including spikes, holes, or slopes to form a continues film with more uniform thickness. It is likely that the grain size, grain connectivity and uniformity of the deposited film has more complex relations with other parameters of the thermal evaporation such as vacuum level and distance between source and target. After a series of systematic evaporations of thin film of gold on PI and glass substrates, we finalized the configuration shown in Figure 2.14, where the source is ~12 cm away from the samples (rotary center of the samples) and 45 degrees angle with respect to the surface of the substrate. The tooling factor for this new configuration was re-calibrated using the same method presented in section 2.5.    34   Figure 2.13 Schematic diagrams of perpendicular evaporation (top) and side angle evaporation (bottom). In both graphs, materials (green layer) are deposited on a rough surface (pink) from a line of sight direction (green arrows). “A, B, C, D” represent surface topographical areas of spikes, slopes, valleys and flat regions 35  respectively. The blue lines in top graph at A,B,C,D denote the film thickness measured in the line of sight direction. The red lines in in both graphs denote the actual film thickness.    Figure 2.14 Schematic diagram of the changed configuration of the evaporator to achieve the side-angle evaporation.   At the optimum condition the average sheet resistance of the 15 nm thick Au film of OLED fabricated on glass substrate is measured to be 5.5 Ω/□ with optical transmittance ~54% at 550 nm, while Au film of OLED fabricated on PI substrate is measured to be 4.9 Ω/□ with optical transmittance of ~53% at 550 nm. The corresponding SEM images of the surface of the Au as top electrode of the OLEDs fabricated on the glass and PI substrate are shown in Figure 2.15. As 36  can be seen the Au film on the surface of the OLED fabricated on the PI is much more continuous than that deposited on PI with perpendicular evaporation shown in Figure 2.11, which agrees with the sheet resistance measurement results. However, the mechanism responsible for this observation is not conclusive at this point. More investigation is necessary to find the underlying mechanism.    Figure 2.15 SEM images of the Au anode of the OLED fabricated on glass (left two images) and on PI (right two images).   37  Using the proposed side-angle configuration at optimum conditions we successfully fabricated OLEDs on PI and glass (for reference) substrates. It should be noted that the OLEDs made on PI substrate work as well as those made on the glass substrate. The typical OLEDs made on the PI substrate are shown in Figure 2.16.    Figure 2.16 From left to right, the photos of the PI OLEDs which are bended in stress, tensile, and around a thin metal edge.  38  Chapter 3: OLED on PI 3.1 Flexibility The OLEDs fabricated on PI (PI OLEDs) by the side-angle evaporation demonstrates excellent flexibility. Figure 3.1 shows a typical fabricated PI OLEDs that can be easily folded. The bending radius of curvature of the folded OLED is estimated to be ~200 μm. Such number is calculated by mapping the photo and measuring the ratio of bending diameter (vertical dimension of the folded area) to the active area width (horizontal dimension of the folded area, 2 mm by design). No degradation of performance in brightness, color and uniformity are observed during the multiple folding, bending and restoring process.   Figure 3.1 OLED folded into an equivalent bending radius of curvature ~200 μm with no visible degradation in brightness, color and uniformity before and after the bending. Scale bar is 2.0 mm.   Given the thickness of the substrate (50 um) and the bending radius of curvature (~200 um), the bending tensile strain applied on the OLED is estimated by the equation shown in Figure 3.2, 39  where in this case R=200 um and r=25 um, assuming the neutral axis (zero strain) is in the middle of the PI substrate because the PI substrate is much thicker than the OLED layers total thickness. Therefore the tensile strain for the OLED shown in Figure 3.1 is estimated to be ~12.5%. However since some of the OLED materials (e.g. Ag, Au) have larger Young’s modulus than PI, the actual neutral axis could be shifted slightly towards the OLED side, and the actual strain on the OLED would be smaller than above estimated value.    Figure 3.2 Strain calculation of a bended OLED, assuming OLED films are on the outermost of the substrate and the middle line of the substrate is the neutral axis with zero strain.   The level of strain the OLED experienced is very high and likely to cause cracks on the films. The folded area of the OLEDs were examined by SEM to investigate the effect of the applied strain on the morphology of the films. As can be seen in Figure 3.3 that high level of strain in the folded area creates a lot of cracks parallel to the folding line. Despite the presence of the sever cracks on films, the device is still working well without any visible degradation in its 40  performance, probably because the cracks do not propagate throughout the film. The cracks do not extend overall from top to bottom nor completely block the current flow from left to right, which is probably why the device is still working after such folding.   Figure 3.3 SEM images of the folded area of the OLED in Figure 3.1. In the images the vertical direction corresponds to the line of folding.   The effect of ductility of thin metal films on polymer substrates have been studied in several published literature [45][46]. It is possible that similar mechanics applies to the case of folded OLED here. In general the adhesion of a metal film on polymer substrate is stronger when the polymer has higher surface roughness such as PI, which could be the possible reason why the OLED films sticks to the substrate well and do not completely crack or peel off from PI. We tested the adhesion of the bottom silver layer to the PI substrate by simply scratching the films by a sharp metal object with strong forces. And as shown in Figure 3.4 even after more than 10 times scratching on the surface, the silver is still in good adhesion to the PI substrate and no silver flakes are scratched off. For comparison, the same thickness of silver deposited on glass is very easily scratched off.  41   Figure 3.4 Photo showing a PI OLED that has been scratched by a tweezers head in the silver areas where the arrows point. After scratching more than 10 times the silver is still in strong adhesion to the PI substrate.   There are many highly flexible OLEDs and OLED displays reported in the literature, some of which are at cm-order radius of curvature [47][48] and some can reach mm-order radius of curvature [21][25][37][38]. In 2013, a research group reported estimated radius of curvature as small as 10 um which is the most flexible and stretchable OLED ever reported [20]. Such small bending radius of curvature, however, is theoretically estimated based on the strain tolerance and the thickness of the 1.4 μm PET substrate, as well as by mapping the wrinkling of the device rather than actually bended like the folding of OLED demonstrated in Figure 3.1. The strain applied on OLED devices on such a thin PET substrates is very low even at small radius of curvatures. The OLED reported in the literature is not phosphorescent small molecules OLED but all polymer based by using stretchable emitting polymers and polymer transparent electrode, which are more flexible than phosphorescent small molecules. Considering that the OLED we fabricated are phosphorescent and based on small molecules as well as reaching bending radius 42  of curvature to 200 μm, we can claim that the OLED made on PI presented in this thesis is the most flexible phosphorescent OLED (Ph-OLED) ever reported in the literature.   Figure 3.5 shows the approach to bezel-free display proposed in Figure 1.2 and Figure 1.3 in the thesis objective – the displaying area can occupy 100% of the front screen by hiding all the electrodes and wiring to the side or back of the OLED, without damaging the device or affecting the performance thanks to the high flexibility of the OLED as well as the excellent mechanical stability of the PI substrate. The ~200 μm bending radius of the OLEDs also satisfies the level of curving required to make the displays (R=0.2 mm at the edges) in Figure 1.3.   Figure 3.5 Approach to edge-free displaying whose displaying area can occupy 100% of the screen by hiding the electrical wiring and the rest of the OLED to the side or back.   3.2 Bending Test A bezel-free display or edge display requires highly flexible OLEDs, but to make rollable and foldable flexible displays the OLEDs need to tolerate repeated bending and storage under bending over a long period of time. Therefore the OLEDs need to maintain their performance 43  while enduring high level of stress as a result of mechanical deformations. To investigate the stability of the OLEDs with repeated mechanical deformations, the fabricated devices were characterized by the bending test. We designed the bending test apparatus as shown in Figure 3.6. The OLED device (orange colored) is held between a fixed holding arm and a rotating arm that is driven by a stepper motor. The stepper motor is programed by Arduino (source code shown in Chapter 5:Appendix A  ) to rotate the bending arm by 100 degrees and back to its original position as one full bending cycle, at a rate of ~25 cycles per minute. A molybdenum wire of diameter 0.2 mm is held by two metal arms in-axis with the stepper motor’s rotary axis. The relative positions of the parts can be adjusted to suite different sizes of devices and achieve different bending angles and radius of curvature. In our case the bending radius of curvature is maintained at ~2 mm.   Figure 3.6 The 3D drawing of the bending test apparatus. The texts mark the major parts of the setup.   44  Figure 3.7 shows the bending test setup in a typical test. During the bending test, the OLED’s active area are centered w.r.t the position of the molybdenum wire to achieve equal stress on both halves of the emitting area. The two clips holding the devices are in contact with the electrodes of the OLEDs, and also connected to power adapters via wiring to provide operating voltage to the OLEDs if needed.   Figure 3.7 Photo of the bending test apparatus holding two powered on OLED devices.   Figure 3.8 shows the bending test of two PI OLEDs. The left OLED is constantly powered on at a voltage of 4.5V throughout the test; the right OLED is not powered on during the bending test but just turned on at 4.5V for taking the photograph. We can see that after 5000 cycles, there is 45  still no visible degradation of brightness, uniformity, or color change of the OLEDs whether the device is powered on or not during the period of bending. The tensile strain in each bending is estimated to be ~1.25%. The bending result surpasses what have been reported in other literatures both in terms of bending radius (typically 4 mm, 5 mm, or 10 mm in other literatures) and bending cycles (typically 1000 cycles) [49][50][51][52][53].   Figure 3.8 In the order from upper-left to bottom-right, photos for bending test of two PI OLEDs after 0, 1000, 2000, 3000, 4000, 5000 cycles.   3.3 Mechanical Strength It is believed that the high flexibility and long bending lifetime of the PI OLED greatly benefit from the thin substrate thickness as well as the excellent mechanical stability of the PI substrates. As shown in Figure 3.9, the PI OLED is repeatedly poked in the active area from the back at different regions. The tip applies a large pointy stress to the poked spot and causes a black spot 46  in the emitting area. When the pointy stress is removed the black dot disappears, leaving no visible degradation in emission brightness, color, uniformity, nor does it damage or burn the OLED.   Figure 3.9 Photos of a PI OLED being repeatedly poked in the active area from the back. a), b), c) showing the OLED being punched in the active area from the back at different locations, where a black dot appears (circled), and when the pointy stress is removed the black dot disappears. d) After a series of poking in different regions, the OLED shows not damage or degradation in performance.    47  The poked spots are examined under SEM and the images are shown in Figure 3.10. Similarly to the folded area SEM images shown in Figure 3.3, the poked areas have cracks of films which however do not affect the performance of the device, probably because of the same reason that the cracked area are not completely disconnected or peeled off, and the current flow into the cracked areas is not completely blocked.   Figure 3.10 SEM images of the poked areas in the pointy stress test.   3.4 Encapsulation and Luminance The OLEDs were encapsulated inside the glovebox using thermoplastic laminating sheets and wired using thin aluminum ribbons as shown in Figure 3.11. The OLEDs were then taken outside the glovebox for optical and electrical characterization using a Lisun LMS-9000 spectrodiometer paired with a 5 cm diameter-integrating sphere. The relative spectrum of a typical fabricated OLED is shown in Figure 3.12 which has a peak wavelength at 522 nm, dominant wavelength at 547 nm. Figure 3.13 shows the OLED’s CIE index of x=0.3008 y=0.6160 and color temperature of 6032 k. The luminance efficiency is measured to be 0.22 lm/W at 6.5 V which is very low compared to the numbers reported in other literatures. Obviously some more optimization in 48  organic materials and band alignment is required in order to push up the luminance efficiency. It is also believed that the low luminance efficiency is also caused by the quick oxidation of the calcium layer of the OLED, because the thermal plastic is not yet the optimized method for encapsulating the OLED. It turns out oxygen and humidity still penetrates to the devices as the OLEDs completely stop working usually within an hour, sometimes within a few minutes. A more enduring encapsulation method such as coating a layer of low permeability material is needed. There are few suitable encapsulation materials that can be thermally evaporated inside the glovebox. So we tried to coat a layer of parylene on the OLED, which is an excellent encapsulation polymer. But the parylene coater is outside the glove box, and the OLEDs need to be taken out of the glovebox and then transferred to parylene coating chamber for deposition, which exposes the devices temporarily in air for a few minutes. And that short exposure time is long enough to completely oxidize the calcium layer of the OLEDs. Therefore an encapsulation inside the glovebox is needed, or the Ca/Ag layers of the OLED need to be replaced by LiF/Al which is less sensitive to oxygen or humidity.   49   Figure 3.11 OLED encapsulated between two pieces of thermal plastic with aluminum foil as electrical contact.   Figure 3.12 The relative color spectrum of the OLED.   -0.200.20.40.60.811.2380 430 480 530 580 630 680 730 780relative spectrum (a.u.) wavelength (nm)relative spectrum vs. wavelength50   Figure 3.13 CIE index of the OLED.   51  Chapter 4: OLED on Various Rough Substrates & Future Work Having seen the effect of the side-angle evaporation in fabricating OLED on rough substrate such as PI, we explored the possibility of making OLED on other readily available materials such as 3M Scotch tape, fiber glass sheets, transparency sheets and glossy paper that have not been considered as suitable substrates for OLED fabrication due to their roughness. Here, we present working OLED fabricated on the above substrates.   4.1 OLED on Scotch Tape Scotch tape is being commonly used for office work, packaging, and daily life applications. For the fabrication, a piece of Scotch tape is first taped on a 1*3 inch microscope glass slide for handling and patterning. The tape surface is then wiped and cleaned with methanol for a few times and transferred to the evaporation chamber for evaporation process. OLED layers are patterned and evaporated on the tape by the side-angle evaporation method as described earlier in section 2.6. Finally, the tape with deposited OLED devices is peeled off from the glass slide for further characterization. Figure 4.1 shows a typical set of OLEDs fabricated on Scotch tape. The OLED can be turned on at voltage >4.5 V and powered at voltage <7.5V as when the operating voltage goes too high the devices are burned likely due to the surface defects and roughness.   52   Figure 4.1 OLED devices fabricated on Scotch tape. Powered at 6 V  The sheet resistance of the Au top anode is measured to be 6.5 Ω/□, similar to the order of Au anode on PI and glass substrates. The surface roughness of the Scotch tape OLED are examined under SEM. Figure 4.2 shows the SEM images of Scotch tape surface before and after making OLED on top. We can see that both before and after making the OLED, the Scotch tape surface shows roughness in the order of microns. The side-angle evaporation method effectively deals with large surface roughness and defects and was able to make uniform coating of OLED materials on the surface and fabricate operating OLED devices on top. 53   Figure 4.2 SEM micrographs of the Scotch tape surface before (top two images, pre-sputtered with 6 nm of gold) and after (bottom two images) the OLED fabrication. All the SEM images are taken from a 45 degrees tilt.   A challenge encountered during the fabrication process is that when peeling off the tape off the glass slide, tape experiences very large stress to overcome the adhesion force between the tape and the glass surface. As it turns out the OLEDs stop working after the tape is peeled off. To solve this problem the tape adhesive side is passivated priory to the fabrication, by taping a piece of food wrap on the adhesive side. After OLED fabrication is complete, the Scotch tape can be easily taken off from the glass slide thanks to the passivation. Another method that has been tried is to suspend the piece of tape a few millimeters above the glass slide using two spacers at the 54  two ends. The tape however turns out to deform a lot under the heat during the thermal evaporation, causing badly patterned electrodes and malfunction of the devices. It is promising that in the future OLED can be pre-fabricated on rolled up tapes and be ready to cut, stick and use like regular tapes, which opens applications of OLED in markers, labeling, packing, and other tape-type applications.   4.2 OLED on Fiberglass Sheet Fiberglass sheet, a sheet made from a composite of epoxy resin and reinforced by glass fiber, is another candidate as substrate for OLED fabrication. The solid sheet fiberglass (natural color, ~125 um thick) was purchased from ACP composite. To fabricate OLED on a fiberglass sheet, the substrate is cut and cleaned in methanol and IPA each for 10 minutes. Then the fiber glass goes though the same fabrication steps as in previous section to fabricate the OLED on top. Figure 4.3 shows the OLED devices made on fiberglass sheet.  55   Figure 4.3 OLED fabricated on fiber glass substrate powdered at 4.5 V.   The sheet resistance of the Au top electrode is measured to be 4.8 Ω/□, which is at the same order of sheet resistant of PI OLED. Figure 4.4 shows the SEM images of the fiber glass surface before and after making the OLED on top. Very like a moon surface, the fiber glass surface has topography such as pits, valleys and micro-porous holes with size varying from less than a micron to a few tens of microns. The side-angle evaporation effectively handles this high level of topography and successfully fabricated OLEDs on such rough surface with uniform emission. Unlike the OLEDs made on Scotch tape, the OLEDs made on fiber glass has excellent mechanical strength thanks to the enhanced mechanical stability of the substrate. But a problem 56  occurs when the OLEDs are bended that the emission quickly fades and darkens. We believe it is due to the heating generated by damaged and cracked films because of the bending stress. The fiber glass is thicker (125 um) than the PI and Scotch tape and therefore has larger stain during bending. Although it is the thinnest fiber glass we find available in the market, to fabricate both strong and flexible OLED thinner fiber glass substrates are more favorable.    Figure 4.4 SEM images of fiber glass before (top two images, pre-sputtered with 12 nm of gold) and after (bottom two images) fabrication of OLED.   57  4.3 OLED on Transparency Sheets Another interesting substrate we tried on making OLED on is the transparency films for writing and printing class notes in lecturing. We select the transparency sheets designed for laser jet printing because they are more thermally stable. Figure 4.5 shows the OLED made on transparency sheets. Same fabrication procedures are used to fabricate the OLEDs.   Figure 4.5 OLED made on transparency sheet substrate powered at 6 V.  58   Figure 4.6 SEM images of the transparency films surface before (top two) and after (bottom two) OLED fabricated on top. The films without OLED are sputtered with 6 nm of gold for SEM imaging.   The sheet resistance of the Au top anode is measured to be 5.3 Ω/□, similar order as PI. Figure 4.6 shows the SEM images of the transparency films surface before and after the OLED are fabricated on top. The transparency sheets are generally flat but have topography like hills and holes on the order of a few microns. Some craters structures are also observed, which could be air bubbles forming hills on the surface and pop up or melt under the heat during thermal evaporation.  59   4.4 OLED on Glossy Paper OLED made on paper substrates have been reported in a few literatures in which the fabrication process usually involves coating a thick buffer layer such as parylene on the paper to smoothen the rough surface. [54][55] In our case we tried to directly fabricate OLED on glossy paper without any buffer layer. The results turn out to give a very low yield of devices, i.e. occasionally one or two out of a batch of 18 fabricated OLED devices work for only a few seconds and burn before we can take any photos for the devices. We examine the surface of paper and the malfunction paper OLED under SEM which are shown in Figure 4.7.  60   Figure 4.7 SEM images of glossy paper surface before (top two images) and after (bottom two images) fabricating OLED on top. The glossy paper without OLED are sputtered 6 nm of gold for imaging.    From the SEM images we can see that the glossy paper surface have extremely high topologies like ruins, much rougher than the surfaces of PI, Scotch tape, fiberglass and transparency films. These are probably the inorganic coating on the cellulose paper fibers. After depositing the OLED materials the surface is smoothened by all the layers but still have holes and discontinuous areas. The sheet resistance of the Au top anode is measured to be ~70k Ω/□, which is why the paper OLED required a high voltage of >9V to power on and quickly burns even powered on for a few seconds.   61  4.5 Future Work Future work will be focused on more systematic study of side-angle evaporation method in parameters such as source distance, chamber pressure, angles dependences, deposition rate, rotary speed, some of which have been or partially been investigated for the manuscripts being written for potential publications. Other future work involve fabricating OLEDs on other rough substrates we can find, such as different types of tapes and plastics. A better encapsulation method is also needed for the luminance characterization of the OLEDs.  62  Chapter 5: Conclusion  The need for fabricating rollable and foldable displays as well as bezel-free and edge displays have driven the motivation to fabricate super flexible OLEDs. In this thesis, highly flexible top-emitting phosphorescent OLEDs have been fabricated on PI substrates which demonstrate superior flexibility, mechanical stability, and as the approach for bezel-free displays. The OLED structure is systematically designed by considering the materials for each layer of the OLED,                                              including the selection of thin film of gold as semi-transparent electrode, 50 μm PI as flexible substrate, phosphorescent emitter and small molecules transporting materials, as well as Ca/Ag as EIL and bottom cathode. A new side-angle-evaporation method is proposed, studied and optimized to certain conditions in order to deposit thin films on rough substrates with more uniform thickness and better films continuity. The side-angle evaporation method successfully fabricates OLEDs on rough PI substrates with sheet resistance of ~5 Ω/□ and optical transmission of ~53% at 550 nm for the 15 nm gold top electrode, which is comparable to the performances of the thin film of gold on glass substrate.   The resulting PI OLED can be folded to ~200 μm bending radius and endure approximately ~12.5% strain without any visible degradation in brightness, uniformity and color spectrum, which to the best of our knowledge is the most flexible phosphorescent OLED ever reported. The PI OLED can also survive 5000 times of bending at R=2 mm radius of curvature while powered on at 4.5V throughout the test, as well as withstand poking and pointy stress in the active area, both showing no visible degradation in performances. Further SEM investigations in the folded 63  and poked areas of the PI OLEDs show cracks appearing under the large folding or poking strain which however do not malfunction the devices.   In conclusion, the excellent flexibility and mechanical stability of the PI OLEDs are believed to have the following reasons. First, the thin film of gold as semi-transparent electrode has excellent ductility, which maintains good continuity and conductivity during small radius bending and repeatedly bending. Secondly, the thin thickness of the 50 μm PI substrate makes the OLEDs experience less strain during bending while still maintaining strong enough mechanical stability to support and protect the OLED films from damages and stress. Thirdly, although the OLED films crack under the extreme strains at conditions such as folding and poking, the good adhesion between the rough PI surface and the OLED films (especially bottom silver layer) possibly limits the chances of cracks propagating throughout the surface and prevent the films from peeling off, which maintain certain level of local connectivity of the films and allows the charge carriers to delivery to the cracked areas and give emissions. And most importantly, all above are made possible because the side-angle evaporation enables the fabrication of OLEDs on rough PI substrates, which is otherwise very difficult to be fabricated on as OLED substrate by other methodologies.   The side-angle evaporation is also proved effective in fabricating OLEDs on various types of extremely rough substrates including Scotch tape, fiber glass and transparency sheets, which are much rougher than the PI films. The side-angle evaporation method can be generalized to the deposition of other organic small molecules (emitters, ETL, HTL) and applicable to other rough substrates expecting devices of various functions and applications.  64  Bibliography  [1] W. Helfrich and W. G. Schneider, “Recombination Radiation in Anthracene Crystals,” Phys. Rev. Lett., vol. 14, no. 7, pp. 229–231, Feb. 1965. [2] J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns, and A. B. Holmes, “Light-emitting diodes based on conjugated polymers,” Nature, vol. 347, no. 6293, pp. 539–541, Oct. 1990. [3] K. Saxena, V. K. Jain, and D. S. Mehta, “A review on the light extraction techniques in organic electroluminescent devices,” Opt. Mater. (Amst)., vol. 32, no. 1, pp. 221–233, Nov. 2009. [4] K. Hong and J.-L. Lee, “Review paper: Recent developments in light extraction technologies of organic light emitting diodes,” Electron. Mater. Lett., vol. 7, no. 2, pp. 77–91, Jun. 2011. [5] W. H. Koo, S. M. Jeong, F. Araoka, K. Ishikawa, S. Nishimura, T. Toyooka, and H. Takezoe, “Light extraction from organic light-emitting diodes enhanced by spontaneously formed buckles,” vol. 4, no. April, 2010. [6] Y.-H. Cheng, J.-L. Wu, C.-H. Cheng, K.-C. Syao, and M.-C. M. Lee, “Enhanced light outcoupling in a thin film by texturing meshed surfaces,” Appl. Phys. Lett., vol. 90, no. 9, p. 091102, 2007. [7] K. Leo, S. Reineke, F. Lindner, G. Schwartz, N. Seidler, and K. Walzer, “White organic light-emitting diodes with fluorescent tube efficiency,” vol. 459, no. May, 2009. [8] M. C. Gather, A. Köhnen, and K. Meerholz, “White organic light-emitting diodes.,” Adv. Mater., vol. 23, no. 2, pp. 233–48, Jan. 2011. [9] G. Schwartz, M. Pfeiffer, S. Reineke, K. Walzer, and K. Leo, “Harvesting Triplet Excitons from Fluorescent Blue Emitters in White Organic Light-Emitting Diodes,” Adv. Mater., vol. 19, no. 21, pp. 3672–3676, Nov. 2007. [10] J.-S. Park, H. Chae, H. K. Chung, and S. I. Lee, “Thin film encapsulation for flexible AM-OLED: a review,” Semicond. Sci. Technol., vol. 26, no. 3, p. 034001, Mar. 2011. [11] S. Chen, L. Deng, J. Xie, L. Peng, L. Xie, Q. Fan, and W. Huang, “Recent developments in top-emitting organic light-emitting diodes.,” Adv. Mater., vol. 22, no. 46, pp. 5227–39, Dec. 2010. 65  [12] S. Graphene, J. Wu, M. Agrawal, A. Becerril, Z. Bao, Z. Liu, Ќ. Y. Chen, and P. Peumans, “Organic Light-Emitting Diodes on,” vol. 4, no. 1, pp. 43–48, 2010. [13] T.-H. Han, Y. Lee, M.-R. Choi, S.-H. Woo, S.-H. Bae, B. H. Hong, J.-H. Ahn, and T.-W. Lee, “Extremely efficient flexible organic light-emitting diodes with modified graphene anode,” Nat. Photonics, vol. 6, no. 2, pp. 105–110, Jan. 2012. [14] J. Li, L. Hu, L. Wang, Y. Zhou, G. Grüner, and T. J. Marks, “Organic light-emitting diodes having carbon nanotube anodes.,” Nano Lett., vol. 6, no. 11, pp. 2472–7, Nov. 2006. [15] D. Zhang, K. Ryu, X. Liu, E. Polikarpov, J. Ly, M. E. Tompson, and C. Zhou, “Transparent, conductive, and flexible carbon nanotube films and their application in organic light-emitting diodes.,” Nano Lett., vol. 6, no. 9, pp. 1880–6, Sep. 2006. [16] B. M. G. Helander, Z. Wang, M. T. Greiner, Z. Liu, J. Qiu, and Z. Lu, “Oxidized Gold Thin Films : An Effective Material for High-Performance Flexible Organic Optoelectronics,” pp. 2037–2040, 2010. [17] Z. B. Wang, M. G. Helander, J. Qiu, D. P. Puzzo, M. T. Greiner, Z. M. Hudson, S. Wang, Z. W. Liu, and Z. H. Lu, “Unlocking the full potential of organic light-emitting diodes on flexible plastic,” vol. 5, no. October, pp. 753–757, 2011. [18] H. Kim, J. S. Horwitz, G. P. Kushto, Z. H. Kafafi, and D. B. Chrisey, “Indium tin oxide thin films grown on flexible plastic substrates by pulsed-laser deposition for organic light-emitting diodes,” Appl. Phys. Lett., vol. 79, no. 3, p. 284, 2001. [19] G.-F. Wang, X.-M. Tao, and R.-X. Wang, “Flexible organic light-emitting diodes with a polymeric nanocomposite anode.,” Nanotechnology, vol. 19, no. 14, p. 145201, Apr. 2008. [20] M. S. White, M. Kaltenbrunner, E. D. Głowacki, K. Gutnichenko, G. Kettlgruber, I. Graz, S. Aazou, C. Ulbricht, D. a. M. Egbe, M. C. Miron, Z. Major, M. C. Scharber, T. Sekitani, T. Someya, S. Bauer, and N. S. Sariciftci, “Ultrathin, highly flexible and stretchable PLEDs,” Nat. Photonics, vol. 7, no. 10, pp. 811–816, Jul. 2013. [21] S.-W. Cho, J.-A. Jeong, J.-H. Bae, J.-M. Moon, K.-H. Choi, S. W. Jeong, N.-J. Park, J.-J. Kim, S. H. Lee, J.-W. Kang, M.-S. Yi, and H.-K. Kim, “Highly flexible, transparent, and low resistance indium zinc oxide–Ag–indium zinc oxide multilayer anode on polyethylene terephthalate substrate for flexible organic light light-emitting diodes,” Thin Solid Films, vol. 516, no. 21, pp. 7881–7885, Sep. 2008. [22] W. Ji, J. Zhao, Z. Sun, and W. Xie, “High-color-rendering flexible top-emitting warm-white organic light emitting diode with a transparent multilayer cathode,” Org. Electron., vol. 12, no. 7, pp. 1137–1141, Jul. 2011. 66  [23] K.-H. Choi, H.-J. Nam, J.-A. Jeong, S.-W. Cho, H.-K. Kim, J.-W. Kang, D.-G. Kim, and W.-J. Cho, “Highly flexible and transparent InZnSnO[sub x]∕Ag∕InZnSnO[sub x] multilayer electrode for flexible organic light emitting diodes,” Appl. Phys. Lett., vol. 92, no. 22, p. 223302, 2008. [24] T. Baron, J. L. Autran, S. Deleonibus, J. H. Cheon, J. H. Choi, J. H. Hur, and J. Jang, “Active-Matrix OLED on Bendable Metal Foil,” vol. 53, no. 5, pp. 1273–1276, 2006. [25] S. Purandare, E. F. Gomez, and A. J. Steckl, “High brightness phosphorescent organic light emitting diodes on transparent and flexible cellulose films.,” Nanotechnology, vol. 25, no. 9, p. 094012, Mar. 2014. [26] J. W. Kim, S. C. Ryu, Q. Hung Vu, J. Yong So, S.-M. Lee, N. Thi Mai, and L. S. Park, “Synthesis and Performance of Polyimide Films for the Flexible Organic Light Emitting Diodes,” Mol. Cryst. Liq. Cryst., vol. 513, no. 1, pp. 214–226, Nov. 2009. [27] J.-S. Park, T.-W. Kim, D. Stryakhilev, J.-S. Lee, S.-G. An, Y.-S. Pyo, D.-B. Lee, Y. G. Mo, D.-U. Jin, and H. K. Chung, “Flexible full color organic light-emitting diode display on polyimide plastic substrate driven by amorphous indium gallium zinc oxide thin-film transistors,” Appl. Phys. Lett., vol. 95, no. 1, p. 013503, 2009. [28] F. So, J. Kido, and P. Burrows, “O rganic Light- Emitting Devices for Solid-State Lighting Introduction : The Potential for,” Mrs Bull., vol. 33, no. July, pp. 663–669, 2008. [29] M. A. Baldo and S. R. Forrest, “Highly efficient phosphorescent emission from organic electroluminescent devices,” vol. 395, no. September, pp. 151–154, 1998. [30] C. Adachi, M. a. Baldo, M. E. Thompson, and S. R. Forrest, “Nearly 100% internal phosphorescence efficiency in an organic light-emitting device,” J. Appl. Phys., vol. 90, no. 10, p. 5048, 2001. [31] B. Mi, Z. Gao, Z. Liao, W. Huang, and C. H. Chen, “Molecular hosts for triplet emitters in organic light-emitting diodes and the corresponding working principle,” Sci. China Chem., vol. 53, no. 8, pp. 1679–1694, Aug. 2010. [32] S. Reineke, K. Walzer, and K. Leo, “Triplet-exciton quenching in organic phosphorescent light-emitting diodes with Ir-based emitters,” Phys. Rev. B, vol. 75, no. 12, p. 125328, Mar. 2007. [33] R. C. Evans, P. Douglas, and C. J. Winscom, “Coordination complexes exhibiting room-temperature phosphorescence: Evaluation of their suitability as triplet emitters in organic light emitting diodes,” Coord. Chem. Rev., vol. 250, no. 15–16, pp. 2093–2126, Aug. 2006. 67  [34] M. a. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, and S. R. Forrest, “Very high-efficiency green organic light-emitting devices based on electrophosphorescence,” Appl. Phys. Lett., vol. 75, no. 1, p. 4, 1999. [35] Q. Yang, Y. Hao, Z. Wang, Y. Li, H. Wang, and B. Xu, “Double-emission-layer green phosphorescent OLED based on LiF-doped TPBi as electron transport layer for improving efficiency and operational lifetime,” Synth. Met., vol. 162, no. 3–4, pp. 398–401, Mar. 2012. [36] G. M. Farinola and R. Ragni, “Electroluminescent materials for white organic light emitting diodes.,” Chem. Soc. Rev., vol. 40, no. 7, pp. 3467–82, Jul. 2011. [37] H. Cho, C. Yun, J.-W. Park, and S. Yoo, “Highly flexible organic light-emitting diodes based on ZnS/Ag/WO3 multilayer transparent electrodes,” Org. Electron., vol. 10, no. 6, pp. 1163–1169, Sep. 2009. [38] J. Lewis, S. Grego, B. Chalamala, E. Vick, and D. Temple, “Highly flexible transparent electrodes for organic light-emitting diode-based displays,” Appl. Phys. Lett., vol. 85, no. 16, p. 3450, 2004. [39] Z. Wang, Y. Lou, S. Naka, and H. Okada, “Highly simplified small molecular phosphorescent organic light emitting devices with a solution-processed single layer,” AIP Adv., vol. 1, no. 3, p. 032130, 2011. [40] M. T. Greiner, L. Chai, M. G. Helander, W.-M. Tang, and Z.-H. Lu, “Metal/Metal-Oxide Interfaces: How Metal Contacts Affect the Work Function and Band Structure of MoO 3,” Adv. Funct. Mater., vol. 23, no. 2, pp. 215–226, Jan. 2013. [41] K. Hinode, “Whiskers grown on aluminum thin films during heat treatments,” J. Vac. Sci. Technol. A Vacuum, Surfaces, Film., vol. 14, no. 4, p. 2570, Jul. 1996. [42] N. G. Semaltianos, “Thermally evaporated aluminium thin ® lms,” vol. 183, no. September 2000, pp. 223–229, 2001. [43] M. A. Baldo, M. E. Thompson, and S. R. Forrest, “Phosphorescent materials for application to organic light emitting devices *,” vol. 71, no. 11, pp. 2095–2106, 2009. [44] R. H. Jordan, L. J. Rothberg, A. Dodabalapur, and R. E. Slusher, “Efficiency enhancement of microcavity organic light emitting diodes,” vol. 69, no. April 1996, 1997. [45] T. Li and Z. Suo, “Ductility of thin metal films on polymer substrates modulated by interfacial adhesion,” Int. J. Solids Struct., vol. 44, no. 6, pp. 1696–1705, Mar. 2007. [46] Y. Xiang, T. Li, Z. Suo, and J. J. Vlassak, “High ductility of a metal film adherent on a polymer substrate,” pp. 1–4, 2005. 68  [47] J.-S. Yoo, S.-H. Jung, Y.-C. Kim, S.-C. Byun, J.-M. Kim, N.-B. Choi, S.-Y. Yoon, C.-D. Kim, Y.-K. Hwang, and I.-J. Chung, “Highly Flexible AM-OLED Display With Integrated Gate Driver Using Amorphous Silicon TFT on Ultrathin Metal Foil,” J. Disp. Technol., vol. 6, no. 11, pp. 565–570, Nov. 2010. [48] J. K. Jeong, D. U. Jin, H. S. Shin, H. J. Lee, M. Kim, T. K. Ahn, J. Lee, Y. G. Mo, and H. K. Chung, “Flexible Full-Color AMOLED on Ultrathin Metal Foil,” IEEE Electron Device Lett., vol. 28, no. 5, pp. 389–391, May 2007. [49] J.-W. Kang, W.-I. Jeong, J.-J. Kim, H.-K. Kim, D.-G. Kim, and G.-H. Lee, “High-Performance Flexible Organic Light-Emitting Diodes Using Amorphous Indium Zinc Oxide Anode,” Electrochem. Solid-State Lett., vol. 10, no. 6, p. J75, 2007. [50] M. Noda, N. Kobayashi, M. Katsuhara, A. Yumoto, S. Ushikura, R. Yasuda, N. Hirai, G. Yukawa, I. Yagi, K. Nomoto, and T. Urabe, “An OTFT-driven rollable OLED display,” J. Soc. Inf. Disp., vol. 19, no. 4, pp. 316–322, 2011. [51] W. Kim, S. Kwon, S.-M. Lee, J. Y. Kim, Y. Han, E. Kim, K. C. Choi, S. Park, and B.-C. Park, “Soft fabric-based flexible organic light-emitting diodes,” Org. Electron., vol. 14, no. 11, pp. 3007–3013, Nov. 2013. [52] F. Li, Z. Lin, B. Zhang, Y. Zhang, C. Wu, and T. Guo, “Fabrication of flexible conductive graphene/Ag/Al-doped zinc oxide multilayer films for application in flexible organic light-emitting diodes,” Org. Electron., vol. 14, no. 9, pp. 2139–2143, Sep. 2013. [53] X. Wu, F. Li, W. Wu, and T. Guo, “Flexible white phosphorescent organic light emitting diodes based on multilayered graphene/PEDOT:PSS transparent conducting film,” Appl. Surf. Sci., vol. 295, pp. 214–218, Mar. 2014. [54] D.-Y. Yoon, T.-Y. Kim, and D.-G. Moon, “Flexible top emission organic light-emitting devices using sputter-deposited Ni films on copy paper substrates,” Curr. Appl. Phys., vol. 10, no. 4, pp. e135–e138, Nov. 2010. [55] D.-Y. Yoon and D.-G. Moon, “Bright flexible organic light-emitting devices on copy paper substrates,” Curr. Appl. Phys., vol. 12, pp. e29–e32, Sep. 2012.   69  Appendices  Appendix A    Arduino source code for driving stepper motor ////////////////////////////////////////////////////////////////// //The stepper motor is standard 1.8 degrees steps, 12V, 2 Phaser //The stepper motor driver is EasyDriver by Sparksfun //use rotate and/or rotateDeg to controll stepper motor //speed is any number from .01 -> 1 with 1 being fastest -  //Slower Speed == Stronger movement ///////////////////////////////////////////////////////////////// #define DIR_PIN 2 #define STEP_PIN 3  void setup() {    pinMode(DIR_PIN, OUTPUT);    pinMode(STEP_PIN, OUTPUT);  }   void loop(){     //rotate a specific number of degrees    rotateDeg(100, .2);    delay(1000);  70     rotateDeg(-100, .2);  //reverse   delay(1000);   }  void rotateDeg(float deg, float speed){    //rotate a specific number of degrees (negitive for reverse movement)   //speed is any number from .01 -> 1 with 1 being fastest - Slower is stronger   int dir = (deg > 0)? HIGH:LOW;   digitalWrite(DIR_PIN,dir);     int steps = abs(deg)*(1/0.225);   float usDelay = (1/speed) * 70;    for(int i=0; i < steps; i++){      digitalWrite(STEP_PIN, HIGH);      delayMicroseconds(usDelay);       digitalWrite(STEP_PIN, LOW);      delayMicroseconds(usDelay);    }  } 

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
http://iiif.library.ubc.ca/presentation/dsp.24.1-0165557/manifest

Comment

Related Items