Open Collections

Tully, Alexandra Bennett

University of British Columbia

Organic photovoltaics (OPVs) have the potential to serve as flexible and transparent solar cells, comprised of inexpensive materials and created through scalable and energy efficient processes. However, poor power conversion efficiency (largely due to inefficient charge separation) prevents them from being a commercial competitor to today’s conventional solar cells. This thesis marks the first results of a collaborative research project at the University of British Columbia’s Stewart Blusson Quantum Matter Institute investigating the physics underlying poor charge transfer in OPVs. The first step in engineering more efficient charge separation is understanding the fundamentals of the charge transfer process. When an electron is excited in an OPV, it remains coulombically bound with its hole, forming an exciton. Excitons in organic solar cells are more tightly bound with larger binding energies than those in conventional solar cells, making them harder to dissociate into free charges. The nature of these excitons plays a role in the generation of free carriers in OPVs, but it remains an open question that can benefit from the temporal and spatial information provided by time- and angle-resolved photoemission spectroscopy (TR-ARPES). We work with neat C60 thin films due to the material’s foundational status as a prototypical OPV electron-acceptor material and because it is a photo-absorbing excitonic semiconductor, with a manifold of optically-accessible excitonic states. The importance of long-range order to conducting spectroscopic analysis of a thin film’s electronic structure prompted our development of a generalizable growth technique that consistently results in single-domain C60 films of controllable thicknesses, using Au(111) as an epitaxially well-matched substrate. To better understand the evolution of the optically-excited carriers, we use TR-ARPES (supported by low-energy electron diffraction and scanning tunneling microscopy) to investigate and to map the energy landscape of C₆₀ thin films in both space and time (with ångström and femto- to picosecond resolution, respectively). This thesis presents the time-evolution of excitonic states and their angular dispersion. Our results include what is, to our knowledge, the first angular-resolved data within the first Brillouin zone of the excited states of C₆₀.

Text

2024-01-02

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/87013

Physics

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/