Measuring Nanoscale Exciton Transport and Carrier Recombination in Organic Solar Cells

Abstract

Organic photovoltaic devices (OPVs) have the potential to provide low cost solar energy to unique applications which are not accessible by traditional photovoltaics. These devices are made from abundant materials and can be deposited on lightweight flexible substrates with low cost roll-to-roll manufacturing techniques. However, to date they have suffered from relatively low power conversion efficiencies compared to their inorganic counterparts. As such, a deeper understanding of the fundamental processes which govern photoconversion in OPVs is needed in order to better inform materials and device design and realize more efficient devices. Optical absorption in OPVs leads to the formation of Coulombically bound electron-hole pairs, called excitons, which must be dissociated into free carriers to collect photocurrent. Inefficiencies in these devices result from the short diffusion length (LD) of excitons and the subsequent recombination of generated carriers. In this dissertation, material parameters which affect the magnitude of L¬D and new techniques to quantify and decouple transport and recombination mechanisms will be presented. Covered topics include the effects of molecular impurities on L¬D, techniques to measure LD and the exciton lifetime in previously inaccessible dark (non-luminescent) materials through photovoltage measurements, methodologies to quantitatively decouple recombination mechanisms at device relevant operating conditions, and the effects of polycrystalline grain size on LD in singlet fission materials, which are capable of producing two excitons per absorbed photon. These studies provide tools to better understand the underlying physics which govern photoconversion and material parameters which can be manipulated to enhance exciton transport to realize more efficient devices.