On the Excited States Formed During Interfacial Charge Transfer and Recombination in Organic Bulk Heterojunction Photovoltaic Devices

Abstract

The optimization and implementation of organic photovoltaic (OPV) devices as low-cost and highly modular power sources are complicated by the intricacies of materials design and processing. Small variations in molecular structure and packing can lead to drastic changes in the photophysical pathways that enable or detract from optical-to-electrical energy conversion. This thesis outlines progress in understanding functional OPV device operation by characterizing these photophysical pathways using optical spectroscopy. With both optical and electrical perturbations to the photovoltaic systems, we relate the spatial properties of organic semiconductors with their thermodynamic energy landscapes and the kinetic processes that mediate energy conversion. We probe interactions between electron-donating and electron-accepting molecules whose energy level offsets must overcome coulombic binding energies of excitonic states in the low dielectric media, driving separation of charge carriers to produce power through generation of photocurrent and photovoltage. We highlight the roles of energetics and kinetics, while further elucidating the influences of electronic coupling, spin character, excited-state density, and delocalization for optimizing interfacial charge transfer and lessening photovoltaic loss through recombination.