EFFECT OF CHARGE TRANSFER STATE ENERGETIC DISORDER ON SPECTRAL RESPONSE, RECOMBINATION AND ENERGY LOSS OF ORGANIC SOLAR CELLS

Abstract

Organic photovoltaics (OPVs) have demonstrated rapid progress in the last decade, with power conversion efficiencies comparable with thin-film silicon-based solar cells. Despite this improvement in performance, which for the most part originated from material innovations, and advanced interface and optical design, fundamental understanding of the carrier generation-transport-recombination processes in OPVs are still lacking. Specifically, the exact role of charge transfer (CT) states, an intermolecular state unique to organic heterojunctions that is believed to be a crucial mediator for most physical processes in an OPV, is still under debate. The inherently disordered nature of OPV materials, originating from molecular conformation and packing, and localized variations in the dielectric environment, mandates that CT-density of states is broadened by energetic (static) disorder. Despite that, current analytical models and analysis of experimental results often fail to explicitly include the effect of CT-state disorder. In this thesis, we seek to address that gap and discuss the influence of CT-state energetic disorder on spectral response, carrier generation-recombination processes, and energy-loss of OPVs. We show how energetic disorder in CT-states, coupled with the existence of multiple electronic CT-states, lead to ultra-broad sub-gap (CT) absorption spectra, which extend to the near-infrared. Once the CT-absorption reaches near-infrared, we demonstrate a new non-radiative recombination mechanism involving energy transfer between CT and polaron states. Our work suggests that this recombination process is fundamental and explains numerous experimental observations regarding energy-loss in OPVs. Further characterization of CT-state disorder utilizing temperature dependent external quantum efficiency spectra shows strong correction between the CT-state energetic disorder and non-radiative energy loss in a wide range of OPV material systems. Based on the aforementioned experimental findings, we suggest design rules to minimize the non-radiative loss, and also suggest a redefinition of energy-loss such that the effect of CT-state disorder is incorporated. We believe our findings will enhance the general understanding of the effect of CT-state disorder on OPV performance and aid to bridge the gap between the experimental and thermodynamic efficiency limits of OPVs.