Dependence of charge separation efficiency on the exciton-charge transfer offset and Gaussian disorder in organic solar cells

Abstract

State-of-the-art organic solar cells increasingly rely on low-offset semiconductor blends, challenging the traditional requirement of a large energetic driving force for efficient charge separation. In these systems, the energetic offset $\Delta E_{\mathrm{LE-CT}}$ between local exciton (LE) and charge-transfer (CT) states approaches the thermal energy, making exciton-CT hybridization and thermal repopulation of the exciton level critical to device performance. In this work, we directly compare a macroscopic two-state rate model with three-dimensional kinetic Monte-Carlo (kMC) simulations to investigate microscopic charge separation dynamics and the role of Gaussian energetic disorder. We demonstrate that in the absence of disorder, the analytical rate model accurately reproduces kMC predictions for the whole range of $\Delta E_{\mathrm{LE-CT}}$. Specifically, the macroscopic model successfully explains horizontal shifts in the internal quantum efficiency curves that arise depending on how the energetic offset is physically realized in the constituent molecules. We show that these variations can be captured entirely through the ratio of degeneracies of the LE and CT states, respectively. Introducing Gaussian energetic disorder into the kMC simulation reveals a distinct crossover behavior depending on $\Delta E_{\mathrm{LE-CT}}$. While disorder is mostly detrimental at large offsets, it can significantly boost efficiency at intermediate and low offsets. Thermalization of charge carriers within the disorder-broadened density of states creates an effective driving force allowing charge separation even at zero or negative energetic offsets.