Abstract
Geminate pair separation is one of the successive steps determining the performance of organic photovoltaic devices (OPVs). Its analytical description is challenging. The Onsager model, developed in 1930s, is still widely used to describe this process. However, it suffers from severe limitations when applied to disordered organic semiconductors, and, as a result, its predictions deviate from experimental data and kinetic Monte Carlo (kMC) simulations. We employ kMC modelling to investigate the geminate pair separation probability as a function of disorder (energetic within Gaussian disorder model + off-diagonal), temperature, electric field, hopping center concentration, and carrier localization radius, with particular attention to variable-range hopping and the initial energy distribution of mobile carriers. Our results show that non-equilibrium geminate pairs at low temperatures exhibit much higher separation probabilities than the Onsager model predicts, consistent with previous work. However, for initially thermalized pairs with weak carrier localization, long jumps from low-lying states cause efficiency to drop drastically with decreasing temperature, even below the Onsager model predictions. We also vary system density together with carrier localization radius to mimic materials with partial intramolecular charge delocalization and D-A blends. The effect of off-diagonal disorder on pair separation probability is minor in conventional systems with mean hop length ~1 nm, but becomes noticeable in diluted media.