Nonideal parasitic resistance effects in bulk heterojunction organic solar cells

A common assumption in both experimental measurements and device modeling of bulk heterojunction (BHJ) organic solar cells is that parasitic resistances are ideal. In other words, series resistance (Rsr) is near zero while shunt resistance (Rsh) approaches infinity. Relaxation of this assumption affects device performance differently depending on the chosen BHJ material system. Specifically, the impact of nonideal Rsr is controlled by the electric field dependence of the probability of charge transfer (CT) state dissociation (PCT). This is demonstrated by evaluating the experimental current density versus voltage response within the framework of a drift/diffusion model for two BHJ systems that strongly differ in PCT. Second, light intensity measurements of devices with nonideal Rsr and Rsh are shown to convolute the scaling of short-circuit current and open-circuit voltage with light intensity, which is a common technique to study BHJ device physics. Finally, we show the connection between the drift/diffusion and equivalent circuit model with regard to each model's treatment of CT state dissociation. In particular, the equivalent circuit model utilizes a light intensity dependent Rsh to describe this dissociation process and predicts a photocurrent under reverse bias that exceeds the photocurrent permitted by light absorption.