Nonideal parasitic resistance effects in bulk heterojunction organic solar cells

摘要

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 (R_(sr)) is near zero while shunt resistance (R_(sh)) approaches infinity. Relaxation of this assumption affects device performance differently depending on the chosen BHJ material system. Specifically, the impact of nonideal R_(sr) is controlled by the electric field dependence of the probability of charge transfer (CT) state dissociation (P_(CT))- 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 P_(CT). Second, light intensity measurements of devices with nonideal R_(sr) and R_(sh) 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 R_(sh) to describe this dissociation process and predicts a photocurrent under reverse bias that exceeds the photocurrent permitted by light absorption.