Charge extraction and photocurrent in organic bulk heterojunction solar cells

Abstract

The pseudo point-symmetry of the photocurrent-voltage characteristics ${J}_{\text{ph}}(V)$ (defined as the difference between light and dark currents) has been utilized to determine important properties of organic bulk-heterojunction (BHJ) solar cells, e.g., contact recombination velocities, the dominant charge recombination mechanism, or the presence of a field dependent exciton dissociation process. In order to improve the theoretical understanding of the photocurrent generation in BHJ solar cells, we apply a numerical drift-diffusion model to investigate the effect of injection barriers, selective contacts, different recombination mechanisms, and series resistances on ${J}_{\text{ph}}(V)$. We show the consistency of the model with experimental data from literature and reduce different experimental observations to a single, fundamental mechanism in solar cells with intrinsic absorber layers: position dependent equilibrium concentrations and lifetimes of the charge carriers. Based on this result, we discuss the special points of the photocurrent-voltage characteristic such as the point of symmetry and the compensation voltage.