Charge extraction and photocurrent in organic bulk heterojunction solar cells

The pseudo point-symmetry of the photocurrent-voltage characteristics J(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(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.