Coupled optoelectronic simulation of organic bulk-heterojunction solar cells: Parameter extraction and sensitivity analysis

A comprehensive optoelectronic device model for organic bulk-heterojunction solar cells is presented. First the optical incoupling into a multilayer stack is calculated. From the photon absorption profile a charge transfer exciton profile is derived. In this study we consider the Onsager–Braun mechanism to calculate the dissociation of the CT excitons into free charge carriers. These free charge carriers then migrate toward the electrodes under the influence of drift and diffusion. A general problem arising in computer simulations is the number of material and device parameters, which have to be determined by dedicated experiments and simulation-based parameter extraction. In this study we analyze measurements of the short-circuit current dependence on the active layer thickness and current-voltage curves in poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester based solar cells. We have identified a set of parameter values including dissociation parameters that describe the experimental data. The overall agreement of our model with experiment is good, however, a discrepancy in the thickness dependence of the current-voltage curve questions the influence of the electric field in the dissociation process. In addition transient simulations are analyzed which show that a measurement of the turn-off photocurrent can be useful for estimating charge carrier mobilities.