Optimum mobility, contact properties, and open-circuit voltage of organic solar cells: A drift-diffusion simulation study

We investigate the role charge carrier mobility plays for loss mechanisms in organic bulk heterojunction solar cells. For this purpose, we perform drift-diffusion calculations for several recombination models and properties of the contacts. We show that in case of selective contacts, higher mobilities increase device efficiency, independent of injection barrier heights, energy level bending at the contacts, and the amount of background dark carriers in the device. Nonselective contacts provide a source of photocarrier loss at the wrong electrode. This is evident from a decrease of the open-circuit voltage (V-oc) with an increased role of charge carrier diffusion, which originates from a higher mobility or from interface barriers reducing the built-in potential. In this case, V-oc furthermore depends on the device thickness. Considering the effect of different recombination models, a too high mobility of one charge carrier decreases V-oc significantly for Langevin recombination. That is why balanced mobilities are desirable for high efficiency in this case. In presence of recombination via CT states, V-oc is mainly governed by the dynamics of the charge transfer state. Based on these differentiations we show that the existence of an optimum mobility derived from simulation depends strongly on the assumptions made for contact and recombination properties and obtain a comprehensive picture how charge carrier mobility influences the parameters of organic solar cells.