We consider models of organic bulk heterojunction photovoltaics with spatially varying compo- sition of donor/acceptor materials. We find analytic expressions for the current-voltage relation in simplifed cases for two blend geometries, and find the effect of varying composition on charge transport is minimal. We then present numerical results for various blend compositions, including the experimentally relevant composition of a donor-rich region near the cathode (a "skin layer" of donor material). We find the primary effect of this variation on device performance derives from its effect on photocharge generation, and that the effect of a skin layer on device performance is small.
Applied Physics Letters