Reciprocal carrier collection in organic photovoltaics

Buffer layers between the acceptor and cathode can perform several functions in organic photovoltaic devices, such as providing exciton blocking, protection of active layers against damage from cathode deposition, and optical spacing to maximize the electric field in the active device region. Here, we study electron collection by replacing the common buffer layer, bathocuproine, with a series of six, substituted tris(beta-diketonato)Ru(III) analogues in the structure: indium-tin-oxide/copper phthalocyanine/C-60/buffer/Ag. These buffer layers enable collection of photogenerated electrons by transporting holes from the cathode to the C-60/buffer interface, followed by recombination with photogenerated electrons in the acceptor. We use a model for free-polaron and polaron-pair dynamics to describe device operation and the observed inflection in the current-voltage characteristics. The device characteristics are understood in terms of hole transfer from the highest occupied molecular orbital energy levels of several Ru-complexes to the acceptor.