Bulk heterojunction photovoltaic cells with low donor concentration

High-efficiency organic photovoltaic (OPV) cells are mostly based on a bulk heterojunction ^[1] (BHJ) structure, which is essentially a thin film of mixed electron donor and acceptor. With few exceptions, the acceptor component is a fullerene-based material such as C ₆₀ ^[2] and PCBM, ^[3] whereas a large variety of materials has been found to be useful as the donor component. ^[4] In order to achieve high power conversion efficiency, the donor-acceptor composition of the BHJ needs to be optimized with respect to light absorption and charge generation, including for instance the use of a low bandgap donor, ^[5] to complement the absorption of the acceptor and selecting a donor with proper highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels to match that of the acceptor. In this work, we show that it is possible to achieve a large open-circuit voltage ( V _oc > 1.0 V) in a fullerene-based OPV with almost any donor, provided that the donor is present in a small concentration and that MoO _x is used as the Schottky barrier contact ^[6] to the BHJ. With fine tuning of the donor concentration to overcome the hole-transport limitation in the BHJ, high power conversion efficiency (η _PCE > 5%) has been realized.