Enhanced performance of P3HT/PCBM bulk heterojunction photovoltaic devices by adding spin ½ radicals

We have studied bulk heterojunction organic photovoltaic devices (solar cells) based on polymer/fullerene blends using both electrical and magneto-optical methods. We show that adding spin ½ galvinoxyl radicals to the device active layer, consisting of a regio-regular polythiophene/fullerene derivative [P3HT/PCBM] blend, significantly improves the device performance. Compared to pristine photovoltaic devices, the radical-rich devices show improved short-circuit current density, fill factor, and power conversion efficiency. The enhanced device performance is attributed to a reduced geminate recombination rate and improved carrier transport, both of which result from spin-spin interactions between the radical impurities and the photogenerated carriers. Optically detected magnetic resonance, a technique that is sensitive to spin-lattice relaxation rates, is used to verify the proposed mechanism.