Origin of the enhanced performance in poly(3-hexylthiophene): 6,6-phenyl C61-butyric acid methyl ester solar cells upon slow drying of the active layer

The origin of the enhanced performance of bulk heterojunction solar cells based on slowly dried films of poly(3-hexylthiophene) (P3HT) and 6,6-phenyl C61-butyric acid methyl ester is investigated, combining charge transport measurements with numerical device simulations. Slow drying leads to a 33-fold enhancement of the hole mobility up to 5.0×10−7m2V−1s−1 in the P3HT phase of the blend, thereby balancing the transport of electrons and holes in the blend. The resulting reduction of space-charge accumulation enables the use of thick films (∼300nm), absorbing most of the incoming photons, without losses in the fill factor and short-circuit current of the device.