Enhancing the Efficiency of Organic Photovoltaics by a Photoactive Molecular Mediator

High boiling-point solvent additives, such as 1,8-diiodooctane, have been widely used to tune nanoscale phase morphology for increased efficiency in bulk heterojunction organic solar cells. However, liquid-state solvent additives remain in the active films for extended times and later migrate or evaporate from the films, leading to unstable device performance. Here, a solid-state photoactive molecular mediator, namely N(BAI)₃, is reported that could be employed to replace the commonly used solvent additives to tune the morphology of bulk heterojunction films for improved device performance. The N(BAI)₃ mediator not only resides in the active films locally to fine tune the phase morphology, but also contributes to the additional absorption of the active films, leading to ∼11% enhancement of power conversion efficiency of P3HT:PC₆₀BM devices. Comparative studies are carried out to probe the nanoscale morphologies using grazing incidence wide-angle X-ray scattering and complementary neutron reflectometry. The use of 1 wt% N(BAI)₃ is found to effectively tune the packing of P3HT, presumably through balanced π-interactions endowed by its large conjugated π surface, and to promote the formation of a PC₆₀BM-rich top interfacial layer. These findings open up a new way to effectively tailor the phase morphology by photoactive molecular mediators in organic photovoltaics.