Self-imitating oligomeric additives enable 19% efficiency in benzo[1,2-b:4,5-b′]difuran polymer-based organic solar cells

The development of power conversion efficiency (PCE) for organic solar cells (OSCs) based on polymer donors with benzo[1,2-b:4,5-b′]-difuran building block is slower than that of those based on benzodithiophene due to uncontrollable aggregation behavior. However, the former is expected to be more promising in realizing environmentally friendly and high-performance devices. Thereby, a smart aggregation tuning strategy is needed for boosting the efficiency of this type of OSCs. Here we report solid additives designed by self-imitation strategy, which aims to control the aggregation of the donor D18-Fu, and regulate the domain expansion of the acceptor L8-BO. Three oligomeric additives, with or without halogenation, can uniformly reduce the energy loss and enhance charge generation compared to an additive-free control device. This improvement is demonstrated through a series of morphological characterizations, photophysical analyses and theoretical simulations, indicating strong interaction between additive molecules and donor & acceptor. As a result, a 19% PCE is reported in binary OSCs, which also represents the highest level for devices based on benzo[1,2-b:4,5-b′]-difuran core contained polymer donor. Apart from high performance, our study provides new insights into manipulating the competition between the donor and acceptor’s pure phase formation through new additive design methods.