TiO2 nanoparticles via simple surface modification as cathode interlayer for efficient organic solar cells

Cathode interlayers (CILs) play important roles in promoting high-performance organic solar cells (OSCs). Among the commonly used CILs, metal oxides, such as ZnO, TiO2 and SnO2, display suitable work function and high conductivity. However, they are usually processed under high temperature and in most cases, they are adopted in the inverted OSCs. Since high-performing non-fullerene OSCs are usually fabricated with conventional configuration, it is interesting to explore the potential application of these metal oxide materials as CIL in these cells. An efficient strategy is to use metal oxide nanocrystals as CIL to deposit onto photoactive layers. However, the as-synthesized nanocrystals have the difficulty to form smooth and dense films due to the serious aggregation. In this work, we developed a method via the surface modification of TiO2 nanoparticles by using 4-dimethylamino benzoic acid to reduce the aggregation, so that smooth TiO2–N film can be formed on top of the photoactive layer. Besides, TiO2–N displays lower work function, which is helpful for better energy levels alignment. All these factors contribute to the improved charge generation and transport, leading to the power conversion efficiencies enhanced from 13.68% to 15.90% for PM6:BTP-4Cl solar cells.