Non-fullerene acceptor pre-aggregates enable high efficiency pseudo-bulk heterojunction organic solar cells

Pseudo-bulk heterojunction (BHJ) fabricated by sequential casting of donor and acceptor layers has been recently demonstrated a superior structure to prepare organic solar cells (OSCs) with enhanced efficiency compared to the conventional BHJ OSCs cast from a common solution of donor and acceptor. However, molecular diffusion and aggregation within the pseudo-BHJ layer bring great challenges to fully realize the advantage of pseudo-BHJ structure. Herein, a solution-incubated pre-aggregation strategy is employed to tune the nanoscale aggregates of non-fullerene acceptor (NFA) BTP-eC11 and N3 to substantially enhance device power-conversion efficiency (PCE). NFA pre-aggregates are incubated in solutions via aging or adding anti-solvent, and then sequentially cast onto D18 fibrillar network, which then penetrate to form a pseudo-BHJ structure with appropriate domain sizes to ensure superior charge mobilities. While the conventional pseudo-BHJ OSCs obtain inferior PCEs below 17% compared with normal BHJ OSCs, NFA pre-aggregates help to achieve remarkable PCEs of 17.7% and 17.5% for D18/BTP-eC11 and D18/N3 pseudo-BHJ OSCs. This work demonstrates that the solution-incubated nanoscale pre-aggregation is an efficient approach to regulate molecular diffusion and aggregation to guarantee high performance pseudo-BHJ OSCs.