Layer-by-layer processed binary all-polymer solar cells with efficiency over 16% enabled by finely optimized morphology

Optimal active layer morphology is a prerequisite for high-efficiency all-polymer solar cells (all-PSCs). Herein, we report that the vertical phase separation as well as microstructures of the polymer donor and acceptor can be finely optimized in layer-by-layer (LbL) processed all-PSCs. By using 1-chloronaphthalene as the solvent additive during the deposition of the polymer acceptor in the top layer and applying thermal annealing on the entire active layer, bulk-heterojunction like morphology with favorable vertical composition distribution, improved lamellar ordering of the polymer donor (PBDB-T), and the formation of polymer fibrils of the polymer acceptor (PYT) have been realized simultaneously. This favorable morphology led to greatly enhanced exciton splitting efficiency, reduced trap density, improved charge transport, and suppressed charge recombination loss. As a result, the LbL processed all-PSCs of PBDB-T/PYT afforded a power conversion efficiency (PCE) of 16.05%, which is one of the highest PCEs for binary all-PSCs. Moreover, a fill factor (FF) of 0.77 has been obtained, which is the highest value for all-PSCs based on polymerized small molecule acceptors up to date. This work demonstrates an effective strategy for morphology optimization of LbL processed all-PSCs, which will greatly contribute to efficiency breakthrough