High‐Performance Additive‐/Post‐Treatment‐Free Nonfullerene Polymer Solar Cells via Tuning Molecular Weight of Conjugated Polymers

Abstract In recent years, rapid advances are achieved in polymer solar cells (PSCs) using nonfullerene small molecular acceptors. However, no research disclosing the influence of molecular weight ( M n ) of conjugated polymer on the nonfullerene device performance is reported. In this work, a series of polymers with different M n s are synthesized to systematically investigate the connection between M n and performance of nonfullerene devices for the first time. It is found that the device performance improves substantially as the M n increases from 12 to 38 kDa and a power conversion efficiency (PCE) as high as 10.5% is realized. It has to be noted this PCE is achieved without using any additives and post‐treatments, which is among the top efficiencies of additive‐ and post‐treatment‐free PSCs. Most importantly, the variation trend of the optimal active layer thickness and morphology is significantly different from the device with fullerene as acceptor. The findings clarify the effect of M n on the performance of nonfullerene PSCs, which would benefit further efficiency improvement of nonfullerene PSCs.