High‐Performance Binary All‐Polymer Solar Cells with Efficiency Over 18.3% Enabled by Tuning Phase Transition Kinetics

Abstract All‐polymer solar cells (All‐PSCs) are considered to be the most promising candidates for realizing efficient and stable organic solar cells (OSCs). However, the challenge of controlling morphology has hindered the performance of All‐PSCs. Compared to small molecule acceptors, polymer acceptors play a more crucial role in obtaining an ideal morphology for All‐PSCs. The molecular weight of polymer acceptors is one of the key factors determining the morphological and mechanical properties as well as the interactions with donors. Herein, by using a monomer of PYIT (PYIT1) and PYITs with low (PYIT2) and high (PYIT3) molecular weights, the impact of molecular weight of the polymer acceptor is systematically investigated on the phase transition process, morphological, and photovoltaic properties. It is found that tuning the molecular weight effectively regulates the phase transition process of the polymer acceptor and its interaction with the polymer donor. This induces significant effects on the aggregation behaviors of the polymers. Appropriate molecular weight polymer acceptors can facilitate favorable phase separation morphology. With PBQx‐Cl as the donor and PYIT2 as acceptor, a high‐performance binary All‐PSC is achieved with an efficiency of 18.39%. This study provides deep insights into the performance enhancement of All‐PSCs through rational polymer acceptor design.