Central Core-Twisted Conformation Acceptors Achieving 20.60% Efficiency via Suppression of Nonradiative Losses without Sacrificing Current and Fill Factor in Binary Organic Solar Cells

Designing acceptors with low nonradiative energy losses without compromising short-circuit current density and fill factor remains a critical challenge for achieving high-efficiency organic solar cells. In this study, we design and synthesize two acceptors, named a-Th2Cl and a-Th2Br, featuring a halogenated thiophene unit grafted via a single bond onto the central core, an approach that extends beyond conventional central core conjugation extension acceptor design. The rotation around the single bond can result in twist conformation, and the aggregation-caused quenching effect during the transition from solution to film is effectively suppressed, favoring increasing photoluminescence quantum yields. X-ray crystal structure analysis reveals that a-Th2Br exhibits unusual molecular packing behavior with strong <i>J</i>-aggregation. Theoretical simulation demonstrates that a-Th2Br aggregates exhibit a reduced extent of excited-state charge transfer and enhanced fluorescent oscillator strength compared to the benchmark acceptor L8-BO, rationalizing the observed high photoluminescence quantum yields and low nonradiative energy losses for the two acceptors films and corresponding organic solar cells. Moreover, when blended with PM6, both acceptors yield favorable bulk heterojunction morphologies. As a result, binary organic solar cells based on PM6:a-Th2Cl and PM6:a-Th2Br deliver power conversion efficiencies of 19.87 and 20.60% (certified 20.05%), with impressively low nonradiative energy losses values of 0.202 and 0.194 eV, respectively. These results highlight the potential of central core engineering in simultaneously suppressing nonradiative losses and maintaining a high short-circuit current density and fill factor in high-performance organic solar cells.