Polymer Molecular-Weight Engineering Enables 19.56%-Efficient Organic Solar Cells Based on Y6 Acceptors

As the prototypical Y-series nonfullerene acceptor, Y6 exhibits outstanding optoelectronic properties but its limited solubility and strong crystallization tendency have long been regarded as constraints on the device performance. Here, we propose a strategy distinct from conventional acceptor molecular engineering: by tuning the molecular weight of the donor polymer D18, the film formation kinetics of Y6 can be substantially reshaped, enabling highly efficient devices without relying on molecular redesign of the acceptor. We find that an intermediate polymer molecular weight effectively slows down the rapid crystallization of Y6 during spin-coating, prolongs the duration of the intermediate metastable state, suppresses premature phase separation, and facilitates the formation of a more uniform and interpenetrating bulk heterojunction morphology. In situ UV–vis absorption and fluorescence spectroscopy provide direct evidence for this kinetic control mechanism. The device based on this strategy achieved 19.56% photoelectric conversion efficiency, indicating that polymer molecular-weight engineering can overcome the limitation of low solubility acceptors.