Dissecting the structure-stability relationship of Y-series electron acceptors for real-world solar cell applications

Despite striking progress toward improving the photovoltaic (PV) performance of organic solar cells (OSCs) with recent Y-series non-fullerene acceptors (Y-NFAs), knowledge about their outdoor performance under real-world conditions and photodegradation mechanisms remains elusive, which is urgently needed to close the lab-to-fab gap of OSCs. Herein, for the first time, we study the structure-outdoor-stability relationship of Y-NFAs. We show that Y-NFAs with long internal side-chains exhibit high energy barriers for photoisomerization, and fluorinated end-groups can enhance the structural confinement to inhibit the photodegradation pathway and thereby improve device stability. Furthermore, the performance loss of Y-NFA-based OSCs under illumination is mainly driven by increased trap-assisted recombination over time. The structure-stability correlation and demonstration of outdoor performance of these state-of-the-art Y-NFA cells provided in this study highlight molecular engineering of device stability control to minimize power output losses in real-world climates.