Thermal-Induced Performance Decay of the State-of-the-Art Polymer: Non-Fullerene Solar Cells and the Method of Suppression.

Improving thermal stability is of great importance for the industrialization of polymer solar cells (PSC). In this paper, we systematically investigated the high-temperature thermal annealing effect on the device performance of the state-of-the-art polymer:non-fullerene (PM6:Y6) solar cells with an inverted structure. Results revealed that the overall performance decay (19% decrease) was mainly due to the fast open-circuit voltage (, 10% decrease) and fill factor (FF, 10% decrease) decays whereas short circuit current () was relatively stable upon annealing at 150 °C (0.5% decrease). Pre-annealing on the ZnO/PM6:Y6 at 150 °C before the completion of cell fabrication resulted in a 1.7% performance decrease, while annealing on the ZnO/PM6:Y6/MoO films led to a 10.5% performance decay, indicating that the degradation at the PM6:Y6/MoO interface is the main reason for the overall performance decay. The increased ideality factor and reduced built-in potential confirmed by dark - curve analysis further confirmed the increased interfacial charge recombination after thermal annealing. The interaction of PM6:Y6 and MoO was proved by UV-Vis absorption and XPS measurements. Such deep chemical doping of PM6:Y6 led to unfavorable band alignment at the interface, which led to increased surface charge recombination and reduced built-in potential of the cells after thermal annealing. Inserting a thin C layer between the PM6:Y6 and MoO significantly improved the cells' thermal stability, and less than 2% decay was measured for the optimized cell with 3 nm C.