Abstract
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 (V(OC), 10% decrease) and fill factor (FF, 10% decrease) decays whereas short circuit current (J(SC)) 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(3) films led to a 10.5% performance decay, indicating that the degradation at the PM6:Y6/MoO(3) interface is the main reason for the overall performance decay. The increased ideality factor and reduced built-in potential confirmed by dark J - V curve analysis further confirmed the increased interfacial charge recombination after thermal annealing. The interaction of PM6:Y6 and MoO(3) 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(60) layer between the PM6:Y6 and MoO(3) significantly improved the cells' thermal stability, and less than 2% decay was measured for the optimized cell with 3 nm C(60).