Synergistic strain engineering of the perovskite films for improving flexible inverted perovskite solar cells under convex bending

Flexible perovskite solar cells (fPSCs) have demonstrated commercial viability because of their promising lightness, flexibility, and low-cost advantages. However, in most applications, the fPSCs suffer from constant external stress, such as being kept at a convex bending state, imposing external stress on the brittle perovskite films and causing the fPSCs long-term stability problems. Overcoming these issues is vital. Herein, we propose an effective way to enhance the stability of the fPSCs under convex bending by modulating the residual stress of perovskite film for the first time. Specifically, we have carefully designed a synergistic strain engineering to toughen the perovskite films by introducing 1-butyl-3-methylimidazolium tetrafluoroborate, citric acid, and a novel cross-linker, 5-(1,2-dithiolan-3-yl) pentanoate into perovskite films simultaneously. Besides passivating the perovskite films, the multiple additives effectively convert the residual stress within the perovskite films from tensile to compressive type to alleviate the detrimental impact of bending on the flexible perovskite films. As a result, the optimal efficiencies of triple-additive modified fPSCs have achieved 22.19% (0.06 cm²) and 19.44% (1.02 cm²). More importantly, the strategy could significantly improve the stability of the perovskite films and fPSCs at a convex bending state. Our approach is inductive for the future practical field applications of high-performance fPSCs.