Negative Thermal Expansion Material‐Mediated Stress Engineering for Enhancing the Mechanical and Thermal Stability of Flexible Perovskite Solar Cells

ABSTRACT Flexible perovskite solar cells (FPSCs) offer lightweight, high‐efficiency, flexibility, residual stress from substrate‐perovskite thermal expansion coefficients (CTE) /lattice mismatch drives grain‐boundary cracking, defect rise, and delamination, limiting stability and efficiency gains. Substrates with a low CTE limit the lattice contraction of perovskites during the cooling process, which leads to the generation of tensile strain. In this paper, a strategy for regulating the CTE was developed by adding potassium pyrophosphate (KPP) to perovskite precursors. The P─O bonds in KPP not only form coordination bonds with Pb 2+ , but also form hydrogen bonds with FA + , reducing the defect state density and improving the stability of the device. More importantly, KPP with the characteristic of “thermal contraction and cold expansion” can significantly relieve the residual stress in the perovskite thin film. After 100 thermal cycles between 25°C and 100°C, its thermal cycling stability remains at 90%, while that of the control group is only 70.9%. Therefore, the optimized FPSCs achieved a power conversion efficiency (PCE) of 25.41%. In addition, unpackaged devices exhibit mechanical robustness at T 92 > 10 ,000 bending cycles (with a bending radius of 5 mm), operational stability at T 91 >1000 h.