Engineering intrinsic flexibility in polycrystalline perovskite film by grain boundary stitching for high mechanical endurance

A metal halide perovskite (MHP) thin film processed at low temperature is uniquely suited for flexible perovskite solar cells owing to its characteristic low formation energies. However, the intrinsic brittleness (low toughness) of MHP crystals restrict the mechanical endurance, particularly in polycrystalline MHP films enriched with grain boundaries (GBs). To address this issue, a mechanically flexible film of highly crystalline MHP is achieved via a novel soft stitching strategy of GBs, in which a multifunctional sticky elastomer (s-ELA) is used to connect the rigid crystallite grains. The s-ELA also acts as a scaffold in the MHP crystallization process, thereby passivating the structural defects at GBs and improving charge transport properties. This soft-rigid structural design endows excellent mechanical endurance and preserves the morphology after 10,000 deformation cycles of bending at radii as small as 2 mm and stretching of 20%. Furthermore, the hydrophobicity of the s-ELA protects against ingress of moisture and oxygen. The soft-rigid device design represents a new approach towards wearable MHP films with good mechanical endurance, charge transport properties and environmental stability.