In Situ Polymerized Scaffolds Imposing Large Compressive Stress for High‐Performance Perovskite Solar Cells

Metal halide perovskite solar cells have achieved remarkable efficiencies but remain constrained by operational instability, intrinsically linked to mechanical strain within the polycrystalline films. Existing strategies primarily mitigate harmful tensile stress, yet lack the capacity to introduce substantial, beneficial compressive strain. Here, we develop a universal "thermo-mechanical anchoring" strategy that imposes large compressive stress via in situ polymerization of a designed monomer in the perovskite matrix. This method enables broad-range control of film stress, converting substantial tensile stress into pronounced compressive strain (up to 36.05 MPa), experimentally validating a positive correlation between compressive stress and device performance. The polymer network, with a high thermal expansion coefficient and strengthened adhesion, contracts more than the perovskite during cooling, mechanically pulling grains inward. Using a non-polymerizable analogue, we decouple chemical passivation from strain regulation, demonstrating compressive strain as the key factor enabling a champion power conversion efficiency (PCE) of 26.53% for FAMA perovskite, alongside 98% efficiency retention after 1000 h of continuous illumination. Compatible with various additives and perovskite systems, this strategy delivers a champion PCE of 27.07% for CsFAMA triple-cation devices, with a certified reverse-scan efficiency of 26.81%. This establishes in situ polymerization as a foundational design principle for strain-engineering perovskite semiconductors.