Cross-Linked Polymer Framework to Release Residual Strain of Perovskite Film by an Atomic Scale

The intrinsic thermally induced residual strain of formamidinium-based lead triiodide has been a great challenge for the instability. This intrinsic residual strain induces significant lattice distortion, particularly in the near-surface region of the perovskite active layer, while the mechanisms of stress regulation are still not well-understood. Here, we developed a photoinduced in situ polymerization strategy to alleviate the residual strain. The self-shrinkage behavior of polymers modulates the atomic-scale interactions within the perovskite lattice, effectively tuning its mechanical properties. Our study reveals the fundamental mechanisms underlying micro-to-macro mechanical property evolution. The cross-linked device exhibited reduced lattice distortion, achieving a power conversion efficiency of 25.56%. Additionally, the unencapsulated PSCs showed exceptional heat stability, retaining 90% of their initial efficiency even after 1000 h of exposure at 85 °C in an N₂ atmosphere. This polymer-based approach provides a promising platform for an in-depth investigation of gradient stress regulation in PSCs.