Sustained‐Release Regulation on FAPbI 3 Crystallization to Enhance Photo‐Thermal Stability of Perovskite Solar Cells

Controlling crystallization kinetics of FAPbI 3 perovskite is crucial for improving crystal quality, device performance and stability. Herein, we report a sustained‐release strategy to regulate the PbI 2 ‐to‐FAPbI 3 conversion. PbI 2 lattice expansion arising from the interaction between PEI (oligomeric poly(ethyleneimine)) and iodide species suppresses nonuniform PbI 2 crystallization and facilitates FAI infiltration. Moreover, the weak interaction between oligomeric PEGDE (poly(ethylene glycol) diglycidyl ether) and FAI slows the reaction between FAI and PbI 2 , leading to homogeneous phase conversion during annealing. Consequently, enlarged FAPbI 3 grains are obtained with improved crystallographic orientation, reduced strain and suppressed nonradiative recombination. Furthermore, released PEI and PEGDE undergo in situ crosslinking to form a three‐dimensional network that passivates grain boundaries, stabilizes microstructure and affords inner encapsulation. The resulting champion n‐i‐p device presents a power conversion efficiency of 26.6% (certified 26.17%). Both films and devices exhibit enhanced photothermal stability. The PEI/PEGDE‐treated device maintains 92% of its initial efficiency at 85 °C for over 1,100 h, 96% after continuous illumination (∼100 mW cm − 2 ) and maximum‐power‐point (MPP) tracking for over 1,500 h, and 89% after MPP tracking at 85 °C over 12 days. This work offers a polymerization‐assisted route for synchronizing crystallization control, defect passivation and inner encapsulation to enhance perovskite solar‐cell stability.