Abstract
ABSTRACT
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.