Control of Lattice Strain in α‐FAPbI ₃ Film

Control of residual lattice strain has recently emerged as a key strategy to not only govern the phase stability of α‐formamidinium lead triiodide (α‐FAPbI 3 ) but also manipulate its band structure. Here, we employed an interlayer (IL) between SnO 2 and perovskite, where the heteroepitaxial crystal growth of perovskite film with varied residual lattice strain was enabled by tuning the interplanar lattice spacing ( d ) corresponding to the (002) plane of IL, ranging from 0.329 nm for Cl‐based IL to 0.340 nm for Br‐based IL, as evidenced by transmission electron microscopy. The minimum d ‐mismatch between IL and α‐FAPbI 3 was monitored from the Br‐based IL, effectively relieving the residual tensile strain across the perovskite film and thus resulting in the enhanced phase stability of α‐FAPbI 3 . The employment of Br‐based IL indeed induced a beneficial energy level alignment with defect passivation, which effectively suppressed the interfacial charge recombination and led to a notable increase in open‐circuit voltage. Therefore, the improved power conversion efficiency (PCE) of the strain‐engineered device was well maintained over 2000 h in ambient air, in contrast to a PCE drop by ≈27% for the control device without IL over 1000 h.