Strain Regulation through In Situ 1D/3D Heterojunction Construction with an Optimal Alkyl Chain Length for Efficient and Stable Perovskite Solar Cells

Residual strain in the perovskite lattice significantly affects device performance by modifying interfacial carrier dynamics. However, simultaneously achieving both strain enhancement and homogenization remains a major challenge. Herein, we construct one-dimensional (1D)/three-dimensional (3D) perovskite heterojunctions by depositing a series of quaternary ammonium iodides with varying alkyl chain lengths, including tetrabutylammonium iodide (TBAI), tetrahexylammonium iodide (THAI), and tetraoctylammonium iodide (TOAI), onto 3D perovskite films. This allows us to investigate the influence of alkyl chain length on the strain state of the 3D perovskite. Notably, THAI, which possesses the optimal alkyl chain length, promotes the formation of 1D THAPbI₃ through octahedral coordination. Furthermore, THAI enables stable anchoring of formamidinium (FA⁺) cations at the heterointerface via hydrogen-bonding interactions, thereby suppressing the outward diffusion of FA⁺ and alleviating strain inhomogeneity. Consequently, the homogenized strain field alleviates local strain concentration and significantly suppresses nonradiative recombination, while the optimized interfacial structure passivates surface defects. Benefiting from this mechanically and electronically coupled optimization, the champion device achieves a power conversion efficiency (PCE) of 25.06% and retains 90% of its initial performance after 1400 h in humid environments (40 ± 5% RH).