Facet Orientation-Dependent Strain Relaxation Stabilizes\nFAPbI<sub>3</sub> Perovskite Solar Cells

The\nexclusive orientation of a singular perovskite facet during\ncrystallization can lead to significant strain accumulation in solution-processed\nfilms, compromising the performance of perovskite solar cells (PSCs).\nTo address this issue, we propose a facet-complementarity strategy\nthrough crystallization modulation utilizing a formamidine-based additive\n1-(2-fluorobenzyl)-1<i>H</i>-pyrazolo[3,4-<i>b</i>]pyridine-3-carboximidamide hydrochloride (FPPC) to control strain\naccumulation. The FPPC ligand shows strong adsorption on the (111)\nfacet of perovskite via chemical interactions with octahedra lattice,\npromoting the formation of a more stable (111) facet against the preferred\n(100) orientation and achieving facet complementarity in the FAPbI<sub>3</sub> film. Additionally, the increased dissociation barrier of\nthe ligand-PbI<sub>2</sub> bonding extends the crystallization growth\nwindow, allowing for a better lattice rearrangement and a balanced\ncrystallization rate from bulk to surface. As a result, the facet-complementary\ndevices achieve champion efficiencies of 25.63 and 24.26% on 0.09\n and 1.0 cm<sup>2</sup> scales, respectively, with greatly enhanced\ndevice longevity.