Facet Orientation-Dependent Strain Relaxation Stabilizes FAPbI₃ Perovskite Solar Cells

The exclusive orientation of a singular perovskite facet during crystallization can lead to significant strain accumulation in solution-processed films, compromising the performance of perovskite solar cells (PSCs). To address this issue, we propose a facet-complementarity strategy through crystallization modulation utilizing a formamidine-based additive 1-(2-fluorobenzyl)-1H-pyrazolo[3,4-b]pyridine-3-carboximidamide hydrochloride (FPPC) to control strain accumulation. The FPPC ligand shows strong adsorption on the (111) facet of perovskite via chemical interactions with octahedra lattice, promoting the formation of a more stable (111) facet against the preferred (100) orientation and achieving facet complementarity in the FAPbI3 film. Additionally, the increased dissociation barrier of the ligand-PbI2 bonding extends the crystallization growth window, allowing for a better lattice rearrangement and a balanced crystallization rate from bulk to surface. As a result, the facet-complementary devices achieve champion efficiencies of 25.63 and 24.26% on 0.09 and 1.0 cm2 scales, respectively, with greatly enhanced device longevity.