Formamidinium lead iodide (FAPbI3) is a highly attractive absorber for photovoltaics, but the photoactive black α-phase is intrinsically metastable at room temperature and commonly requires compositional engineering or high-temperature processing for stabilization.[1,2] However, these strategies may introduce additional degradation pathways.[3] Here, we use pressure-assisted annealing as a physical processing route to promote phase-pure α- FAPbI3 formation in thin films without relying on alloying. FAPbI3 layers are crystallized under controlled temperature and uniaxial pressure. Combining heat and pressure lowers the temperature required for α-phase formation and suppresses residual PbI₂/δ- FAPbI3 signatures, yielding compact black α- FAPbI3 films over a broad processing window.

Structural analysis by GIWAXS and XRD shows that pressure-assisted annealing enhances the crystallite size, reduces microstrain, and induces preferential texturization. These changes indicate that pressure does not only accelerate phase conversion but also modifies the crystallization pathway and lattice relaxation of FAPbI3. Complementary nanoscale measurements reveal a pronounced morphological improvement: rough reference films with large height variations are transformed into smooth, densely packed films with substantially reduced surface roughness. Kelvin probe force microscopy further shows a narrower contact-potential distribution after pressing, consistent with a more electronically homogeneous surface.

The structural and morphological improvements translate into enhanced optoelectronic quality. Photoluminescence imaging shows a more homogeneous emission response after pressure-assisted annealing, while spectral shifts and increased photoluminescence quantum yield indicate reduced non-radiative recombination and modified local bandgap/strain landscapes. These improvements are consistent with larger grains, fewer electronically active grain boundaries, and reduced disorder at the film surface and buried interfaces. When implemented in inverted solar cells, pressure-treated FAPbI3 absorbers increase the open-circuit voltage and fill factor, indicating reduced recombination losses and improved charge extraction. Finally, operational testing under illumination shows improved stability for pressure-treated devices compared with reference FAPbI3 cells under accelerated aging conditions.

Pressure-assisted annealing therefore provides a direct physical handle to control FAPbI3 phase formation, crystallinity, surface morphology, electronic homogeneity, and device stability. This work establishes mechanical processing as a complementary strategy to chemical strain engineering and highlights pressure as a parameter for stabilizing high-quality pure- FAPbI3 perovskite solar cells.