Bypassing the yellow phase for extremely stable formamidinium lead iodide perovskite solar cells

Using modeling and structural studies, we show that chloride incorporation in formamidinium lead iodide (FAPI) perovskites alters the energetics of both the formation and degradation pathways. We fabricated films with two coadditives [15 mole % FA chloride (FACl) and 0.5 mole % BA₂PbI₄, where BA is butylammonium)], in which FACl ensures chloride incorporation and both additives collectively create a compressive lattice strain that stabilizes the FAPI black phase and bypasses the formation of a yellow phase during degradation. The coadditive strategy revealed a favorable transition from face-sharing 2H, 4H, 6H, and 8H phases to the corner-sharing 3C black phase. Photovoltaic devices with a p-i-n architecture had an average power conversion efficiency (40 devices) of 24.1% and lost only 2% of their efficiency after 1200 hours at 85° ± 5°C, 1-sun illumination, and open-circuit conditions. Upon stressing at 15-sun illumination at 90°C for >400 hours, the stabilized black 3C phase degraded not through the 2H-PbI₂ phase but through the energetically uphill 3R-PbI₂ phase.