Facilitating High-Quality Crystal Growth of Two-Step Sequential Processed FAPbI3 Perovskite for Efficient Photovoltaics

The controllable two-step deposition of perovskite films is advantageous for large-area manufacturing. However, the incomplete reaction between the initially deposited lead iodide (PbI2) layer and subsequent organic ammonium salts remains a significant challenge, which ultimately degrades the perovskite film quality. Differing from the conventional PbI2 porosity regulation or PbI2-based intermediate phase engineering, this study innovatively introduces N-methyl pyrrolidone (NMP) and isopropylamine hydrochloride (IPACl) in the organic salt solution to synergistically modulate the crystal growth of formamidinium lead triiodide (FAPbI3) perovskite films. The research findings suggest that the coordinative solvent, NMP, can partially dissolve the initially deposited compact PbI2 layer while simultaneously in situ forming the PbI2-NMP adducts, thereby promoting the formation of alpha-phase perovskite. Further integration of IPACl plays a role in the van der Waals interaction, enhancing the oriented growth of alpha-phase perovskite and suppressing the delta-phase formation during thermal annealing. Consequently, highly crystalline FAPbI3 films featuring enlarged grain size, more uniform morphology, smoother surface, and lower defect density are demonstrated, rendering a champion efficiency of 25.50% for two-step processed perovskite solar cells, along with significantly improved stability. Moreover, the doctor-blade coated solar mini-modules demonstrate champion efficiencies of 20.30% and 16.06% with aperture areas of 16 and 100 cm2, respectively.