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Black-phase formamidinium lead iodide perovskite (FAPbI3), whilst the most promising species for efficient perovskite photovoltaics, is energetically unfavored at room temperature, and is thus always accompanied by undesirable yellow phases during crystallization 1,2,3,4 . The challenge to formulate the fast crystallization process of perovskite has limited the community in deriving unified guidelines for governing the formation of black-phase FAPbI3 5,6 . Here, through in-situ monitoring of the perovskite crystallization process, we report an oriented nucleation mechanism that acted as the key to avoid undesirable phases. This concept was applicable to improving the photovoltaic device performance under different film-processing scenarios. The small-area device demonstrated a power conversion efficiency of 25.4% (certified 25.0%), and the module (27.83 cm 2 ) achieved a champion aperture efficiency of 21.4% (certified). MainFormamidinium lead iodide perovskite (FAPbI3) features desirable bandgap and thermal resistance, and has thus emerged as the most promising candidate among the perovskite family for highly efficient photovoltaic devices 1,2,3,7,8 . However, the photoactive black-phase FAPbI3 is not energetically favorable at room temperature 4,9,10 .Polytype formation and other intermediate non-photoactive phases can readily occur, which undermines the photovoltaic performance. A few approaches have been developed to promote the formation of black-phase FAPbI3 at room temperature, such as adduct formation with PbI2 and solvent engineering using ionic liquids 11,12 .

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