Phase transformation barrier modulation of CsPbI₃ films via PbI₃⁻ complex for efficient all-inorganic perovskite photovoltaics

Cesium lead iodide (CsPbI₃) has gained great attention due to its thermal stability and appropriate bandgap (≈1.73 eV) at black (γ) phase potentially suitable for tandem solar cells. However, it is challenging to obtain CsPbI₃ film with desired black phase. Herein, we fabricate kinetically favorable γ-CsPbI₃ thin films by stoichiometry modulation, where in-situ 2D GIWAXS measurement was innovatively performed to illustrate the phase transition process of the precursor films, to aid a full picture study on the entire film evolution process. Conceptually different from introducing other extrinsic species, the cogenetic doping by excessive cesium iodide is found to tailor energy barriers for phase transformations during both the film formation and ageing process simultaneously. During film growth, excessive CsI affects the formation of Pb−I complex in the precursor solution, which facilitates the δ to γ phase transformation. Also, the Cs-rich resultant film could suppress γ to δ phase transformation. The corresponding CsPbI₃ solar cells deliver a PCE of 16.68% without performance loss at continuous maximum power point output (MPP) for ~175 h under continuous illumination in a N₂ glovebox. This work highlights the importance of precursors chemistry and provides guidelines to adjust the phase transformation barrier in CsPbI₃ films without any foreign additives.