Achieving γ-Phase CsPbI₃ by Reducing Underlayer Surface Roughness

Cesium lead iodide (CsPbI₃) exhibits great potential in developing photovoltaic cells due to suitable optical bandgap and thermal stability. However, the photoactive γ-phase normally exists at high-temperatures ≈180 °C, and it is challenging to obtain γ-phase CsPbI₃ at room temperature. Here, it discovers that γ-phase CsPbI₃ is achievable by reducing the underlayer surface roughness to a certain level. This method is universal as demonstrated on the surface of poly(3,4-ethylenedioxythiophene) poly(styrene sulfonate) (PEDOT:PSS), poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA), polystyrene (PS), and silicon substrates. Moreover, it is found that lower surface roughness resulted in smaller crystallite size in the CsPbI₃ film, which is an important reason for achieving γ-phase because the decrease in crystallite size will increase grain surface energy to suppress tilting of PbI₆ octahedra and lattice distortion. This study offers a universal approach to obtain γ-phase CsPbI₃ for the development of high-performance all-inorganic perovskite solar cells and other optoelectronic devices.