Achieving γ‐Phase CsPbI₃ by Reducing Underlayer Surface Roughness

Cesium lead iodide (CsPbI 3 ) 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 CsPbI 3 at room temperature. Here, it discovers that γ‐phase CsPbI 3 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 CsPbI 3 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 PbI 6 octahedra and lattice distortion. This study offers a universal approach to obtain γ‐phase CsPbI 3 for the development of high‐performance all‐inorganic perovskite solar cells and other optoelectronic devices.