Toward Uniaxially Textured CsPbIBr2 Perovskite Thin Films with Twin Domains by Potassium Incorporation

With the recent success of solution-processed inorganic halide perovskites in optoelectronic applications, the ability to control crystal grain morphology, size, and orientation is of great interest to further optimize material properties and performance. Here, by tuning the cation composition of CsPbIBr2 precursor solutions via potassium halide, we obtain uniaxially textured, smooth-surface polycrystalline thin films with a grain size of up to eight micrometers and alignment parallel to ⟨101⟩. The potassium-induced crystallization and derivative phases are analyzed. It is postulated that the conversion of nonperovskite phases to perovskite at higher temperatures relates to the observed differences in crystallization behavior in the presence of potassium halide. The lattice contraction and the band-gap enlargement are also found to suggest partial potassium incorporation into the crystal lattice. We also observe the coexistence of two types of twin domains with boundaries parallel to {121} and (010)/(101̅) that are optimally aligned parallel to the ⟨101⟩ transport direction in the potassium-ion-treated CsPbIBr2. Finally, perovskite solar cells with enhanced performance are demonstrated based on the K+-incorporated CsPbIBr2 with a textured morphology. These findings illustrate how potassium ion incorporation can be used as a means to control the crystallinity, microstructure, and hence the device performance of inorganic lead halide perovskites CsPbIBr2.