Impact of non-stoichiometric ratios of the KI on the structure stability and optical properties of α-CsPbI3 perovskite thin films

Abstract

Inorganic CsPbI₃ perovskite has emerged as a promising light-absorbing material for photovoltaic applications, offering a suitable band gap for solar energy conversion and greater stability in ambient conditions compared to organic–inorganic halide perovskites. Nevertheless, the photoactive α-phase of CsPbI₃ remains stable only at higher temperatures, with a rapid phase transition to the inactive δ-phase occurring under room temperature conditions. Herein, we introduce a non-stoichiometric ratio of KI into CsPbI₃ film to stabilize the α-phase at room temperature. (XRD) analysis shows that the prepared CsPbI₃:x%KI, x = 0.0, 0.3, 0.5, 0.7, 1.0%, films exhibit the α-phase, notably, with 0.7% KI enhancing stability under ambient conditions. Also, scanning electron microscopy (SEM) showed that the optimal morphology was achieved with 0.7% KI. Grain size decreased with 0.7%KI then slightly increased with the addition of 1%, suggesting a suitable ratio for photovoltaic applications. Additionally, UV–Vis spectroscopy was employed to analyze the optical properties, demonstrating enhanced light absorption in the doped films. In the visible region, the absorption reduced sharply to its lowest value for the CsPbI₃:0.3%KI film and then increased as the KI ratio continued to rise. The optical energy gap (E_g) value of pristine CsPbI₃ film is 1.692 eV, which is reduced slightly and irregularly as the amount of KI doping increased, with the 0.5% KI-doped film attaining the lowest E_g value of 1.681 eV. Additionally, the photoluminescence (PL) spectroscopy showed higher intensity with 0.7%KI film, indicating fewer traps /defects and delayed recombination, improving the cell performance. These results suggest that KI doping significantly enhances the stability and optical properties of CsPbI₃, making it promising for perovskite solar cells.