Tuning optoelectronic properties of CsPbI2Br perovskite films via octadecyl ammonium iodide substitution

Abstract Metal halide perovskites such as CsPbI 2 Br are promising candidates for next-generation optoelectronic devices due to their tuneable bandgaps, high light absorption, and solution processibility. However, their practical application is hindered by structural purity, high defect densities, and non-radiative carrier losses, all of which limit efficiency and long-term stability. Recent strategies to mitigate these challenges involve compositional engineering with bulky organic cations and careful control of post-deposition annealing, but the combined influence of thermal processing and partial organic substitution on CsPbI 2 Br remains insufficiently understood. In this work, we report the synergistic impact of thermal annealing and partial organic cation substitution on the structural, morphological, and optoelectrical properties of CsPbI 2 Br perovskite thin films. Varying the annealing temperature and the substitution of Cs + with octadecyl ammonium iodide (ODAI) can tune the crystallinity, defect density, and carrier recombination properties. X-ray diffraction analysis revealed a tetragonal crystal structure with optimal crystallinity at 240 ℃ and signs of phase transition at 300 ℃. Field emission scanning electron microscopy images confirm enhanced grain size and uniformity upon substitution with 15 mol% ODAI. The films with low ODAI concentration (ODA 0.15 ) exhibit reduced micro strain and dislocation density, maintaining structural integrity while passivating defects. Ultraviolet–visible absorption spectra show bandgap narrowing with moderate substitution, while photoluminescence measurements demonstrate significantly enhanced emission intensity in ODA 0.15 film, indicating suppressed non-radiative recombination. Excessive ODAI (ODA 0.25 or higher) leads to lattice distortion, phase inhomogeneity, and reduced optical performance. These results highlight the importance of precise ODAI control and annealing conditions for optimizing CsPbI 2 Br perovskites, providing a viable route toward efficient and stable optoelectronic devices.