Researchers have developed thermodynamically stable β-CsPbI₃-based perovskite solar cells with efficiencies exceeding 18%. The study addresses the challenge of stabilizing β-CsPbI₃, which has a favorable band gap for tandem solar cells. By using choline iodide (CHI) surface treatment, the team improved the charge carrier lifetime and energy level alignment between the β-CsPbI₃ absorber layer and carrier selective contacts. This treatment filled cracks and pinholes in the perovskite layer, enhancing the structural and optical properties of the material. The resulting solar cells showed highly reproducible and stable efficiencies of 18.4% under ambient conditions. The β-CsPbI₃ films were found to have a band gap of 1.68 eV, which is redshifted compared to previously reported values. The study also confirmed the all-inorganic nature of the perovskite films through various analytical techniques. The CHI treatment improved the energy level alignment and reduced nonradiative recombination, leading to enhanced performance. The solar cells demonstrated excellent thermal and humidity stability, with a certified PCE of 18.3%. The research highlights the potential of β-CsPbI₃-based perovskite solar cells for high-efficiency and stable photovoltaic applications.Researchers have developed thermodynamically stable β-CsPbI₃-based perovskite solar cells with efficiencies exceeding 18%. The study addresses the challenge of stabilizing β-CsPbI₃, which has a favorable band gap for tandem solar cells. By using choline iodide (CHI) surface treatment, the team improved the charge carrier lifetime and energy level alignment between the β-CsPbI₃ absorber layer and carrier selective contacts. This treatment filled cracks and pinholes in the perovskite layer, enhancing the structural and optical properties of the material. The resulting solar cells showed highly reproducible and stable efficiencies of 18.4% under ambient conditions. The β-CsPbI₃ films were found to have a band gap of 1.68 eV, which is redshifted compared to previously reported values. The study also confirmed the all-inorganic nature of the perovskite films through various analytical techniques. The CHI treatment improved the energy level alignment and reduced nonradiative recombination, leading to enhanced performance. The solar cells demonstrated excellent thermal and humidity stability, with a certified PCE of 18.3%. The research highlights the potential of β-CsPbI₃-based perovskite solar cells for high-efficiency and stable photovoltaic applications.