From unstable CsSnI₃ to air-stable Cs₂SnI₆: A lead-free perovskite solar cell light absorber with bandgap of 1.48 eV and high absorption coefficient

All-inorganic and lead-free cesium tin halides (CsSnX₃, X=Cl, Br, I) are highly desirable for substituting the organolead halide perovskite solar cells. However, the poor stability of CsSnX₃ perovskites has so far prevented the fabrication of devices that can withstand sustained operation under normal conditions. In this paper, a two-step sequential deposition method is developed to grow high-quality B-γ-CsSnI₃ thin films and their unique phase change in atmosphere is explored in detail. We find the spontaneous oxidative conversion from unstable B-γ-CsSnI₃ to air-stable Cs₂SnI₆ in air. Allowing the phase conversion of the CsSnI₃ film to evolve in ambient air it gives the semiconducting perovskite Cs₂SnI₆ with a bandgap of 1.48 eV and high absorption coefficient (over 10⁵ cm⁻¹ from 1.7 eV). More importantly, the Cs₂SnI₆ film, for the first time, is adopted as a light absorber layer for a lead-free perovskite solar cell and a preliminary estimate of the power conversion efficiency (PCE) about 1% with open-circuit voltage of 0.51 V and short-circuit current of 5.41 mA/cm² is realized by optimizing the perovskite absorber thickness. According to the bandgap and the Shockley-Queisser limit, such inorganic perovskite solar cell with higher efficiency and pronounced stability can be expected by material quality improvement and device engineering.