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 (CsSnX 3 , X=Cl, Br, I) are highly desirable for substituting the organolead halide perovskite solar cells. However, the poor stability of CsSnX 3 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-CsSnI 3 thin films and their unique phase change in atmosphere is explored in detail. We find the spontaneous oxidative conversion from unstable B-γ-CsSnI 3 to air-stable Cs 2 SnI 6 in air. Allowing the phase conversion of the CsSnI 3 film to evolve in ambient air it gives the semiconducting perovskite Cs 2 SnI 6 with a bandgap of 1.48   eV and high absorption coefficient (over 10 5   cm −1 from 1.7   eV). More importantly, the Cs 2 SnI 6 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/cm 2 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.