Unraveling Structural Transformations and Degradation Mechanisms of Pb-Sn Mixed Perovskite Thin films

Perovskite solar cells (PSCs) have attracted much attention due to their potential to transform the photovoltaic industry with higher efficiency and lower material usage. However, their intrinsic instability under conditions such as moisture, light, and heat poses a significant obstacle to their growth, making them unsuitable for long-term use under ambient conditions. MAPbI₃ is the most studied compound for PSC applications, however, the presence of Pb causes toxicity. Hybrid MAPb_(1-x)Sn_xI₃ is a potential replacement for this compound to reduce the toxicity concerns. In this study, MAPbI₃ and MAPb_0.8Sn_0.2I₃ perovskites were designed and studied for their stability under ambient conditions with 60% RH (relative humidity). Various characterization techniques, including SEM, XRD, and UV-vis-NIR spectroscopy, were employed to analyze the structural, morphological, and optical properties of the films over 28 days. SEM analysis revealed that both MAPbI₃ and MAPb_0.8Sn_0.2I₃ films underwent significant morphological alterations upon exposure to humid conditions. The initially well-defined grain shapes gradually degraded into porous structures with coarse surfaces over time. All of the samples formed well-defined tetragonal phases with distinct XRD peak intensities on the zeroth day. The XRD results indicated that MAPbI₃ rapidly degraded into MAPbI₃·H₂O within 7 days of exposure. Even after 28 days, it existed as MAPbI₃·H₂O. UV-vis-NIR spectroscopy showed that the incorporation of Sn into the perovskite structure led to a red shift in the absorption edge and a slight reduction in the band gap, from 1.56 eV for MAPbI₃ to 1.38 eV for MAPb_0.8Sn_0.2I₃. The investigation of humidity-induced degradation in MAPbI₃ and MAPb_0.8Sn_0.2I₃ perovskite thin films has yielded valuable insights into their stability and degradation mechanisms at 60% RH.