Hybrid lead halide perovskites have shown remarkable progress in next-generation solar cells, with power conversion efficiencies (PCEs) increasing from 3.8% to 25.5% in a decade. However, their lead content raises environmental and health concerns, driving the search for lead-free alternatives. Tin halide perovskites emerge as the most promising lead-free option due to their lower toxicity and high PCEs (exceeding 13%). Despite their superior semiconductor properties (broader absorption range, near-ideal bandgaps, higher carrier mobilities), their poor stability under ambient conditions, mainly due to the facile oxidation of Sn²⁺ to Sn⁴⁺, remains a significant challenge. This oxidation introduces p-type self-doping, leading to high rates of recombination and poor solar cell performance. While strategies like SnX₂ additives and low-dimensional phases have been explored, a complete understanding of the decomposition pathways is crucial for effective stability improvement. Existing reports on tin perovskite degradation mechanisms are limited compared to lead-based analogues, making a detailed investigation of the degradation pathways, particularly the role of SnI₄, necessary.

Studies have shown that ASnX₃ perovskites can decompose in air to form A₂SnX₆ and SnO₂, or SnI₄, SnO₂, and FAI/PEA in the presence of oxygen. However, these studies lack a detailed understanding of the degradation mechanism and the role of intermediate products like SnI₄, which is highly reactive with water and oxygen. This paper aims to fill this gap by providing a comprehensive investigation into the degradation mechanism of tin perovskites.

The researchers used a combination of diffraction (XRD), spectroscopy (UV-Vis, PL, NMR, XPS), and ab initio simulation techniques (DFT) to investigate the degradation mechanism of ASnI₃ perovskites (20% PEA and 80% FA). They varied the SnI₄ content in the perovskite films by controlling the SnI₄ concentration in the precursor solution and through precursor purification. The impact of SnI₄ on optoelectronic properties, device performance, and stability was evaluated through UV-Vis absorption, time-resolved photoluminescence (TRPL), current density-voltage (J-V) curves, and absorbance decay measurements. The role of SnI₄ in degradation was studied by analyzing the reactions with moisture and oxygen, employing ¹H-NMR and UV-Vis spectroscopy to identify intermediate products (HI and I₂). Density Functional Theory (DFT) calculations were used to simulate the adsorption of O₂ and I₂ on the FASnI₃ perovskite surface to gain insights into the reaction mechanisms. The influence of hole transport layers (HTLs) – NiO, CuSCN, and PEDOT:PSS – on perovskite stability was examined using UV-Vis spectroscopy and steady-state PL. Optical degradation tests under dry/moist air/nitrogen flow were conducted to further support the proposed degradation mechanism. Device fabrication involved a typical inverted architecture (ITO/PEDOT:PSS/(PEA)_0.2(FA)_0.8SnI₃/PC₆₀BM/BCP/Ag).

The study identified SnI₄ as a major contributor to the degradation of tin iodide perovskites. SnI₄, a direct product of perovskite decomposition in air, evolves into iodine (I₂) through a two-step process: hydrolysis with H₂O to form HI, followed by oxidation of HI by O₂. Iodine, a highly aggressive species, further oxidizes the perovskite to produce more SnI₄, establishing a cyclic degradation mechanism. Increased SnI₄ content in the perovskite films led to poorer optoelectronic properties (reduced PL lifetime, increased bandgap), decreased device performance (lower PCE, V_oc, and J_sc), and faster degradation rates. DFT calculations supported the experimental findings, showing favorable energies for the reactions involving oxygen, water, and iodine. The study demonstrated that the stability of (PEA)_0.2(FA)_0.8SnI₃ films strongly depends on the HTL substrate. Improved hole withdrawal by HTLs (PEDOT:PSS and CuSCN showing better results than NiO) chemically reduces the perovskite, mitigating its sensitivity to oxidizing species and thus improving stability. Experiments under controlled humidity and oxygen levels confirmed the crucial roles of both oxygen and moisture in the degradation process.

The findings address the research question by detailing the cyclic degradation mechanism of tin perovskites, explaining their instability under ambient conditions. The identification of SnI₄ and I₂ as key players in this mechanism provides crucial insights into the degradation process. The study's significance lies in highlighting the importance of controlling SnI₄ impurities and employing efficient hole-accepting HTLs to enhance the stability of tin perovskite solar cells. These findings contribute to a deeper understanding of the stability limitations of lead-free perovskites, paving the way for the design and optimization of more stable and efficient devices.

This study provides a detailed understanding of the degradation mechanisms in tin perovskites, highlighting the crucial roles of SnI₄ and I₂. The findings emphasize the importance of precursor purification to reduce SnI₄ impurities and the use of efficient hole transport layers to enhance stability. Future research should focus on developing strategies to further suppress the formation of I₂ and explore new HTLs with even greater hole-extraction capabilities for enhanced stability and performance of tin-based perovskite solar cells.

While the study provides a detailed degradation mechanism, the influence of other factors, such as HTM/perovskite interface passivation, chemical reactivity between HTM and perovskite, and substrate-dependent point defect formation, on stability wasn't fully explored. Further investigation into these factors is needed for a comprehensive understanding of perovskite stability. The DFT calculations utilized a simplified model (FASnI3 instead of the stoichiometric PEAFASnI used experimentally) for some reaction energy calculations, which may introduce some uncertainty in the precise energetic values.