Light-induced degradation of methylammonium tin iodide absorber layers

Light-induced degradation of tin-based organic inorganic halide perovskites (Sn-HOIP) absorbers is a major barrier for their deployment in photovoltaic applications. Sn-HOIP are believed to intrinsically degrade due to the tendency of tin to change the oxidation state from (2+) to (4+). So far, most studies have been performed on absorbers that were synthesized with solvent-based techniques. The solvents themselves have been associated with Sn-HOIP degradation. Here, we show that in solvent-free coevaporated methylammonium tin iodide (\MASI) films exposed to white light, no Sn(4+) could be detected, even after almost 100\,h of exposure. To understand the degradation mechanism, the chemical composition at the surface of \MASI was measured by X-ray photoelectron spectroscopy after different illumination intervals. The measurements showed that \MASI decomposed into tin iodide (SnI$_2$) and a minimal amount of metallic tin. The SnI$_2$ phase at the surface increases as a function of light exposure. Despite the strong degradation, light-induced decomposition was not accompanied by the formation of Sn(4+). In addition, the stability of SnI$_2$ under illumination was studied and compared to that of PbI$_2$. Here, SnI$_2$ did not show any degradation, contrarily to PbI$_2$ which degraded into metallic lead. Our results show that the tendency of tin to be in multiple oxidation states is not triggered by light. Instead, the critical point is the choice of an organic component (methylammonium) that leaves the perovskite crystal during illumination. These results show that it is essential to retain the organic component in the perovskite lattice, either by including additives or by replacing the organic component.