Metastable Oxygen-Induced Light-Enhanced Doping in Mixed Sn-Pb Halide Perovskites

Mixed Sn-Pb perovskites are promising solar cells materials for single- and multi-junction solar cells thanks to the possibility of tuning the bandgap energy down to 1.2-1.3 eV. However, tin-containing perovskites are adversely affected by multiple factors leading to doping. In this work, we investigated metastable oxygen-induced doping in ASn_xPb_1-xI₃ (where A is methylammonium or a mixture of formamidinium and cesium) by means of microwave conductivity, structural and optical characterization techniques. We observe that longer oxygen exposure times lead to progressively higher dark conductivities ranging from <∼1 to ∼40 S/m, which slowly decay back to their original levels over days when the layers were stored under N₂. Allegedly, oxygen acts as an electron acceptor, leading to tin oxidation from Sn²⁺ to Sn⁴⁺ and creation of free holes which effectively p-dope the perovskite. Additionally, the metastable oxygen-induced doping is enhanced by exposing the perovskite simultaneously to oxygen and light. Presumably, on illumination electrons are excited to the conduction band of perovskite and the reaction with oxygen is substantially favored in comparison to valence band electrons, explaining the enhanced conversion rate by light. Next, we show that doping not only leads to a reduction in the photoconductivity signal but also induces long-term effects even after loss of doping, which is thought to derive from consecutive oxidation reactions leading to the formation of defect states. In addition to that, we revealed that short-term exposure to oxygen immediately impairs the charge carrier dynamics of the perovskite, while structural and optical changes are only noticeable upon long-term exposure to oxygen and light. Indeed, on prolonged exposure the perovskite crystal structure deteriorates due to a build-up of SnO_xspecies and loss of iodide due to the release of I₂ near the surface. We emphasize that, although the exposure to oxygen is relatively short, this is sufficient to cause immediate and enormous changes in the charge carrier dynamics. Basically, we state that the defect density arising from short-term exposure to oxygen immediately impairs the solar cell opto-electronic properties, while perovskite structural and optical properties degradation only emerges upon accumulation of oxidation products. We believe that the exposure to oxygen of mixed Sn-Pb perovskites solar cells during production and operation should be strictly prevented to improve their performance and lifetime. Moreover, understanding the oxygen-induced degradation processes is of fundamental importance to select the best materials, device architectures and encapsulation, and appropriate fabrication conditions.