Metastable\nOxygen-Induced Light-Enhanced Doping in\nMixed Sn–Pb Halide Perovskites

Mixed Sn–Pb\nhalide perovskites are promising absorber materials\nfor solar cells due to the possibility of tuning the bandgap energy\ndown to 1.2–1.3 eV. However, tin-containing perovskites are\nsusceptible to oxidation affecting the optoelectronic properties.\nIn this work, we investigated qualitatively and quantitatively metastable\noxygen-induced doping in isolated ASn<sub><i>x</i></sub>Pb<sub>1–<i>x</i></sub>I<sub>3</sub> (where A is\nmethylammonium or a mixture of formamidinium and cesium) perovskite\nthin films by means of microwave conductivity, structural and optical\ncharacterization techniques. We observe that longer oxygen exposure\ntimes lead to progressively higher dark conductivities, which slowly\ndecay back to their original levels over days. Here oxygen acts as\nan electron acceptor, leading to tin oxidation from Sn<sup>2+</sup> to Sn<sup>4+</sup> and creation of free holes. The metastable oxygen-induced\ndoping is enhanced by exposing the perovskite simultaneously to oxygen\nand light. Next, we show that doping not only leads to the reduction\nin the photoconductivity signal but also induces long-term effects\neven after loss of doping, which is thought to derive from consecutive\noxidation reactions leading to the formation of defect states. On\nprolonged exposure to oxygen and light, optical and structural changes\ncan be observed and related to the formation of SnO<sub><i>x</i></sub> and loss of iodide near the surface. Our work highlights that\neven a short-term exposure to oxygen immediately impairs the charge\ncarrier dynamics of the perovskite, while structural perovskite degradation\nis only noticeable upon long-term exposure and accumulation of oxidation\nproducts. Hence, for efficient solar cells, exposure of mixed Sn–Pb\nperovskites to oxygen during production and operation should be rigorously\nblocked.