Mixed Sn-Pb halide perovskites are promising absorber materials for solar cells due to the possibility of tuning the bandgap energy down to 1.2-1.3 eV. However, tin-containing perovskites are susceptible to oxidation affecting the optoelectronic properties. In this work, we investigated qualitatively and quantitatively metastable oxygen-induced doping in isolated ASn_xPb_1-xI₃ (where A is methylammonium or a mixture of formamidinium and cesium) perovskite thin films by means of microwave conductivity, structural and optical characterization techniques. We observe that longer oxygen exposure times lead to progressively higher dark conductivities, which slowly decay back to their original levels over days. Here oxygen acts as an electron acceptor, leading to tin oxidation from Sn²⁺ to Sn⁴⁺ and creation of free holes. The metastable oxygen-induced doping is enhanced by exposing the perovskite simultaneously to oxygen and light. Next, we show that doping not only leads to the 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. On prolonged exposure to oxygen and light, optical and structural changes can be observed and related to the formation of SnO_x and loss of iodide near the surface. Our work highlights that even a short-term exposure to oxygen immediately impairs the charge carrier dynamics of the perovskite, while structural perovskite degradation is only noticeable upon long-term exposure and accumulation of oxidation products. Hence, for efficient solar cells, exposure of mixed Sn-Pb perovskites to oxygen during production and operation should be rigorously blocked.

Many colloidal quantum dot (QD)-based devices involve charging of the QD, either via intentional electronic doping or via electrical charge injection or photoexcitation. Previous research has shown that this charging can give rise to undesirable electrochemical surface reactions, leading to the formation of localized in-gap states. However, little is known about the factors that influence the stability of charged QDs against surface oxidation or reduction. Here, we use density functional theory to investigate the effect of various ligands and solvents on the reduction of surface Cd in negatively charged CdSe QDs. We find that X-type ligands can lead to significant shifts in the energy of the band edges but that the in-gap state related to reduced surface Cd is shifted in the same direction. As a result, shifting the band edges to higher energies does not necessarily lead to less stable electron charging. However, subtle changes in the local electrostatic environment lead to a clear correlation between the position of the in-gap state in the bandgap and the energy gained upon surface reduction. Binding ligands directly to the Cd sites most prone to reduction was found to greatly enhance the stability of the electron charged QDs. We find that ligands bind much more weakly after reduction of the Cd site, leading to a loss in binding energy that makes charge localization no longer energetically favorable. Lastly, we show that increasing the polarity of the solvent also increases the stability of QDs charged with electrons. These results highlight the complexity of surface reduction reactions in QDs and provide valuable strategies for improving the stability of charged QDs.