Mixed Sn-Pb perovskites are promising solar cell materials for single- and multi-junction devices 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 how doping in Cs_0.25FA_0.75Sn_0.5Pb_0.5I₃ is induced by the presence of Sn⁴⁺ in the spin-coating solvent and secondly by exposure of these layers to oxygen. For these measurements we used time-resolved and steady-state microwave conductivity techniques (TRMC and SSMC), structural and optical characterization.

First, we performed a quantitative analysis how the back ground doping and defect density in these spin-coated layers is affected by addition of SnF₂ ranging from 0 to 20 mol%. Optical spectroscopy is used to determine the fraction of Sn⁴⁺ to Sn²⁺ in the spin-coating solution, which varies from 0.012% to 0.032%. By applying SSMC, we observe a large decrease in dark conductivity from ~ 100 to < ~ 1 S m^-1 in the spin-coated layers on going from 0 to 2 mol% SnF2, with no further measurable reduction for higher SnF₂ concentrations. We demonstrate that the minimum SnF₂ addition required to achieve this reduction in dark conductivity is not absolute, but highly dependent on the extent of oxidation of the SnI2 precursor. The dynamics of laser-induced excess carriers show progressively longer carrier lifetimes with higher SnF2 concentrations. By fitting intensity-dependent photoconductivity signals, we find that upon SnF₂ addition the concentrations of doping and defects concomitantly decrease by an order of magnitude. It is inferred that in the spin-coating solution a ~ 100 times excess of SnF₂ is required to scavenge all Sn4+ (SnI4) and obtain nearly intrinsic lead tin perovskites.

Then we observed that exposure of these layers to oxygen leads to progressively higher dark conductivities, 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 the 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. We emphasize that, although exposure to oxygen is relatively short, this is sufficient to cause immediate and permanent changes in the charge carrier dynamics measured by TRMC. Basically, the defect density arising from short-term exposure to oxygen immediately impairs the solar cell properties, while changes in the structural and optical properties only emerge upon prolonged exposure leading to accumulation of oxidation products.