Last year’s mixed Sn–Pb perovskites have been applied as low-bandgap absorbers in efficient solar cells. However, the performance is still limited by tin oxidation, resulting in doping and defects. Here we perform a quantitative analysis on how tin oxidation affects the optoelectronic properties of spin-coated Cs_0.25FA_0.75Sn_0.5Pb_0.5I₃with varying SnF₂additions ranging from 0 to 20 mol %. First, optical spectroscopy is used to determine the fraction of Sn⁴⁺in the spin-coating solution, which varies depending on the purity of the starting SnI₂precursor. By applying steady-state microwave conductance, a large decrease in the dark conductivity from ∼100 to <∼1 S m^–1in the spin-coated films on going from 0 to 2 mol % SnF₂is observed. We conclude that, without SnF₂, ∼12% of the Sn⁴⁺in solution leads to mobile carriers in the form of free holes, p₀, in the perovskite layer. Upon SnF₂addition, p₀decreases to <1 × 10¹⁶cm^–3. We infer that a ∼70 times excess of SnF₂over the initial concentration of Sn⁴⁺in solution is required to scavenge the Sn⁴⁺and obtain layers with reduced doping. Although the reduction of p₀and defects results in increased carrier lifetimes, higher SnF₂additions are also required to decrease the surface defects, leading to even longer lifetimes close to 200 ns. The reduced doping of these perovskite films with SnF₂makes them ideal candidates for efficient solar cells; however, SnF₂also induces compositional heterogeneity and accumulation of SnO_xat the surface.

Metal-halide perovskites deposited by wet-chemical deposition have demonstrated great potential for various electronic applications, including solar cells. A remaining question is how light-induced excess charges become distributed over such polycrystalline material. Here, we examine the local conductive properties of MAPbI₃ and CsFAPbI₃ by using scanning microwave microscopy (sMIM) in the dark and light. sMIM is an atomic force microscopy (AFM)-based technique measuring variations of the in-phase and out-of-phase signals due to changes in the tip-sample interaction, yielding MIM-Re and MIM-Im images, respectively. Combining this information leads to a picture for CsFAPbI₃ in which excess charges are distributed evenly over the grains, but due to local defect-rich areas, possibly related to different crystal facets, local perturbations in carrier concentration exist. For solar cells, this distribution in carrier concentration under illumination leads to variation in the local Fermi level splitting, which should be suppressed to reduce the voltage deficit.