Quantitative Analysis of the Doping and Defect Density in Mixed Sn–Pb Perovskites Mediated by SnF2

Abstract

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_0.5I₃ with varying SnF₂ additions 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 SnI₂ precursor. By applying steady-state microwave conductance, a large decrease in the dark conductivity from ∼100 to <∼1 S m^–1 in the spin-coated films on going from 0 to 2 mol % SnF₂ is 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 SnF₂ addition, p₀ decreases to <1 × 10¹⁶ cm^–3. We infer that a ∼70 times excess of SnF₂ over the initial concentration of Sn⁴⁺ in solution is required to scavenge the Sn⁴⁺ and obtain layers with reduced doping. Although the reduction of p₀ and defects results in increased carrier lifetimes, higher SnF₂ additions 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 SnF₂ makes them ideal candidates for efficient solar cells; however, SnF₂ also induces compositional heterogeneity and accumulation of SnO_x at the surface.