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

High Resolution Image Download MS PowerPoint Slide 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 0.25 FA 0.75 Sn 0.5 Pb 0.5 I 3 with varying SnF 2 additions ranging from 0 to 20 mol %. First, optical spectroscopy is used to determine the fraction of Sn 4+ in the spin-coating solution, which varies depending on the purity of the starting SnI 2 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 % SnF 2 is observed. We conclude that, without SnF 2, ∼12% of the Sn 4+ in solution leads to mobile carriers in the form of free holes, p 0, in the perovskite layer. Upon SnF 2 addition, p 0 decreases to <1 × 10 16 cm –3 . We infer that a ∼70 times excess of SnF 2 over the initial concentration of Sn 4+ in solution is required to scavenge the Sn 4+ and obtain layers with reduced doping. Although the reduction of p 0 and defects results in increased carrier lifetimes, higher SnF 2 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 SnF 2 makes them ideal candidates for efficient solar cells; however, SnF 2 also induces compositional heterogeneity and accumulation of SnO x at the surface.