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

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 Cs0.25FA0.75Sn0.5Pb0.5I3 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, p 0, in the perovskite layer. Upon SnF2 addition, p 0 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 p 0 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.