Here, we investigated the separate and combined effects of guanidinium (Gua⁺) and/or thiocyanate (SCN⁻) ions on the opto-electronic properties of mixed Sn–Pb perovskites, which are used as absorber layers in photovoltaics. Therefore, we spin-coated Cs_0.25FA_0.75Sn_0.5Pb_0.5I₃ thin films with GuaI, Pb(SCN)₂ or GuaSCN, in the absence or presence of SnF₂. By comparing the (micro)structural and opto-electronic properties of the perovskite films, we elucidated the functions of both ions. We found that SCN⁻ suppresses tin oxidation and doping, reduces crystal defects and improves the carrier transport properties, regardless of SnF₂ addition. We demonstrate that this is due to coordination with Sn²⁺ and scavenging of Sn⁴⁺ in the spin-coating solution, resulting in a pile-up of SnO_x at the film surface. Gua⁺ is incorporated to a limited extent into the 3D cubic perovskite structure. Gua⁺ cannot suppress tin oxidation and doping, making this additive useless without SnF₂. Conversely, when combined with SnF₂, Gua⁺ enhances the carrier transport properties. Combining Gua⁺ and SCN⁻ until a maximum addition of 4 mol% and SnF₂ results in large grains and pinhole-free films with superior charge carrier transport properties, leading to a substantial increase in the pseudo-open circuit voltage of 50 mV. Addition of >4 mol% GuaSCN leads to the formation of Gua-based 2D perovskites, including Gua_xFA_2−xSn_yPb_1−yI₄ and Gua_xFA_3−xSn_yPb_2−yI₇, which do not improve the carrier dynamics. In short, we observe a synergistic effect on addition of Gua⁺ and SCN⁻ ions, which leads to improved structural and opto-electronic properties, which is promising for implementation in solar cells.

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.