Stabilization of Perovskite Lattice and Suppression of Sn²⁺/Sn⁴⁺ Oxidation via Formamidine Acetate for High Efficiency Tin Perovskite Solar Cells

Abstract

Tin halide lead‐free perovskite solar cells (TPSCs) have received tremendous research interest recently due to their nearly ideal bandgap, broad light absorption, non‐toxicity, and environmental friendliness. However, the uncontrollable crystallization process and the facile oxidation of Sn2+ limit the further increase of power conversion efficiency (PCE). To solve these problems, a series of acetates are introduced into the perovskite precursor solution to regulate the crystallization process. It is revealed that formamidine acetate (FAAc) has strong COSn coordination with Sn2+ compared with acetic acid (HAc) and methylammonium acetate (MAAc), which can stabilize the lattice structure, minimize defect states and suppress the oxidation of Sn2+. Meanwhile, benefiting from this coordination ability, it not only leads to large‐size colloidal clusters in precursor but also slows down the crystallization process and improves the crystallinity of tin halide perovskite films. The device with FAAc achieved an increased PCE from initially 9.84% to 12.43%, and it could maintain 94% of its initial value for 2000 h in N2 atmosphere. This work provides a feasible strategy for depositing high‐quality tin perovskite films with low defect density and lattice distortion, which will be crucial for related photovoltaics and other optoelectronic devices.

Origins and Suppression of Sn(II)/Sn(IV) Oxidation in Tin Halide Perovskite Solar Cells

“…[ 68,69 ] More importantly, the DOS of the TPSCs treated with the antioxidant strategy was degraded, which meaning the defect states in the tin perovskite crystals gradually increase as the oxidation occurs, leading to the creation of various degrees of nonradiative recombination centers between energy levels. [ 69,70 ] As shown in Figure 4e, according to Equation (17) [ 68,71 ] $$$$\begin{eqnarray} \def\eqcellsep{&}\begin{array}{*{20}{c}} {\Delta {V}_{{\rm{OC}}} = \ \frac{{L\ {q}^2}}{{{\rm{DOS}}*\Delta Q}}\ } \end{array} \end{eqnarray}$$$$where L is the thickness of perovskite film, Δ Q is the short‐circuit photogenerated charge, ∆ V oc is the V oc rise generated under gradient intensity of transient light irradiation. [ 71,72 ] With the density of states gradually increases, ∆ V oc will reduce, implying that more photogenerated carriers will undergo nonradiative recombination between the valence and the conduction bands, which indirectly explains the V loss in TPSCs.…”

“…It was demonstrated that the CO bond in FAAc not only stabilized the lattice more effectively, but also had the effect of inhibiting Sn(II) oxidation. [ 70 ] Kim et al. reported that formamidinium thiocyanate (FASCN) had a strong interaction with Sn 2+ , which greatly prevents the oxidation of quasi‐2D tin perovskite during film formation.…”

“…It was demonstrated that the C=O bond in FAAc not only stabilized the lattice more effectively, but also had the effect of inhibiting Sn(II) oxidation. [70] Kim et al reported that formamidinium thiocyanate (FASCN) had a strong interaction with Sn 2+ , which greatly prevents the oxidation of quasi-2D tin perovskite during film formation. [103] Liu and co-workers showed that guanidine thiocyanate (GASCN) additives can form Lewis base-acid adducts with uncoordinated Sn(II) atoms, inhibiting the oxidation of Sn(II)/Sn(IV) and charge recombination.…”

Pseudohalide-Modulated Crystallization for Efficient Quasi-2D Tin Perovskite Solar Cells with Minimized Voltage Deficit

“…8−10 Furthermore, the rapid crystallization process of TPSCs is identified as a key factor affecting crystallinity and orientation, structural and phase distribution, film morphology, and Sn-related defects in tin perovskite films. 11−14 Many strategies have been implemented to address these issues for TPSCs, which include the attempt of reducing additives (e.g., metallic Sn powder, 15 hydrazine vapor, 16 pyrazine, 17 and SnF 2 12 ), dimensional regulation with more stable 2D phase, 11,18,19 and solvent engineering. 20 Therein, dimensional-mixed tin halide perovskites show an alleviative ptype self-doping effect owing to the quantum confinement effect.…”

“…Many strategies have been implemented to address these issues for TPSCs, which include the attempt of reducing additives (e.g., metallic Sn powder, hydrazine vapor, pyrazine, and SnF 2 ), dimensional regulation with more stable 2D phase, ,, and solvent engineering . Therein, dimensional-mixed tin halide perovskites show an alleviative p-type self-doping effect owing to the quantum confinement effect. , Introducing bulky organic spacers such as phenylethylammonium (PEA + ), butylammonium (BA + ), ethylammonium (EA + ), and 5-ammoniumvaleric acid (5-AVA + ) in tin perovskites gives a reduced dimensionality with a slower crystallization and retarded crystal growth rate. − The quasi-2D tin perovskites usually have a range of phases with different layer numbers ( n ) from 1 to ∞. , These perovskites could enhance the water and oxygen resistance, thus leading to robust device stability.…”