Conventional Solvent Oxidizes Sn(II) in Perovskite Inks

By M. Saidaminov, I. Spanopoulos, Jehad Abed, Weijun Ke, Joshua Wicks, M. Kanatzidis, Edward H. Sargent. 2020

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The most efficient perovskite solar cell devices use Pb(II); however, the toxicity of lead and its decomposition products represent possible obstacles to their widespread commercial and carbon impact. Pb from hybrid perovskites may enter plantsand consequently the food cycle10× more efficiently than do other Pb sources. Although consumer solar modules are encapsulated, there remain concerns about their distribution, maintenance, and recycling. Replacing lead with more environmentally benign metals, without compromising device efficiency, is of utmost importance. Recent high-throughput computational studies evaluating the electronic properties of thousands of ABX3 perovskites revealed that only lead and tin perovskites can deliver solar cell relevant bandgaps and absorption coefficients. Replacing Pb with Sn is a promising path; however, Sn(II) undergoes undesired oxidation forming Sn(IV) and charge carrier recombination centers, leading to a significant loss of photovoltage in solar cells. Innovative approaches, including dopant additives, a reducing atmosphere, and comproportionation, have increased the record power conversion efficiency for Sn-based devices, approaching currently only half of that for Pb-based solar cells. In this Viewpoint, we report that the solvent conventionally used in the fabrication of perovskite films, dimethyl sulfoxide (DMSO), oxidizes Sn(II) to Sn(IV). Using the combination of nuclear magnetic resonance (H NMR) and X-ray absorption near-edge spectroscopy (XANES), we find that the DMSO/ Sn(II) pair undergoes an irreversible redox reaction forming dimethylsulfide (DMS) and Sn(IV) in solution. Recently, Hamill et al. showed that DMSO induces the conversion of methylammonium into dimethylammonium cation in CH3NH3I + PbI2 solutions at 150 °C. 12 DMSO is also known as an oxidation agent for Sn(II) in acidic medium; given that perovskite organic cations are Lewis acids, we hypothesized that DMSO can oxidize Sn(II) in perovskite solutions. To check this hypothesis and provide insight into reaction/ decomposition pathways in Sn(II)-containing perovskite solutions, we performed a series of experiments. We prepared a perovskite solution in DMSO by dissolving FAI (where FA is formamidinium cation) and SnI2, sealed the vial, and heated it at 120 °C for 5 h, all in an inert atmosphere. We noticed a significant change in color (Figure 1); the solution became much darker, indicating the oxidation of Sn(II). On the other hand, the dimethylformamide (DMF)based solution showed an only negligible change in color in the analogous experiment. Though perovskite solutions are generally not heated for such a long time, this accelerated test can provide insights about the processes that occur in solution when heating the solution to dissolve the precursors or annealing spin-coated film with the leftover DMSO. It does not take much Sn doping (<0.1%) in ASnI3 perovskites to turn the material’s p-type conductors and render them inactive as solar photovoltaic absorbers. H NMR measurements of the corresponding solutions revealed the presence of DMS (at 2.07 ppm) even after 10 min of heat treatment at 120 °C (Figure 2a,b). Longer heat treatment led to more DMS formation (Figure S1). A blank sample without FAI and SnI2 was treated in the same way, and it showed no DMSO decomposition (Figure S2). Notably, FAI or PbI2 DMSO-based heat-treated solutions under the same conditions exhibited trace amounts of DMS (Figures S3−S5). Variable-temperature H NMR measurements showed that DMS formation commences at 100 °C in the presence of Sn ions (Figures S6 and S7). We then conducted X-ray absorption near-edge spectroscopy (XANES) on the solutions before and after heat treatment. The XANES spectra of S K-edge confirmed that DMSO was

Subfields: Electrical and Electronic Engineering, Materials Chemistry

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