Continuous MASnI₃ Perovskite Thin Films Deposited by Spin Coating without Antisolvent Treatment

Organic-inorganic metal halide perovskite materials have emerging photovoltaic properties, thus high potential for applications in solar cells. Also, they are good competitors to the current market-dominant silicon solar cell as they are cheaper to manufacture. The most efficient perovskite solar cells show a power conversion efficiency of more than 24%. However, they contain lead (Pb), which is harmful to the human body as well as the environment. The most suitable substitute for the lead in the perovskite is tin (Sn) which is less toxic. However, most tin-based perovskite materials require an anti-solvent treatment to start the crystallization for getting a fully covered perovskite thin film. These anti-solvents steps add complexity to the deposition process and are anti-solvents are also often toxic. So, it is also necessary to eliminate the anti-solvent step from the solar cell fabrication process to make it more ecofriendly. In this work, we deposited the methylammonium tin iodide (MASnI₃) without any antisolvent and obtained fully covered perovskite thin films. The crystallization and coverage are highly dependent on the spin speed, spin time and concentration of the perovskite ink. The ink was prepared by dissolving 1 mole of methylammonium iodide (MAI), 1 mole of tin iodide (SnI₂), and 15 mol% of tin fluoride (SnF₂) into a mixed solvent. The mixed solvent contains N, N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) of a ratio of 4:1. The perovskite ink was spin-casted at 1000 rpm for 10 s and 6000 rpm for 110s on glass and glass/ITO substrates. The deposited films were cured at 70° C for 10 min on a hot plate inside the glovebox in a nitrogen (N₂) atmosphere. The optical properties of the thin films were determined from UV-Vis, Photoluminescence (PL), ellipsometry and the physical properties from scanning electron microscopy (SEM), atomic force microscope (AFM), and X-ray diffraction (XRD). XRD q-2q scans show that the deposited films have an a-phase MASnI₃ crystal structure with a predominant (001) texture. SEM images show continuous films with large crystallites up to 40 um. The bandgap determined from UV-Vis measurements is approximately 1.25 eV which matches up with the bandgap determined from the PL measurements (1.26 eV) and is similar to what others have found for MASnI₃. The effect of 0, 25 mol%, and 100 mol% MAAc additive in the ink on the properties of the spin cast films was investigated. The MAAc additive increased the PL signal but did not affect the film’s morphology observed by SEM images.<br/><br/><br/>This work was in part funded by the Department of Navy’s HBCU/MI Program through ONR grant number N00014-19-1-2576 and in part by NSF through STTR Phase II grant 1927020. The authors would like to thank Dr. Holtz of Texas State for training access and help with the PL measurements.