The effect of the CH3NH3PbClxI3-x perovskite layer thickness and grain size on its electrophysical and optical properties

Abstract

Lead halide perovskite CH₃NH₃PbCl_xI_3-x thin films are widely used as photoactive layers in perovskite solar cells. CH₃NH₃PbCl_xI_3-x is a low band gap semiconductor with a broad absorption spectrum and a high conductivity showing excellent compatibility with exciting hole and electron selective layers in terms of electronic energy alignment, which provide efficient charge generation, separation and transport in perovskite solar cells. In this paper, CH₃NH₃PbCl_xI_3-x layers were deposited on the TiO2 surface by one step spin-coating technique from a methylammonium iodide (MAI) and lead chloride (PbCl₂) solution. To prepare the perovskite solution, PbCl₂ (Sigma-Aldrich) 230 mg of PbCl₂ and 394 mg of MAI were dissolved in 1 ml of N, N-Dimethylformamide (Sigma-Aldrich) solvent. As expected, the elevation of the spin-coating rate resulted in CH₃NH₃PbCl_xI_3-x thickness reduction, which should lead to a decrease in the R₃ resistance in CH₃NH₃PbCl_xI_3-x. However, the impedance spectroscopy revealed that with thickness reduction from 955 nm to 753 nm, the R₃ resistance of CH₃NH₃PbCl_xI_3-x declines from about 2590 Ώ to 2258 Ώ reaching the minimum value at 505 nm. The further decrease in CH₃NH₃PbCl_xI_3-x thickness increased CH₃NH₃PbCl_xI_3-x film resistance. The study of CH₃NH₃PbCl_xI_3-x absorbance and luminescence spectra showed that the change in CH₃NH₃PbCl_xI_3-x defect density occurred, which explains the decrease in CH₃NH₃PbCl_xI_3-x resistance. According to the absorbance and luminescence spectroscopy study, the CH₃NH₃PbCl_xI_3-x thickness reduction led to a decrease in the density of interstitial CH₃NH₃PbCl_xI_3-x⁺ defects. CH₃NH₃PbCl_xI_3-x⁺ species form deep levels trapping free electrons and as a result, increasing CH₃NH₃PbCl_xI_3-x resistance. The PSCs based on a 505 nm thick CH₃NH₃PbCl_xI_3-x layer showed the highest performance with the improved short current density and fill factor. The champion device had a power conversion efficiency of 9.92 %.