The effect of the CH3NH3PbClxI3-x perovskite layer thickness and grain size on its electrophysical and optical properties
Keywords:Perovskite solar cells, photoactive layer, CH3NH3PbClx I3-x, copper phthalocyanine, iodine ions, impedance meter, electron transport layer, hole transport layer
Lead halide perovskite CH3NH3PbClxI3-x thin films are widely used as photoactive layers in perovskite solar cells. CH3NH3PbClxI3-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, CH3NH3PbClxI3-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 CH3NH3PbClxI3-x thickness reduction, which should lead to a decrease in the R3 resistance in CH3NH3PbClxI3-x. However, the impedance spectroscopy revealed that with thickness reduction from 955 nm to 753 nm, the R3 resistance of CH3NH3PbClxI3-x declines from about 2590 Ώ to 2258 Ώ reaching the minimum value at 505 nm. The further decrease in CH3NH3PbClxI3-x thickness increased CH3NH3PbClxI3-x film resistance. The study of CH3NH3PbClxI3-x absorbance and luminescence spectra showed that the change in CH3NH3PbClxI3-x defect density occurred, which explains the decrease in CH3NH3PbClxI3-x resistance. According to the absorbance and luminescence spectroscopy study, the CH3NH3PbClxI3-x thickness reduction led to a decrease in the density of interstitial CH3NH3PbClxI3-x+ defects. CH3NH3PbClxI3-x+ species form deep levels trapping free electrons and as a result, increasing CH3NH3PbClxI3-x resistance. The PSCs based on a 505 nm thick CH3NH3PbClxI3-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 %.