Abnormal absorption onset shift of CH3NH3PbI3 film by adding PbBr2 into its precursor and its effect on photovoltaic performance

Abstract

High quality CH3NH3PbI(3-x)Brx perovskite film with full surface coverage and fewer trap states was prepared using a single-step deposition process and PbBr2 doped perovskite precursor solution. Specifically, the thin film surface becomes rougher and the grain size gets larger as the PbBr2 concentration was increased from 1% to 10 mol%. Ultraviolet–visible spectroscopy (UV–vis) and incident photon-to-current efficiency (IPCE) spectra show that abnormal absorption onset shifts to longer wavelength with increasing the content of PbBr2, because the larger grain size can reduce the stress accumulated in the lattice over the Pb–I bond angle. It is surprising to see that when 5 mol% of PbBr2 was used, the CH3NH3PbI3 (MAPbI3) shows the best thermal stability, with the decomposition onset increased by about 20 °C. The photoluminescence (PL) peak of the CH3NH3PbI(3-x)Brx film shifts to smaller wavelength as more PbBr2 was used owing to decreased trap states in perovskite films. It is found that at the optimal PbBr2 content of 5 mol%, the CH3NH3PbI(3-x)Brx based solar cell gives open-circuit voltage of 1.08 V with power conversion efficiency (PCE) as high as 18.97%, about 16.6% higher than the device based on the MAPbI3 without Br.