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

High quality CH₃NH₃PbI_(3-x)Br_x 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)Br_x 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)Br_x 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.