The Photochemical Stability of PbI2 and PbBr2: Optical and XPS and DFT Studies

We investigated the photochemical stability of PbX2 (X = I and Br) halides by optical and X-ray photoelectron spectroscopy (XPS). The optical absorbance displayed a strong reduction for PbI2 with light soaking and permanent behavior for PbBr2. The XPS survey spectra showed a sharp drop in the I:Pb ratio for PbI2 from 1.63 to 1.14 with exposure time from 0 to 1000 h while for PbBr2, it remains practically unchanged (1.59–1.55). The measurements of the XPS Pb 4f and Pb 5d spectra have shown the partial photolysis of PbI2 with the release of metallic lead whereas PbBr2 demonstrated remarkable photochemical stability. According to the density functional theory (DFT), calculations of the metal and iodide vacancy formation energies for PbBr2 are higher than for PbI2 which confirms the better stability to light soaking. The high photochemical stability of PbBr2 means that it can be used as excess under MAPbBr3 perovskite synthesis to improve not only the power conversion efficiency but also stability to light soaking.