In situ optical absorption spectroscopy of annealing behaviours of quench-deposited films in the binary system CsI-PbI2

We have investigated annealing behaviours of quench-deposited films of the binary CsI-PbI2, system by in situ optical absorption spectroscopy. Various films composed of multiple crystalline phases of CsPbI3 and/or Cs4PbI6 as well as of CsI and/or PbI2 are obtained, depending on the mixing ratio of the CsI and PbI2, It is difficult to prepare films purely composed of a single CsPbI3 or Cs4PbI6 phase alone. However, it is possible to obtain films where crystallites of either CsPbI3 or Cs4PbI6 coexist with the CsI phase. Using such films, we measure the fundamental optical absorption spectrum of CsPbI3 and Cs4PbI6 for the first time. Cs4PbI6 exhibits stronger oscillator-like absorption compared to CsPbI3, due to the localized nature of both the Pb 6s and 6p states.