Spatial Phase Distributions in Solution-Based and Evaporated Cs-Pb-Br Thin Films

In recent years, inorganic cesium-lead-halide perovskites, CsPbX3 (X = I, Br, Cl), have attracted interest for optoelectronic applications such as highly efficient thin-film light-emitting diodes or wide-gap absorber materials for photovoltaics. However, phase segregation and secondary phases in as-deposited thin films are still considered to be limiting factors for devices based on CsPbX3. Here, we report a correlative electron microscopy and spectroscopy approach for the identification of secondary phases and their distributions in Cs-Pb-Br thin films, deposited by solution based and coevaporation methods on various substrates. We identified phases by their compositional, structural, and optoelectronic properties, using X-ray diffraction, spectroscopy, and a variety of microscopy techniques. We found that the Cs-Pb-Br films contain ternary Cs4PbBr6 and CsPb2Br5 phases in addition to CsPbBr3, a finding consistent with calculations of formation enthalpies by means of the density functional theory showing that these values are very similar for the three ternary phases. We find that these phases can exhibit different spatial distributions inside the film and discuss the influence of the deposition method and synthesis parameters on the resulting phase composition of the Cs-Pb-Br layers.