The Effect of Halide Composition on the Luminescent Properties of Ternary Cesium–Copper Halide Pseudo‐Perovskite Films

Abstract Ternary copper halide pseudo‐perovskites are in the forefront of research as potential active materials in light emission applications. The optoelectronic properties of these compounds can be fine‐tuned by the preparation of mixed‐halide compositions. After irradiation, self‐trapped excitonic states are formed in these materials. However, the emission from these self‐trapped states is not yet fully understood. In this work, mixed‐halide Cs 3 Cu 2 X 5 films (where X: I and/or Br) are prepared by a simple spray‐coating method. Using ultraviolet photoelectron spectroscopy, the changes in optoelectronic properties are linked to the electronic structure of these materials. It is revealed that the incorporation of bromide into the lattice makes the emission process of these materials more vulnerable to trap states. By combining the different spectroscopic characterization techniques, the exact band structure of these compounds is determined, and the different processes are translated to the absolute energy scale. As an alternative excitation mechanism of self‐trapped states, α ‐particles are used to induce radioluminescence response. The Cs 3 Cu 2 X 5 films exhibit composite decay patterns, most likely attributed to a multitude of different trap state‐mediated recombination processes.