Tuning the luminescence of thin nanocrystalline CsPbBr$_3$ perovskite films during the in situ anion exchange reaction

Inorganic perovskite CsPbX$_3$ nanocrystals (PNCs), where X is a halide anion, are currently promising materials for a wide range of optoelectronic devices. One of the key tasks to be solved before they are used in practice is to obtain stable thin PNC films whose luminescence wavelength could be finely tuned. The chemical composition of CsPbX$_3$ PNCs is the main parameter determining their band gap width and, hence, the position of their photoluminescence maximum. Variation of the PNC composition in the course of their synthesis or postsynthetic treatment in solution makes it possible to obtain CsPbBr$_{(3-x)}$I$_x$ and CsPbBr$_{(3-y)}$Cl$_y$ materials emitting in the entire visible spectral range. In addition, these PNCs are more structurally stable than CsPbCl$_3$ and CsPbI$_3$ ones. However, most exchange reactions in solution reported in published studies are spontaneous and poorly controllable. In this study, the anion exchange reaction is proposed to be carried out directly on the formed thin film of CsPbBr$_3$ incorporated in the matrix of a copolymer of methyl and lauryl methacrylates. The exchange reactions with octadecylammonium iodide and PbI$_2$ leading to a shift of the photoluminescence maxima to longer wavelengths by 130 and 137 nm within 15 and 6 min, respectively. The study also shows the possibility of carrying out an ion exchange reaction on a substrate mimicking the real structure of a light-emitting diode