Pseudo-perovskites[1] is a rapidly expanding materials family, derived from metal-halide perovskites. Compared to their predecessors, copper-halide pseudo-perovskites are less toxic and possess an excellent stability under ambient conditions. These properties paired with high photoluminescence quantum yield (PLQY)[2] and a trap-state resistant emission mechanism (self-trapped-exciton emission) made this class of materials popular in optoelectronic applications like photodetectors, LEDs, X-ray and ionizing radiation detection. The cost-effective and diverse synthetic methods allow their preparation in various forms, from single crystals to polycrystalline thin films and even nanocrystals. A few years ago, only inorganic pseudo-perovskites were prevalent in the literature with different stoichiometries (e.g., CsCuX, RbCuX, X=Cl, Br, I). As organic-inorganic hybrid pseudo-perovskites have recently emerged, an even more rapid expansion of the available compositions can be envisioned.
Their optoelectronic properties (e.g., large Stokes shift, high PLQY, long PL lifetime) can be related to a self-trapped exciton emission mechanism, which results from the distortion of their soft crystal lattice. After excitation, different electronic states can be formed, where excitons can be trapped. From these lower energy states, the radiative recombination of trapped exciton results in a bright PL emission. To better understand the electronic and band structure differences between different stoichiometries, easily accessible and fast methods are necessary to determine the midgap states within the band gap. By understanding the band structure, we can transfer this knowledge to optoelectronic device design.
In my presentation, the band structure mapping of different pseudo-perovskite thin films will be discussed, as determined by spectroelectrochemical and optical techniques. We investigated fully inorganic (Cs₃Cu₂I₅) and hybrid ((Gua)₃Cu₂I₅, where Gua = Guanidinium)[3] compositions, synthesized by a spray coating method. For the spectroelectrochemical measurements, the steady-state PL signal was monitored as a function of the applied potential. This method enabled the mapping of the band structure of these materials. The in-depth understanding of the band structure was achieved by analyzing the wavelength-dependent photoluminescence lifetime.