Fine-tuning of optical band gap in mixed halide aziridinium lead perovskites†

Hybrid halide perovskites form a promising class of light-absorbing materials. Among the numerous 3D semiconducting perovskites, there is a group of emerging aziridinium-based hybrids that are considered to be prospective materials for optoelectronic applications. In this work, we report the mixed halide aziridinium perovskites of (AzrH)PbBr_xI_3−x series (AzrH = aziridinium). Small changes in the composition of perovskites are shown to have a defining impact on the optoelectronic properties of the reported materials. Halogen substitution allowed a variation in band gap values of these compounds, ranging from 1.57 to 2.23 eV, as established using electronic spectroscopy. Crystal structures of (AzrH)PbBr_xI_3−x perovskites were studied using single crystal and powder X-ray diffraction analysis. The lattice constant had a linear dependence on the Br content in the structure, thus strictly following Vegards's law. Importantly, the reported compounds displayed a preferential inclusion of iodine upon synthesis, revealing that the mixed halide perovskite composition cannot be estimated based on the precursors’ ratio only, and it should be post-synthetically checked. The reported results expand the range of hybrid perovskites with tuneable band gaps beyond the conventional methylammonium and formamidinium-based perovskites and offer a new series of metal-halide hybrids suitable for photovoltaic and other optoelectronic applications.