Targeted Crystallization of 3D/Polytype Phase Heterostructures in High-Bromide Content Perovskites

Mixed-dimensionality metal-halide perovskite films comprising heterostructured interfaces, grain boundaries or surfaces primarily using ‘2D’ phases enable control over optoelectronic properties, interfaces and stability in functional devices. Less explored are low-dimensional polytype phases maintaining ABX₃ stoichiometry, but with mixtures of face- and corner-sharing octahedral arrangements. Here we present our latest data exploring deliberate introduction of targeted polytype phases via both surface treatment and bulk solution additive approaches applied primarily to high-bromide content methylammonium-free perovskite compositions with 1.7 eV and 1.8 eV band gaps. By blade-coating solutions and monitoring using synchrotron-based <i>in situ</i> grazing-incidence wide-angle X-ray scattering (GIWAXS) we look at the impact on the crystallization process of different additives. Drawing on the chemical structure and stability of formamidinium, these include ethylenediamine, imidazolium and guanidinium derivatives. From this we find significant differences in the heterogeneity of the perovskite phase and demonstrate enhancements in the ambient stability of these materials. Combining solution characterisation methods including nuclear magnetic resonance (NMR) spectroscopy, we establish a clear understanding of the bridge between solution chemistry, precursor phase formation and resulting material properties across a range of perovskite compositions. Our work provides important insight into the controlled growth of stable perovskite materials by engineering the composition of benign secondary phases.