Halide Perovskites, rising star materials in photovoltaics, with the general formula ABX3, where A=methylammonium (MA), formamidinium (FA) or Cs, B=Pb and X=halide have remarkable electronic and optical characteristics. Still, much remains unknown regarding the connection between their physical and chemical properties. The identity of the X component influences greatly the band gap of the material, and mixed halide compositions have been used for band gap-tuning. Halide exchange was previously reported in thin-films and in micron-sized single crystals.
We demonstrate halide exchange in mm-sized MAPbX3 single crystals, achieved by diffusion. In these macro-sized crystals, the effect of surface and defects is significantly smaller than in the microcrystals and polycrystalline thin films studied previously. Therefore, they are better suited to examine the fundamental exchange process(es), unencumbered by possible grain boundary and surface diffusion effects. Initially, the halide exchange creates normal concentration gradients of the out-going and in-coming halides, on a scale of a few microns to a few hundred microns. The depth (from the surface) of the substituted area depends on the halide pair and the role of each halide (which one is being exchanged and which one is exchanging). As expected the concentration gradient of the in-coming halides decreases from the surface of the crystal towards its inner core and vice versa for the out-going halides. While these gradients do cause a lattice parameter change and may cause a symmetry change, we show that if the interchanged halide pair is such that their sizes are relatively similar (e.g., Br- and Cl-, Br- and I-, not Cl- and I-) the product crystal remains surprisingly single crystalline. This finding is valid, no matter which one of the two halides is being exchanged. Our results suggest that for these similar-sized halide pairs, this exchange occurs through a solid-state chemical reaction such that the resulting crystal orientation is determined by that of the initial crystal (a topotactic reaction). This is demonstrated by scanning microscope images, electron dispersive spectroscopy and X-ray diffraction measurements. This phenomenon is crucial when one wishes to achieve a specific composition of halide perovskite crystals/crystallites, using multi-synthesis steps without compromising the degree of crystallinity and orientation.
