Rational Synthesis of Lead Bromide Perovskites with Various Crystallite Sizes

The goal of this project was to synthesize methylammonium (MA), formamidinium (FA), and cesium (Cs) lead bromide single crystal and microplate perovskites with high phase purity. Perovskites are a family of semiconductors with the specific crystal structure ABX₃ (A=MA, FA, or Cs; B=Pb; X=Br). They have desirable properties including long diffusion lengths, easy fabrication, and low trap-state density. In order to accurately measure their fundamental properties, the perovskites must be synthesized with minimal defects and high phase purity. Single crystal and microplate perovskites offer fewer grain boundaries, which can improve these properties relative to bulk crystals.

Microplates were synthesized by a dissolution recrystallization method. A lead precursor was spin coated or drop cast onto a glass slide and then reacted for several hours in an ABr solution to form a thin of perovskite microplates. Scanning electron microscopy was used to determine the sizes of the microplates and assess the microscopic coverage of the films, which changes depending on the reaction conditions. All samples consisted of microplates ranging from 1 to 5 microns in size, and the coverage varied depending on the precursor deposition. The single crystals were grown over the course of several days using an antisolvent vapor diffusion method. The resulting crystals were millimeters in size.

The XRD patterns for the microplates and the single crystals match well with the accepted patterns found in literature. The peak width is inversely proportional to the crystallite size, so the full width at half maximum is greater for the microplates than for the single crystals. Initial Cs microplate samples showed some phase impurities, but these were avoided by reducing thermal processing in subsequent iterations.

We have demonstrated the synthesis of single crystal and microplate MA, FA, and Cs lead bromide perovskites, confirmed by XRD and SEM. Single crystal growth by the anti-solvent diffusion method requires a high precursor concentration (0.2 to 0.6 M) in the initial solution. The microplate shape, size, coverage, and purity depends on the reaction times and temperature, reactant concentrations, and precursor deposition method.