A detailed investigation into the synthesis of green-emitting powders of Cs 4 PbBr 6 and CsPbBr 3 materials by antisolvent precipitation from CsBr-PbBr 2 precursor solutions in dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) is reported. Various solvated lead bromide and polybromide species (PbBr 2 , [PbBr 3 ] − , [PbBr 4 ] 2− , and possibly [PbBr 5 ] 3− or [PbBr 6 ] 4− ) are detected in the precursor solutions by optical absorbance and emission spectroscopies. The solvodynamic size of the species in solution is strongly solvent-dependent: ~1 nm species were detected in DMSO, while significantly larger species were observed in DMF by dynamic light scattering. The solvodynamic size of the lead bromide species plays a critical role in determining the Cs-Pb-Br composition of the precipitated powders: smaller species favor the precipitation of Cs 4 PbBr 6 , while larger species template the formation of CsPbBr 3 under identical experimental conditions. The powders have been characterized by 133 Cs and 207 Pb solid-state nuclear magnetic resonance, and 133 Cs sensitivity toward the different Cs environments within Cs 4 PbBr 6 is demonstrated. Finally, the possible origins of green emission in Cs 4 PbBr 6 samples are discussed. It is proposed that a two-dimensional Cs 2 PbBr 4 inclusion may be responsible for green emission at ~520 nm in addition to the widely acknowledged CsPbBr 3 impurity, although we found no conclusive experimental evidence supporting such claims.

The fabrication of high-quality cesium (Cs)/formamidinium (FA) doublecation perovskite films through a two-step interdiffusion method is reported. Cs x FA 1-x PbI 3-y(1-x) Br y(1-x) films with different compositions are achieved by controlling the amount of CsI and formamidinium bromide (FABr) in the respective precursor solutions. The effects of incorporating Cs + and Bron the properties of the resulting perovskite films and on the performance of the corresponding perovskite solar cells are systematically studied. Small area perovskite solar cells with a power conversion efficiency (PCE) of 19.3% and a perovskite module (4 cm 2 ) with an aperture PCE of 16.4%, using the Cs/FA double cation perovskite made with 10 mol% CsI and 15 mol% FABr (Cs 0.1 F A 0.9 PbI 2.865 Br 0.135 ) are achieved. The Cs/FA double cation perovskites show negligible degradation after annealing at 85 °C for 336 h, outperforming the perovskite materials containing methylammonium (MA).

Under mild mechanical pressure, halide perovskites show enhanced optoelectronic properties. However, these improvements are reversible upon decompression, and permanent enhancements have yet to be realized. Here, we report antisolvent-assisted solvent acidolysis crystallization that enables us to prepare methylammonium lead bromide single crystals showing intense emission at all four edges under ultraviolet light excitation. We study structural variations (edge-vs-center) in these crystals using micro-X-ray diffraction and find that the enhanced emission at the edges correlates with lattice compression compared to in the central areas. Time-resolved photoluminescence measurements show much longer-lived photogenerated carriers at the compressed edges, with radiative component lifetimes of ∼1.4 μs, 10 times longer than at the central regions. The properties of the edges are exploited to fabricate planar photodetectors exhibiting detectivities of 3 × 1013 Jones, compared to 5 × 1012 Jones at the central regions. The enhanced lifetimes and detectivities correlate to the reduced trap state densities and the formation of shallower traps at the edges due to lattice compression.

The solvent acidolysis crystallization technique is utilized to grow mixed dimethylammonium/methylammonium lead tribromide (DMA/MAPbBr3) crystals reaching the highest dimethylammonium incorporation of 44% while maintaining the 3D cubic perovskite phase. These mixed perovskite crystals show suppression of the orthorhombic phase and a lower tetragonal‐to‐cubic phase‐transition temperature compared to MAPbBr3. A distinct behavior is observed in the temperature‐dependent photoluminescence properties of MAPbBr3 and mixed DMA/MAPbBr3 crystals due to the different organic cation dynamics governing the phase transition(s). Furthermore, lateral photodetectors based on these crystals show that, at room temperature, the mixed crystals possess higher detectivity compared to MAPbBr3 crystals caused by structural compression and reduced surface trap density. Remarkably, the mixed‐crystal devices exhibit large enhancement in their detectivity below the phase‐transition temperature (at 200 K), while for the MAPbBr3 devices only insignificant changes are observed. The high detectivity of the mixed crystals makes them attractive for visible‐light communication and for space applications. The results highlight the importance of the synthetic technique for compositional engineering of halide perovskites that governs their structural and optoelectronic properties.