Solution-phase transition zones govern crystallographic perfection in metal halide perovskite single crystals.

Despite rapid progress in solution-grown metal halide perovskite single crystals for optoelectronic applications, further enhancement of their intrinsic crystallographic quality through solution engineering remains challenging. In particular, the fundamental links between precursor solution properties and crystal quality, as well as a universal mechanism governing crystallization from solution, remains poorly understood. Here, using in-situ microscopic spectroscopy, we identify key solution properties of viscosity, concentration, coordination number and growing temperature as critical determinants of crystal quality through their modulation of solution-phase transition zones, which are directly observed experimentally located 10 - 60 μm away from the crystal-solution interface. We reveal a direct correlation between crystal quality, quantified by the carrier mobility-lifetime product, and the width of the transition zone, providing a clear guideline for improving crystallographic quality by widening this zone. As a result, FAPbBr3 single crystals with markedly improved crystallographic perfection are achieved via solution engineering, demonstrating a high charge-collection efficiency of 99% and an energy resolution of 1.32% for 662 keV γ-rays from a 137Cs source.