Correlating Defect Structures and Optical Performance in Bridgman-Grown Cs₃Cu₂I₅ Crystals: A Defect Engineering Approach for Enhanced Blue Emission

Cs₃Cu₂I₅, a nontoxic broad emitter with exceptional blue-emissive properties, holds great promise for optoelectronic applications. However, achieving large-size, high-quality single crystals with minimal defects remains a critical challenge. We report a defect engineering strategy to optimize the growth of high-quality Cs₃Cu₂I₅ single crystals by systematically controlling the growth parameters and characterizing defects. Using the Bridgman method with precisely controlled molar ratios (CuI/CsI = 39%:61%) and thermal conditions (G = 16 K/cm, v = 0.45 mm/h), we demonstrate that constitutional supercooling-induced eutectic decomposition (L → Cs₃Cu₂I₅ + CsCu₂I₃) leads to the formation of interlaced inclusions exhibiting a globule-to-lamellar transition morphology. Advanced microstructural and optical characterization revealed that these defect structures induced lattice expansion and competitive photon absorption, reducing the photoluminescence quantum yield by approximately 40% and lowering optical transmittance from 89 to 84%. Our defect-controlled growth approach achieves a 30% higher PLQY compared with conventional methods, establishing critical correlations between growth conditions, defect evolution, and optoelectronic performance in this promising lead-free blue-emitting perovskite system. These findings provide fundamental insights and practical guidelines for developing high-performance perovskite single crystals for advanced optoelectronic applications.