Composition-dependent plasmon-enhanced emission in lead-free Cs3Cu2X5 halide LEDs: a DFT-FDTD study

Abstract

Lead-free Cs₃Cu₂X₅ (X = Cl, Br, I) halides show high photoluminescence quantum yields (PLQYs) and good ambient stability, yet LEDs based on these materials still suffer from poor optical outcoupling. In this work, we combine density functional theory (DFT) and finite-difference time-domain (FDTD) simulations to optimize plasmonic enhancement in Cs₃Cu₂X₅ LEDs using composition-specific optical constants. First-principles calculations provide a wavelength-dependent refractive index and extinction coefficient data for each halide. These values are then used in FDTD to model a complete device stack with Ag/SiO₂ core–shell nanostructures. Out of the three halides, Cs₃Cu₂Cl₅ performs best with 4.4× Purcell enhancement and 30% light extraction using optimized nanorods. The chloride outperforms the others due to its lower refractive index (n ≈ 1.9). Cs₃Cu₂Br₅ has the highest spectral overlap (95.5%) but only moderate extraction efficiency (26%) because of increased optical confinement. For Cs₃Cu₂I₅, a nanosphere geometry is required due to its emission wavelength. But, the extraction efficiency remains limited to approximately 10% even with moderate Purcell enhancement. The optimal separation between the emitter and plasmon depends on the material composition. For Cs₃Cu₂Br₅, this distance is 8–12 nm, while Cs₃Cu₂Cl₅ requires approximately 15 nm. These results provide composition-specific design guidelines for plasmon-enhanced lead-free LEDs.