An integrated DFT–FDTD design of plasmon-enhanced lead-free CsSnxGe1−xI3 perovskite LEDs

Abstract

CsSn_xGe_1−xI₃ as lead-free perovskites are promising for next generation NIR emitting perovskite light emitting diodes (PeLEDs) due to their tunable bandgaps and stability. However, they suffer from poor light extraction efficiency (LEE), and accurate composition-specific optical data for these materials remain scarce. This study presents a density functional theory (DFT) informed finite-difference time-domain (FDTD) framework to optimize light extraction via compositional tuning and plasmonic enhancement. First, DFT calculations were performed to obtain composition-specific complex refractive index and extinction coefficient values for x = 0, 0.25, 0.5, 0.75, and 1. Results showed that the bandgap increased from 1.331 eV for CsSnI₃ to 1.927 eV for CsGeI₃ with increasing Ge content, while the refractive index ranged from 2.2 to 2.6 across compositions. These optical constants were then used as inputs for FDTD simulations of a PeLED structure with optimized Au/SiO₂ core–shell nanorods for plasmonic enhancement. A 12.1-fold Purcell enhancement was achieved for CsSn_0.25Ge_0.75I₃, while LEE reached 25% for CsSn_0.5Ge_0.5I₃. A LEE enhancement of 36% was obtained for CsSnI₃, and spectral overlap between emitter and plasmon resonance reached 96% for Sn-rich compositions. Among the studied compositions, CsSn_0.5Ge_0.5I₃ provides the best balance between emission enhancement, light extraction efficiency (25%), Purcell enhancement (5.3×), spectral matching (93%), and oxidation stability, while Ge-rich alloys exhibit stronger spontaneous emission rate enhancement. These results establish composition-aware design guidelines for lead-free perovskite emitters targeting flexible and wearable optoelectronic applications.