Composition-Dependent Plasmon-Enhanced Emission in Lead-Free Cs$_3$Cu$_2$X$_5$ Halides: A DFT--FDTD Study

Abstract

Lead-free Cs$_3$Cu$_2$X$_5$ (X = Cl, Br, I) halides exhibit high photoluminescence quantum yields and excellent ambient stability, yet light-emitting devices based on these materials remain limited by poor optical outcoupling. In this work, we develop an integrated density functional theory (DFT) and finite-difference time-domain (FDTD) framework to establish quantitative links between halide composition, wavelength-dependent optical constants, and plasmonic enhancement. First-principles calculations are used to obtain composition-specific refractive index (n) and extinction coefficient (k) spectra, which are directly implemented into three-dimensional FDTD simulations of a complete PeLED stack incorporating Ag/SiO$_2$ core--shell nanostructures. Among the investigated compositions, Cs$_3$Cu$_2$Cl$_5$ demonstrates the strongest plasmonic response, achieving a 4.4$\times$ Purcell enhancement and 30\% light extraction efficiency (LEE) using optimized nanorods. The superior performance originates from its lower refractive index, which reduces dielectric screening and improves near-field coupling. Cs$_3$Cu$_2$Br$_5$ exhibits the highest spectral overlap ($J_{\mathrm{cos}} = 0.955$) but yields moderate extraction (26%) due to increased optical confinement. Cs$_3$Cu$_2$I$_5$ requires a nanosphere geometry and shows limited enhancement, with LEE restricted to 10%. Distance-ependent analysis reveals composition-specific optimal emitter--plasmon separations, ranging from 8--12 nm for Cs$_3$Cu$_2$Br$_5$ to approximately 15 nm for Cs$_3$Cu$_2$Cl$_5$. These results provide composition-dependent design guidelines for plasmon-enhanced lead-free PeLEDs and highlight the critical role of accurate optical constants in predictive device optimization.

Figures (12)

• Figure 1: Crystal structure of Cs3Cu2X5 (a) X = Cl, (b) X = Br, and (c) X = I. The green, brown, violet, sky blue, and blue atom colors correspond to Cl, Br, I, Cs, and Cu atoms, respectively.
• Figure 2: Overview of the DFT-FDTD modeling approach adopted in this work. Optical constants from first-principles calculations feed into device-level electromagnetic simulations that extract key performance metrics.
• Figure 3: (a) Schematic of the simulated PeLED stack. (b) Energy band alignment showing hole injection from ITO through Spiro-OMeTAD and electron injection from Ag through ZnO into the perovskite active layer.
• Figure 4: Placement of the dipole source and plasmonic nanostructures within the LED stack. A core--shell Ag/SiO2 nanorod (variable radius 8--25 nm, length 30--70 nm) is used for Cs3Cu2Cl5 and Cs3Cu2Br5, while a nanosphere geometry is employed for Cs3Cu2I5. The 5 nm SiO2 shell prevents non-radiative quenching while maintaining near-field coupling.
• Figure 5: Calculated electronic band structures and total density of states (DOS) of Cs_3Cu_2X_5 compounds: (a) Cs_3Cu_2Cl_5, (b) Cs_3Cu_2Br_5, and (c) Cs_3Cu_2I_5. The band structures are plotted along the high-symmetry directions of the Brillouin zone, as indicated on the horizontal axis, with the Fermi level set to 0 eV (red dashed line). The corresponding total DOS is shown in the right panel of each subfigure. All compounds exhibit direct band gaps of 2.42 eV, 2.15 eV, and 2.34 eV for Cs_3Cu_2Cl_5, Cs_3Cu_2Br_5, and Cs_3Cu_2I_5, respectively, as marked in the band structure plots.