Unveiling the impact of trivalent metal cation transmutation on Cs$_{2}$AgM(III)Cl$_{6}$ double perovskites using many-body perturbation theory

TL;DR

This work investigates Cs2AgM(III)Cl6 lead-free double perovskites by replacing Pb2+ with trivalent M(III) cations across a broad set. Using first-principles DFT/DFPT and many-body perturbation theory (HSE06, GW, BSE), it characterizes structural stability, electronic structure, optical response, excitons, and polaronic transport. The results show a robust cubic framework with GW-BSE bandgaps spanning 1.47–6.20 eV and notable excitonic effects (EB 0.17–0.60 eV) along with intermediate-to-strong carrier–phonon coupling, yet with negligible ionic screening for dielectric response. These insights indicate that trivalent cation transmutation can yield stable, tunable, Pb-free HDPs with promising optoelectronic properties for flexible devices.

Abstract

Lead-free halide double perovskites A$_{2}$M(I)M(III)X$_{6}$ have garnered significant attention in the past decade as promising alternatives to CsPbX$_{3}$ perovskites, addressing concerns related to lead toxicity and material instability. In this work, we employ a trivalent metal cation transmutation strategy to design a series of inorganic Pb-free halide double perovskites Cs$_{2}$AgM(III)Cl$_{6}$ and perform a comprehensive investigation into their potential for applications in optoelectronic devices. Our first-principles calculations, rooted in density functional theory, demonstrate that these materials possess a face-centered cubic lattice structure while showcasing remarkable thermodynamic, dynamical, and mechanical stability. The G$_{0}$W$_{0}$@PBE electronic bandgap ranges from 1.47-6.20 eV, while the Bethe-Salpeter equation (BSE) indicates strong optical absorption spanning near-infrared to ultraviolet regions for these compounds. Furthermore, the excitonic properties suggest that these perovskites exhibit intermediate exciton binding energies (0.17 to 0.60 eV) and generally longer exciton lifetimes, except for the materials with M(III) = Sc, Y, Tb, and Lu. The Fröhlich model indicates that these materials exhibit intermediate to strong carrier-phonon interactions, with hole-phonon coupling more prominent than electron-phonon coupling. Interestingly, the charge-separated polaronic states are found to be less stable than the bound exciton states, with higher polaron mobility for electrons (4.92-29.03 cm$^{2}$V$^{-1}$s$^{-1}$) than for holes (0.56-8.69 cm$^{2}$V$^{-1}$s$^{-1}$) in these materials. Overall, our study demonstrates that trivalent metal cation transmutation in Cs$_{2}$AgM(III)Cl$_{6}$ enables the creation of stable and lead-free halide double perovskites with exceptional, tunable optoelectronic properties, making them ideal for flexible optoelectronic applications.

Figures (3)

• Figure 1: (a) Polyhedral view of the Cs$_{2}$AgM(III)Cl$_{6}$ HDPs (space group $Fm\bar{3}m$), where blue, red, black, and green balls represent Cs, Ag, M(III), and Cl atoms, respectively. Also, phonon dispersion curves of (b) Cs$_{2}$AgAlCl$_{6}$, (c) Cs$_{2}$AgScCl$_{6}$, and (d) Cs$_{2}$AgGaCl$_{6}$ HDP, calculated with the DFPT method.
• Figure 2: Electronic band structures of Cs$_{2}$AgM(III)Cl$_{6}$ HDPs obtained using HSE06 xc functional, where M(III) = Al, Sc, Ga, As, Y, Rh, In, Sb, Tb, Lu, Tl, and Bi. Band structures of considered HDPs are obtained on the following path (in crystal coordinates): W (0.5, 0.25, 0.75) - L (0.5, 0.5, 0.5) - (0, 0, 0) - X (0.5, 0, 0.5) - W (0.5, 0.25, 0.75) - K (0.375, 0.375, 0.75). The Fermi level is set to be zero and marked by the red line.
• Figure 3: Imaginary part [Im($\varepsilon$)] of the dielectric function for Cs$_{2}$AgM(III)Cl$_{6}$ HDPs obtained using BSE@G$_{0}$W$_{0}$@PBE method, where M(III) = Al, Sc, Ga, As, Y, Rh, In, Sb, Tb, Lu, Tl, and Bi.