# Theoretical exploration of As-based mixed halide double perovskites A3AsI6 (A = K, Rb, and Cs) for photovoltaics applications using a DFT approach

## TL;DR

This paper uses computer simulations to study new lead-free materials that could be used in solar cells, finding they have good properties for capturing light and converting it into energy.

## Contribution

The study introduces and evaluates As-based mixed halide double perovskites A3AsI6 (A = K, Rb, Cs) as promising lead-free materials for photovoltaic applications.

## Key findings

- Band gap energies decrease with increasing alkali metal ionic radii, with values around 2.56-2.76 eV.
- K3AsI6 shows better thermal stability compared to Rb3AsI6 and Cs3AsI6.
- Materials exhibit strong optical interactions, making them suitable for optoelectronic applications.

## Abstract

In this study, we explore the structural, electronic, optical, and elastic features of environmentally friendly lead-free mixed-halide double perovskites with the general composition A3AsI6 (A = K, Rb, and Cs), which are comprehensively analyzed using density functional theory (DFT). Our calculations reveal that the optimized lattice constants increase from 12.42 Å for K3AsI6 to 12.99 Å for Cs3AsI6, which is consistent with the progressive enlargement of the alkali metal ionic radii. To evaluate the electronic band structures, the Tran–Blaha-modified Becke–Johnson (TB-mBJ) potential was applied, with and without incorporating spin–orbit coupling (SOC), to achieve reliable estimations of the band gaps. The results reveal a consistent trend of decreasing band gap energies: 2.763 eV (mBJ) and 2.566 eV (mBJ + SOC) for K3AsI6 (indirect), 2.821 eV (mBJ) and 2.607 eV (mBJ + SOC) for Rb3AsI6, and 2.829 eV (mBJ) and 2.621 eV (mBJ + SOC) for Cs3AsI6. The density of states analyses further clarify the orbital contributions to the occupied and unoccupied bands. Elastic constants (Cij) confirm the mechanical stability of the materials, while Poisson's and Pugh's ratios indicate brittle behavior. Moreover, the calculated Debye temperatures suggest that K3AsI6 could better withstand thermal stresses induced by lattice vibrations than its Rb and Cs analogues. The optical characteristics, such as the dielectric function ε(ω), absorption coefficient α(ω), reflectivity R(ω), and refractive index n(ω), were comprehensively examined, revealing robust interactions with incident electromagnetic radiation. These comprehensive results underscore the potential of A3AsI6 (A = K, Rb, and Cs) double perovskites as viable candidates for next-generation optoelectronic applications, particularly in environmentally benign, lead-free technologies.

Density functional theory (DFT) is used to study the structural, electronic, optical, and photovoltaic properties of As-based double perovskites A3AsI6 (A = K, Rb, Cs), identifying promising candidates for high-efficiency solar cells.