# First-principles calculations to investigate structural, electronic, optical, elastic, mechanical and phonon properties of novel Q3GaBr6 (Q = Na and K) for next-generation lead-free solar cells

## TL;DR

This paper explores new lead-free perovskites for solar cells, showing they are stable and efficient with promising electronic and optical properties.

## Contribution

The study introduces a new class of vacancy-ordered perovskites and integrates DFT with device modeling for efficient material screening.

## Key findings

- Q3GaBr6 compounds show stable cubic structures with direct band gaps suitable for solar cells.
- High absorption and low reflectivity in the visible range suggest strong optoelectronic potential.
- Device modeling predicts a theoretical efficiency of 22.21% under ideal conditions.

## Abstract

Lead-free halide perovskites have emerged as promising alternatives to toxic Pb-based photovoltaic absorbers, yet many candidates suffer from poor stability or unfavorable electronic properties. In this work, we present the first comprehensive first-principles and device-level investigation of the novel vacancy-ordered perovskites Q3GaBr6 (Q = Na, K) to evaluate their potential for next-generation optoelectronic and solar-cell applications. Density functional theory (DFT) calculations confirm that both compounds crystallize in a stable cubic Fm3̄m phase with negative formation energies, favorable tolerance factors, and strong Ga–Br bonding within rigid octahedral frameworks. Electronic-structure analysis reveals direct band gaps of 1.445 eV (K3GaBr6) and 1.991 eV (Na3GaBr6), with Br-4p states dominating the valence band and Ga-/Q-site orbitals contributing to the conduction band. Optical studies show high absorption (>104 cm−1 in the visible region), low reflectivity, strong dielectric response, and pronounced UV absorption, indicating suitability for broadband optoelectronics. Mechanical and phonon analyses further confirm mechanical stability, moderate stiffness, and absence of imaginary phonon modes, while AIMD simulations validate excellent thermal robustness at elevated temperatures. Incorporating DFT-extracted parameters into SCAPS-1D device modeling demonstrates promising photovoltaic performance, with efficiency, current density, and fill factor strongly influenced by absorber thickness, defect density, and doping concentration. Under ideal simulated conditions, the device shows a theoretical upper-limit efficiency of 22.21%. The proposed DFT–SCAPS integrated approach provides an efficient and computationally economical route to screen and optimize lead-free perovskite absorbers, significantly reducing experimental trial-and-error while enabling accurate prediction of photovoltaic performance.

Lead-free halide perovskites have emerged as promising alternatives to toxic Pb-based photovoltaic absorbers, yet many candidates suffer from poor stability or unfavorable electronic properties.