DOI: https://doi.org/10.36347/sajb.2026.v14i04.003

Pages: 296-319

The environmental and health hazards associated with lead-based perovskites have accelerated the search for sustainable, lead-free alternatives with comparable or superior functional performance. Among these, double perovskites such as Cs₂AgBiBr₆ and Cs₂AgFeCl₆ have emerged as promising candidates; however, a comprehensive and consistent comparative understanding of their optoelectronic and thermoelectric behavior remains limited. In this study, a systematic first-principles investigation based on Density Functional Theory (DFT) is conducted using the generalized gradient approximation (GGA-PBE) within the Quantum ESPRESSO framework, while transport properties are evaluated via the BoltzTraP code. The optimized structures reveal thermodynamic stability with negative formation energies. Electronic band structure analysis indicates an indirect bandgap of approximately 1.85 eV for Cs₂AgBiBr₆ and a comparatively narrower gap of ~1.62 eV for Cs₂AgFeCl₆, suggesting enhanced visible-light absorption in the latter. Optical spectra further confirm strong absorption coefficients in the visible region, positioning both materials as viable optoelectronic absorbers. Thermoelectric evaluation demonstrates a maximum Seebeck coefficient of ~280 μV/K for Cs₂AgBiBr₆ and ~320 μV/K for Cs₂AgFeCl₆ at elevated temperatures, with corresponding figure of merit (ZT) values reaching 0.72 and 0.89, respectively, at 800 K. These findings highlight the superior thermoelectric efficiency and competitive optoelectronic response of Cs₂AgFeCl₆, underscoring its potential for integration into next-generation energy harvesting systems, including photovoltaic and thermoelectric devices. This work provides critical design insights for the development of high-performance, lead-free perovskite materials tailored for sustainable energy applications.