Comprehensive prediction of lead-free mixed-valence Cs2AgIAuIIIX6 (X = Cl, Br, I) halide double perovskites for high-efficiency photovoltaics

Abstract

In light of their suitability for optoelectronic and photovoltaic devices, lead-free halide double perovskites (HDPs) have been extensively concerned in recent years. However, most HDPs have large and indirect band gaps (> 2 eV), which is unfavorable for perovskite solar cells. Here, the structural stability, optoelectronic response, and photovoltaic performance of mixed-valence HDPs Cs2AgIAuIIIX6 (X = Cl, Br, I) have been theoretically examined. The thermodynamic, dynamical, and mechanical stability are verified in terms of the decomposition energy, phonon dispersion, and elastic constants. The computed indirect-gaps are 1.44 eV for Cs2AgIAuIIICl6, 1.25 eV for Cs2AgIAuIIIBr6, and 1.22 eV for Cs2AgIAuIIII6, respectively. These mixed-valence Cs2AgIAuIIIX6 (X = Cl, Br, I) HDPs possess more suitable optical band gaps in comparison with that of Cs2AuIAuIIIX6. The role of different metal cations in determining electronic properties is further elucidated. Furthermore, the optical analysis discloses that three mixed-valence HDPs exhibit high visible-light absorption coefficients. Additionally, the photovoltaic response of Cs2AgIAuIIIX6 (X = Br, I) is further substantiated since the predicted efficiency is beyond 28 %. Overall, our study reveals that mixed-valence Cs2AgIAuIIIX6 (X = Cl, Br, I) HDPs shows good stability and superior photovoltaic performance, which is an efficient candidate for single-junction perovskite solar cells.