Comprehensive Prediction of Lead-Free Mixed-Valence Cs2AgIAuIIIX6 (X = Cl, Br, I) Halide Double Perovskites for High-Efficiency Photovoltaics

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 (Cs2AgAuX6)-Au-I-X-III (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 (Cs2AgAuCl6)-Au-I-Cl-III, 1.25 eV for (Cs2AgAuBr6)-Au-I-Br-III, and 1.22 eV for (Cs2AgAuI6)-Au-I-I-III, respectively. These mixed-valence (Cs2AgAuX6)-Au-I-X-III (X = Cl, Br, I) HDPs possess more suitable optical band gaps in comparison with that of (Cs2AuAuX6)-Au-I-X-III. 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 (Cs2AgAuX6)-Au-I-X-III (X = Br, I) is further substantiated since the predicted efficiency is beyond 28 %. Overall, our study reveals that mixed-valence (Cs2AgAuX6)-Au-I-X-III (X = Cl, Br, I) HDPs shows good stability and superior photovoltaic performance, which is an efficient candidate for single-junction perovskite solar cells.