Hydrogen Disproportionation Reduces the Bandgap and Prolongs the Carrier Lifetime in Cs₂AgBiBr₆: Quantum Dynamics...

Cs₂AgBiBr₆ is a promising lead-free double perovskite with excellent stability, but it suffers from a large bandgap for optoelectronic applications. Hydrogenation is reported not only to reduce the bandgap but also to prolong the carrier lifetime in Cs₂AgBiBr₆, improving device efficiencies significantly. In order to elucidate the mechanisms underlying these phenomena, we combine density functional theory and nonadiabatic molecular dynamics and reveal the role of hydrogen (H) atoms in Cs₂AgBiBr₆. We demonstrate that H atoms spontaneously undergo a disproportionation reaction to form H⁺ and H^-. The H^- anions form chemical bonds with the Bi³⁺ cations and introduce midgap states to reduce the bandgap, with the calculated bandgap change comparable to the experimental observation. Moreover, these states facilitate electron-hole separation in Cs₂AgBiBr₆, suppressing their recombination and, thus, extending the carrier lifetime, despite the reduced bandgap. Our results rationalize the experimental phenomena and provide crucial insights into the development of novel lead-free perovskite materials.