White light emission with unity efficiency from Cs2Na1-xAgxIn1-yBiyCl6 double perovskites: the role of bismuth and silver

Double perovskites Cs2Na1-xAgxIn1-yBiyCl6 can emit warm white light with almost unity quantum efficiency and are thus among the most promising materials for solid-state lighting. The emission spectrum is reproducible and the materials themselves are robust against degradation, as well as fabricated from earth-abundant, non-toxic precursors. The emission efficiency is however sensitive to the materials composition, with small variations modifying the photoluminescence quantum yield by more than an order of magnitude. We provide here a comprehensive, systematic study of the optical properties as a function of composition and identify the microscopic mechanism linking the presence of Bi and Ag to the high emission quantum yield. A 0.1% minimum fraction of Bi and Ag is found to trigger efficient emission of warm white light. Ag alloying is fundamental to obtain high emission yields. Continuous-wave spectroscopy measurements are complemented with ab initio computation of the electronic band structure in demonstrating that Bi strengthens optical absorption at the band gap edge of the double perovskites. In addition, it is found by combining time-resolved photoluminescence and transient absorption spectroscopy that Ag and Bi promote the formation of bright (radiative) self-trapped excitons, while inhibiting exciton relaxation into long-lived dark states.