Charge Injection from Excited Cs₂AgBiBr₆ Quantum Dots into Semiconductor Oxides

Lead-free double perovskites such as Cs2AgBiBr6 are gaining attention because of their environmental friendliness compared to the lead halide perovskites. In order to establish their photoactivity, we have probed the excited-state behavior of Cs2AgBiBr6 nanocrystals and charge injection from their excited state into different metal oxides (TiO2, ZnO). The electron-transfer rate constants determined from ultrafast transient absorption spectroscopy were in the range of 1.2–5.2 × 1010 s–1. Under steady-state photolysis (ambient conditions), the electrons injected into TiO2 are scavenged by atmospheric oxygen, leaving behind holes which accumulate within the quantum dots (QDs). These accumulated holes further induce oxidation of QDs, resulting in the overall photodegradation of perovskite films. Annealed films of Cs2AgBiBr6 nanocrystals, when employed as an active layer in solar cells, delivered photocurrent under visible-light excitation.