Boosting the Upconversion and Near-Infrared Emission via Alloying Bi3+ in Cs2NaErCl6 Double Perovskite

Recently, lanthanide-doped halide perovskites have attracted increasing attention due to their unique optical properties, such as upconversion (UC) and near-infrared (NIR) emission. However, these emitters suffer from low efficiency, limiting further practical applications. Herein, a Bi 3+ -alloyed double perovskite, Cs 2 NaEr 0.5 Bi 0.5 Cl 6 , with strong NIR-triggered UC and NIR-II (1000–1700 nm) emission is reported. Cs 2 NaEr 0.5 Bi 0.5 Cl 6 exhibits efficient green UC emission with a high quantum yield up to 2.1% and NIR-II emission peaking at 1541 nm with a quantum yield of 37.9% under 980 nm excitation. The strong UC emission is related with the efficient energy transfer between Er 3+ and self-trapped exciton states induced by corporation of Bi 3+ ions. In addition, Bi 3+ ions can act as diluters, suppressing the energy loss caused by undesirable successive energy transfer among Er 3+ ions, enhancing the UC and NIR-II emission. Furthermore, a water-resistant Cs 2 NaEr 0.5 Bi 0.5 Cl 6 /polydimethylsiloxane film is prepared. Excitingly, Cs 2 NaEr 0.5 Bi 0.5 Cl 6 not only shows the potential as a UC thermometry with high sensitivity but also exhibits a high signal to noise ratio in NIR imaging.