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

Abstract 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.