Dark Singlet Exciton Sensitized Triplet Energy Transfer Across the CsPbBr3 Nanoplate‐Organic Interface

Abstract Semiconductor nanocrystals (NCs) sensitized triplet energy transfer (TET) provides a promising alternative for achieving efficient triplet‐triplet annihilation up‐conversion (TTA‐UC). However, the study on the role of NC's exciton fine structure in TET, which is important to further enhance the TET efficiency, is still limited. Herein, the respective contribution of bright/dark exciton to TET using 1‐naphthoic acid (mediator) decorated 2 nm thick CsPbBr 3 nanoplates as the model system is studied. The well‐characterized and unique exciton fine structure and large bright‐dark splitting energy of CsPbBr 3 nanoplates provide an ideal platform to unambiguously explore the TET between dark singlet exciton and mediator. By using time‐resolved temperature dependent optical spectroscopy, the direct participation of dark singlet state in the TET process is proved and also point out that the dark singlet exciton to mediator TET is the major energy transfer pathway, which is due to the ill‐defined nature of exciton's spin in strongly confined nanoplates and dark exciton's long intrinsic lifetime. Finally, in combination with 2,5‐diphenyloxazole as annihilator, mediator decorated CsPbBr 3 nanoplates can convert visible to ultraviolet light with a maximum photon gain of 0.75 eV under a low threshold of ≈0.76 W cm −2 .