Defect engineering of inorganic sensitizers for efficient triplet–triplet annihilation upconversion

In hybrid triplet–triplet annihilation upconversion (TTA-UC) systems, the triplet state of a molecular acceptor can be sensitized by an inorganic sensitizer at a longer wavelength. The TTA will generate singlet emission at a shorter wavelength, leading to UC.Triplet energy transfer (TET) from inorganic sensitizer to organic acceptor is one of the critical processes to determine the efficiency of TTA-UC.Vacancies, anti-/interstitial-site atoms, extrinsic doping, ligand exchange, and shell coating can generate defects in sensitizers. They can form different depths of traps to influence the distribution, transfer, and recombination process of charges of the sensitizer.Defect engineering by the selective formation of intrinsic traps, external doping-induced traps, and surface states can mediate TET, lower the threshold, and increase UC efficiency.The nature of defects has yet to be fully understood. There need to be more methods to get controllable defects.