Employing Core‐Shell Quantum Dots as Triplet Sensitizers for Photon Upconversion

A new family of surface‐functionalized CdSe/ZnS core‐shell quantum dots (csQD) has been developed, which work as triplet sensitizers for triplet‐triplet annihilation‐based photon upconversion (TTA‐UC). The surface modification of csQD with acceptor molecules plays a key role in the efficient relay of the excited energy of csQD to emitter molecules in the bulk solution, where the generated emitter triplets undergo triplet‐triplet annihilation that leads to photon upconversion. Interestingly, improved UC properties were achieved with the core‐shell QDs compared with core‐only CdSe QDs (cQD). The threshold excitation intensity, which is defined as the necessary irradiance to achieve efficient TTA process, decreases by more than a factor of four. Furthermore, the total UC quantum yield is enhanced more than 50‐fold. These enhancements should be derived from better optical properties of csQD, in which the non‐radiative surface recombination sites are passivated by the shell layer with wider bandgap.