Number of Surface-Attached Acceptors on a Quantum Dot Impacts Energy Transfer and Photon Upconversion Efficiencies

Composites of organic molecules and inorganic quantum dots (QDs) have emerged as attractive photon upconversion systems that use triplet–triplet annihilation upconversion (TTA-UC). However, the upconversion efficiency of such systems is still far from reaching their theoretical potential. The number of acceptor molecules directly coordinated on a QD (n) should determine triplet–triplet energy transfer (TTET) efficiency (Φ_TTET), which consequently affects the efficiency of TTA-UC, but the research focusing on the n value has been limited. In the present report, the effect of n on TTET from CdSe or CdTe QDs to perylene-3-carboxylic acid (Pe; i.e., acceptor) were systematically investigated. The TTET and TTA-UC efficiencies increase with increasing n. The regulation of n on a QD could provide a straightforward means to realize high-performance TTA-UC. For the molecule/QDs systems, small QDs with a wide band gap are favorable for intrinsic TTET (i.e., TTET in a one-to-one QD-Pe composite system), because intrinsic TTET efficiency is detemined by the triplet energy of QDs. On the other hand, the small QDs limit the n due to the small surface area. Therefore, the proper choices of QDs and acceptors providing both sufficient free energy change for TTET and large n are important to achieve efficient TTA-UC.