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 (Phi(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.