DOI: 10.1002/cphc.201101005 Design of a Water-Soluble Hybrid Nanocomposite of CdTe Quantum Dots and an Iridium Complex for Photoinduced Charge Transfer Yu Wang, [a] Steve Li, [b] Stephen V. Kershaw,* [a] Frederik Hetsch, [a] Anthony Y.Y. Tam, [c] Guangcun Shan, [a] Andrei S. Susha, [a] Chi-Chiu Ko, [b] Vivian Wing-Wah Yam, [c] Kenneth K. W. Lo,* [b] and Andrey L. Rogach [a] 1. Introduction Interest in charge recombination in semiconductor-quantum- dot (QD) based nanostructures has intensified in recent years due to their many potential applications in light-emitting and photovoltaic devices. [1–5] Multiple exciton generation (MEG) by one absorbed photon in some QDs [6–8] provides exciting possi- bilities to improve the conversion efficiencies of QD-based solar cells by decreasing the loss of high-energy carriers. [9] However, it is necessary for application of the MEG process to dissociate the exciton by ultrafast charge transfer to electron donors and acceptors before the exciton–exciton annihilation process in photoexcited QDs, which occurs on the 10 to 100 ps timescale. [10] There have been a few reports of ultrafast exciton dissociation in QDs by electron transfer or by hole transfer to adsorbed acceptors [11–15] along with a series of re- ports where energy transfer in inorganic–organic nanocompo- sites has been found to be dominant. [16–18] However, for photo- voltaic-related applications of QDs, [5] efficient charge separa- tion still remains a challenge owing to poor understanding of the mechanism and rate of charge transfer. [19–23] Recently, con- siderable progress has been made in the synthesis and design of type II heterojunction nanostructures by combining QDs with luminescent metal complexes to spatially separate photo- excited electrons and holes. [14, 15] Klimov and co-workers [15] re- ported photoexcitation of QD semiconductor sensitized Ru complexes with efficient hole transfer from the nanocrystals to the absorbed complex with a 5 ps time constant. Lian and co- workers [14] reported that excitons in the CdSe QDs dissociate by electron transfer to the adsorbed Re-bipyridyl complex, and the exciton dissociation half-time is about 2.3 ps in QDs within the first exciton band. For the Ru-polypyridine dyes [15] (in ben- zonitrile organic solvent), the physical coupling between the dye and quantum dot makes use of the fact that carboxylate functional groups inserted on the heterocyclic dye rings show some ligand-like affinity for the QDs, and a short time after mixing the authors observe evidence (not least the quenching of dye and QD luminescence) that the dye has started to bind at QD surfaces. They report improved charge transfer for dyes with multiple carboxylate moieties. In our work, we have made our Ir-dye water soluble and moreover by conjugating to a poly-ethylenimine water-soluble polymer we ensure good dispersion of the dye throughout the globules of polymer in solution. In our case, the binding interaction between QDs and dye/polymer conjugate is via amine groups which also are known to have an affinity for the CdTe QD surface. Although our present paper does not address hot-carrier ex- traction in the context of QD photovoltaic solar-cell design, many other groups are addressing the challenge of efficient carrier transfer and attempting to improve the physical cou- pling of QDs with other parts of their device structures (e.g. wide-bandgap-semiconductor substrates or molecular dyes) to facilitate efficient (and perhaps eventually fast and hot) carrier transfer. The SILAR method (successive ionic layer adsorption We report the use of an organo-iridium dye conjugated with a water-soluble copolyethylenimine polymer, allowing the hybrid material to be used in combination with thioacid- coated CdTe quantum dots in an aqueous medium. When they are combined, hot carrier cooling observed in the pure quan- tum-dot case is heavily suppressed indicating fast (ps) electron transfer on a timescale that competes with non-radiative (Auger) relaxation. [a] Y. Wang, Dr. S. V. Kershaw, F. Hetsch, G. Shan, Dr. A. S. Susha, Prof. A. L. Rogach Department of Physics and Materials Science and Centre for Functional Photonics (CFP) City University of Hong Kong (Hong Kong SAR) Fax: (+ 852) 3442-0538 E-mail : skershaw@cityu.edu.hk [b] Dr. S. Li, Prof. C.-C. Ko, Prof. K. K. W. Lo Department of Biology and Chemistry Centre for Functional Photonics (CFP) City University of Hong Kong (Hong Kong SAR) Fax: (+ 852) 3442-0538 E-mail : BHKENLO@cityu.edu.hk [c] A. Y. Y. Tam, Prof. V. Wing-Wah Yam Institute of Molecular Functional Materials Areas of Excellence Scheme University Grants Committee (Hong Kong) and Department of Chemistry University of Hong Kong (Hong Kong SAR) ChemPhysChem 0000, 00, 1 – 8  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim &1& These are not the final page numbers! ÞÞ