Wave-Function Engineering of CdSe/CdS Core/Shell Quantum Dots for Enhanced Electron Transfer to a TiO2 Substrate

In this paper, we have synthesized a series of core/shell quantum dots (QDs) for the purpose of enhancing the electron transfer from the dots to a TiO2 substrate. We make use of the fact that CdSe is a small-bandgap material compared with CdS; therefore, in a common CdSe/CdS core/shell QD, the photo-excited electron is confined deeply in the CdSe core. By special construction of the CdS/CdSe core/shell QDs, referred as reversed type-I, the electron wave function will distribute largely in the shell region. This facilitates the transfer of the electron from the QD to the TiO2 substrate, resulting in significantly improved electron-injection efficiency. Such an enhanced electron-injection efficiency was confirmed by fluorescence lifetime decay measurements, showing the largest lifetime reduction after that the QDs were adsorbed on the TiO2 surface. The reversed type-I CdS/CdSe QDs show a much higher photon-to-current conversion efficiency than type-I CdSe/CdS and CdSe QDs without shell. Furthermore, by chemical-bath depositing of CdS on the QD-sensitized electrode to form a quantum-well structure, the electron recombination between the QDs and the redox couple was reduced, hence further enhancing the electron-injection efficiency. The absorbed-photon-to-current efficiency of the quantum well CdS/CdSe/CdS sensitized solar cells reaches a value as high as 60%.