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All-Quantum Dot Multilayer Using Layer-by-Layer Assembly Methods in Quantum Dot-Sensitized Solar Cells

All-Quantum Dot Multilayer Using Layer-by-Layer Assembly Methods in Quantum Dot-Sensitized Solar Cells
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Quantum dots (QDs)-based solar cells have received great attention as a cost-effective and highly efficient alternative to conventional solar cells. In addition, QDs are considered to have potential for opening a new way to utilize hot photogenerated carriers or for generating multiple charge carriers with a single photon. This thesis describes the formation of multilayer quantum dots assembly based on layer-by-layer technique and their application for quantum dot-sensitized solar cells (QDSSCs). Two different layer-by-layer (LBL) assembly methods were applied for multilayer assembly, one is based on electrostatic interaction and the other is based on metal ligand coordination. For the electrostatic LBL assembly of quantum dots, strong polyelectrolyte quantum dots were used as assembly building units. The strong polyelectrolyte quantum dots surfaces with sulfonates or quaternary ammoniums can endow quantum dots with excellent colloidal stability independent of the pH and ionic strength and can be exploited to achieve stable and flexible electrostatic LBL assembly. To maximize the absorption of incident light and the generation of excitons by CdSe QDs within a fixed thickness of TiO2 film, a multilayer of CdSe QDs was prepared on the mesoporous TiO2 film by electrostatic LBL assembly and the experimental conditions of QD deposition were optimized by controlling the concentration of salt and repeating the LBL deposition a few times. A double-layer assembly of QDs using the electrostatic interaction is exploited for the study of Förster resonance energy transfer (FRET) in QDSSCs. Donor (overcoated CdSe/CdS/ZnS QDs) and acceptor QDs (bare CdSe QDs) are prepared with oppositely charged surface ligands. They showed energy transfer with FRET efficiency of 76% from the photoluminescence decay lifetime of donor QDs which dramatically decreased in films with acceptor QD layer. The double-layer of QDs based on energy transfer improved light harvesting by funneling photons energy from the donor QD layer to the acceptor QD layer through non-radiative transfers of the excited state energy in QDSSCs. For the metal-ligand coordination-induced layer-by-layer assembly of quantum dots, metal chalcogenide complexes ligand modified quantum dots were used as assembly building units. Before applying metal-ligand coordination-induced assembly in QDSSCs, it was needed to replace mesoporous TiO2 to aligned 1D nanostructures since small pore size of mesoporous TiO2 make infiltration of colloidal quantum dots into mesopore structure difficult and ineffective and are easily blocked while depositing QDs. In addition, the vertically aligned 1D nanostructures, ZnO nanowires, suggested to improve electron transport by avoiding the particle-to-particle hopping or percolation through a random polycrystalline network usually happened in the mesoporous TiO2 network. The overcoating shell, TiO2, could also improve overall of photovoltaic properties reducing the recombination processes by the formation of an energy barrier at the core oxide surface or passivate recombination centers on the core oxide surface. Molecular metal chalcogenide complexes could serve as capping ligands for QDs and provide colloidal stability of QDs. The outward terminal sites of metal chalcogenide complexes were still able to participate in metal-ligand coordination. As a result, the quasi all-inorganic multilayer assembly of QDs was successfully formed using metal-ligand coordination between metal chalcogenide surface ligand and metal ion on ZnO/TiO2 core/shell nanowires film. The metal chalcogenide surface ligand-metal ion-metal chalcogenide surface ligand bridges was expected to behave as a highly conductive continuous channels between QDs. It would result in the decrease of interparticle spacing and the enhancement of electronic coupling and electron transportation efficiency between QDs.
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