| Summary: | 博士 === 國立臺灣科技大學 === 化學工程系 === 99 === A method that does not employ hot injection techniques has been developed for the size-tunable synthesis of high-quality CdSe quantum dots (QDs) with a zinc blende structure. In this environmentally benign synthetic route, which uses relatively less toxic precursors, solvents, and capping ligands, CdSe QDs that absorb visible light are obtained. The size of the as-prepared CdSe QDs and, thus, their optical properties can be manipulated by changing the microwave reaction conditions. In this approach, the reaction is conducted in open air and at a much lower temperature than in hot injection techniques. The use of microwaves in this process allows for a highly reproducible and effective synthesis protocol that is fully adaptable for mass production and can be easily employed to synthesize a variety of semiconductor QDs with the desired properties.
A strategy with respect to band gap engineering by controlling the composition of CdSe quantum dots (QDs) is reported. After the CdSe QDs are prepared, their compositions can be effectively manipulated from equimolar to Cd-rich and then to Se-rich QDs using an ultrasonic-enhanced chemical bath deposition technique. To obtain Cd-rich CdSe QDs, Cd was deposited on equimolar CdSe QDs. Further deposition of Se on Cd-rich CdSe QDs produced Se-rich CdSe QDs. By changing the QDs composition, the overall optical properties of the CdSe QDs can be manipulated. It was found that as the composition of the CdSe QDs changes from equimolar CdSe to Cd-rich and then Se-rich CdSe, the band gap decreases along with a red shift of UV-vis absorption edges and PL peaks. Clearly, the band gap decreases as a function of composition change from equimolar to Cd-rich and then to Se-rich CdSe QDs. The quantum yield also decreases with surface composition from equimolar to Cd-rich and then to Se-rich, largely due to the changes in the surface state. Because of the involvement of the surface trapping state, the carrier life time also increased from the equimolar CdSe QDs to the Cd-rich to the Se-rich CdSe QDs. We have shown that the optical properties of CdSe QDs can be controlled by manipulating the composition of the surface atoms. The as prepared QDs are characterized by XRD, TEM, EDS, UV-vis, FTIR, time-resolved fluorescence spectroscopy, and fluorescence spectrophotometry. These highly controlled synthesis method, and band gap engineering strategy along with tunable-optical properties via manipulating composition can potentially be extended to other semiconductor nanocrystals.
Following the preparation, colloidal CdSe QDs are assembled on a new architecture of polytetrafluoroethylene (PTFE) framed TiO2 electrode for QDSSC for the first time. CdSe QDs are attached on the surface of the film using linker molecules (3-mercaptopropionic acid, MPA) and absorbed directly without MPA. The as-prepared electrode is composed of a TiO2 compact and a PTFE framed structure layer with average thickness of 2μm and 23μm (to 28μm) respectively. UV-vis absorption spectra show that more CdSe quantum dots are anchored on the surface of TiO2 film modified with MPA than direct absorption. Energy conversion efficiency up to 0.18% can be achieved with a cell prepared from TiO2 (25 μm)/MPA/CdSe QD electrode. Electrochemical impedance measurement shows that the recombination resistance is higher for a cell assembled with TiO2 (25 μm)/MPA/CdSe QD photoanode than TiO2 (25 μm)/CdSe QD resulting in an increase of cell efficiency. Although further investigation and optimization of the new photoanode for QDSSC is needed, the PTFE framed structure along with the compact layer is a new approach for QDSSC application and provides a tunable film thickness and cost-effective preparation for large scale production of the photoanode.
Key words: CdSe QDs, microwave, mass production, oleic acid, band gap, surface modification, composition, chemical bath deposition, Quantum dot-sentisitized solar cell, TiO2 film, PTFE framed, compact layer, structure layer, average life time.
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