| The aqueous colloidal synthesis of semiconductor nanocrystals is an advantageous alternative to the widely used organometallic route. In comparison to the latter, the aqueous method (1) employs the most widespread environmentally friendly and biocompatible solvent,(2) is usually under relatively low reaction temperature and easy operated,(3) provides various functionalizations of the nanocrystals by applying an appropriate capping lignd which in turn may be further functionlized by electrostatic of colvalent linking,(4) permits an efficient shape control of nanocrystals in water route.Chalcogenide semiconductor nanocrystals, as a class of widely used semiconductor nanoparticles, have attracted people’s attention in physical, chemical, optoelectronic, biomedical areas. Excitation and emission of the quantum dot are highly tunable, because the size of the crystals can be controlled during synthesis process. Meanwhile the advantages of quantum dots such as narrow fwhm (full-width at half maximum), high quantum efficiency, and excellent dispersion have attracted extensive interest due to potential application in the biological labeling, photovoltaics, photocatalysis and so. In order to further optimize the preparation process of ZnSe quantum dots(QDs) and to improve their fluorescent properties, in this paper, we prepared ZnSe colloidal QDs and ZnSe/ZnS colloidal QDs in aqueous route, and ZnSe QDs as core martials were fully purified before shell epitaxial overgrowth. Compared to traditional aqueous solution method, this method can avoid hybrid structure formation of ZnSeS QDs for shell directly growth in ZnSe precursor solution. Meanwhile, ZnSe:Eu/ZnS quantum dots(QDs) were successfully synthesized by two-step wet chemical method:nucleation-doping and epitaxial shell-growing. The influences of different shell thicknesses and Eu doping concentrations in ZnSe/ZnS core-shell QDs on Eu sensitization enhancement by energy transfer were systematically investigated. The prepared ZnSe QDs and ZnSe/ZnS QDs employed as luminescent down-shifting layer, were spin-coated onto the upper surface of manufactured Si solar cells. The main results obtained are listed as follows:1. In aqueous solution, monodisperse and highly luminescent ZnSe colloidal QDs and ZnSe/ZnS colloidal core/shell QDs were synthesized. When the reaction time is30min, the photoluminescence(PL) spectrum and absorption spectrum of ZnSe QDs exhibited a strong near band-edge emission (NBE) peak centered at399nm and the first excitonic absorption peak is at393nm. The position of NBE peak in the room-temperature PL spectra showed a red-shift ranging from399nm to416nm with reaction time increasing from0.5h to5h. When reaction time increased to5h, ZnSe QDs were into Ostwald mature stage and the PL intensity is strongest. The size of ZnSe QDs can be controlled through different reaction time. The ZnSe QDs exhibit excellent quality and high PL intensity with2monolayer(ML). A20nm red-shift was appeared when the shell thickness increased from OML to4ML. The reason for this red-shift was variation of exciton binding energy induced by leakage of electron wavelength into shell region. Within the framework of effective-mass approximation, exciton states confined in ZnSe QDs and ZnSe/ZnS QDs are investigated. Our theoretical results are in agreement with the experimental measurements2. Eu-doped in ZnSe:/ZnS quantum dots (formed as ZnSe:Eu/ZnS QDs) were successfully synthesized by two-step wet chemical method:nucleation-doping and ZnSe/ZnS core/shell quantum are studied in details by means of photoluminescence (PL), spectrum photoluminescence excitation spectra(PLE) and time-resolved PL spectroscopy. The emission intensity of Eu ions is enhanced and that of ZnSe QDs is decreased, implying that energy was transferred (ET) from the excited ZnSe host materials(the donor) to the doped Eu ions(the acceptor). The PLE reveals that the ZnSe QDs act as an antenna for the sensitization of Eu ions through an energy transfer process. The dynamics of ZnSe:Eu/ZnS core/shell quantum dots with different shell thicknesses and doping concentrations are studied via PL spectra and fluorescence lifetime spectra. The maximum phosphorescence efficiency is obtained when the doping level of Eu is approximately6%and the sample showed strong white light under UV lamp illuminated. By surface modification with a ZnS shell layer, the intensity of the Eu related PL emission is increased approximately three times compared with that without modification. The intensity ratio of band edge emission of ZnSe QDs (IB) and characteric emission of Eu ions(I613) obtained from emission spectra is consistent with the theoretical calculation based on energy transfer dynamic model and lifetime measurements.3. Here ZnSe quantum dots (QDs), prepare in aqueous route method and employed as luminescent down-shifting layer, were spin-coated onto the upper surface of manufactured Si solar cells. ZnSe QDs film exhibited porous structure, which can effectively enhance the absorption of incident light. Measurements under standard test conditions (AM1.5,100mW/cm2) show that the efficiency of ZnSe QDs/Si hybrid solar cell is increased from11.48%to12%with100nm thickness of ZnSe QDs film. The improvement of ZnSe QDs/Si hybrid solar cell is ascribed to efficient down shifting process of ZnSe QDs, which enhances spectra response (400nm-600nm) for Si solar cell. The mechanism of this optical coupling and efficiency enhancement is investigated in detail. We employed ZnSe/ZnS QDs with high resistant ability for photobleaching and high quantum efficiency as down-shifting materials to form ZnSe/ZnS QDs-Si hybrid solar cell. The short circuit current density of the cells increased from26.67mA/cm2to30.2mA/cm2, and EQE is increased from31%to39%in the UV wavelength region. All these results indicate that ZnSe QDs and ZnSe/ZnS QDs are promising down-shifting materials for application in Si-based solar cell and CdTe thin film solar cell... |
PDF Full Text Request