| Colloidal semiconductor quantum dots (QDs) are important in fundamental and application research, due to their size-and shape-dependent optical properties and excellent solution-processing chemistry. Quantum dot based solar cells have drawn a lot of attention during past few years because of the possibility of boosting the energy conversion efficiency beyond the traditional Shockley and Queisser limit of32%for Si based solar cells. The development of various chemical synthetic methods for high quality quantum dots will lay a foundation for their industrialized application. At present, the state-of-the-art methods for synthesizing the high quality quantum dots include the organometallic approach and its alternatives. However, some toxic and expensive reagent as raw materials and solvents were used in these methods. Moreover, the key part of these synthetic methods is the injection of room-temperature organometallic precursors solution into well-stirred, hot organic solvents. In the industrial case, the rapid precursor injection and strong stirring is very difficult to achieve. Therefore, the injection-based synthetic method is not suitable for large-scale, industrial preparation of nanocrystals. To overcome this difficulty, new environmentally friendly synthetic methods for high-quality nanocrystals that do not require the injection of precursors and has mild operating conditions with low-cost have to be developed.In this dissertation, we mainly developed greener and lower cost synthesis methods for the large-scale synthesis of II-VI semiconductor QDs, discussed the nucleation and growth mechanism of QDs, and studied the manufacturing technology and the transformation mechanism of the light and electricity about the new inorganic/organic hybrid solar cell based on II-VI semiconductor QDs/conjugated polymer with the aim of providing new way for solar cell based on QDs. The main research results obtained as follows:1. The CdSe magic-sized nanoclusters (MSNs) ensembles in very small size were prepared via a noninjection one-pot approach. Sharp absorption onset, persistent absorption spectrum at given wavelength and narrow full width at half-maximum of absorption spectrum indicated that the three MSNs were single-sized ensembles. In this one-pot approach, N-oleoylmorpholine was used as reaction medium and solvent, cadmium acetate dehydrate as Cd resource, and Se powder or selenium dioxide as Se resource. By changing reaction temperature, reaction time, the feed ratios of Cd to Se, and the kinds of precursors to control the activity of monomer, the three MSNs of CdSe with different magic number were prepared, which were named as families392,461, and511, based on their first absorption peak at the lowest energy in wavelength of nm. When certain long-chain fatty acid was added into reaction system at different temperature to passivate the surface of MSNs, the high-quality CdSe QDs with strong white-light emission could be obtained. The white-light QDs with a maximum quantum yield (QY) up to27%can be stable for at least a2-month during storage period.2. A conventional non-injection one-pot method without nucleation initiators for high-quality CdS QDs was developed by simply mixing cadmium stearate and S powder into new solvent N-oleoylmorpholine. The resulting CdS QDs with observed quantum size-dependent effects possessed narrow size distribution and high crystallinity. The influence of reaction temperature, reaction time, the feed ratios of Cd to S and the concentration of precursor on the size, size distribution and crystallinity of CdS QDs was investigated. The study showed that a lower reaction temperature favored the synthesis of CdS QDs with wider tunable size range and narrower size distribution. A strong absorption peak near308nm, which was attributed to "magic-sized" nanoclusters (MSCs) of CdS as the reaction intermediaries, developed at an early stage of the synthesis of regular CdS NCs. An overlapped nucleation-growth stage followed by a dominated growth stage was observed. Taking into consideration of the properties of the cluster intermediate, we proposed a plausible mechanism for CdS QDs nucleation and growth. A low growth temperature (180℃) and favorable parameters (ratio of2Cd/S and [S] of0.05M) were selected to operate a scale-up synthesis. The obtained CdS QDs had similar spectral properties, size distribution, monodispersity_and crystallinity to these obtained in the small scale batch. A significant PL improvement and a continuous QY increase for the CdS QDs were observed during a long storage time in air and in a glovebox under room temperature. A slow surface reconstruction was proposed to be the cause for the PL enhancement of CdS QDs. The whole operation was carried out in the open air. This low-cost, green, and reproducibly non-injection one-pot method is in favour of large-scale industrial production for high-quality CdS quantum dots (QDs).3. Highly luminescent blue-emitting CdS/ZnS core/shell quantum dots (QDs) were synthesised in N-oleoylmorpholine by two facile steps:first, the CdS core QDs was prepared via a simple one-pot method; second, ZnS shells were successively overcoated on CdS core with a single-source precursor as the shell precursor, whose thickness could be precisely tuned by controlling drip feed speed and amount of shell precursor. The obtained CdS/ZnS core/shell QDs showed the maximum photoluminescent quantum yield of54.8%and narrow spectra bandwidth, exhibiting high monodispersity, good color purity and long fluorescent lifetimes. The CdS/ZnS core/shell QDs with tunable PL emission peak from424nm to470nm could be obtained by tuning the size of cores or the thickness of shells, which are promising materials for blue light-emitting devices. The PL efficiency of QDs passivated with appropriate thickness of shell will greatly enhance. In addition, the water-soluble CdS/ZnS core/shell QDs coated by glutathione was successfully prepared by ligand exchange method. Moreover, the water-soluble QDs still remained the same morphological features, good crystallinity and monodispersity as oil-soluble QDs.4. The stearate-capped CdTe QDs have been first prepared via al ess phosphonic route, in which cadmium stearate was used as Cd precursor, TOP-Te as Te precursor and N-oleoylmorpholine as solvent and reaction medium. It was found that N-oleoylmorpholine not only could readily dissolve precursors cadmium stearate as well as TOP-Te at a relative low temperature, but could disperse the precursor and QDs and control the nucleation and growth of QDs well. The method greatly reduced the use of phosphonic compounds. The as-prepared CdTe QDs exhibited size-dependent optical properties, steep absorbance edge and narrow photoluminescence full width at half maximum (fwhm). The high-resolution transmission electron microscopy (HRTEM) images and X-ray diffraction (XRD) revealed that the highly monodisperse CdTe QDs possessed regular spherical morphology, zinc blende structure, narrow size distribution, and high crystallinity. The investigation results of storage stability also demonstrated that the stearate-capped CdTe QDs had an unexpected good oxidation resistance. Fourier transform infrared transmission spectra (FTIR) confirmed the existence of stearate on the CdTe QDs surfaces.5. It was found that the PL of P3HT was increasingly quenched upon the addition of CdS QDs, indicated that the exciton could be separated rapidly at the donor-acceptor interface. For the first time, the hybrid solar cells based on CdS-QDs/conjugated polyme were fabricated. Herein, the CdS QDs from one-pot method acted as electron acceptors, and the conjugated polymer poly-3(hexylthiophene)(P3HT) was used as an electron donor. The influence of various factors on the device performance was investigated in detail, including the synthesis method, the solvent system, the amount of post-treatment of QDs by pyridine, and ethanedithiol treatment on photoactive layer. The best device showed an Isc of3.97mA/cm2, Voc of0.87V, FF of25%, the power conversion efficiency (PCE) of0.86%under AirMass (A.M.)1.5Global solar conditions. Additionally, the morphology of the photoactive layer treated by ethanedithiol was roughly studied through atomic force microscopy (AFM), field-emission scanning electron microcopy (FESEM) and electron energy loss spectroscopy (EELS). |
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