Synthesis Of Monodisperse Quantum Dots (QDs) Via A Hot-bubbling Route And Its Application In Solar Cells

Semiconductor quantum dots (QDs) has its unique optical properties, which makesQDs promising materials for a variety of applications such as solar cells, opticalbiomarkers, light-emitting devices (LED) and other fields. This dissertation proposes amethod, hot-bubbling, to synthesize oil-soluble quantum dots (II-V, II-Ⅵ family), andfocuses on theirs application in quantum-dots sensitized solar cells (QDSSCs). Themain achievements are as follows:1. A new method named “hot-bubbling” is proposed where the gas-liquid reactionswere carried out in a high-boiling olefins. A series of quantum dots have been preparedwhich include Cd6P7, Zn3P2, CdS and their complex dots (shell@core structuredZnO@Zn3P2, doped Mn:Cd6P7QDs). This strategy has a faster mass transfer due to thepresence of gas reactants, thus combines the advantages of gas-liquid aqeous synthesis(Henglein method) and hot-injection method affording the possibility for highcrystallinity. This method also has its own features of low-cost, possibility of large-scalesynthesis, and applicability for fluidized bed in present industrial equipments. Morethan5.0g luminescent quantum dots (QDs) have been obtained for a laboratory-scalesynthesis. Results showed that the reactions occur at the gas-liquid interface, andsupersaturated regions are easer to be obtained compared to those of hot-injection orHenglein method. The reason lies in the fact that high temperatures were employed, andthe gas reactants were used, which thus provides a rapid nucleation as well as avoidOstwald growth. Synthetic parameters including the gas flow rate and usage of ligandswere studied to control the growth process precisely. Two types of CdS “magic-sized”nanoclusters (MSNs) emitting at313nm and323nm were synthesized at temperaturelower than80°C. The absorption wavelength of CdS QDs was found to be tunable from313nm to467nm. Complex QDs such as manganese doped cadmium phosphide(Mn:Cd6P7) and ZnO@Zn3P2shell@core QDs were also successfully synthesized bythis means. A wider emission with wavelength up to710nm, enhanced photo/chemicalstability, extended fluorescence lifetime (430ns) and improved photocurrent density (8nA·cm-2) are achieved due to the doping effects or shell protection that shares a suitableband alignment at the interfaces.2. Ligand-exchange has been employed to improve the adhensive properties when theQDs were deposited on the substrate electrodes (TiO2mesoporous film). It was found that a pH=10-12is opticmal value for the ligand exchange where the trioctylphosphineoxide (TOPO) were replaced by mercaptopropionic acid (MPA), and an aqueoussolution of QDs was prepared. The uniform QDs derived TiO2films were prepared bydrop-casting the aqueous QDs solution on an electrode. The QDSSCs were fabricatedby using QDs as sensitizers. The photovoltaic performances have been studied. It wasfound that the co-sensitized SCs show higher efficiency (η=1.55%) than thesingle-component (CdS MSNs) sensitized SCs (η=0.53%) or the CdS regular QDssensitized SCs (η=0.98%). The incident photon-to-current conversion efficiency ofco-sensitized SCs exhibis two peaks at370nm and460nm which resemble theabsorption peaks of the two different sized clusters. This demonstrates that a broadspectral response can be obtained when two different sizes of CdS QDs are used tosensitize the solar cells. When Mn:Cd6P7QDs were used as sensitizers, the photon tocurrent efficiency doubles compared to the counterpart sensitized with the undoped QDs(Cd6P7). It is indicated that the manganese ions are favourable for transferring the photoelectrons to the out circuit due to the midgap states and the long lifetime created by Mndoping.