A Sonochemical Approach To Synthesis Of CdS Quantum Dots For Quantum Dot Sensitized Solar Cells

The preparation of quantum dots have received a great concern in quantum dot sensitized solar cells, and the primary method is successive ion layer adsorption reaction(SILAR). Howerer, SILAR selects a strong ionized precursor solvent that may lead to an aggregation on the top surfaces of rods and requires a time-consuming rinsing operation, as well as poor reproducibility. So in this academic dissertation, sonochemical synthesis of CdS QDs was introduced into the fabrication of CdS/ZnO core-shell nanostructure in QDSSCs and the optimal ultrasonic time was investigated. Furthermore, ZnO surface passivation and annealing process were explored in order to improve the performance of solar cells. The results obtained were as follows:(1) Based on ZnO seed layer, ZnO nanorod arrays were prepared by a simple hydrothermal approach, and CdS/ZnO core-shell nanorod arrays fabricated by sonochemical synthesis method. The microstructures were characterized by FESEM, EDX and XRD analyses. The results presented that well-aligned Zn O nanorod arrays were prepared and showed a hexagonal wurtzite structure and a preferential orientation along the c-axis. CdS were generated and showed a cubic phase crystal structure.(2) The annealing temperature of seed layer and ultrasonic time were researched to achieve proper parameters. The results showed short circuit current density increased with annealing temperature of seed layer, but photoelectrochemical performance got worse. Because the increase of naonorod arrays density enhances recombination dynamics between photo-generated electrons and electrolyte. Meanwhile, it is observed the thickness of CdS shell increased with ultrasonic time, and absorption of visible light was enhanced. However, the power conversion efficiency enhanced before 10 min due to the increase of light harvesting efficiency, and decreased after 10 min due to the recombination.(3) In order to improve the photoelectrochemical performance, ZnO/ZnS nanostructure was prepared to research the passivation of ZnO surface, and annealing progress was used to study the performance of solar cells. The results showed that the fill factor and the power conversion efficiency was enhanced, due to the surface defects passivation of ZnO by ZnS and suppression of recombination. For annealing process, it is can be observed that power conversion efficiency showed dropping trend after ascendant, and achieved a best in a certain temperature. Annealing treatment improved the photoelectrochemical performance because the crystal structure of CdS changed from cubic to hexagonal phase, and crystal surface binding properties were enhanced. So crystal surface defects and interfacial combination were decreased. Meanwhile the transmission spectrum had a red shift increasing the absorption of visible light.