الفهرس 
Chapter I Introduction and literature ReviewOnly 14 pages are availabe for public view
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Abstract
The World’s conventional energy resources (oil, natural gas, and coal) have shown signs of deficiency beside their negative impact on the environment. Therefore, searching for renewable energy resources is taking place. One of the most prominent clean energy resources is the sun, which is a clean and infinite external energy source.
As the solar cell is a device used to convert solar energy into electricity directly by the photovoltaic effect. In 1954, the first solar cell made of silicon was developed. Solar cells can be classified into different types such as thin-film cells, nanocrystalline, organic cells and so on. Although the highest efficiency of solar cells is around 40% in the Lab; the cost of its production is still high and the technology is not commercially available. As a result, low-cost solar cells (such as Dye-Sensitized Solar Cells (DSSCs)) have been studied extensively to get low-cost cells with considerable efficiency. DSSCs have many advantages such as inexpensive material cost, easy fabrication, and relatively high efficiency.
A typical DSSC consists of a working electrode (photoanode) which is a transparent conducting oxide (TCO) substrate covered with a film of dye-sensitized semiconductor oxides, a counter electrode and an electrolyte which is sandwiched between the two electrodes. Upon light irradiation, dye molecules are photoexcited and the excited electron is then injected into the conduction band of the semiconductor oxide. Then, the injected electron migrates to the TCO of the photoanode. Afterward, the electron reaches the counter electrode after performing electrical work on the way. The electron is then transferred to the electrolyte where it reduces the oxidant species. Subsequently, the original state of the dye is restored by electron donation from the reduced species in the electrolyte to complete the circuit. Continuous efforts have been invested around the world to enhance the efficiency of DSSC.
In this work, we prepared and characterized the performance of ZnO nanoparticles films employed as the working electrodes of dye-sensitized solar cell (DSSCs) devices. Then, we investigated the effect of using ZnS as a surface modifier on the performance and efficiency of the manufactured DSSCs. This was carried out by measuring the parameters of the solar cells under simulated solar radiation.
The thesis consists of three chapters:
Chapter I: Introduction and literature review
This chapter contains an introduction to the history, structure, operation of DSSCs, etc. and a literature review.
Chapter II: Experimental Work
ZnO nanoparticles photoelectrodes adsorbed with different SILAR cycles of ZnS QDs were prepared. The crystal structure of the obtained ZnO photoelectrodes were carried out by using XRD, and SEM. UV-VIS spectroscopy was used to evaluate the optical transparency of the ZnS QDs adsorbed on the ZnO nanoparticles at different SILAR cycles n. The electrical characteristics of DSSCs were achieved by current density-voltage (J-V) measurements, in addition to the open-circuit voltage decay (OCVD) was measured to estimate electron lifetime.
Chapter III: Experimental Results: Discussion and conclusion
In this chapter, the Zinc Sulfide quantum dots (ZnS QDs) were chosen as a surface modifier for the ZnO photoanode. The obtained X-ray Diffraction (XRD) spectra were studied and confirm the existence of the wurtzite hexagonal type structure of ZnO with crystallite size equal to 24 nm and lattice parameters: a=3.205Å and c=5.122Å. A wide hump was observed for all X-ray diffraction spectra, which corresponds to the hexagonal phase ZnS and it’s broadening implies a small particle size with an average diameter equal to 6 nm.
The top-view Scan Electron Microscope (SEM) images were used to study the surface morphologies of the ZnO nanoparticles adsorbed with different SILAR cycles of ZnS QDs. The study of the obtained top-view Scan Electron Microscope (SEM) images confirm that the ZnS QDs sit in the porous structure of ZnO nanoparticles at its surface, and the size of the gaps between the nanoparticles was decreased by increasing the number of SILAR cycles.
The results of the UV-VIS spectroscopy illustrate that the adsorption of ZnS QDs reduces the optical transparency.
Results of the photocurrent density-voltage curves of DSSC with different SILAR cycles n of ZnS QDs on the surface of ZnO nanoparticles photoelectrode; indicates that the Voc shifts slightly towards higher values by increasing the SILAR cycles n from that of the reference photoelectrode with n=0. The highest value of Voc was achieved for photoelectrode with n=5 as an optimum value with 7% higher than that for photoelectrode with n=0.
The high ability of ZnS to stay on the surface of ZnO, even the trap state on the ZnO surface can be repaired, leading to the amount of adsorbed dye molecules on the ZnO electrode decreased compared with that of the ZnO electrode with n=0, indicating that the modification of the photoelectrode surface could reduce the light-harvesting efficiency (LHE). Therefore, in order to minimize the reduction of light-harvesting efficiency (LHE), it is necessary to reduce the amount of ZnS on the ZnO surface with a critical value.
from the results of The Open Circuit Voltage Decay (OCVD); it was found that the electron lifetimes in the DSSC with n=5 were longer than those in the reference device with n=0, which indicated that recombination in the DSSC with the ZnS-modified electrode was retarded, and thereby induced the prolonged electron lifetime, so as this enhancement with electron lifetime contributed to the increase in the Φcol value. It seems that the reduction in LHE overrides the improvement in the electron collection efficiency Φcoll and thus the Jsc value was decreased by ZnS modification. The results also indicate that ZnS QDs is an effective material for Voc improver in DSSCs.