Preparation And Characterization Of CdS And Cu_xTe Thin Films And Their Applications

CdTe poly crystalline thin film solar cells have wildly attracted the people's interest due to the well-known advantages, such as high efficiency, low cost, feasibility to be industrialized fabrication, and have become one of focuses in the research of thin film solar cells. Nowadays, the preparation of CdTe cells with 16.5% efficiencies has been achieved in the laboratory, and the cells with 13.38% efficiencies have been fabricated in our country. A pilot production line with 0.3MW in production capacity has been established. With the increase of industrialization course, there are some further studies to be conducted. The fabrications of window layer and back contact layer for CdTe solar cells are two key procedures. In this thesis, the studies have aimed at the preparations and characterization of the two layers. And the results have been applied in the fabrication of CdTe cells in order to improve the cells performance. There are two aspects in our work, one is the preparation, characterization and applications of CdS polycrystalline thin films, the window layer, deposited with closed-space sublimation (CSS) technology, and the other is the preparation, characterization and applications of Cu_xTe thin films, the back contact layer, with vacuum evaporation technology. The main results obtained during the research are listed below.1. The deposition conditions of CdS polycrystalline thin films have been investigated. And the effect of deposition conditions on the structure, composition, morphology as well as optical, electrical properties of CdS thin films have been studied systemically. We have found that the structure of CdS polycrystalline thin films deposited with CSS were of hexagonal phase and the composition of them were rich in S, and with the increase of oxygen content in deposition atmosphere the chemical component of CdS thin films was similar to the ideal stoichiometric proportion. Then the effect of annealing condition on structure, morphology as well as optical, electrical properties of CdS thin films have been investigated, and a relationship have been set up between the annealing condition and the properties of CdS thin films.2. The structures and optical properties of Cu_xTe thin films deposited with vacuum evaporation have been investigated. We have found that the non-crystalline structures are dominate in Cu_xTe thin films, though the polycrystalline structures of cubic phase are exist in some Cu_xTe with small x value. Then the effects of annealing conditions on structure of Cu_xTe thin films have been studied. The result indicated that the structures of Cu_xTe thin films are dependent upon the variation of x value. Especially, when x=0.99 or 1.25, the structure transformation from non-crystalline to crystalline structure occurred in Cu_xTe thin films, and high crystallinity in these thin films was observed. It is indicated that the crystallization temperature of Cu_xTe thin films with small x value is lower than that of CdTe thin film with large x value.3. The application of CdS thin films have been studied, including the effects of annealing conditions and thickness of CdS thin films on the performance of CdS/CdTe hetero-junction solar cells. The result indicated that the saturated reverse dark current density decreased by one order of magnitude and diode ideal factor was reduced after heat treatment at the temperature range from 350℃to 420℃. Annealed at 385℃, diode ideal factor was smaller than the others. Due to the saturated reverse dark current density and diode ideal factor decreased, the dark current diminished. Therefore, the annealing process had advantage of improving the performance of CdTe solar cells.4. The performance of CdTe solar cells with Cu_xTe buffer layer has been studied. The result showed that the introduction of Cu_xTe buffer layer was favor of eliminating Roll over phenomenon and reducing the contact barrier between Au and CdTe. When x=1.25, the solar cells with Cu_xTe buffer layer have less saturated reverse dark current density and diode ideal factors, and the doping concentration of CdTe layer increased by 14%, and short current density enhanced by 25%. These changes were probably due to the diffusing of Cu into the CdTe layer, which was confirmed with the complex interface states appeared at high annealing temperature because of the over diffusion of Cu.5. The effects of thickness, doping concentration and recombination centre density of CdS thin films on the performance of CdTe cells were simulated. The methods to improve the performance of CdTe cells by optimizing the properties of CdS have been found. Meanwhile, the effects of thickness, doping concentration of Cu_xTe thin films on the performance of CdTe cells were simulated. The results about the effects of doping concentration were not accord with the real performance of CdTe Cells. Then we modified the model by adding a p~+-CdTe layer and simulated the performance of CdTe cells by this model again. The simulated results were consistent with the real performance of cells...