Research And Preparation Of High Efficiency CdTe Thin Film Solar Cells

CdTe is a II-VI direct band-gap compound semiconductor. It’s one of the main members of the second generation high efficient low cost solar cells, due to its nearly ideal band structure and high absorption coefficient. Currently CdTe has the highest module efficiency of thin film solar cells. It was reported that a new efficiency record as high as18.7%for small-area and16.1%for commercial-scale modules of CdTe thin film solar cell have been achieved recently. This achievement expands its application prospect and enhances the competitiveness of CdTe solar cell greatly. However, a lot of problems must be solved to obtain high energy conversion efficiency. The main structure of CdTe thin film solar cells is an heterogeneous p-n junction formed by CdS and CdTe films, and both of which are polycrystalline thin films and have high crystalline defects than the corresponding single crystals; the high work function of CdTe makes it rather difficult to prepare an ohmic contact with a CdTe film; CdTe has very low carrier concentration and is very difficult to be doped; the diffusion of doping elements will seriously affect the stability of CdTe solar cells. This thesis is specifically focused on how to prepare high efficient CdTe thin film solar cells and also carried out some research about the mechanism of CdTe thin film solar cells.In Chapter I, we first discussed the application prospect and the industry status quo of photovoltaic industry. Then we introduced the basic principle and the performance analysis of solar cells. We reviewed the development of CdTe thin film solar cell and had a discussion on the main structure and principle of CdTe thin film solar cell in recent research stage.In Chapter Ⅱ, we carried out researches on film preparation and the heat treatment process. The research on chemical bath deposition of CdS film improved both the film quality and the operability of experimentation. It enabled us to prepare high quality CdS films of nearly any thickness. Some researches were focused on the main factors that affect the deposition of CdTe films with close-spaced sublimation method. CdTe films with uniform and smooth surface morphology were prepared, taking into account the need for device fabrication. A vapor CdCl2heat treatment system and process was successfully established. This process can significantly speed up the re-crystallization of CdS thin films, and can effectively prevent excessive oxidation on the surface. The CdS film treated with this method has an excellent morphology with uniform densely compacted and large scale mono-grain layer. Duo to the high quality precursor CdS film and the effective treatment process, we were able to prepare high quality p-n junction while the thickness of windows layer was reduced to only80nm. This laid the groundwork for the preparation of high efficient CdTe thin film solar cells. The research on CdTe film treatment with vapor CdCl2was also carried out. We found that the inter-diffusion of CdS and CdTe films was greatly promoted because of high CdCl2vapor pressure, and an n-type CdSxTe1-x was formed because of the inter-diffusion reaction. Through the optimization of the heat treatment conditions, preliminary results were achieved for CdTe cells with CdTe films treated by vapor CdCl2.In Chapter Ⅲ, the research was focused on the back contact preparation of CdTe solar cells. We first studied the etching process of CdTe film with mixed nitric and phosphoric acids (NP). We found that the Te-rich surface of CdTe films etched by NP solution can effectively reduce the barrier of the contact even through there was no doping of Cu. We also systematically studied the doping of Cu and the stability problems of cells because of the diffusion of Cu. We found that a proper doping of Cu with controlled amount will increase the carrier density, and improve the fill factor and open current voltage. But as the doping or diffusion was increased, the films would be doped with increased compensation. The resistivity of films will increase and then reduce the fill factor of cells. At last when there was too much Cu in CdTe film, a lot of Cu would gather at grain boundaries, and shunt the solar cell. A variety of back contact was fabricated to form an ohmic contact with CdTe films, such as: carbon paste, Cu-Au alloy and MoO3buffer layer. Because of the high quality precursor films, effective CdCl2treatment and specially controlled doping of Cu, we finally achieved high efficiencies of13.0%,14.6%and14.2%with the three back contacts, respectively.In Chapter Ⅳ, we systematically analyzed the Ⅰ-Ⅴ curves measured under high temperatures, and did researches on cells with different distribution of Cu at varied temperature (Ⅰ-Ⅴ-T). The influences of series resistance, parallel resistance and ideal factor and reverse saturation current on Ⅰ-Ⅴ curves were studied through mathematical modeling. Through the Ⅰ-Ⅴ-T measurement, we studied the conductive mechanism and the stability of cells due to the diffusion, concentration and defects of Cu. Through the analysis of variation of open-circuit voltage with temperature, we found that when the cells were measured in a lamped condition the dominant recombination mechanism was the recombination in the space charge region and at the metallurgical interface of the n layer and p layer. Through the analysis of reverse saturation current, we verified the widespread existence of a deep trap level in CdTe thin film.