Design And Property Investigations Of New High-efficiency Solar Cells

Solar energy is an important renewable energy resource. Solar cells, which can convert solar irradiation to electric power, is clean, pollution-free, and important to alleviate the increasingly serious energy crisis and environment pollutions. Since the energy crisis in 1970s, people have paid great attention to solar cells. The technology and applications of solar cells have been developed rapidly, accompanying the launching of various well-known projects such as,"Million Solar Roofs Initiative"in USA,"Sunlight Program"in Japan,"Hundred Thousands of Roofs Plan"in Germany, and"Light Works of No- Electric Rural of Western Provinces and Autonomous Regions"in China.In this thesis, based on in-depth understanding of the basic concepts, theories and development trend of solar cells, detailed studies on the physical model, structure and performance parameters of most widely used silicon-based solar cells have been carried out by simulation.Conventional process of crystalline silicon solar cell is simple and suitable for mass production, so they have occupied the vast majority of current commercial market. However, their photo-electrical conversion efficiency is limited by the process of diffusion and metallization. The diffusion process normally requires low-doping in order to reduce the carrier recombination, while the metallization process requests heavy-doping to form good ohmic contacts. Therefore, a heavy doping, which makes the surface sheet resistance reaching about 40Ω/ , is usually required. As a result, the conversion efficiency of photovoltaic solar cells are greatly limitted. Selective-emitter technology, with high-concentration dopant in electrode-contacts region and low concentration doping in the light absorption region, can overcome the above problem. The core of selective-emitter is heavily/lightly-doped emitter zone in the cell surface. In this thesis, detailed study and analysis of the typical selective emitter solar cell were performed. Using a two-dimensional model established by TCAD software MEDICI, we investigated effects of cell parameters on its performance, and predicted the influence of emitter-depth, doping concentration. Besides, we designed a high-performance selective emitter solar cell. By optimizing the model parameters, the conversion efficiency of the solar cell has been significantly improved. Even without the texture structure in the front surface, its maximum conversion efficiency reaches about 19.16%, exceeding that of conventional and current high-efficiency silicon cells (16.5% and 18.8%, respectively).Amorphous silicon (a-Si) thin film solar cells, which can be processed under low temperatures and on a variety of low-cost large-area substrates, have a great cost advantage. However, their low photoelectrical conversion efficiency and S-W effect, as well as their internal structural defects, make it difficult to understand the mechanism and result in many application limitations. In this work, we studied the relationship between the structure parameters and the performance of a-Si solar cells by using one- and two-dimensional simulation software (AMPS-1D and MEDICI, respectively). By setring up a model consistent with the experiment reports in MEDICI and comparing several a-Si defect models, a good model in better agreement with the real situation was proposed. Furthermore, we systematically optimized and improved the structure parameters of the model (including the active regions, ITO, etc), increasing the conversion efficiency from 13% to nearly 18%. Then, starting from the single junction a-Si solar cell, solar cells with the tandem structure were proposed and studied. The conversion efficiency reaches more than 15%, exceeding the high level recently obtained from triple-junction cells, which is about 14.6%.In summary, reducing defects in materials, decreasing carrier recombinations and increasing the carriers'lifetime, are the key points to enhance the cell performance greatly. The fact that our simulations are constructed on the basis of comparisons with experiments, makes the structure optimization and performance prediction relatively believable. Thus the results could serve as useful guidance and reference for practical fabrications of new high-efficiency and low-cost solar cells.