Modeling, design and fabrication of thin-film microcrystalline silicon solar cells

The modeling, design and fabrication of low-cost thin-film microcrystalline silicon (μc-Si) solar cells is studied in this thesis. The cell, considered in this investigation, utilizes low-cost glass as the substrate and microcrystalline Si (μc-Si) as the active layer. A comprehensive refractive index (n) and extinction coefficient (k) model of silicon as function of doping, temperature and wavelength is developed to assist the optical design of the cell. In order to obtain acceptable short circuit current density (Jsc) from the cell, it is found that the thickness of the silicon thin film should be more than 10μm. To get the best light trapping effect, the surface of the cell should be double-side or front-side textured. The density of the texture pits should be as high as possible and the bottom angle of the texture pits should be as small as possible. However, the depth of the texture pits does not have too much influence on the overall performance of the cell.; A model and corresponding software are developed to investigate the electronic properties of the material/device built using low cost μc-Si. This model divides the defect regions inside the material into different categories according to the defect levels in them. Therefore, this model is able to deal with various kinds of defects and defect clusters. The software uses finite element method to solve time-dependent continuity equations with different boundary conditions to get the carrier distribution inside the materials and hence the I-V characteristics of the devices. It is found that, to get satisfying thin film μc-Si thin film cell, the grain size of the film should be about 10μm, and the surface recombination velocities at the grain boundaries should be less than 1000cm/s. The requirement on the minority carrier lifetime is not rigid because of better tolerance of thin-film solar cells to this property.; Some critical fabrication steps in making such a thin film solar cell are also investigated. An Al-involved crystallization/grain enhancement procedure using optical processing is used to get large-grain μc-Si thin films. This process involves both a-Si/Al reaction and Al diffusion inside Si. It can produce μc-Si at temperatures lower than the softening point of low-cost glass within a much shorter duration compared with other crystallization/grain enhancement techniques. Crystallization of Si film can start at temperatures as low as 200°C when Al is involved. However, to get strong crystallization and grain enhancement, the processing temperature should be more than 450°C. At temperatures around 500°C, the crystallization becomes much stronger. The local melting at the Si-Al interface may cause this crystallization.