| dc.description.abstract | Currently, the effects of physical properties on silicon-based thin film solar cells have been extensively researched as well as numerous investigation results have yielded a considerable amount of information about hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (µc-Si:H) solar cells. In this thesis, we focus on the electrical performances of several structural parameters and two-dimensional device modeling for two types p-i-n thin film solar cells which utilize a-Si:H and µc-Si:H as the material of i-layer (i.e. active layer) are carried out by using Silvaco TCAD simulation program.
First, we demonstrate a a-SiC:H/a-Si:H/a-Si:H thin film solar cell. Some characteristic behaviors are influenced by various parameters, such as the thicknesses, the doping concentration, and the density of states. The optimal efficiency of 9.92% is achieved with thickness of 40/300/10 (nm) and doping concentration of 1×1018/1×1017/1×1020 (cm-3). Nevertheless, from the results of the cells with different i-layer thickness and density of dangling bond states, we can conclude that the defect density is thickness dependent in poor quality samples. The light-induced degradation effect occurs more obviously in the thick samples. As a result, using a cell with thinner active layer thickness will yield better performance with poor quality materials.
µc-Si:H is a complex material composed of microcrystal grains in an amorphous matrix plus voids/cracks with crystal grain sizes smaller than 20-30 nm. A modulated crystalline volume fraction model in µc-Si:H thin film solar cells is established by consideration of the columnar crystal growth, which can be regarded as an array of crystalline and amorphous silicon regions with grain boundaries between them. The approach of utilizing two defective a-Si:H-like material as a columnar grain boundaries and an a-Si:H region which are inserted into the middle of the crystalline-like (namely, highly crystallized µc-Si:H) active layer is realized in order to demonstrate crystalline volume fraction (Xc) in a more realistic and complete way. The electrical properties of microcrystalline silicon solar cells with different i-layer thickness, grain size, crystalline volume fraction (Xc), and density of dangling bond states are presented. According to the simulation results, the efficiency of high and low Xc samples is 9.02 % and 7.55 % with small grain size (=20 nm) when i-layer thickness is 4 µm, respectively. However, the samples with larger grain size (>30 nm) exhibits better performance is due to the fact that it is a critical point for microcrystalline silicon transits to polycrystalline silicon with grain size equals to 30 nm, which. Nevertheless, the higher the density of states and Xc of intrinsic layer is, the less light-induced degradation produced by a-Si:H in solar cell. Some external quantum efficiency results are also presented and indicates that the light-induced degradation effect only occurred in the short wavelength range which corresponds to a-Si:H absorption spectrum and we can further conclude that a-Si:H fraction in µc-Si:H is responsible for this effect.
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