Study On High-Rate Deposition And The Stability Of Microcrystalline Silicon Thin-Films

High-rate deposition process plays an important role in reducing production cost of microcrystalline silicon (μc-Si) solar cells. It is also an access to industrialize the amorphous/microcrystalline silicon tandem solar cells. High-rate deposition of microcrystalline silicon films and solar cells were realized in this paper using very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) technique combined with high pressure. After a series of experiments and analyses, impact factors on deposition rate and material quality ofμc-Si films and solar cells were discussed in detail. Research contents and main innovations were described follow.The influences of factors including pressure, power, silane concentration, temperature, total flow rate and the distance space between the cathode and the substrate to the deposition rate ofμc-Si:H were systematically investigated. On-line optical emission spectra (OES) measuring was used to detect the growth rate changes. It can be concluded that high pressure and a large distance space between the cathode and the substrate contribute to increasing the deposition rate of the microcrystalline silicon films. Increasing pressure can not only accelerate the silane decomposition, but also be in favor of ions diffusion from bulk plasma to the growth surface and simultaneously decrease the bombardment of electrons with high energy on the silicon films. As a result, the deposition rate is increased. A large space between the cathode and the substrate will enlarge the reaction space, more reactive ions could reach the growth surface, and therefore the deposition rate is increased. By increasing pressure and raising the space distance, highest deposition rate of over 23?/s was achieved in our experiments.Then factors that affect the material quality were investigated and the following conclusions were obtained. Our experiments demonstrated for the first time that the long resident time has great effect on the defect density of the silicon films. When the pressure or the electrode space is increased without changing the total flow rate, the gas residence time will be prolonged, which will lead to poor film quality with high defect density. So the total gas flow rate should be increased as well as a high pressure and a large distance space between the cathode and the substrate were applied to prepare high quality materials with a high deposition rate. Accordingly, device-qualityμc-Si:H films with crystalline fractions over 40%,μτaround 10-4cm2/V, and the <220> preferential orientation have been obtained at high deposition rate of 23?/s by optimizing the deposition conditions.The influence of different deposition rates on the properties ofμc-Si:H thin films and the profermance of their solar cells was studied carefully. Compared with the low deposition rate materials, the high deposition rate ones have a thicker amorphous incubation layer, worse uniformity of vertical (growth direction) structure, larger grains, looser microstructure and a rougher surface on top.The magnitude of relative light induced degradation is closely related to material structure. Amorphous fraction is the key determining factor to light induced degradation. The results showed clearly that the magnitude of relative efficiency degradation is increase with amorphous fraction. The more amorphous fraction located in material, the more degradation was been found. With better structure and optical properties, Microcrystalline silicon with transition region is more suitable for the manufacturing of stable Microcrystalline silicon solar cells due to the structure and optical properties.In summary, device-quality high-rate depositionμc-Si:H films and solar cells have been realized in this dissertation. Various methods to improve the deposition rate and material quality were investigated in detail. All these results will be valuable for the reduction of production cost of theμc-Si:H solar cells in the future. Amorphous fraction is the key determining factorto light induced degradation.,the magnitude of relative efficiency degradation is increase with amorphous fraction.