| 其他摘要 | Si-based Thin Film Solar Cells, as the most promising solar cells since now, have been studied intensively and applied to many areas，because of their advantages, such as the very abundant storage of Si in the earth, the environmental friendly characteristic, and the very low manufacturing cost. But the low efficiency and bad stability still confined their development. In this work, laser crystallization method was used to crystallize the surface layer of the a-Si thin films, and the resulting μc-Si/a-Si composite films were proposed to be as the intrinsic absorbing layer in thin film solar cells, so as to improve the efficiency and stability. Since the microstructure of the crystallized layer directly determines the performance of solar cells, in this work we tried to build up relationships between the microstructure of the crystallized layer and laser parameters by studying the crystallization behaviors of a-Si thin films induced by different excimer lasers as well as making numerical simulations of the temperature field within the films. The aim of this work is to try to provide experimental and theoretical basis for making μc-Si /a-Si composite films by laser surface crystallization method as well as making efforts for the development of the third generation of high-efficient and stable solar cells.
In this work, the crystallization behaviors of a-Si thin films induced by KrF and ArF were studied, including the surface crystallization threshold of the as-deposit a-Si thin films, the influences of laser fluence and laser shots on films’ crystallinity, the surface morphology and crystallizing layers' distributions along depth, and so on. The results indicated that the laser surface crystallization threshold of the as-deposit a-Si thin films was near 110mJ/cm2, which was independent of shot numbers. The crystallinity could be increased mainly by increasing laser fluence, however the shot numbers also had an obvious effect on it when laser fluence was low. SEM and TEM view of the crystallizing layer showed that the crystallinity and mean grain size were both increased as laser fluence was increased; but when laser fluence was over a certain value, the uniformity of the grains lowered and some bigger grains showed up, leading to bad film qualities. TEM cross-section view of the ArF excimer laser crystallized thin films showed that the crystallized μc-Si layer was deepened by increasing laser fluence, and followed the parabola law. By comparing the experimental results we found that, under the same conditions, surface crystallization of KrF excimer laser resulted in higher crystallinity and deeper crystallized μc-Si layer, and ArF excimer laser resulted in more uniform grain sizes and higher surface quality. So it was better when laser fluence was 110~150 mJ/cm2 for KrF excimer laser crystallization, and 120~180mJ/cm2 for ArF excimer laser crystallization.
Temperature field simulations were carried out by numerical method, focusing on the influences of laser wavelength, laser fluence and substrate temperature on the temperature field within the films. The simulation results showed that the surface melting threshold of a-Si thin films was increased as laser wavelength and pulse duration increased. And when pulse duration was constant, the lowest surface melting threshold was obtained at wavelength of 300~350nm. Surface melting duration, melting layer depth and surface highest temperature of the films were all increased as laser fluence was increased; and under the same conditions, the effects of KrF(248nm) laser were more obvious than that of ArF(193nm) laser. Substrate temperature mainly had an influence on surface melting duration of the films, that was to say, the surface melting duration time would increased quickly by increasing substrate temperature, and would have less effects on melting depth and surface temperature of the films. By comparing the simulation results with experimental results of this work and with other researching results, the accuracy and applicability of the numerical simulation were verified. |
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