| At the end of the 21st century, Solar cells will generate more than half of the world's total electricity and become to the main power as the new generation clean energy, the proportion of thin-film solar cell in photovoltaic device is rising year by year. Currently, the consumption of daily energy is mainly provided by burning fossil fuels. As is known, the process of burning fossil fuel will produce pollution, which will damage the environment and as an energy source,the fossil fuels can not be regenerated. Due to inexhaustible renewable and environmental friendliness features, the solar energy will gradually replace fossil fuels in the future. However,the solar cells have the disadvantages of high preparation cost and the complex prepare process.Several mature compounds in solar cells, such as gallium arsenide, cadmium telluride and copper indium gallium selenium have been constrained in the development to civil nature direction because the arsenic and cadmium belong to heavy metals and have seriously environment pollution, the sources of Copper, indium, gallium, selenium material are limited.These reasons have been restricting the development of solar cells. As a new type of semiconductor material, As a new type of semiconductor material, ZnO has a broad application prospects including the piezoelectric sensors, displays, light-emitting devices and solar cells, and ZnO is environmentally friendly material and abundant in nature has great potential for applications in solar cells especially.The investigation mainly studied ZnO solar cells, firstly, by using the first-principles theory,the n-type and p-type dopant materials and related proportion concentration of ZnO were analyzed to find the ZnO doping type which is suitable for the n-type layer, the middle layer and the p-type layer of the solar cells.Then the n-type, p-type and middle layer were prepared by magnetron sputtering method. The effects of sputtering power, working pressure, substrate temperature, substrate type, and N2O: Ar ratio of p-type ZnO on the crystal quality, surface morphology, conductivity and transmittance of the visible region were analyzed. Finally, the ZnO homojunction and heterojunction solar cells were prepared. The effects of different intermediate layers and front electrodes on the photovoltaic properties of solar cells were studied.The main work and research results of the investigation are as follows:1. Theoretical models of A1 doped ZnO and In doped ZnO were built. It was found that both two structures were n-type degenerate semiconductor which obey Fermi dwastribution. It was also found that the A1 doped ZnO was with higher conductive carrier concentration 2.81×1021cm-3 and theoretical transmission rate could be as high to 90% in visible light area, so A1 doped ZnO are suitable for n junction and transparent conducting electrode in solar cell.When conducting the p-ZnO fabrication, it was found that impurity energy levels are lower in A1-2N co-doped structure, and the self-compensation of acceptors were decreased, indicating the p-ZnO fabrication was realized. It was the first time to investigate the effect of Eu doping to absorption rates of AZO in visible light area and explore the possibility of application of Eu doped AZO structure in interlayer of solar cells.2. Using magnetron sputtering to prepare A1 doped n-type ZnO films, Al, N Codoped p-type films and Eu, A1 Codoped ZnO which are worked as a solar cell interlayer film. Combined with the theoretical results of the simulation, the following results are obtained:(1) The key process parameters on the film preparation were studied, finding that the crystalline quality and optical properties were better when the sputtering power is 120W, the working pressure is 2.0Pa and the substrate temperature is 150 degrees centigrade; in addition,the experimental values of the lattice constant, carrier concentration and transmittance are in good agreement the simulation results;(2) The transformation mechanism of p-type ZnO was analyzed, and it was found that the acceptor-donor codoped method could effectively reduce the crystal madelung energy. The p-type ZnO thin films fabricated under certain N2O atmosphere and the avarage transmissivity was about 90% within the visible region; what's more, it was found that the p-type ZnO thin films exhibited good optical performance with the ratio of A1 and N was about 1: 2, which was in good agreement with the simulation results.(3) The Eu and A1 co-doped ZnO films depodited on the ITO substrate had good electrical conductivity and transmittance, and there was an absorption peak in the visible region, which was consistent with the results in the theoretical calculations.3. Based on above theoretical and experimental results, the effects of different interlayers and front electrode on the photovoltaic properties of ZnO solar energy structures were studied.(1 ) The homojunction (p-type/ Al, N-doped ZnO / n-type Al-doped ZnO) and heterojunction (p-type Si/ n-type Al-doped ZnO) solar energy with Ti/Ag as the front electrode were studied. The results showed that solar cell of Eu, A1 co-doped ZnO as the middle layer had the highest photoelectric conversion rate, among which the heterojunction solar cell was 1.467%,the homojunction solar cell was 0.426% , the forecast was consistent with the theoretical result;(2) The effect of different metal and AZO / metal / AZO front electrode on the photoelectric conversion rate of p-Si/Eu, A1 co-doped ZnO/n-AZO heterojunction solar cells was studied. It was found that Cr / Ni metal used as the front electrode battery had the photoelectric conversion rate of 6.68%, AZO/Cr/AZO as the front electrode of the solar cell had the photoelectric conversion rate as high as 7.11%;The above researches not only make a systematic study on the experimental process,preparation mechanism, crystal structure, electrical properties and optical properties of ZnO thin films, but also provides a theoretical basis for the practical application of ZnO thin film solar cells. |
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