Solar Cell Materials Cuins <sub> 2 </ Sub> Thin Films And Their Nanorod Arrays

As a renewable and clean energy, solar energy is a promising replaceable energy to traditional energy and its application research has been one of the main areas in the energy development, owing to its advantages of enormous reserves, inexhaustibility and being friendly to environment. Solar cell, on the base of photovoltaic effect, is an important form of utilization of solar energy. As the core of the photovoltaic systems, solar cell has been produced in large scale over the past 10 years, with an increase of more than 30% annually. However, compared with traditional thermal power generation and hydropower, solar cell has the problem of higher cost. To solve that, the key is to find methods to reduce production cost and improve the conversion efficiency of solar cell.CuInS2 (CIS), an importantⅠ-Ⅲ-Ⅵ2 ternary semiconductor materials, has emerged as one of the promising absorber materials for thin film solar cells, due to its notable advantages of high absorption coefficient, nearly ideal band gap value of 1.55eV, excellent chemical stablility and low toxicity. As the low cost techniques, electrodeposition, spray pyrolysis, paste coating techniques, screen printing and other non-vacuum technologies have recently been extensively applied to the fabrication of CIS absorber layer, while paste coating method and screen printing technologies using the polycrystalline CIS compound as the precursor, and single-source thermal evaporation and close-spaced sublimation method using the CIS powder to prepare high-quality CIS films. Thus, in the paper, it is systematically studied of the homogenized polycrystalline CIS compound sintered at low temperatures, which is crucial to the preparation of CIS thin film sintered from ball-milled precursors on glass substrates at low cost.As we all know, the photoelectric conversion efficiency of the CIS thin film solar cell is up to 27%-32% theoretically, while the highest conversion efficiency reported was only 12.2%, so there is a lot of potential to improve the efficiency of solar cells. On the other hand, the nanostructure of absorbing layer of solar cell can increase the light absorption, and improve the photo-induced carriers separation and transportation as well, which is effective to improve the efficiency of solar cell.Mo thin film is used as the back electrode material of solar cell by more and more researchers, because of its good properties of heat stability, low resistivity and a good ohmic contact between Mo thin film and the upper CIS thin film. Meanwhile, its performances have a direct effect on the crystalline orientation, surface morphology and interfacial properties of CIS thin film prepared exactly on Mo thin film, resulting in a huge impact on the performances of CIS thin film solar cell. Additionally, the solar cell reported with the highest conversion efficiency is also based on Mo thin film as bottom electrode.In this dissertation, the Mo thin films were deposited as the bottom electrode of solar cell by DC magnetron sputtering, while the chalcopyrite CIS powders were synthesized by vacuum sintering at low temperature, and the CIS thin films were prepared by single-source thermal evaporation and solid-state sulfurization method. The new CIS nanorod array thin film was also researched, by means of solid-state sulfurization method. The research results are summarized as follows:1. The obsoleted equipment was refited into a DC magnetron sputtering device, which was used to prepare Mo thin film, and the effects of sputtering conditions on the Mo film deposition rate, structure, morphology and electrical performance were systematically studied. In sputtering process, the Mo thin film was grown with (211) preferred orientation at 150℃, while (110) preferred orientation in other temperature conditions.2.The CIS powder was synthesized by vacuum sintering with both the precursors including Cu,In and S powders, and CuS,In2S3 powders. The chalcopyrite CIS powder was synthesized with ball-milled precursors including Cu, In and S powder in 10-1Pa at low temperature (350℃), with the product exhibiting a grain size of about 250nm, which is suitable for preparing CIS thin film by paste coating and single-source thermal evaporation method.3. The CIS thin film was deposited on soda-lime glass substrate by single-source thermal evaporation technique, with the synthesized CIS powder used as raw material. After annealing at 250℃-450℃the sample exhibited a (112) preferred orientation, with N type conductivity and a band gap of 1.50 eV.4.The evaporated Cu/In thin films on soda-lime glass had two phase transformations at 153℃and 314℃, confirmed by DSC test, and it was concluded that the monoclinic Cu11In9 and triclinic Cu7ln3 alloys were formed by XRD characterization, respectively. With further solid-state sulfurization, the two alloys were transformed to CIS thin films, which had P type conductivity and band gaps of 1.34eV and 1.39eV, respectively. However, the CIS thin film sulfurized from the precursor of Cu7ln3 alloy also contained a small amount of CuxS binary phase. In addition, the CIS thin film deposited on the glass/Mo substrate had better crystallization properties than that on the Mo foil substrate.5. The new CIS nanorod array thin film was prepared by the solid-state sulfurization method. It can be observed that the CIS nanorod arrays were grown vertically on the thin film by SEM, and the sample was near stoichiometric amount of CIS compound by EDS. The nanostructure of absorbing layer of solar cell is used to increase the absorption for light and improve the photo-induced carriers separation and transportation, which is an effective way to improve the efficiency of the solar cell.Besides, we have studied another importantⅠ-Ⅲ-Ⅵ2 group semiconductor material, CuInSe2 compound. The CuInSe2 powder was synthesized by the vacuum sintering method, and the thin film was deposited on the Mo foil substrate by the solid-state selenization method.In short, in this dissertation, we focus on the fabrication of solar cells with high efficiency and low cost. Many researches were performed on the preparation of CIS powders, thin film and new nanorod arrays thin film, as well as Mo thin film, with a certain amount of research achievement.