| With the depletion of fossil fuels and environmental issues becoming serious, looking for a more sustainable long-term clean energy is urgent for people. Solar energy is a inexhaustible green energy which could be one of the future power alternatives. Solar cell is the most direct way to use solar energy which could convert sunlight directly into electricity. Thin film solar cells have many advantages like less consumption of materials, high conversion efficiency, high absorption coefficient, optical band gap is optimizing with the solar spectrum, etc. It will be the candidate of Silicon solar cells. But common CIGS thin film solar cells faces a shortage of Indium resources, and couldn’t be largely used. We will study a new Copper based compound thin solar cells which is called Copper zinc tin selenium-Cu2ZnSnSe4(CZTSe) thin film solar cells. This CZTSe solar cell not only has the usual advantage of thin films solar cells, but also has a absorber which consists of abundant and non-toxic element.In this paper, we will refer to the past numerous research experience in CIGS to study the CZTSe solar cells. In order to avoid high vacuum equipment cost in CIGS fabrication industry, we intend to select the electrochemical deposition technology which has high utilization ratio of material, low energy consumption and low equipment cost to prepare the absorbing layer of CZTSe thin film solar cell. In the meantime, we measured the band offset in CdS/CZTSe heterojunction to know whether CdS is the ideal buffer layer of CZTSe solar cells. The research content mainly includes three parts:In the first part, we systematically study of the electrochemical deposition copper tin zinc precursors and the process of selenization of tin copper zinc precursors. The CZTSe solar cells absorber layer is CZTSe thin film, its composition and crystallization quality will influence intensely with the performance of solar cells. In the experiments we control different deposition potential, selenization temperature and the optimization of the selenization time, achieving the one-step electroplating of copper tin zinc precursors and successful selenizing copper zinc tin into CZTSe. The film components is corresponding to the stoichiometric proportion of CZTSe. Considering the zinc and Tin content will be loss during the annealing process, we deposite a high ratio of zinc and Tin in the copper tin zinc precursors. In this high zinc and Tin components of the precursors, we study the selenization process of copper zinc tin systematically. At550℃we could get a good crystallization CZTSe thin film in Se atmosphere. We measure the different transition phase of CZTSe with temperature int the experiment, study every temperature period for each possible reaction mechanism, in order to further control of CZTSe into second phase and inhibit the generation of miscellaneous phase. We also tried using H2S to sulfurize of copper tin zinc precursors, and study the influnce of the different proportion of Ar:H2S ratio to CZTS formation.The second part mainly discusses the influence of different zinc content to the electro-optical properties of CZTSe film, explores the impact of zinc proportion to the defects of CZTSe film and the performance of CZTSe solar cells. Considering the large influence of intrinsic defects relaied on Zn/Sn ratio, we compare CZTSe film and electro-optical properties with different Zn/Sn proportion in order to get a high performance CZTSe solar cell. Finally we fabricate CZTSe solar cells with different Zn/Sn ratio and compare the Ⅰ-Ⅴ character of CZTSe soalr cells. We look for the influnce of solar cells by measure the electronic parameter of different solar cells. Using the optimized CZTSe film as the absorbor layer, we prepared the CZTSe solar cells with structure of the traditional CIGS solar cells which is Glass/Mo/CZTSe/CdS/ZnO/AZO. Finally, in the zinc-rich CZTSe solar cell we achieve a photoelectric conversion efficiency of1.7%, much higher than the zinc-poor solar cells. The area of zinc-rich CZTSe solar cell is0.8cm2, the open voltage is172mV, the short circuit current is28.4mA.cm-2, the fill factor is35.7%, series resistor is5.2Ω.cm and the shunt resistance is960Ω..In the third part we study the band offset in CdS/CZTSe by X-ray photoelectron spectroscopy. Semiconductor heterojunction band offset is one of the most important parameters in heterojunction device. It affects the transport property of carriers, the recombination of carriers, the Fermi energy split and so on. But at present only theoretical calculation of CdS/CZTSe heterojunction band structure has been reported, there is still lack of the experiment report. We intend to use X-ray photoelectron spectroscopy to measure the band offset in CdS/Cu2ZnSnSa and ZnS/Cu2ZnSnS4heterojunction, and study the influence of the different buffer layer to photocarrier transport in CZTSe thin film solar cells. It has been found in experiments that CdS/CZTSe belongs to the type-Ⅱ semiconductor heterojunction, photo-generated minority can across from CZTSe into the conduction band of CdS buffer layer without any trouble. But because the interface bandgap was reduced, photocarrier recombination from CZTSe to CdS buffer layer is increased at the interface. This should be avoided in solar cell’s designment, so CuInSe2-based solar cell are all designed to the type-I semiconductor heterojunction, and accquired a high photoelectric conversion efficiency. Using the PLD technology deposition, CdS/CZTSe heterojunction has the type-1semiconductor heterojunction, but due to the lack of surface modification in chemical bath solution and the danmage during the PLD process, it is not a proper way to the produce solar cells. The CZTSe heterojunction formed with the ZnS will form a larger spikes at the interface, electrons from CZTSe tunneling to will have a larger potential barrier, and will reduce the photo current in solar cells. Other optimal buffer layer for CZTSe solar cell need further study. |
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