The study on the CIGS solar cell by physical vapor deposition and selenization process

• Main Authors: Lin, You-Chuan, 林祐全
• Other Authors: Hung, Shao-Kang
• Format: Others
• Language: zh-TW
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/57252581258003037776

博士 === 國立交通大學 === 機械工程系所 === 102 === ABSTRACT Copper indium gallium diselenide (CIGS) is a member of the I–III–VI2 group chalcopyrite semiconductors materials that can be used to make low-cost photovoltaic devices. Thin films of CIGS are considered as the most promising material for application as an absorber in high-efficiency thin film solar cells because of band gap energy of 1.02–1.69 eV, optical absorption coefficient over 104 cm-1 and extremely high stability. In this study, the CIGS cell setup consists of soda-lime glass (SLG, substrate), Mo (back contact), CIGS (absorber layer), In2S3 (buffer layer), ZnO, and TiO2-doped ZnO (window layer), each layer with different role in the working cell. For Molybdenum (Mo) thin film, prepared onto soda-lime glass substrates, by direct current (dc) magnetron sputtering, using a metal Mo target in an argon gas environment. A Taguchi method with an L9 (33) orthogonal array, the signal-to-noise ratio and analysis of variances were employed to examine the performance characteristics of the coating operations. The main sputtering parameters, such as working pressure (Pa), dc power (W) and substrate temperature (°C), were optimized, with reference to the adhesion strength, structural features, surface morphology and electrical properties of the Mo films. The experimental results demonstrate that the working pressure strongly affects the Mo films resistivity performance characteristics. The bilayer molybdenum thin film has both low resistivity and good adhesion, single layer is not. The first layer was prepared at a high pressure of 1.07Pa that to get a good adhesion and the second layer was prepared at a low pressure of 0.4 Pa, to provide with lower resistivity, totally was 500 nm. The multilayer precursors films of In/CuGa/In are deposited onto Mo-coated SLG substrates, using dc magnetron sputtering of CuGa alloy with 30wt.% Ga and elemental In targets and the thermal evaporation of Se. Thickness of the In layer was adjusted by adjusting the deposition time, to produce precursors with Cu/(In +Ga) atomic ratios of 0.808, 0.912, 1.035 and 1.125. The precursors was designed to have atomic compositions of Cu/(In+Ga)= 0.912, that is well matched for higher efficient CIGS-based solar cell. Further, the precursors were then selenized in a vacuum ambient (1.33 Pa), using one-step and three-step annealing at a constant temperature of 560 °C in a tube furnace. It was found that the SLG/Mo/CIGS quality (good microstructure, no crack or peel-off of the films) could be improved by using three-step selenization process. For window layer (ZnO:TiO2 = 98:2 wt %, TiO2-doped ZnO transparent conducting oxide films, TZO), and the TiO2-doped ZnO target was prepared by the solid state reaction method. All TZO thin films exhibited strong (0 0 2) diffraction peaks of hexagonal wurtzite structure. The optimal deposition parameters for the sputtering of TZO thin films were a deposition time of 70 min, a substrate temperature of 300oC, and rf power of 130 W, a sputtering pressure of 2 Pa. Using a Ti buffer decreases the resistivity and optical transmittance of the AZO films. The crystalline and microstructure characteristics of the TZO films are improved by annealing. The solar cell device with this film showed the power conversion efficiency (η) of 8.99% with an open-circuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF) of 0.4548 V, 31.335 mA/cm2, and 63.10 %, respectively.