以物理氣相沉積法與硒化法製備CIGS太陽能電池元件研究

Abstract

摘 要 銅銦鎵二硒化物(CIGS) 是I-III-VI2族黃銅礦半導體，用於製造便宜的光伏元件。CIGS薄膜為應用於高效率吸收膜太陽能電池最被看好的材料，CIGS帶隙能量為1.02–1.69 eV，光吸收係數高於104 cm-1，且有極高的穩定性高。本研究探討Soda-lime glass (SLG, substrate), Mo (back contact), CIGS (absorber layer), In2S3 (buffer layer), ZnO, 及TiO2-doped ZnO (window layer) 薄膜結構的光、電特性。 鉬背電極是使用直流磁控濺鍍沉積鉬(Molybdenum, Mo) 背電極於蘇打玻璃，應用田口實驗設計(Taguchi methods)、L9 (33) 混合直交表，觀察濺鍍參數(製程壓力、濺直流功率、基板溫度) 對鉬金屬薄膜的影響，分析鉬薄膜附着力、微結構，導電性與反射率。實驗結果顯示沉積壓力為影響鉬薄膜電阻率的重要因子。單層鉬薄膜擁有較低的電阻率，但無法同時兼具良好的附著性；因此使用雙層結構鍍製鉬薄膜，可同時兼備低電阻率與良好附著性。第一層鉬薄膜於高壓(1.07 Pa)環境下鍍製，此層具有良好的附著性；第二層鉬薄膜在低壓(0.4 Pa)環境下鍍製，此層具有較低的電阻率，兩層薄膜厚度控制在500 nm厚。 吸收層是使用Cu70Ga30靶及In靶，以直流磁控濺鍍沉積CIG金屬前驅層，前驅層以三明治結構方式鍍製，固定上下銦層的厚度，改變中間銅鎵層厚度，觀察不同Cu/(In+Ga)比例(分別為0.808、0.912、1.035、1.125) 對金屬前驅層硒化後CIGS吸收層的影響。研究結果發現，Cu/(In+Ga) 比例為0.912時，適合作為太陽能電池元件。此外、探討SLG/Mo/CIGS硒化過程，包抬:一階段及三階段不同持溫時間，對CIGS吸收層影響。研究顯示一階段硒化處理，由於升溫速度過快，造成硒層快速流失，導致薄膜整體原子數配比與理想值差異過大，使得薄膜剝落。三階段硒化處理，在第三階段硒化溫度560 °C時持溫時間20分鐘，可得到CIGS黃銅礦結構，經拉曼光譜量測，未發現有Cu2Se二次相結構產生。 透光層 (ZnO:TiO2 = 98:2 wt %, TiO2-doped ZnO 透明導電膜)，使用自製靶材。所有TZO薄膜顯示為(0 0 2) hexagonal wurtzite structure 繞射結構。研究結果顯示，濺鍍參數為: 沉積時間70 min，基板溫度300 °C，濺鍍功率130 W，沉積壓力2 Pa，可得到最低的電阻率7.98×10-3 Ω-cm，光穿透率為83.95 %。此外、以Ti金屬薄膜(約10 nm) 為緩衝層(基板與導電膜間) 其電阻率下降至6.81×10-3 Ω-cm。再進行真空退火 (450 °C)，顯示TiO2-doped ZnO透明導電膜光穿透率為85.21 %，電阻率下降至3.76×10-3 Ω-cm 整合製備CIGS薄膜太陽能電池，元件經光照後量測其I-V curve，得開路電壓(Voc)= 0.4548 V、短路電流密度(Jsc)= 31.335 mA/cm2、填充因子(FF)= 63.10 %、效率(η)= 8.99 %。

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.