物理氣相沉積法製備CIGS太陽能電池吸收層與AZO透明導電膜之研究

Abstract

本研究以射頻磁控濺鍍法沉積AZO透明導電膜及Mo薄膜背電極(back contact.)於蘇打玻璃，再以熱蒸鍍法製備CIG金屬前驅層，最後應用固態硒化法使CIG金屬層硒化成為CuInGaSe2薄膜。實驗中分別對CIGS太陽能電池內薄膜以個別特性做研究分析，將AZO透明導電膜鍍製於玻璃與塑膠基材(PET)，觀察不同基材對電阻率與可見光穿透率影響。此外，於基材上加入Al金屬緩衝層，觀察Al金屬緩衝層對於AZO透明導電膜之光、電性質影響。 金屬電極Mo薄膜應用田口實驗規劃及變異數分析，觀察不同濺鍍參數(沉積功率、製程壓力、沉積時間、基板溫度)對電性影響。CIG金屬前驅層以熱蒸鍍法製備，並以固態硒化法成長出具有黃銅礦相結構CIGS薄膜，薄膜內元素比例變化時將造成CIGS薄膜特性改變，且所製備的CIGS薄膜半導體特性為p-type。實驗中分別藉由表面輪廓儀(α-step)、原子力顯微鏡(AFM)、場發射電子顯微鏡(FE-SEM)觀察薄膜表面形貌，利用X-ray繞射儀(XRD)、穿透電子顯微鏡(TEM)來分析所製備的薄膜結構，薄膜的光、電及半導體特性分別使用可見光光譜分析儀(UV-VIS)、四點探針(Four-Point Probe)、霍爾效應(Hall-Measurement)、光子激發光譜儀(Photoluminescence)來測量。 本論文所製備最低透明導電膜電阻率為2.22×10-3Ω-cm(AZO/glass)與9.20×10-3Ω-cm(AZO/PET)，可見光穿透率為77.82% (AZO/glass)與78.41% (AZO/PET)，加入Al金屬緩衝層可得到最低電阻率為9.46×10-4Ω-cm AZO/Al(2 nm)/glass與9.10×10-3Ω-cm AZO/Al(2 nm)/PET，可見光穿透率為77.78 % AZO/Al(2 nm) /glass與78.40 % AZO/Al(2 nm)/PET。實驗中金屬Mo電極應用田口實驗規劃法，其製程條件為A3B1C3D2 (沉積功率125W、製程壓力0.26 Pa、沉積時間50min、基板溫度200℃)厚度644 nm，可得到最低電阻率1.92×10-5Ω-cm。以蒸鍍法製備CIG金屬前驅層配合固態硒化法所生成的CIGS薄膜，導電形態為p-type，因硒化溫度提高後硒含量不足，無足夠硒元素與CIG金屬前驅層反應，致使CIGS薄膜(112)繞射波峰並未提高。

Transparent conductive films of Al-doped (2 wt.%) zinc oxide (AZO) and Mo films as a back contact were deposited by radio frequency magnetron sputtering system. Furthermore, the structure and performance of CIGS films prepared using thermal evaporation on Mo coated glass with the optimal properties, with a two-stage method were determined, as a function of selenization temperature (vacuum atmosphere). AZO films were prepared onto soda-lime glass and flexible polyethylene terephthalate (PET) substrates at room temperatures. To optimize the electrical conductivity of the back contact, the influence of deposition parameters such as RF power, working pressure, deposition time, and substrate temperature, on the electrical, structural and morphological properties of Mo films were investigated, using the Taguchi quality design concept. Then, CIGS films were deposited, using thermal evaporation, followed by selenization (the so called two-stage process). These were formed on the substrate, which was coated with Mo with optimal properties. The structural and electrical properties of the CIGS films were also determined. The electrical characteristics (Hall effect and resistivity as a function of argon sputtering pressure), optical transmittance, structure (X-ray diffraction (XRD) and transmission electron microscopy (TEM)) and surface morphology (SEM, JEOL, JSM-6500F) were examined. The resistivity was 9.22×10-3 Ω-cm with a carrier concentration of 4.64×1021 cm-3, Hall mobility of 2.68 cm2/V-s and visible range transmittance about 80% on the PET substrates. By applying Al buffer the resistivity of the AZO films decreases, but those of the optical transmittance also decreases. The crystalline and microstructure quality of the AZO films can be improved after annealing. On the other hand, the lowest resistivity of 9.46×10-4Ω-cm (sheet resistance∼37.87 Ω/sq. for a thickness∼250 nm) and a high transmittance (80 %) were obtained by applying a 2 nm thick Al buffer layer on glass substrates at room temperature. Molybdenum (Mo) is widely used as a back contact material in CIGS solar cells. The Mo films deposition parameters for the sputtering system, use RF power (125 W), sputtering pressure (0.26 Pa), deposition time (50 min), substrates temperature (200 ℃) thickness (644 nm), are recommended. The XRD patterns indicate that the intensity of the CIGS (112) diffraction peak (located at 2θ∼26.8°) becomes stronger, as the selenization temperature increases, and that the film crystalline quality is improved by raising the selenization temperature. The lower band gap indicates that the CIGS films deviate little from the ideal single chalcopyrite structure. This is partly due to the presence of secondary phases and the porous morphological surfaces of these films, because of the lower selenization temperatures and a lack of sufficient Se, during CIGS growth.