Growing Cu(In,Ga)Se₂ thin film solar cells with high efficiency and low production costs.

• Other Authors: Yang, Shihang.
• Format: Others
• Language: English; Chinese
• Published: 2012
• Subjects: Photovoltaic cells > Design and construction; Solar cells > Design and construction; Thin films
• Online Access: http://library.cuhk.edu.hk/record=b5549502; http://repository.lib.cuhk.edu.hk/en/item/cuhk-328395

銅銦鎵硒薄膜太陽能電池因為其高效率及相對低廉的成本，商業應用已經開始陸續出現。我們自主研發的集成式銅銦鎵硒薄膜電池生產系統可以全程製作襯底大小為10cm x 10cm 的電池及剃型組件。本研究工作主要分為兩個方向:第一個方向是研究及測試生長高效率太陽能電池及組件的具體條件。通過儀器改進及電池每層鍍膜的條件優化，能夠重複的生長高效率電池及組件; 第二個方向是通過減少銅銦鎵硒吸光習的厚度從而達到降低電池生產成本的目的。 === 銅銦鎵硒採用三步共蒸法製備吸收層。第一步先蒸發銦、鎵、硒三種元素形成n型硒化銦(鎵)薄膜;第二步蒸發銅、硒形成銦鎵硒半導體薄膜; 第三步蒸發一層額外的型硒化銦(鎵)薄膜保證整體電池是p型半導體。三步期間的襯底溫度經過小心調試，以使得合適的鎵梯度能夠在吸收層裹形成。通過每一層的條件優化我們能夠生長出高光電轉換效率的太陽能電池(17%)及組件(12%)。 === 太陽能電池的變溫測試及弱光測試對瞭解其應用潛能存在非常重要的作用。通過多組對比實驗發現銅銦鎵硒電池的溫度係數可以通過增加鎵在吸收層的組分而得到改善。同時，電池的弱光表現可以通過減少銅的量得到很大的提高。STM 的研究發現弱光表現得到改善是因為吸收層顆粒介面電阻的增加而導致的。 === 減少吸收層的厚度有利於進一步減少太陽能電池的材料成本。當電池的吸收層厚度小於一微米時，開路電壓跟短路電流都明顯有所減少，從而導致太陽能電池效率降低。更薄電池效率的提高可以從兩個方面來實現:氧化鋅表面的陷光結構及更加合適的鎵含量的使用。通過這兩艇改進方法，電池效率被提高到14%以上，使得超薄電池有更好的應用前景。 === Cu(In,Ga)Se₂ (CIGS)-based thin film solar cell has been commercialized recently due to its high energy conversion efficiency. We have designed an integrated satellite deposition system for producing CIGS solar cell with substrate size of 10cm x 10cm. This work mainly contains two parts with first part focusing on growing and characterizing high quality baseline solar cells and solar modules and second part concentrating on further reducing the material costs by growing thinner absorber layer with high efficiency. === The most difficult part in growing high quality CIGS solar cells originate from the absorber layers which contain p-type chalcopyrite structures with four different elements: Cu, In, Ga and Se. The widely used three-stage process is employed to co-evaporate In, Ga and Se first, then Cu and Se are evaporated to form the chalcopyrite CIGS structure and additional In, Ga and Se are deposited in the end to ensure an overall Cu deficiency, which is important for getting p-type semiconductors. The substrate temperatures during these three stages are carefully adjusted to introduce proper gallium gradients which is important for collecting electrons efficiently. Together with optimizing other layers we are able to get cell efficiency (area around 0.5 cm²) over 17%. To produce CIGS mini-modules, laser scribing as well as mechanical scribing are employed for series interconnection of individual cells using monolithic integration. The power and speed of laser together with the condition of mechanical scriber are carefully adjusted to ensure a minimum dead area in the module. Module (area around 80 cm²) with efficiency over 12% is produced. === Solar cells were fabricated and tested under varied temperature and weak light conditions. Temperature coefficient is compared between CIGS solar cells and other types of solar cells. Temperature coefficient is improved a lot with higher gallium content in the absorber layer. Weak light performance is shown to be increased a lot when copper percentage is lowered down. In order to examine the origin of beneficial effects from Cu-poor absorber, solar cells are grown with comparable grain sizes using our technique and I-V performances are examined under STM in grain/atomic scale. Leakage current is found to be mainly originates from boundary area. CIGS solar cells with Cu-poor absorber benefit from the reduced leakage from boundary area. === CIGS solar cells with thinner absorber thickness are studied and compared with conventional CIGS solar cells. We have found that high conversion efficiency solar cells can be grown for absorber thickness as thin as 1.5μm. Further reduction in absorber thickness deteriorates solar cell performances in both V∝ and Jsc resulting in conversion efficiency as low as 11%. === Two major approaches are performed to improve solar cell performances. Light trapping by etching AZO top contact for creating pyramid-structures to enhance light scattering. Efficiency is increased by more than 1.5% for solar cells with etched AZO surfaces. Solar cells with efficiency larger than 13% can be grown by using AZO etching. Another approach is by using suitable Ga content in absorber layer. Solar cells with efficiency as high as 14.17% are grown which makes thinner CIGS solar cells very competitive. === Detailed summary in vernacular field only. === Detailed summary in vernacular field only. === Detailed summary in vernacular field only. === Detailed summary in vernacular field only. === Yang, Shihang = 高效率、低成本銅銦鎵硒薄膜太陽能電池的製造 / 楊世航. === Thesis (Ph.D.)--Chinese University of Hong Kong, 2012. === Includes bibliographical references (leaves 101-109). === Abstract also in Chinese. === Yang, Shihang = Gao xiao lu, di cheng ben tong yin jia xi bo mo tai yang neng dian chi de zhi zao / Yang Shihang. === Chapter 1 --- Introduction to Photovoltaics --- p.1 === Chapter 1.1 --- Energy crisis --- p.1 === Chapter 1.2 --- Physics of solar cells --- p.4 === Chapter 1.2.1 --- Light Absorption --- p.4 === Chapter 1.2.2 --- Charge Carrier Separation --- p.8 === Chapter 1.2.3 --- Solar Cell I-V Characteristics --- p.9 === Chapter 1.3 --- Classifications of Solar Cells --- p.11 === Chapter 1.3.1 --- Crystalline silicon solar cell --- p.11 === Chapter 1.3.2 --- Thin film solar cells --- p.12 === Chapter 1.3.3 --- Organic and polymer solar cells --- p.13 === Chapter 1.4 --- Cu(In,Ga)Se₂ (CIGS) based Solar Cells --- p.13 === Chapter 1.4.1 --- State of the art --- p.13 === Chapter 1.4.2 --- Material properties and structures --- p.14 === Chapter 1.4.3 --- CIGS advantages --- p.17 === Chapter 2 --- Integrated CIGS deposition system and fabrication process optimization --- p.21 === Chapter 2.1 --- Introduction to vacuum deposition system --- p.21 === Chapter 2.1.1 --- Integrated CIGS solar cell deposition system --- p.21 === Chapter 2.1.2 --- Ni-Al top grid evaporation system --- p.23 === Chapter 2.2 --- Fabrication processes --- p.23 === Chapter 2.2.1 --- Substrate treatment --- p.23 === Chapter 2.2.2 --- Molybdenum back contact deposition --- p.24 === Chapter 2.2.3 --- CIGS absorber layer formation --- p.26 === Chapter 2.2.4 --- Hetero-junction formation --- p.31 === Chapter 2.2.5 --- Window layer optimization --- p.32 === Chapter 2.2.6 --- Laser and mechanical scribing for mini-modules fabrication --- p.37 === Chapter 2.3 --- Equipment improvements --- p.42 === Chapter 2.3.1 --- Heating uniformity of substrate --- p.42 === Chapter 2.3.2 --- Use of pyrometer for improved control of absorber thickness/composition --- p.43 === Chapter 2.3.3 --- Se cracking unit --- p.45 === Chapter 2.4 --- Characterization of CIGS solar cells --- p.47 === Chapter 2.4.1 --- Morphology, composition and crystallinity --- p.47 === Chapter 2.4.2 --- Depth profile of CIGS --- p.49 === Chapter 2.4.3 --- Electrical property measurements --- p.51 === Chapter 2.5 --- Conclusion --- p.54 === Chapter 3 --- Performance of CIGS solar cells under non-standard test conditions --- p.56 === Chapter 3.1 --- Temperature coefficient measurement of CIGS --- p.57 === Chapter 3.1.1 --- Equipment set-up --- p.57 === Chapter 3.1.2 --- Temperature coefficients for different types of solar cells . --- p.60 === Chapter 3.1.3 --- CIGS solar cells with varied Ga/III composition --- p.65 === Chapter 3.2 --- Weak Light Performance of CIGS --- p.69 === Chapter 3.2.1 --- Introduction --- p.69 === Chapter 3.2.2 --- Experiment --- p.72 === Chapter 3.2.3 --- Results and discussion --- p.73 === Chapter 3.3 --- Conclusion --- p.81 === Chapter 4 --- CIGS solar cells with lower fabrication cost --- p.83 === Chapter 4.1 --- Fabrication cost analysis for commercial CIGS solar cells --- p.83 === Chapter 4.2 --- Thinner CIGS absorber layer --- p.84 === Chapter 4.2.1 --- Solar cell performances with different absorber thicknesses --- p.84 === Chapter 4.2.2 --- Performance improvement for thinner solar cell --- p.87 === Chapter 4.3 --- Conclusion --- p.96 === Chapter 5 --- Conclusion --- p.98 === Chapter 5.1 --- Summary of previous researches --- p.98 === Chapter 5.2 --- Future work --- p.99 === Bibliography --- p.101