臺灣博碩士論文加值系統

在本論文中，我們主要研究了銅銦鎵硒（CIGS）薄膜晶體管以及不均勻性對CIGS太陽能電池模組性能的影響。CIGS是一種非常好的薄膜太陽能電池材料，它具有高吸收係數以及較高的載子遷移率，可以有效降低光伏元件的製造成本。現如今，CIGS太陽能電池產業已經相當完善，但仍有一些困難困擾著這個產業，如薄膜均勻性，薄膜形態不穩定，以及產量低等因素極大影響了電池的質量，進而降低CIGS太陽能電池的市場份額以及其與其他種類的太陽能電池材料的競爭性。另外，除了在太陽能電池領域的應用以外，CIGS在其他許多領域也有著很大的潛力，如感光元件，光通訊元件以及薄膜晶體管元件。通過解決CIGS太陽能電池所面臨的困難，以及擴展此材料在其他領域的應用，可以有效提升CIGS材料的價值。
本論文的第一部分介紹了我們在CIGS薄膜晶體管領域的研究。我們成功製備了第一個CIGS薄膜晶體管，其飽和載子遷移率約為1.8 cm2/V-s，開關比超過了1000倍。我們採用了新型的圓形圖案來設計薄膜晶體管，從而簡化了製程，並避免了傳統薄膜晶體管結構中所面臨的問題。在CIGS薄膜晶體管結構中，我們將Al2O3通過原子層沉積法成長與CIGS薄膜上。由於Al2O3具有較高的介電常數，所製備的薄膜晶體管也具有較高的導通電流。此外，Al2O3對CIGS薄膜具有鈍化作用，可以有效降低界面處的缺陷，從而使薄膜晶體管的飽和載子遷移率進一步提高。我們還利用蝕刻的方式降低原本用於太陽能電池應用的CIGS薄膜的厚度，從而使其達到薄膜晶體管所需要的條件。在成功製備CIGS薄膜晶體管后，我們對其工作性能進行了量測和研究。在研究過程中，我們發現蝕刻的過程對薄膜晶體管的載子遷移率有提升作用，我們猜測這是由於處於底層位置的薄膜中含有較少的缺陷，而位於薄膜表面的部分缺陷較多。這一假設得到了PL及XRD量測結果的驗證。此外我們還研究了CIGS薄膜中元素比例的變化對薄膜晶體管性能的影響。Cu元素比例越高，薄膜晶體管溝道中的電荷濃度及載子遷移率都越高，這其中可能是Na元素的比例在起作用。
本論文的第二部分闡述了不均勻性對CIGS太陽能電池模組的影響。我們首先研究了CIGS薄膜中殘餘應力對原本有Ga元素不均勻分佈的CIGS太陽能模組的影響。模擬結果可見由於能帶隙的波動被增強，Ga元素不均勻性的影響被殘餘應力放大了。因此在CIGS太陽能電池模組的模擬中，我們將殘餘應力的影響也考慮進去，從而使模擬結果與實際元件的性能更加接近。在模擬中所建立的模型與商業生產中CIGS太陽能電池的結構參數相同，每個模組中有3個電池串聯。在不均勻性研究中，我們考慮了3類不均勻性的分佈，分別為電池內部、電池之間、以及兩者綜合，而不均勻性的種類我們考慮了Ga元素比例不均勻性和CIGS薄膜厚度的不均勻性。在3類不均勻性的考量中，綜合分佈與實際面臨的情況最為接近。而在生產過程中，薄膜厚度的不均勻性往往可以得到很好的控制。模擬的結果顯示，對於Ga元素比例的不均勻性，電池內部的波動主要影響模組的開路電壓，而電池間的波動主要影響模組的短路電流和填充因子。而對於薄膜厚度的不均勻性，不論波動如何分佈，開路電壓受到的影響都較小，而短路電流和填充因子所受的影響與Ga元素比例不均勻性的情況類似。

Cu(Ga, In)Se2 (CIGS) thin film transistors and the inhomogeneity effects on CIGS solar modules are investigated. CIGS is one of the best candidate materials for thin film solar cell due to its strong light absorption as well as its relatively high mobility. It also has the advantage to reduce production cost for photovoltaics devices, and nowadays, the fabrication of CIGS solar cell is mature and commercial. But some problems like uniformity, morphology, and yield still affect the quality of CIGS solar cell and reduce its market in the competition with other kinds of solar cell like Si based solar cell, CdTe solar cell and CZTS solar cell. Besides the application in thin film solar cell, CIGS also has the potential to have a role in other fields, like light sensor, telecommunication, and thin film transistor (TFT). By solving the problems occurred in the fabrication of CIGS solar cell and achieving its application in other fields will promote the value of CIGS.
In the first part of this dissertation, CIGS TFT is investigated. The fabricated CIGS TFT achieves a saturation mobility of ~1.8 cm2/V-s, and the on-off ratio over 3 orders of magnitude for the first time. We use a special ring pattern to simplify the fabrication process and avoided the problems occurred in the traditional TFTs. In the structure of CIGS TFT, Al2O3 is deposited by atomic layer deposition (ALD) on CIGS film as the dielectric layer. With the help of Al2O3 layer, the on current is high due to its high dielectric constant, and moreover, the channel can be passivated by Al2O3 layer, so that defects on the interface of CIGS/Al2O3 decrease, and eventually increase the saturation mobility. We also have applied thin-down process on the CIGS thin film, which was prepared for solar cell fabrication, to meet the required conditions for CIGS TFT. The characteristics of CIGS TFT are investigated consequently, and the improvement of saturation mobility after thin down process is found. This might be due to the different qualities of different layers in CIGS thin film, and is justified by the measurements of photoluminescence (PL) and X-ray diffraction (XRD). Moreover, the performances of CIGS TFTs with different content in CIGS films are investigated. TFT with high Cu/(Ga+In) ratio tends to have both high carrier concentration and saturation mobility. Sodium atoms might play important roles in these properties.
In the second part of this dissertation, the inhomogeneity effects on CIGS solar modules are investigated. We firstly demonstrate the impact of residual strain on the CIGS solar module with Ga content fluctuation using first principle calculations. The simulation results show that the inhomogeneity effect is magnified by residual strain due to the enhanced band gap fluctuations. Then the 3D simulation results of CIGS solar cells are obtained with the residual strain effect incorporated. The parameters used in the model for the simulation are the same to those used in the commercial CIGS solar cell. A module consists of 3 cells in series, while each cell is divided into 3 sections. Intracell inhomo, intercell inhomo, and combined intracell+intercell inhomo are considered within a module, and the effects of both Ga content and thickness fluctuation are investigated. Among these three distributions, intracell+intercell inhomo is closest to the real distribution of inhomogeneity in the fabrication of CIGS solar module. In reality, Ga content fluctuation is serious and thickness fluctuation can be well controlled. The simulation results show that in terms of Ga content fluctuation, Intracell fluctuation causes VOC degradation, and intercell fluctuation causes JSC and fill factor degradations, and in terms of thickness fluctuation, VOC remains the same due to the constant Ga content, JSC is degraded due to the increase of excess carrier recombination, and the tendency of fill factor degradation is similar to Ga content fluctuation.

Contents
誌謝 III
Related Publications:（相關論文發表） IV
摘要 V
Abstract VII
List of Figures XII
List of tables XIV
Chapter 1 1
Introduction 1
1.1 Motivation 1
1.2 Dissertation Organization 2
References: 6
Chapter 2 9
Fabrication of CIGS TFTs 9
2.1 Introduction 9
2.2 Ring electrodes and square electrodes 10
2.3 Thin down process for channels 15
2.4 Al2O3 dielectric layer 17
2.5 I-V characteristics of the CIGS TFTs 21
2.6 Summary 23
References: 24
Chapter 3 27
Material analysis of CIGS channels with and without the thin down process 27
3.1 Introduction 27
3.2 PL measurements for CIGS channels 27
3.3 XRD measurements for CIGS channels 32
3.4 Summary 34
References 35
Chapter 4 37
Electrical characteristics of CIGS TFTs with different element ratios 37
4.1 Introduction 37
4.2 I-V characteristics of CIGS TFTs with different Ga content 37
4.3 I-V characteristics of CIGS TFTs with different Cu content 38
4.4 Sumarry 42
References 43
Chapter 5 45
3D simulation for CIGS solar cells/modules 45
5.1 Introduction 45
5.2 Cell/module structures 46
5.3 Performance of CIGS solar modules with different Ga content and conduction band offset (CBO) 48
5.4 Residual strain effects on the performance of CIGS solar modules 52
5.5 Summary 55
References 57
Chapter 6 61
Inhomogeneity effects on CIGS solar modules 61
6.1 Introduction 61
6.2 Inhomogeneity distributions 61
6.3 CIGS solar modules with Ga fluctuation 63
6.4 CIGS solar modules with thickness fluctuation 67
6.5 Summary 71
References 73
Chapter 7 75
Summary and Future Work 75
7.1 Summary 75
7.2 Future work 77

[1]M. A. Green, K. Emery, Y. Hishikawa, W. Warta, and E. D. Dunlop, “Solar cell efficiency tables (Version 45),” Prog. Photovolt: Res. Appl. Vol. 23, pp. 1–9, 2015.
[2]M. Powalla and B. Dimmler, “Scaling up issues of CIGS solar cells,” Thin Solid Films, vol. 361-362, pp. 540-546, 2000.
[3]C. W. Chen, T. C. Chang, P. T. Liu, H. Y. Lu, K. C. Wang, C. S. Huang, C. C. Ling, and T. Yuen, “High-performance hydrogenated amorphous Si TFT for AMLCD and AMOLED applications,” IEEE Electron Device Lett., vol. 26, pp. 10, 731–733, 2005.
[4]H. Gleskova, and S. Wagner, “Electron mobility in amorphous silicon thin-film transistors under compressive strain,” Applied Physics Letters, vol. 79, pp. 3347 2001.
[5]S. Dinca, G. Ganguly, Z. Lu, E. A. Schiff, V. Vlahos, C. R. Wronski, and Q. Yuan, “Hole Drift-Mobility Measurements in Contemporary Amorphous Silicon,” Materials Research Society Symposium Proceedings, vol. 762, pp. 340-350, 2003.
[6]J. W. Lee, J. D. Cohen, and W. N. Shafarman, “The determination of carrier mobilities in CIGS photovoltaic devices using high-frequency admittance measurements,” Thin Solid Films, vol. 480-481, pp. 336, 2005.
[7]B. Schumann, H. Neumann, A. Temple, G. Kűhn, and E. Nowak, “Structural and electrical properties of CuIn0.7Ga0.3Se2 epitaxial layers on GaAs substrates,” Krist. Tech., vol. 15, pp. 71, 1980.
[8]M. Rusu, P. Gashin, and A. Simashkevich, “Electrical and luminescent properties of CuGaSe2 crystals and thin films,” Sol. Energy Mater. Sol. Cells, vol. 70, pp. 175, 2001.
[9]S. A. Dinca, E. A. Schiff, W. N. Shafarman, B. Egaas, R. Noufi, and D. L. Young, “Electron drift-mobility measurements in polycrystalline CuIn1−xGaxSe2 solar cells,” Applied Physics Letters, vol. 100, pp. 103901, 2012.
[10]D. Kuciauskas, J. V. Li, M. A. Contrera, J. Pankow, and P. Dippo, “Charge carrier dynamics and recombination in graded band gap CuIn1−xGaxSe2polycrystalline thin-film photovoltaic solar cell absorbers,” Journal of Applied Physics, vol. 114, pp. 154505, 2013.
[11]W.-W. Hsu, J. Y. Chen, T.-H. Cheng, S. C. Lu, W.-S. Ho, Y.-Y. Chen, Y.-J. Chien, and C. W. Liu, “Surface passivation of Cu(In,Ga)Se2 using atomic layer deposited Al2O3,” Applied Physics Letters, vol. 100, pp. 023508, 2012.
[12]M. Bouttemy, P. Tran-Van, I. Gerard, and A. Etcheberry, “Thinning of CIGS solar cells: Part I: Chemical processing in acidic bromine solutions,” Thin Solid Films, vol. 519, pp. 7207-7211, 2011.
[13]A. K. Harman, S. Ninomiya, and S. Adachi, "Optical constants of sapphire (alpha-Al2O3) single crystals". Journal of Applied Physics, vol. 76, pp. 8032–8036, 1994.
[14]J. R. Weber, A. Janotti, and C. G. Van de Walle, “Native defects in Al2O3 and their impact on III-V/Al2O3 metal-oxide-semiconductor-based devices,” J. Appl. Phys., vol. 109, pp. 033715, 2011.
[15]T. Kamiya, K. Nomura, and H. Hosono, “Present status of amorphous In–Ga–Zn–O thin-film transistors,” Sci Technol. Adv. Mater., vol. 11, pp. 044305, 2010.
[16]S. Kang, R. Sharma, J.-K. Sim, C.-R. Lee, “Band gap engineering of tandem structured CIGS compound absorption layer fabricated by sputtering and selenization,” Journal of Alloys and Compounds, vol. 563, pp. 207–215, 2013.
[17]W. K. Metzger, I. L. Repins, M. Romero, P. Dippo, M. Contreras, R. Noufi, and D. Levi, “Recombination kinetics and stability in polycrystalline Cu(In,Ga)Se2 solar cells,” Thin Solid Films, vol. 517, pp. 2360-2364 2009.
[18]P. O. Grabitz, U. Rau, and J. H. Werner, “Modeling of spatially inhomogeneous solar cells by a multi-diode approach” phys. stat. sol. (a), vol. 202, No. 15, pp. 2920–2927, 2005.
[19]Synopsys, “Sentaurus Device User Guide”, p. 6, Synopsys, Inc. 2014.
[20]S.-H. Han, F. S. Hasoon, A. M. Hermann, and D. H. Levi, “Spectroscopic evidence for a surface layer in CuInSe2:Cu deficiency,” Appl. Phys. Lett., vol. 91, pp. 021904, 2007.
[21]M. Gloeckler, Ph.D. thesis, “DEVICE PHYSICS OF Cu(In,Ga)Se2 THIN-FILM SOLAR CELLS,” pp. 31, Colorado State University, Colorado, 2002.
[22]W. K. Metzger, I. L. Repins, and M. A. Contreras “Long lifetimes in high-efficiency Cu(In,Ga)Se2 solar cells,” Appl. Phys. Lett., vol. 93, pp. 022110, 2008.
[23]S. Shirakata and T. Nakada “Time-resolved photoluminescence in Cu(In,Ga)Se2 thin films and solar cells,” Thin Solid Film, vol. 515, pp. 6151-6154, 2007.
[24]M. I. Alonso, M. Garriga, C. A. Durante Rincon, E. Hernandez, and M. Leon, “Optical functions of chalcopyrite CuGaxIn1-xSe2 alloys,” Appl. Phys. A, vol. 74, pp. 659-664, 2002.
[25]J. Mann, J. Li, I. Repins, K. Ramanathan, S. Glynn, C. DeHart, and R. Noufi, “Reflection Optimization for Alternative Thin-Film Photovoltaics,” IEEE Journal of Photovoltaics, vol. 3, pp. 472-475, 2013.
[26]M. Elbar, S. Tobbeche, A. Merazga, “Effect of top-cell CGS thickness on the performance of CGS/CIGS tandem solar cell”, Solar Energy, vol. 122, pp. 104-112, 2015.
[27]T. Minemoto , T. Matsui, H. Takakura, Y. Hamakawa, T. Negami, Y. Hashimoto, T. Uenoyama, and M. Kitagawa, “Theoretical analysis of the effect of conduction band offset of window/CIS layers on performance of CIS solar cells using device simulation,” Solar Energy Materials & Solar Cells, vol. 67, pp. 83-88, 2001.
[28]R. - J. Hsu, Master thesis, “IBC and CIGS Based Solar Cell Simulation,” National Taiwan University, 2011.
[29]A. Slobodskyy, T. Slobodskyy, T. Ulyanenkova, S. Doyle, M. Powalla, T. Baumbach, and U. Lemmer, “In-depth analysis of the CuIn 1 − x Ga x Se 2 film for solar cells, structural and optical characterization,” Appl. Phys. Lett., vol. 97, pp. 251911, 2010.
[30]J. H. Werner, J. Mattheis, and U. Rau, “Efficiency limitations of polycrystalline thin film solar cells: case of Cu(In,Ga)Se2” Thin Solid Films, vol. 480-481, pp. 399-409, 2005.
[31]B.-C. Shih, Y. Zhang, W. Zhang,and P. Zhang, “Screened Coulomb interaction of localized electrons in solids from first principles” Phys. Rev. B., vol. 85, pp. 045132, 2012.
[32]S. H. Song, S. A. Campbell, “Heteroepitaxy and the Performance of CIGS Solar Cells” Proc. Photovoltaic Specialists Conference (PVSC), IEEE 39th, p. 2534-2539, 2013.
[33]S. Luo, J-H. Lee, C. W. Liu, J-M. Shieh, C-H. Shen, T-T. Wu, D. Jang, J. R. Greer, “Strength, stiffness, and microstructure of Cu(In,Ga)Se2 thin films deposited via sputtering and co-evaporation”, Appl. Phys. Lett., vol. 105, pp. 011907, 2014.
[34]I. V. Bodnar, A. P. Bologa, L. A. Makovetskaya, G. P. Popelnyuk, “Thermal Expansion and Conductivity of CuGax, In1-xSe2 Solid Solution” Crystal Res. &technol., vol. 20, pp. K29-K33, 1985.
[35]S.-H. Wei, A. Zunger, I.-H. Choi, and P. Y Yu, “Trends in band-gap pressure coefficients in chalcopyrite semiconductors” Phys. Rev. B., vol. 58, pp. R1710--R1713, 1998.
[36]A. Slobodskyy, T. Slobodskyy, T. Ulyanenkova, S. Doyle, M. Powalla, T. Baumbach, and U. Lemmer, “In-depth analysis of the CuIn 1 − x Ga x Se 2 film for solar cells, structural and optical characterization,” Appl. Phys. Lett. vol. 97, pp. 251911, 2010.
[37]J. H. Werner, J. Mattheis, and U. Rau, “Efficiency limitations of polycrystalline thin film solar cells: case of Cu(In,Ga)Se2” Thin Solid Films vol. 480-481, pp. 399-409, 2005.
[38]S. Luo, J-H. Lee, C. W. Liu, J-M. Shieh, C-H. Shen, T-T. Wu, D. Jang, J. R. Greer, “Strength, stiffness, and microstructure of Cu(In,Ga)Se2 thin films deposited via sputtering and co-evaporation”, Appl. Phys. Lett., vol. 105, pp. 011907, 2014.
[39]M. I. Alonso, M. Garriga, C. A. Durante Rincon, E. Hernandez, and M. Leon, “Optical functions of chalcopyrite CuGaxIn1-xSe2 alloys,” Appl. Phys. A, vol. 74, pp. 659-664, 2002.
[40]C. H. Henry, “Limiting Efficiency of Ideal Single and Multiple Energy Gap Terrestrial Solar Cells,” J. Appl. Phys., vol. 51, pp. 4494, 1980.