本研究致力於探討銅銦鎵硒硫[Cu(Inx,Ga1-x)(Sey,S1-y)2, CIGSSe]薄膜太陽能電池元件樣品之成分分佈。為了可以觀測到樣品裡各元素濃度的縱深變化與能帶結構，我們將樣品的薄膜層研磨為一斜面，並利用X光光電子能譜於ZnO:B/CdS/CIGSSe/Mo/Soda-lime glass結構上觀察各重要元素的濃度分布、化學鍵結及電子能帶變化，並輔以X光吸收光譜來探測薄膜介面間之化合物組成特性。
由CdS層與CIGSS層之間的能帶偏移現象可推斷出，導電帶能量差為峭壁型(cliff type)的能量偏移形式，然而較大的峭壁式能量差可能造成增加載子覆合機率與限制開路電壓而影響效率。由B 1s的光電子能譜之擬合(fitting)結果可發現，有效摻雜的硼相對於硼氧化物的強度比例往ZnO底層漸增，而在ZnO層底部靠近CdS層的載子濃度提高則有助於載子的流動。於CIGSSe層的濃度分布方面，銦在CIGSSe層底部有相對較低的比例，鎵則相反，隨著鎵的比例變大則會造成CIGSSe的能隙增高。然而，由光電子能譜量化的結果顯示出在整個CIGSSe層皆具有銅缺乏現象，銅空缺的產生使得電洞濃度提高，CIGSSe層則易形成p型半導體。我們還發現了在CIGSSe/Mo之介面生成了MoSe2與MoS2等化合物，使得介面間形成有利於電子傳輸的歐姆接觸(ohmic contact)，可以增進元件的效率。

In this study, we investigated the depth-dependent compositions and band structure of Cu(In,Ga)(Se,S)2-based solar cell device. In order to measure the elemental composition distribution and the band structure of the multiple-layered films, we polished the CIGSSe-based solar cell with a gradient along the normal direction of the sample to observe the variations of elemental distributions, chemical bonds and electronic band structure by means of X-ray photoemission spectroscopy (XPS). The structural characteristic at the interfaces of layers were also investigated by using X-ray absorption spectroscopy (XAS).
According to the observation of the band offset at the interface between CdS and CIGSSe layers, we can deduce that the conduction band corresponds to the cliff type (Ec= -0.47 eV). Therefore, this type of band structure is possibly increased the recombination probability at the interface and lead to a limitation in the open circuit voltage. The fitting results of B 1s photoelectron spectra reveals that in the bottom of ZnO layer, the concentration of the dopant boron becomes increased with respect to the boron oxide. Because the carrier concentration is increased in the region of ZnO layer near CdS, it would be beneficial to improve the carrier transport. Copper depletion was observed in the whole region of CIGSSe layer, which plays a role of acceptor due to copper vacancies and facilitates the formation of p-type semiconductor. The concentration ratio of In/Ga is decreased from the top to the bottom of the CIGSSe layer. The optimal band-gap distribution could be achieved by controlling the In/Ga ratios. We also found that the Mo(S,Se)2 layer was formed at the interface of CIGSSe and Mo layers, providing an ohmic contact and increasing the open circuit voltage to improve the device performance of the CIGSSe solar cell.

1. Arturo Morales-Acevedo, “Solar Cells - Research and Application Perspectives”, ISBN 978-953-51-1003-3, Published: March 6, (2013).
2. http://en.wikipedia.org/wiki/Sunlight
3. Ngrid Repins, Miguel A. Contreras, Brian Egaas, Clay DeHart, John Scharf, Craig L. Perkins, Bobby To , and Rommel Noufi, “19.9% efficient ZnO-CdS-CuInGaSe2 solar cell with 81.2% fill factor”, Prog. Photovilt: Res. Appl. 16, 235-239, (2008).
4. David W. Niles, Kannan Ramanathan, Falah Hasoon, Rommel Noufi, Brian J. Tielsch, and Julia E. Fulghum, “Na impurity chemistry in photovoltaic CIGS thin films: Investigation with x-ray photoelectron spectroscopy”, J. Vac. Sci. Technol. A 15, 3044, (1997).
5. N.Kohara, S.Nishiwaki, Y.Hashimoto, T.Negami , and T.Wada, “Electrical properties of the Cu(In,Ga)Se2/MoSe2/Mo structure”, Solar Energy Materials and Solar Cells 67, 209–215, (2001).
6. D.Ohashi, T. Nakada, and A. Kunioka, “Improved CIGS thin-film solar cells by surface sulfurization using In2S3 and sulfur vapor”, Solar Energy Materials & Solar Cells, vol. 67, 261–265, (2001).
7. Uwe Rau and Marion Schmidt, “Electronic properties of ZnO/CdS/Cu(In,Ga)Se2 solar cells aspects of heterojunction formation”, Thin Solid Films, 387, 141-146, 2001.
8. Rudmann, D., “Effects of Sodium on Growth and Properties of Cu(In,Ga)Se2 Thin Films and Solar Cells”, in Department of Physics. 2004, Swiss Federal Institute of Technology: Zurich, p.17.
9. Yoshihiro Hamakawa, “Thim-film solar cells：next generation photovoltaics and its applications”, New York: Springer, 2004, p.164-169
10. Zhang S, Wei S, and Zunger A, “Stabilization of ternary compounds via ordered arrays of defect Pairs”, Phys. Rev. Lett. P78, 4059-4062, (1997).
11. M.L. Fearheiley, “The phase relations in the Cu,In,Se system and the growth of CuInSe2 single crystals”, Solar Cells, vol 16, p.91-100, (1986).
12. S. H. Wei, S. B. Zhang, and Alex Zunger, “Effects of Ga addition to CuInSe2 on its electronic, structural, and defect properties”,Appl. Phys. Letters, Vol. 72, 3199, (1998).
13. U. Rau, M. Schmidt, A. Jasenek, G. Hanna , and H.W. Schock, “Electrical characterization of Cu(In,Ga)Se2 thin-film solar cells and the role of defects for the device performance”, Solar Energy Materials & Solar Cells 67, p.137-143, (2001).
14. F. O. Adurodija, S. K. Kim, S. D. Kim, J. S. Song, K. H. Yoon, and B. T. Ahn, “Characterization of co-sputtered Cu-In alloy precursors for CuInSe2 thin films fabrication by close-spaced selenization”, Solar Energy Materials and Solar Cells, 55, 225–236, (1998).
15. A. Niemegeers, M. Burgelman, R. Herberholz, U. Rau, D. Hariskos, and H. W. Schock, “Model for electronic transport in Cu(In,Ga)Se2 solar cells”, Prog. Photovolt. Res. Appl., 6, 407–421, (1998).
16. T. Dullweber, G. Hanna, U. Rau, and H.W. Schock, “A new approach to high-efficiency solar cells by band gap grading in Cu(In,Ga)Se2 chalcopyrite semiconductors”, Solar Energy Materials & Solar Cells 67, 145-150, (2001).
17. Takashi Minemoto, Yasuhiro Hashimoto, Takuya Satoh, Takayuki Negami, Hideyuki Takakura, and Yoshihiro Hamakawa, “Cu(In,Ga)Se2 solar cells with controlled conduction band offset of window/Cu(In,Ga)Se2 layers”, Journal of Applied Physics 89, 8327, (2001).
18. Chang-Soo Lee, Liudmila Larina, Young-Min Shin, Essam A. Al-Ammar and Byung Tae Ahn, “Design of energy band alignment at the Zn1−xMgxO/Cu(In,Ga)Se2 interface for Cd-free Cu(In,Ga)Se2 solar cells”, Phys. Chem. Chem. Phys., 14, 4789–4795, (2012).
19. S.H. Wei and A. Zunger, “Band offsets and optical bowings of chalcopyrites and Zn‐based II‐VI alloys”, J. Appl. Phys., 78, 3846, (1995).
20. Adolf Goetzberger, Christopher Hebling, and H.W. Schock, “Photovoltaic materials, history, status and outlook”, Materials Science & Engineering：R-Reports, 40(1), p.1-46, (2003).
21. T. Wada, N. Kohara, S. Nishiwaki, T. Negami , “Characterization of the Cu(In,Ga)Se2/Mo interface in CIGS solar cells”, Thin Solid Films 387, 118-122, (2001).
22. R. Caballero, C.A. Kaufmann, T. Eisenbarth, M. Cancela, R. Hesse, T. Unold, A. Eicke, R. Klenk, H.W. Schock, “The influence of Na on low temperature growth of CIGS thin film solar cells on polyimide substrates”, Thin Solid Films 517, 2187–2190, (2009).
23. D. Rudmann, G. Bilger, M. Kaelin, F.-J. Haug, H. Zogg, A.N. Tiwari, “Effects of NaF coevaporation on structural properties of Cu(In,Ga)Se2 thin films”, Thin Solid Films 431 – 432, 37–40, (2003).
24. M. Marudachalam, R. W. Birkmire, H. Hichri, J. M. Schultz, A. Swartzlander and M. M. Al-Jassim, “Phases, morphology, and diffusion in CuInxGa1−xSe2 thin films”, J. Appl. Phys. 82, 2896 (1997).
25. M. Gaba´s, N.T. Barrett, J.R. Ramos-Barrado, S. Gota, T.C. Rojas, M.C. Lo´ pez-Escalante, “Chemical and electronic interface structure of spray pyrolysis deposited undoped and Al-doped ZnO thin films on a commercial Cz-Si solar cell substrate”, Solar Energy Materials & Solar Cells 93, 1356–1365, (2009).
26. M. Bär, S. Nishiwaki, L. Weinhardt, S. Pookpanratana, W. N. Shafarman, “Electronic level alignment at the deeply buried absorber/Mo interface in chalcopyrite-based thin film solar cells”, Appl. Phys. Lett. 93, 042110 , (2008).
27. L. Weinhardt, C. Heske, E. Umbach, T. P. Niesen, S. Visbeck, and F. Karg, “Band alignment at the i-ZnO/CdS interface in Cu(In,Ga)(S,Se)2 thin-film solar cells”, Appl. Phys. Lett. 84, 3175, (2004).
28. Liudmila Larina, Donghyeop Shin, Ji Hye Kim, and Byung Tae Ahn, “Alignment of energy levels at the ZnS/Cu(In,Ga)Se2 interface”, Energy Environ. Sci., 4, 3487, (2001).
29. A. D. Katnani and G. Margaritondo, “Microscopic study of semiconductor heterojunctions: Photoemission measurement of the valance-band discontinuity and of the potential barriers”, Phys. Rev. B: Condens.Matter, 28, 1944, (1983).
30. M. Morkel, L. Weinhardt, B. Lohmuller, C. Heske, E. Umbach, W. Riedl, S. Zweigart and F. Karg, “Flat conduction-band alignment at the CdS/CuInSe2 thin-film solar-cell heterojunction”, Appl. Phys. Lett. 79, 4482, (2001).
31. Chang-Soo Lee, Liudmila Larina, Young-Min Shin, Essam A. Al-Ammar, and Byung Tae Ahn, “Design of energy band alignment at the Zn1−xMgxO/Cu(In,Ga)Se2 interface for Cd-free Cu(In,Ga)Se2 solar cells”, Phys. Chem. Chem. Phys., 14, 4789–4795, (2012).
32. L. Weinhardt, C. Heske, E. Umbach, T. P. Niesen, S. Visbeck, “Band alignment at the i-ZnO/CdS interface in Cu(In,Ga)(S,Se)2 thin-film solar cells”, Appl. Phys. Lett. 84, 3175 (2004).
33. M. Bär, S. Nishiwaki, L. Weinhardt, S. Pookpanratana, W. N. Shafarman, and C. Heske, “Electronic level alignment at the deeply buried absorber/Mo interface in chalcopyrite-based thin film solar cells”, Applied Physics Letters 93, 042110, (2008).
34. H. Ishizaki, M. Imaizumi, S. Matsuda, M. Izaki and T. Ito, “Incorporation of boron in ZnO film from an aqueous solution containing zinc nitrate and dimethylamine-borane by electrochemical reaction”, Thin Solid Films 411, 65–68, (2002).
35. Jianhua Hu and Roy G. Gordon, “Deposition of Boron Doped Zinc Oxide Films and Their Electrical and Optical Properties”, J. Electrochem. Soc., Vol. 139, No. 7, (1992).
36. S. Ben Amor, M. Jacquet, P. Fioux and M. Nardin, “XPS characterisation of plasma treated and zinc oxide coated PET”, Applied Surface Science 255, 5052–5061, (2009).
37. David W. Niles, Kannan Ramanathan, Falah Hasoon, Rommel Noufi, Brian J. Tielsch, and Julia E. Fulghum, “Na impurity chemistry in photovoltaic CIGS thin films: Investigation with x-ray photoelectron spectroscopy”, J. Vac. Sci. Technol. A 15, 3044, (1997).
38. Anneli Önsten , Dunja Stoltz, Pål Palmgren, Shun Yu, Thomas Claesson, Mats Göthelid, and Ulf O. Karlsson, “SO2 interaction with Zn(0001) and ZnO(0001) and the influence of water”, Surface Science, 608, 31–43, (2013).
39. G. Hota, S.B. Idage, Kartic C. Khilar, “Characterization of nano-sized CdS–Ag2S core-shell nanoparticles using XPS technique”, Colloids and Surfaces A: Physicochem. Eng. Aspects 293, 5–12, (2007).
40. Nina Siebers, Jens Kruse, Kai-Uwe Eckhardt, Yongfeng Hu, and Peter Leinweber, “Solid-phase cadmium speciation in soil using L3-edge XANES spectroscopy with partial least-squares regression”, J. Synchrotron Rad., 19, 579–585, (2012).
41. Schmid D, Ruckh M, and Schock HW, “Photoemission studies on Cu(In,Ga)Se2 thin films and related binary selenides”, Applied Surface Science, 103(4), 409–429, (1996).
42. R. A. Gordon, D. Yang, E. D. Crozier, D. T. Jiang, and R. F. Frindt, “Structures of exfoliated single layers of WS2 , MoS2 , and MoSe2 in aqueous suspension”, Phys Rev B, 65, 125407, (2002).
43. M. Bär, L. Weinhardt, and C. Heske, “Chemical structures of the Cu(In,Ga)Se2/Mo and Cu(In,Ga)(S,Se)2/Mo interfaces”, Phys Rev B 78, 075404, (2008).
44. D. Li , G.M. Bancroft , M. Kasrai , M.E. Fleet , X.H. Feng , B.X. Yang , K.H. Tan, “S K- and L-edge XANES and Electronic Structure of Some Copper Sulfide Minerals ”, Phys Chem Minerals 21, 317-324, (1994).
45. D. Guay, W. M. R. Divigalpitiya, D. Belanger, X. H. Feng, “Chemical Bonding in Restacked Single-Layer MoS2 by X-ray Absorption Spectroscopy”, Chem. Mater.6, 614-619, (1994).
46. Nils Martensson and Ralf Nyholm, “Electron spectroscopic determinations of M and N core-hale lifetimes for the elements Nb-Te (Z = 41-52)”, PHYSICAL REVIEW B 24,12, (1981).
47. V. Ya. Leonidov and P. A. G. O’Hare, Fluorine Calorimetry Begell House, New York, p. 194, (1999).
48. R. Caballero, M. Nichterwitz, A. Steigert, A. Eicke, I. Lauermann, H.W. Schock, C.A. Kaufmann, “Impact of Na on MoSe2 formation at the CIGSe/Mo interface in thin-film solar cells on polyimide foil at low process temperatures”, Acta Materialia 63, 54–62, (2014).