構成染料感光型太陽能電池光陽極的材料主要屬於多晶n型氧化物半導體，其中有組成奈米結構層的二氧化鈦或氧化鋅以及構成透明電極的氟摻雜二氧化錫，奈米結構層主要的功用為吸附染料以及傳輸光電子。若以能階觀點分析光電子在光陽極中的傳輸過程，則可發現光陽極材料系統中存在著阻礙電子傳輸的介面蕭基能障以及晶界背對背蕭基能障，前者來自於透明電極與奈米結構層能帶位置的差異；後者則是奈米結構層中多晶氧化物半導體的材料本性。若要進一步提升光陽極中光電子的收集效率，必須降低此兩種能障對電子傳輸的影響。
導入鋁摻雜氧化鋅透明電極可以避免介面蕭基能障的產生，藉著溶凝膠法製程以及三階段熱處理程序，可以成功製備出具高透光性、低電阻率的鋁摻雜氧化鋅薄膜，其電傳導性的來源經確認是三階段熱處理程序中第二段高溫結晶處理及第三段氫還原處理的混合效果，前者主要在消除薄膜中的晶體缺陷；後者則是為了釋放傳導用的自由電子。根據近紅外光區穿透光譜以及Drude模型的預測，可以藉著修改薄膜旋鍍製程中的預熱程序，來抑制薄膜的晶粒尺寸與結晶性以增加氫熱處理時氫在薄膜中擴散的效率，進而改善鋁摻雜氧化鋅薄膜電傳導特性。阻抗分析的結果則可以確認製程中關鍵的氫熱處理的作用主要是為了摧毀代表背對背蕭基能障的晶界電容並釋放出傳導用的自由電子，等效電路模型證實晶界電容會由理想電容轉變成代表漏電電容的恆定相單元以及代表自由電子擴散現象的Warburg單元，此一事實說明了阻礙電子傳輸的晶界能障是可以被摧毀的。
透過修正測量試片結構，阻抗分析證實介孔二氧化鈦、介孔氧化鋅與氧化鋅奈米線這三種光陽極奈米結構層都存在著晶界電容，其形態會受到材料微結構與結晶性的影響，三種材料各自對應著三種不同的晶界電容形態。對不同溫度燒結的介孔氧化鋅所建構的等效電路模型證實晶界與表面缺陷濃度以及晶粒尺寸會決定空乏區延伸進晶粒的程度進而影響晶界電容的特性，同時氫熱處理亦被證實可以摧毀光陽極奈米結構層中的晶界能障並因此提升電池元件的短路電流。

The photoanode of dye-sensitized solar cell is composed of polycrystalline n-type oxide semiconductors, including nanostructured TiO2 or ZnO and transparent F-doped SnO2 electrode. The functions of nanostructured layers are to adsorb dye and to transport injected photo electrons. The analysis on electron transport in photoanode according to the energetic perspective illustrates that the interfacial Schottky barrier and grain boundary back-to-back Schottky barrier which restrict electron transport theoretically exist in the photoanodic materials. The former results from the difference of band structure between the nanostructured layer and the transparent electrode, and the latter is attributed to the nature of polycrystalline oxide semiconductor in the nanostructured layer. In order to promote the electron collection efficiency in photoanode, it is necessary to suppress the influence of the energetic barriers on electron transport.
Introducing transparent Al-doped ZnO electrode into photoanode could eliminate the interfacial Schottky barrier. By using sol-gel method with the three-steps annealing procedure, it succeeded in preparing the Al-doped ZnO film with high transmittance and low resistivity. The origin of electric conduction of sol-gel derived AZO films is verified as the combining effect of the high temperature annealing to enhance crystal quality that provides higher mobility of electrons and the reduction annealing to release the localized electrons caused by oxygen absorption. According to the deductions from near infrared transmittance spectra based on Drude model, an effective method to improve electronic conduction is obtained. By modifying the preheating procedure, the grains of Al-doped ZnO films become smaller and more defected, resulting in better electrical conductivity. The results of impedance analysis demonstrated that the function of hydrogen annealing is to destroy the energetic barriers at grain boundaries and releases free electrons which are evidenced by the presence of parallel resistor-capacitor circuit, constant phase element and short Warburg element in equivalent circuits. It manifests that the energetic barrier which restrict electron transport is able to be destroyed.
The existence of grain boundary barriers in the polycrystalline oxide semiconductors applied to photoanodic nanostructured layer (mesoporous TiO2, mesoporous ZnO, and nanowired ZnO) is verified by preparing samples with adequate structures and employing precise impedance analysis. The formation of the grain boundary barriers is dominated by material structural characteristics, such as density of lattice defects, crystalline orientation, and grain sizes. Three types of grain boundary barriers are observed in this study. The analysis for the samples with different sintering temperature showed that the characterization of grain boundary barrier is governed by the depth of depletion region determined by the grain size and defect density at grain boundary. Furthermore, it is also manifested that hydrogen annealing could destroy the grain boundary barriers which restrict injected photoelectron transport in the photoanodic nanostructured layer via impedance spectroscopy. The destruction of barriers seems to increase the short current of dye-sensitized solar cell.

1 A. Luque, and S. Hegedus, “Handbook of photovoltaic science and enginnering”, John Wiley and Sons, (2003).
2 B. O’Regan, and M. Grätzel, Nature 353, 737 (1991).
3 F. Pichot, J. R. Pitts, and B. A. Gregg, Langmuir 16, 5626 (2000).
4 M. G. Kang, N. G. Park, K. S. Ryu, S. H. Chang and K. J. Kim, Sol. Energy Mater. Sol. Cells 90, 574 (2006).
5 R. Gaudiana, J. Macromol. Sci. Part A-Pure Appl. Chem. 39, 1259 (2002).
6 M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Mueller, P. Liska, N. Vlachopoulos, and M. Grätzel, J. Am. Chem. Soc. 115, 6382 (1993).
7 M. Grätzel, J. Photochem. Photobiol. A-Chem. 164, 3 (2004).
8 M. Schaper, J. Schmidt, H. Plagwitz, and R. Brendel, Prog. Photovolt: Res. Appl. 13, 381 (2005).
9 K. Schwarzburg, and F. Willig, J. Phys. Chem. B 107, 3552 (2003).
10 G. Kron, T. Egerter, J. H. Werner, and U. Rau, J. Phys. Chem. B 107, 3556 (2003).
11 B. A. Gregg, J. Phys. Chem. B 107, 13540 (2003).
12 J. Bisquert, J. Phys. Chem. B 107, 13541 (2003).
13 J. Augustynski, J. Phys. Chem. B 107, 13544 (2003).
14 T. Dittrich, E. A. Lebedev, and J. Weidmann, Phys. Status Solidi A-Appl. Mat. 165, R5 (1998).
15 B. O. Aduda, P. Ravirajan, K. L. Choy, and J. Nelson, Int. J. Photoenergy 6, 142 (2004).
16 E. Hendry, M. Koeberg, B. O'Regan, and M. Bonn, Nano Lett. 6, 755 (2006).
17 M. Law, L. E. Greene, J. C. Johnson, R. Saykally, and P. Yang, Nat. Mater. 4, 445 (2005).
18 J. B. Baxter and E. S. Aydil, Appl. Phys. Lett. 86, 053114 (2005).
19 J. Jiu, S. Isoda, F. Wang, and M. Adachi, J. Phys. Chem. B 110, 2087 (2006).
20 G. K. Mor, O. K. Varghese, M. Paulose, K. Shankar, and C. A. Grimes, Sol. Energy Mater. Sol. Cells 90, 2011 (2006).
21 M. Grätzel, Nature 414, 338 (2001).
22 Q. Zhang, T. P. Chou, B. Russo, S. A. Jenekhe, and G. Cao, Adv. Funct. Mater. 18, 1654 (2008).
23 H. M. Nguyen, R. S. Mane, T. Ganesh, S. H. Han, and N. Kim, “Aggregation-Free ZnO Nanocrystals Coupled HMP-2 Dye of Higher Extinction Coefficient for Enhancing Energy Conversion Efficiency”, J. Phys. Chem. C (2009), doi: 10.1021/jp901736w (in press).
24 M. Grätzel, Inorg. Chem. 44, 6841 (2005).
25 R. van de Krol, and H.L. Tuller, Solid State Ion. 150, 167 (2002).
26 M. A. Alim, S. Li, F. Liu, and P. Cheng, Phys. Status Solidi A-Appl. Mat. 203, 410 (2006).
27 J. R. Lee, Y. M. Chiang, and G. Ceder, Acta mater. 45, 1247 (1997).
28 W. Jo, S. J. Kim, and D. Y. Kim, Acta mater. 53, 4185 (2005).
29 S. Anas, R. V. Mangalaraja, M. Poothayal, S. K. Shukla, and S. Ananthakumar, Acta mater. 55, 5792 (2007).
30 M. Tiemann, Chem. Eur. J. 13, 8376 (2007).
31 Y. Shimizu, F. C. Lin, Y. Takao, and M. Egashira, J. Am. Ceram. Soc. 81, 1633 (1998).
32 M. R. Mohammadi, and D. J. Fray, Acta mater. 55, 4455 (2007).
33 H. J. Snaith, and M. Grätzel, Adv. Mater. 18, 1910 (2006).
34 F. Wooten, “Optical properties of solids”, Academic Press, (1972).
35 H. Hosono, Thin Solid Films 515, 6000 (2007).
36 J. G. Lu, S. Fujit, T. Kawaharamura, H. Nishinaka, Y. Kamada, T. Ohshima, Z. Z. Ye, Y. J. Zeng, Y. Z. Zhang, L. P. Zhu, H. P. He, and B. H. Zhao, J. Appl. Phys. 101, 083705 (2007).
37 G. J. Exarhos, X. D. Zhou, Thin Solid Films 515, 7025 (2007).
38 E. Barsoukov, and J. R. Macdonald, “Impedance spectroscopy: theory, experiment, and applications”, John Wiley and Sons, (2005).
39 “Agilent Technology Impedance Measurement Handbook”, Agilent Technology, (2006).
40 “Basics of Electrochemical Impedance Spectroscopy”, Gamry Instruments, (2007).
41 “Basics of Electrochemical Impedance Spectroscopy”, Princeton Applied Research, (2007).
42 R. Schmidt, and A. W. Brinkman, Adv. Funct. Mater. 17, 3170 (2007).
43 K. Ellmer, A. Klein, and B. Rech, “Transparent conductive zinc oxide”, Springer, (2008).
44 Ü. Özgür, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Doğan, V. Avrutin, S. J. Cho, and H. Morkoç, J. Appl. Phys. 98, 041301 (2005).
45 J. G. Li, and T. Ishigaki, Acta mater. 52, 5143 (2004).
46 M. Batzill, and U. Diebold, Prog. Surf. Sci. 79, 47 (2005).
47 T. Kamiya, and H. Hosono, Int. J. Appl. Ceram. Technol. 2, 285 (2005).
48 U. I. Gayaa, and A. H. Abdullah, J. Photochem. Photobiol. C-Photochem. Rev. 9, 1 (2008).
49 J. M. Carlsson, H. S. Domingos, P. D. Bristowe, and B. Hellsing, Phys. Rev. Lett. 91, 165506-1 (2003).
50 桑原誠, 表面科學 6, 340 (1985).
51 S. Ito, T. N. Murakami, P. Comte, P. Liska, C. Grätzel, M. K. Nazeeruddin, and M. Grätzel, Thin Solid Films 516, 4613 (2008).
52 L. Y. Chen, W. H. Chen, J. J. Wang, Franklin C. N. Hong, and Y. K. Su, Appl. Phys. Lett. 85, 5628 (2004).
53 O. Bamiduro, H. Mustafa, R. Mundle, R. B. Konda, and A. K. Pradhan, Appl. Phys. Lett. 90, 252108 (2007).
54 J. Steinhauser, S. Faÿ, N. Oliveira, E. Vallat-Sauvain, and C. Ballif, Appl. Phys. Lett. 90, 142107 (2007).
55 M. Ohyama, H. Kozuka, and T. Yoko, Thin Solid Films 306, 78 (1997).
56 T. Schuler, and M. A. Aegerter, Thin Solid Films 351, 125 (1999).
57 J. H. Lee, and B. O. Park, Thin Solid Films 426, 94 (2003).
58 V. Musat, B. Teixeira, E. Fortunato, R.C.C. Monteiro, Thin Solid Films 502, 219 (2006).
59 S. Mridha, and D. Basak, J. Phys. D: Appl. Phys. 40, 6902 (2007).
60 M. Gabás, S. Gota, J. Ramón, Ramos-Barrado, M. Sánchez, N. T. Barrett, J. Avila, and M. Sacchi, Appl. Phys. Lett. 86, 042104 (2005).
61 Y. J. Lin, Appl. Phys. Lett. 86, 216101 (2005).
62 M. Gabás, S. Gota, J. Ramón, Ramos-Barrado, M. Sánchez, N. T. Barrett, J. Avila, and M. Sacchi, Appl. Phys. Lett. 86, 216102 (2005).
63 O. Bamiduro, H. Mustafa, R. Mundle, R. B. Konda, and A. K. Pradhan, Appl. Phys. Lett. 90, 252108 (2007).
64 L. Schmidt-Mende, and J. L. MacManus-Driscoll, Mater. Today 10, 40 (2007).
65 L. S. Vlasenko, and G. D. Watkins, Physica B 376, 677 (2006).
66 K. Ellmer, and R. Mientus, “Carrier transport in polycrystalline transparent conductive oxides: A comparative study of zinc oxide and indium oxide”, Thin Solid Films (2007), doi:10.1016/j.tsf.2007.05.084 (in press).
67 B. Y. Oh, M. C. Jeong, and J. M. Myoung, Appl. Surf. Sci. 253, 7157 (2007).
68 K. L. Brower, Phys. Rev. B 42, 3444 (1990).
69 M. D. McCluskey, S. J. Jokela, K. K. Zhuravlev, P. J. Simpson, and K. G. Lynn, Appl. Phys. Lett. 81, 3807 (2002).
70 K Ellmer, J. Phys. D 34, 3097 (2001).
71 C. Agashe, O. Kluth, J. Hüpkes, U. Zastrow, B. Rech, and M. Wuttig, J. Appl. Phys. 95, 1911 (2004).
72 B. Z. Dong, H. Hu, G. J. Fang, X. Z. Zhao, D. Y. Zheng, and Y. P. Sun, J. Appl. Phys. 103, 073711 (2008).
73 G. J. Exarhos , and X. D. Zhou, Thin Solid Films 515, 7025 (2007).
74 C. R. M. Grovenor, J. Phys. C: Solid State Phys. 18, 4079 (1985).
75 T. Sugiura, S. Itoh, T. Ooi, T. Yoshida, K. Kuroda, and H. Minoura, J. Electroanal. Chem. 473, 204 (1999).
76 D.H. Zhang and H.L. Ma, Appl. Phys. A 62, 487 (1996).
77 M. Regragui, V. Jousseaume, M. Addou, A. Outzourhit, J.C. Bernéde, and B. El Idrissi, Thin Solid Films 397, 238 (2001).
78 M. I. Mendelson, J. Am. Ceram. Soc. 52, 443 (1969).
79 M. Ohyama, H. Kozuka, and T. Yoko, Thin Solid Films 306, 78 (1997).
80 F. Urbach, Phys. Rev. 92, 1324 (1953).
81 S.W. Xue, X.T. Zu, W.L. Zhou, H .X. Deng, X. Xiang, L. Zhang and H. Deng, J Alloys Compd. 448, 21 (2008).
82 S. Y. Myong, J. Steinhauser, R. Schluchter, S. Faÿ, E. Vallat-Sauvaina, A. Shaha, C. Ballifa, A. Rufenach, Sol. Energy Mater. Sol. Cells 91, 1269 (2007).
83 L. Y. Chen, W. H. Chen, J. J. Wang, Franklin C. N. Hong, and Y. K. Su, Appl. Phys. Lett. 85, 5628 (2004).
84 B. Bayraktaroglu, K. Leedy, and R. Bedford, Appl. Phys. Lett. 93, 022104 (2008).
85 N. Ohashi, T. Ishigaki, N. Okada, T. Sekiguchi, I. Sakaguchi, and H. Haneda, Appl. Phys. Lett. 80, 2869 (2002).
86 S. Y. Myong, and K. S. Lim, Appl. Phys. Lett. 82, 3026 (2003).
87 C. G. Van de Walle, Phys. Rev. Lett. 85, 1012 (2000).
88 R. Shao, S. V. Kalinin, and D. A. Bonnell, Appl. Phys. Lett. 82, 1869 (2003).
89 M. Andres-Verges, and A.R. West, J. Electroceram. 1, 125 (1997).
90 A. Smith, J. F. Baumard, P. Abélard, and M. F. Denanot, J. Appl. Phys. 65, 5119 (1989).
91 J. Jose, and M. A. Khadar, Acta Mater. 49, 729 (2001).
92 J. J. Wu and Daniel K. P. Wong, Adv. Mater. 19, 2015 (2007).
93 S. Westerlund, and L. Ekstam, IEEE Trns. Dielectr. Electr. Insul. 1, 827 (1994).
94 P. Zoltowski, J. Electroanal. Chem. 443, 149 (1998).
95 K. Biswas, S. Sen, and P. K. Dutta, IEEE Trans. Circuits Syst. II-Express Briefs 53, 802 (2006).
96 S. Skale, V. Doleček, and M. Slemnik, Corrosion Sci. 49, 1045 (2007).
97 J. Bisquert, G. Garcia-Belmonte, P. Bueno, E. Longo, and L. O. S. Bulhöes, J. Electroanal. Chem. 452, 229 (1998).
98 V.S. Muralidharan, Anti-Corros. Methods Mater. 44, 26 (1997).
99 C. Ho, I. D. Raistrick, R. A. Huggins, J. Electrochem. Soc. 137, 343 (1980).
100 H. J. Snaith, and L. Schmidt-Mende, Adv. Mater. 19, 3187 (2007).
101 B. C. O’Regan, K. Bakker, J. Kroeze, H. Smit, P. Sommeling, and J. R. Durrant, J. Phys. Chem. B 110, 17155 (2006).
102 J. Bisquert, D. Cahen, G. Hodes, S. Rhle, and A. Zaban, J. Phys. Chem. B 108, 8106 (2004).
103 J. Bisquert, G. Garcia-Belmonte, and F. Fabregat-Santiago, J. Solid State Electrochem. 3, 337 (1999).
104 J. Nelson, and R. E. Chandler, Coord. Chem. Rev. 248, 1181 (2004).
105 L. Han, N. Koide, Y. Chiba, and T. Mitate, Appl. Phys. Lett. 84, 2433 (2004).
106 M. Adachi, M. Sakamoto, J. Jiu, Y. Ogata, and S. Isoda, J. Phys. Chem. B 110, 13872 (2006).
107 T. Hoshikawa, R. Kikuchi, and K. Eguchi, J. Electroanal. Chem. 588, 59 (2006).
108 J. Bisquert, M. Grätzel, Q. Wang, and F. Fabregat-Santiago, J. Phys. Chem. B 110, 11284 (2006).
109 H. M. Cheng, W. H. Chiu, C. H. Lee, S. Y. Tsai, and W. F. Hsieh, J. Phys. Chem. C 112, 16359 (2008).
110 J. E. B. Randles, Discuss Faraday Soc. 1, 11 (1947).
111 Y. M. Chiang, D. Birnie III, and W. D. Kingery, “Physical Ceramics”, John Wiley and Sons, (1997).