臺灣博碩士論文加值系統

染料敏化太陽能電池(以下簡稱染敏電池)在1991年由Michael Grätzel所發明，其具備成本低、製程簡單與環保...等優點，但礙於電池內部的電子再結合反應嚴重使得效率值不易提升，而再結合反應主要發生在摻氟氧化錫(fluorine-doped tin oxide, FTO)/電解液界面與TiO2/電解液界面，藉由加入阻障層(blocking layer)於此界面有助於抑制電子的再結合反應使電池各方面電性質提升。本研究將能階略高於TiO2的Nb2O5阻障層以NbCl5乙醇溶液浸泡沉積在FTO表面，其Nb2O5功能用來抑制FTO/電解液界面的再結合反應。經過穿透式電子顯微鏡(transmission electron microscope, TEM)觀察FTO表面經過3次的浸泡處理後形成厚度大約14奈米的Nb2O5，再利用原子力顯微鏡(atomic force microscpoic, AFM)及掃描式電子顯微鏡(scanning electron microscope, SEM)分析已沉積Nb2O5的FTO表面形貌，此沉積方式會將較為粗糙的FTO表面將之平滑化使表面粗糙度下降，此法有利於TiO2與FTO之間的接觸並降低串聯電阻。定性分析上以X射線光電子能譜(X-ray photoelectron spectroscopy, XPS)與X-射線繞射分析(X-ray Diffraction, XRD)確認Nb2O5的Nb元素所帶的價數為+5與晶型。加入了Nb2O5的染敏電池在1 sun光源並無助於電池效率但在低照度光源時具有最佳的效率值。為了瞭解電池內部各個界面電阻，從暗室與照光情況下利用電化學交流阻抗分析Nb2O5對於抑制再結合的能力具有提升的作用，另一方面也能適當的降低串聯電阻而提高開路電壓以及填充因子。然而過多的Nb2O5阻礙了FTO光穿透度，使TiO2與染料無法有效地吸收光子能量導致短路電流與效率值降低。簡易來說，合宜的阻障層厚度對於電池具有抑制逆反應、增加再結合電阻與降低串聯電阻...等正面的影響力，但沉積過多的阻障層導致無法有效利用光源則會帶來負面的影響。

In 1991, Michael Grätzel has invented the dye-sensitized solar cells (DSSCs) which the efficiency of the solar cells reaches 7 % under 1 sun condition. The DSSCs have many advantages of low cost, environment protection and easy fabrication. DSSCs are limited by the recombination reaction of the electrons from the fluorine-doped tin oxide (FTO) substrate back into the electrolyte and from the TiO2 to electrolyte. In order to suppress the electron recombination at the FTO/electrolyte interface, the Nb2O5 blocking layer which can prevent the electrolyte from direct contact with the FTO substrate is deposited on the FTO to improve the DSSCs performance. The FTO is dipped into NbCl5 ethanol solution for 30 s at room temperature to form Nb2O5 blocking layer after annealing. In transmission electron microscope (TEM) images, the thickness of Nb2O5 blocking layer is about 14 nm through three times dipping cycles. By atomic force microscope (AFM) and scanning electron microscope (SEM) analysis, the surface roughness of FTO decreases with increasing the dipping cycles because Nb2O5 layer forms onto FTO crystal defeat. To make sure the layer composition, the signal of Nb5+ is detected by X-ray photoelectron spectroscopy (XPS) result. Based on the analysis results of transmission line model and electrochemical impedance spectroscopy (EIS), the blocking layer can promote the DSSCs power conversion efficiency (PCE) by increasing the electron recombination resistance and reducing the series resistance. The suppression effect becomes obvious as increasing dipping cycles but a thick blocking layer can resist the illumination income and lower the short circuit current. In other word, the blocking layer has the optimum thickness to enhance the photovoltaic performance and plays a more important role in the indoor or low intensity lighting applications.

摘要 i
Abstract ii
目錄 iii
圖目錄 v
表目錄 viii
第一章 緒論 1
1-1 前言 1
1-2 研究動機與目的 2
第二章 文獻回顧 3
2-1染料敏化太陽能電池的發展 3
2-2染料敏化太陽能電池基本原理 4
2-2-1染料敏化太陽能電池結構簡介 4
2-2-2染料敏化太陽能電池工作原理 5
2-3染料敏化太陽能電池電性 6
2-4低照度染料敏化太陽能電池 7
2-5染料敏化太陽能電池阻障層 12
2-6 交流阻抗分析圖譜 19
2-6-1 交流阻抗分析圖譜基本原理 19
2-6-2 交流阻抗分析應用於染料敏化太陽能電池 22
2-7 開路電壓衰減法 26
第三章 實驗步驟與分析方法 28
3-1 實驗儀器與設備 28
3-2 實驗藥品 28
3-3 染料敏化太陽能電池各部位元件製備 29
3-3-1 光電極基材-透明導電玻璃 29
3-3-2 奈米多孔性二氧化鈦薄膜 30
3-3-3 對電極 30
3-3-4 電解液 31
3-3-5 熱封膜 31
3-3-6 染料N719溶液 31
3-4 實驗流程 31
3-5 染料敏化太陽能電池組裝 32
3-6 太陽能電池元件光電轉換效率測試 32
3-7 電化學交流阻抗圖譜分析 33
3-8 開路電壓衰退法 34
第四章 結果與討論 35
4-1 FTO與FTO-Nb2O5之表面形貌 35
4-1-1 沉積Nb2O5於FTO表面形貌 35
4-1-2 以TEM觀察3 cycles Nb2O5阻障層厚度 39
4-2 鑑定Nb2O5阻障層之組成價數與晶型 41
4-2-1以XPS分析Nb2O5阻障層組成價數 41
4-2-1以XRD分析Nb2O5阻障層晶型 42
4-3 沉積Nb2O5於FTO表面之光電性質 44
4-3-1 在低照度下Nb2O5沉積在FTO表面電流電壓數值 44
4-3-2 在1 sun光源下Nb2O5沉積在FTO表面電流電壓數值 50
4-4 電化學交流阻抗分析 52
4-4-1 電化學交流阻抗分析與電阻計算 52
4-4-2 利用電化學交流阻抗分析傳輸電阻與電子擴散長度 56
4-4-3 利用電化學交流阻抗分析計算β值 58
4-4-4 利用電化學交流阻抗分析不同TiO2厚度與3 cycles Nb2O5 59
4-4-5 電化學交流阻抗分析1 sun與T5螢光低照度之電阻 60
4-5 開路電壓衰退法 61
4-6 Nb2O5阻障層光學性質 63
第五章 結論 65
參考文獻 67

[1]C.-P. Lee, C.-A. Lin, T.-C. Wei, M.-L. Tsai, Y. Meng, C.-T. Li, K.-C. Ho, C.-I. Wu, S.-P. Lau, and J.-H. He, "Economical low-light photovoltaics by using the Pt-free dye-sensitized solar cell with graphene dot/PEDOT:PSS counter electrodes," Nano Energy, Vol. 18, pp. 109-117 (2015).
[2]S. Rühle and D. Cahen, "Electron tunneling at the TiO2/substrate interface can determine dye-sensitized solar cell performance," The Journal of Physical Chemistry B, Vol. 108, pp. 17946-17951 (2004).
[3]H. Choi, C. Nahm, J. Kim, J. Moon, S. Nam, D.-R. Jung, and B. Park, "The effect of TiCl4-treated TiO2 compact layer on the performance of dye-sensitized solar cell," Current Applied Physics, Vol. 12, pp. 737-741 (2012).
[4]E. Palomares, J. N. Clifford, S. A. Haque, T. Lutz, and J. R. Durrant, "Control of charge recombination dynamics in dye sensitized solar cells by the use of conformally deposited metal oxide blocking layers," Journal of the American Chemical Society, Vol. 125, pp. 475-482 (2003).
[5]W. Chen, Y. Qiu, and S. Yang, "A new ZnO nanotetrapods/SnO2 nanoparticles composite photoanode for high efficiency flexible dye-sensitized solar cells," Physical Chemistry Chemical Physics, Vol. 12, pp. 9494-9501 (2010).
[6]L. Li, C. Xu, Y. Zhao, S. Chen, and K. J. Ziegler, "Improving performance via blocking layers in dye-sensitized solar cells based on nanowire photoanodes," ACS Applied Materials & Interfaces, Vol. 7, pp. 12824-12831 (2015).
[7]J. Xia, N. Masaki, K. Jiang, and S. Yanagida, "Sputtered Nb2O5 as a novel blocking layer at conducting glass/TiO2 interfaces in dye-sensitized ionic liquid solar cells," The Journal of Physical Chemistry C, Vol. 111, pp. 8092-8097 (2007).
[8]A. Kay and M. Grätzel, "Dye-sensitized core-shell nanocrystals: improved efficiency of mesoporous tin oxide electrodes coated with a thin layer of an insulating oxide," Chemistry of Materials, Vol. 14, pp. 2930-2935 (2002).
[9]A. K. Chandiran, N. Tetreault, R. Humphry-Baker, F. Kessler, E. Baranoff, C. Yi, M. K. Nazeeruddin, and M. Grätzel, "Subnanometer Ga2O3 tunnelling layer by atomic layer deposition to achieve 1.1 V open-circuit potential in dye-sensitized solar cells," Nano Letters, Vol. 12, pp. 3941-3947 (2012).
[10]F. Fabregat-Santiago, G. Garcia-Belmonte, I. Mora-Seró, and J. Bisquert, "Characterization of nanostructured hybrid and organic solar cells by impedance spectroscopy," Physical Chemistry Chemical Physics, Vol. 13, pp. 9083-9118 (2011).
[11]Q. Wang, J.-E. Moser, and M. Grätzel, "Electrochemical impedance spectroscopic analysis of dye-sensitized solar cells," The Journal of Physical Chemistry B, Vol. 109, pp. 14945-14953 (2005).
[12]H. Meier, "Sensitization of electrical effects in solids1," The Journal of Physical Chemistry, Vol. 69, pp. 719-729 (1965).
[13]R. Memming and H. Tributsch, "Electrochemical investigations on the spectral sensitization of gallium phosphide electrodes," The Journal of Physical Chemistry, Vol. 75, pp. 562-570 (1971).
[14]H. Gerischer, "Electrochemical techniques for the study of photosensitization," Photochemistry and Photobiology, Vol. 16, pp. 243-260 (1972).
[15]C. W. Tang, "Two‐layer organic photovoltaic cell," Applied Physics Letters, Vol. 48, pp. 183-185 (1986).
[16]B. Oregan and M. Gratzel, "A low-cost, high-efficiency solar-cell based on dye-sensitized colloidal TiO2 films," Nature, Vol. 353, pp. 737-740 (1991).
[17]T. Bessho, S. M. Zakeeruddin, C.-Y. Yeh, E. W.-G. Diau, and M. Grätzel, "Highly efficient mesoscopic dye-sensitized solar cells based on donor–acceptor-substituted porphyrins," Angewandte Chemie, Vol. 49, pp. 6646-6649 (2010).
[18]Y.-C. Liu, H.-H. Chou, F.-Y. Ho, H.-J. Wei, T.-C. Wei, and C.-Y. Yeh, "A feasible scalable porphyrin dye for dye-sensitized solar cells under one sun and dim light environments," Journal of Materials Chemistry A, Vol. 4, pp. 11878-11887 (2016).
[19]S. S. Shin, D. W. Kim, D. Hwang, J. H. Suk, L. S. Oh, B. S. Han, D. H. Kim, J. S. Kim, D. Kim, J. Y. Kim, and K. S. Hong, "Controlled interfacial electron dynamics in highly efficient Zn2SnO4-based dye-sensitized solar cells," Chemsuschem, Vol. 7, pp. 501-509 (2014).
[20]S. Qingqing, L. Yafeng, D. Jie, and W. Mingdeng, "Improving the efficiency of dye-sensitized solar cells by photoanode surface modifications," Science China Materials, Vol. 59, pp. 867-883 (2016)
[21]A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, and H. Pettersson, "Dye-sensitized solar cells," Chemical Reviews, Vol. 110, pp. 6595-6663 (2010).
[22]J.-L. Lan, T.-C. Wei, S.-P. Feng, C.-C. Wan, and G. Cao, "Effects of iodine content in the electrolyte on the charge transfer and power conversion efficiency of dye-sensitized solar cells under low light intensities," The Journal of Physical Chemistry C, Vol. 116, pp. 25727-25733 (2012).
[23]A. Luque and S. Hegedus, "Handbook of photovoltaic science and engineering," John Wiley & Sons, (2011).
[24]H.-J. Kim, Y.-C. Kim, J.-T. Hong, M.-J. Kim, H.-W. Seo, J.-W. Park, and J.-Y. Choi, "A study of the photo-electric efficiency of dye-sensitized solar cells under lower light intensity," Journal of Electrical Engineering & Technology, Vol. 2, pp. 513-517 (2007).
[25]F. De Rossi, T. Pontecorvo, and T. M. Brown, "Characterization of photovoltaic devices for indoor light harvesting and customization of flexible dye solar cells to deliver superior efficiency under artificial lighting," Applied Energy, Vol. 156, pp. 413-422 (2015).
[26]Y. S. Tingare, N. S. n. Vinh, H. H. Chou, Y. C. Liu, Y. S. Long, T. C. Wu, T. C. Wei, and C. Y. Yeh, "New acetylene‐bridged 9, 10‐conjugated anthracene sensitizers: application in outdoor and indoor dye‐sensitized solar cells," Advanced Energy Materials, (2017).
[27]S. Rühle, M. Greenshtein, S.-G. Chen, A. Merson, H. Pizem, C. S. Sukenik, D. Cahen, and A. Zaban, "Molecular adjustment of the electronic properties of nanoporous electrodes in dye-sensitized solar cells," The Journal of Physical Chemistry B, Vol. 109, pp. 18907-18913 (2005).
[28]M. Freitag, J. Teuscher, Y. Saygili, X. Zhang, F. Giordano, P. Liska, J. Hua, S. M. Zakeeruddin, J.-E. Moser, and M. Grätzel, "Dye-sensitized solar cells for efficient power generation under ambient lighting," Nature Photonics,Vol. 11, pp. 372-378 (2017).
[29]M. Freitag, F. Giordano, W. Yang, M. Pazoki, Y. Hao, B. Zietz, M. Grätzel, A. Hagfeldt, and G. Boschloo, "Copper phenanthroline as a fast and high-performance redox mediator for dye-sensitized solar cells," The Journal of Physical Chemistry C, Vol. 120, pp. 9595-9603 (2016).
[30]S. G. Chen, S. Chappel, Y. Diamant, and A. Zaban, "Preparation of Nb2O5 coated TiO2 nanoporous electrodes and their application in dye-sensitized solar cells," Chemistry of Materials, Vol. 13, pp. 4629-4634 (2001).
[31]J. Xia, N. Masaki, K. Jiang, and S. Yanagida, "Fabrication and characterization of thin Nb2O5 blocking layers for ionic liquid-based dye-sensitized solar cells," Journal of Photochemistry and Photobiology A: Chemistry, Vol. 188, pp. 120-127 (2007).
[32]J. Xia, N. Masaki, K. Jiang, and S. Yanagida, "Sputtered Nb2O5 as an effective blocking layer at conducting glass and TiO2 interfaces in ionic liquid-based dye-sensitized solar cells," Chemical Communications, pp. 138-140 (2007).
[33]P. Du, L. Song, J. Xiong, Y. Yuan, L. Wang, Z. Xi, D. Jin, and J. Chen, "TiO2/Nb 2O5 core–sheath nanofibers film: Co-electrospinning fabrication and its application in dye-sensitized solar cells," Electrochemistry Communications, Vol. 25, pp. 46-49 (2012).
[34]W. Liu, C. Hong, H.-g. Wang, M. Zhang, and M. Guo, "Enhanced photovoltaic performance of fully flexible dye-sensitized solar cells based on the Nb2O5 coated hierarchical TiO2 nanowire-nanosheet arrays," Applied Surface Science, Vol. 364, pp. 676-685 (2016).
[35]A. K. Chandiran, A. Yella, M. Stefik, L.-P. Heiniger, P. Comte, M. K. Nazeeruddin, and M. Grätzel, "Low-temperature crystalline titanium dioxide by atomic layer deposition for dye-sensitized solar cells," ACS applied materials & interfaces, Vol. 5, pp. 3487-3493 (2013).
[36]吳慧屏, "交流阻抗圖譜應用於染料敏化及鈣鈦礦太陽能電池之研究," 博士, 應用化學系碩博士班, 國立交通大學, 新竹市, (2014).
[37]張育銘, "使用交流阻抗法中傳輸線模組分析氧化鈮阻隔層對染料敏化太陽電池陽極界面逆電流抑制之研究," 碩士, 化學工程學系, 國立清華大學, 新竹市, (2014).
[38]T. Hoshikawa, M. Yamada, R. Kikuchi, and K. Eguchi, "Impedance analysis of internal resistance affecting the photoelectrochemical performance of dye-sensitized solar cells," Journal of the Electrochemical Society, Vol. 152, pp. E68-E73 (2005).
[39]R. Kern, R. Sastrawan, J. Ferber, R. Stangl, and J. Luther, "Modeling and interpretation of electrical impedance spectra of dye solar cells operated under open-circuit conditions," Electrochimica Acta, Vol. 47, pp. 4213-4225 (2002).
[40]F. Fabregat-Santiago, J. Bisquert, G. Garcia-Belmonte, G. Boschloo, and A. Hagfeldt, "Influence of electrolyte in transport and recombination in dye-sensitized solar cells studied by impedance spectroscopy," Solar Energy Materials and Solar Cells, Vol. 87, pp. 117-131 (2005).
[41]Q. Wang, S. Ito, M. Grätzel, F. Fabregat-Santiago, I. Mora-Sero, J. Bisquert, T. Bessho, and H. Imai, "Characteristics of high efficiency dye-sensitized solar cells," The Journal of Physical Chemistry B, Vol. 110, pp. 25210-25221 (2006).
[42]F. Fabregat-Santiago, J. Bisquert, E. Palomares, L. Otero, D. Kuang, S. M. Zakeeruddin, and M. Grätzel, "Correlation between photovoltaic performance and impedance spectroscopy of dye-sensitized solar cells based on ionic liquids," The Journal of Physical Chemistry C, Vol. 111, pp. 6550-6560 (2007).
[43]A. Zaban, M. Greenshtein, and J. Bisquert, "Determination of the electron lifetime in nanocrystalline dye solar cells by open‐circuit voltage decay measurements," ChemPhysChem, Vol. 4, pp. 859-864 (2003).
[44]G. Schlichthörl, S. Huang, J. Sprague, and A. Frank, "Band edge movement and recombination kinetics in dye-sensitized nanocrystalline TiO2 solar cells: a study by intensity modulated photovoltage spectroscopy," The Journal of Physical Chemistry B, Vol. 101, pp. 8141-8155 (1997).
[45]A. Fisher, L. Peter, E. Ponomarev, A. Walker, and K. Wijayantha, "Intensity dependence of the back reaction and transport of electrons in dye-sensitized nanocrystalline TiO2 solar cells," The Journal of Physical Chemistry B, Vol. 104, pp. 949-958 (2000).
[46]J. Bisquert, A. Zaban, and P. Salvador, "Analysis of the mechanisms of electron recombination in nanoporous TiO2 dye-sensitized solar cells. Nonequilibrium steady-state statistics and interfacial electron transfer via surface states," The Journal of Physical Chemistry B, Vol. 106, pp. 8774-8782 (2002).
[47]T. Wei, C. Wan, and Y. Wang, "Poly (N-vinyl-2-pyrrolidone)-capped platinum nanoclusters on indium-tin oxide glass as counterelectrode for dye-sensitized solar cells," Applied Physics Letters, Vol. 88, pp. 103122 (2006).
[48]T. Wei, C. Wan, Y. Wang, C. Chen, and H. Shiu, "Immobilization of Poly (N-vinyl-2-pyrrolidone)-capped platinum nanoclusters on indium− tin oxide glass and its application in dye-sensitized solar cells," The Journal of Physical Chemistry C, Vol. 111, pp. 4847-4853 (2007).
[49]J. L. Lan, C. C. Wan, T. C. Wei, W. C. Hsu, and Y. H. Chang, "Durability test of PVP‐capped Pt nanoclusters counter electrode for highly efficiency dye‐sensitized solar cell," Progress in Photovoltaics: Research and Applications, Vol. 20, pp. 44-50 (2012).
[50]J. Liang, G. Zhang, and W. Sun, "Post-treatment on dye-sensitized solar cells with TiCl4 and Nb2O5," RSC Advances, Vol. 4, pp. 6746-6751 (2014).
[51]R. Fiz, L. Appel, A. Gutiérrez-Pardo, J. Ramírez-Rico, and S. Mathur, "Electrochemical energy storage applications of CVD grown niobium oxide thin films," ACS applied materials & interfaces, Vol. 8, pp. 21423-21430 (2016).
[52]B. Zhang, X. Yu, C. Ge, X. Dong, Y. Fang, Z. Li, and H. Wang, "Novel 3-D superstructures made up of SnO2@C core-shell nanochains for energy storage applications," Chemical Communications, Vol. 46, pp. 9188-9190 (2010).
[53]L. Jiang, L. Sun, D. Yang, J. Zhang, Y.-J. Li, K. Zou, and W.-Q. Deng, "Niobium-doped (001)-dominated anatase TiO2 nanosheets as photoelectrode for efficient dye-sensitized solar cells," ACS Applied Materials & Interfaces, Vol. 9, pp. 9576-9583 (2017).
[54]H.-N. Kim and J. H. Moon, "Enhanced photovoltaic properties of Nb2O5-coated TiO2 3D ordered porous electrodes in dye-sensitized solar cells," ACS Applied Materials & Interfaces, Vol. 4, pp. 5821-5825 (2012).
[55]H. Su, Y.-T. Huang, Y.-H. Chang, P. Zhai, N. Y. Hau, P. C. H. Cheung, W.-T. Yeh, T.-C. Wei, and S.-P. Feng, "The synthesis of Nb-doped TiO2 nanoparticles for improved-performance dye sensitized solar cells," Electrochimica Acta, Vol. 182, pp. 230-237 (2015).
[56]D. B. Menzies, Q. Dai, L. Bourgeois, R. A. Caruso, Y.-B. Cheng, G. P. Simon, and L. Spiccia, "Modification of mesoporous TiO2 electrodes by surface treatment with titanium (IV), indium (III) and zirconium (IV) oxide precursors: preparation, characterization and photovoltaic performance in dye-sensitized nanocrystalline solar cells," Nanotechnology, Vol. 18, pp. 125608 (2007).
[57]F. Hu, Y. Xia, Z. Guan, X. Yin, and T. He, "Low temperature fabrication of ZnO compact layer for high performance plastic dye-sensitized ZnO solar cells," Electrochimica Acta, Vol. 69, pp. 97-101 (2012).
[58]J. Bisquert, "Influence of the boundaries in the impedance of porous film electrodes," Physical Chemistry Chemical Physics, Vol. 2, pp. 4185-4192 (2000).
[59]J. Bisquert, "Theory of the Impedance of Electron Diffusion and Recombination in a Thin Layer," The Journal of Physical Chemistry B, Vol. 106, pp. 325-333 (2002).
[60]C. He, Z. Zheng, H. Tang, L. Zhao, and F. Lu, "Electrochemical impedance spectroscopy characterization of electron transport and recombination in ZnO nanorod dye-sensitized solar cells," The Journal of Physical Chemistry C, Vol. 113, pp. 10322-10325 (2009).
[61]X. Zhang, S.-T. Wang, and Z.-S. Wang, "Effect of metal-doping in TiO2 on fill factor of dye-sensitized solar cells," Applied Physics Letters, Vol. 99, pp. 113503 (2011).
[62]J. Bisquert and I. Mora-Seró, "Simulation of steady-state characteristics of dye-sensitized solar cells and the interpretation of the diffusion length," The Journal of Physical Chemistry Letters, Vol. 1, pp. 450-456 (2009).
[63]G. Kron, U. Rau, and J. Werner, "Influence of the built-in voltage on the fill factor of dye-sensitized solar cells," The Journal of Physical Chemistry B, Vol. 107, pp. 13258-13261 (2003).
[64]A. Yella, H.-W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. K. Nazeeruddin, E. W.-G. Diau, C.-Y. Yeh, S. M. Zakeeruddin, and M. Grätzel, "Porphyrin-sensitized solar cells with cobalt (II/III)–based redox electrolyte exceed 12 percent efficiency," Science, Vol. 334, pp. 629-634 (2011).
[65]F. Xie, S.-J. Cherng, S. Lu, Y.-H. Chang, W. E. Sha, S.-P. Feng, C.-M. Chen, and W. C. Choy, "Functions of self-assembled ultrafine TiO2 nanocrystals for high efficient dye-sensitized solar cells," ACS Applied Materials & Interfaces, Vol. 6, pp. 5367-5373 (2014).