Effect of the Geometry of the Anodized Titania Nanotube Array on the Performance of Dye-Sensitized Solar Cells

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1 Copyright 2010 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 10, , 2010 Effect of the Geometry of the Anodized Titania Nanotube Array on the Performance of Dye-Sensitized Solar Cells Lidong Sun 1, Sam Zhang 1, Xiaowei Sun 2, and Xiaodong He 3 1 School of Mechanical and Aerospace Engineering,, 50 Nanyang Avenue, Singapore , Singapore 2 School of Electrical and Electronic Engineering,, 50 Nanyang Avenue, Singapore , Singapore 3 School of Aeronautics, Harbin Institute of Technology, P.O. Box 3010, Harbin , P. R. China Highly ordered TiO 2 nanotube arrays are superior photoanodes for dye-sensitized solar cells (DSSCs) due to reduced intertube connections, vectorial electron transport, suppressed electron recombination, and enhanced light scattering. Performance of the cells is greatly affected by tube geometry, such as wall thickness, length, inner diameter and intertube spacing. In this paper, effect of geometry on the photovoltaic characteristics of DSSCs is reviewed. The nanotube wall has to be thick enough for a space charge layer to form for faster electron transportation and reduced recombination. When the tube wall is too thin to support the space charge layer, electron transport in the nanotubes will be hindered and reduced to that similar in a typical nanoparticle photoanode, and recombination will easily take place. Length of the nanotubes also plays a role: longer tube length is desired because of more dye loading, however, tube length longer than the electron diffusion length results in low collecting efficiency, which in turn, results in low short-circuit current density and thus low overall conversion efficiency. The tube inner diameter (pore size) affects the conversion efficiency through effective surface area, i.e., larger pore size gives rise to smaller surface area for dye adsorption, which results in low short-circuit current density under the same light soaking. Another issue that may seriously affect the conversion efficiency is whether each of the tube stands alone (free from connecting to the neighboring tubes) to facilitate infiltration of dye and fully use the outer surface area. Keywords: Titanium Dioxide, Nanotube Array, Wall Thickness, Optimal Length, Pore Diameter, Intertube Spacing. CONTENTS 1. Introduction Effect of Wall Thickness Effect of Length Effect of Pore Diameter and Intertube Spacing Summary References and Notes INTRODUCTION Dye-sensitized solar cells (DSSCs) have been vigorously investigated since it was first reported in DSSCs have lots of merits in comparison to inorganic p n junction based solar cells: low cost in fabrication, impurity Author to whom correspondence should be addressed. tolerance, and compatibility with flexible substrate. 2 In a typical dye-sensitized solar cell, photons are intercepted by sensitizers anchored on the surface of TiO 2 (anatase) nanoparticle network to result in electron injection into the conduction band of the TiO 2 photoanode. The injected electrons subsequently percolate to the collecting electrode and migrate through the external load. The oxidized dye molecules are regenerated by electron donation from redox species (usually I /I3 couple) in the electrolyte that permeates the nanoparticle network, and regeneration of the redox species takes place at the platinized counter electrode to complete the circuit. 1 3 It is recognized that the performance of the conventional DSSCs (with nanoparticle based photoanode) is restricted by electron loss during percolation in the J. Nanosci. Nanotechnol. 2010, Vol. 10, No /2010/10/4551/011 doi: /jnn

2 Sun et al. nanoparticle network, since electrons undergo nearly 10 6 trapping/detrapping events before being collected by the electrode; 4 the second restriction comes from the small thickness of the nanoparticle photoanode, which inhibits the absorption of low-energy photons by available dyes especially in long wavelength range. 5 As such, high-aspectratio nanostructures, such as nanotube, 6 9 nanowire, nanorod and nanosheet, make thick photoanode possible to maximize photon absorption, besides reduced electron loss at nanostructure connections. Among these architectures, nanotube structure has received considerable attention as a result of its diverse preparation methods and relatively higher surface area produced by additional inner surface. In particular, highly oriented TiO 2 nanotube arrays (see Fig. 1) prepared by electrochemical anodization are favored, comparing to the randomly packed features presented by sol gel method (Fig. 2). Moreover, geometry of the anodized nanotube array can be tailored by Mr. Lidong Sun received his Bachelor Degree in Metallic Materials Engineering from China University of Petroleum (Beijing), China in 2004 and his Master Degree in Materials Science and Engineering from Beijing University of Aeronautics and Astronautics, China in He is currently pursuing his Ph.D. degree under the supervision of Professor Sam Zhang and Associate Professor Sun Xiaowei in. His research interest is preparation of titania nanotube array and its application in dye-sensitized solar cells. Professor Sam Zhang received his Ph.D. degree in Ceramics in 1991 from The University of Wisconsin-Madison, USA. He is a tenured full professor at the School of Mechanical and Aerospace Engineering,. Professor Zhang serves as Editor-in-Chief for Nanoscience and Nanotechnology Letters (USA), Principal Editor for Journal of Materials Research (USA) and President of the Thin Films Society. Professor Zhang has been in processing and characterization of nanocomposite thin films and coatings for 18 years and has authored/co-authored more than 200 peer reviewed international journal papers, 14 book chapters and edited 9 Journal volumes. He published one book in 2007 (Sam Zhang & Nasar Ali (ed), Nanocomposite Films and Coatings Processing, Properties and Performance, Imperial College Press, UK), one book in 2008 (Sam Zhang, Lin Li, Ashok Kumar, Advanced Characterization Techniques, CRC Press, USA) and 3 books are in press (2009) at CRC Press, USA: Nanocomposite Films and Coatings: Mechanical Properties, Nanocomposite Films and Coatings: Functional Properties, Organic Nanostructured Thin Film Devices and Coatings for Clean Energy. He holds Honorary Professorship at the Institute of Solid State Physics, Chinese Academy of Sciences and Guest Professorship at Zhejiang University and Harbin Institute of Technology. He was featured in the first edition of Who s Who in Engineering Singapore, and in both 26th and 27th editions of Who s Who in the World. He is also a Fellow of the Institute of Materials, Minerals and Mining, UK. Professor Zhang s research interests include Hard yet tough nanocomposite coatings for tribological applications; Measurement of fracture toughness of ceramic films and coatings; Biological Coatings and Drug Delivery Applications; Electronic Thin Films and Films for Energy Applications. Details of Professor Zhang s research and publications are accessible from his home page at Dr. Xiaowei Sun was born in Beijing, China. From 1986 to 1994, he studied at Tianjin University, China, where he received his B. Eng., M. Eng. and Ph.D. degrees all in photonics. From 1994 to 1998, he studied at the Hong Kong University of Science and Technology where he received his second Ph.D. degree in electrical and electronic engineering. He has been with the Division of Microelectronics in the School of Electrical and Electronic Engineering of since 1998 first as an Assistant Professor, and was promoted to Associate Professor in His research interests focus on MOCVD growth of ZnO, display technologies, and nanotechnology. Dr. Sun is a member of IEEE and Society for Information Display (SID). He is the founding Chair of SID Singapore and Malaysia Chapter. His research Interests include MOCVD growth of ZnO., Display technologies: organic lightemitting devices (OLED) and liquid-crystal-on-silicon (LCoS) microdisplayy; Nanotechnology: one-dimensional oxide semiconductors and their applications J. Nanosci. Nanotechnol. 10, , 2010

3 Sun et al. Dr. Xiaodong He received his Bachelor of Science in Mathematics in 1984 from Jilin University, Master of Engineering and Ph.D. in Materials Mechanics from Harbin Institute of Technology (HIT) in 1988 and 1992, respectively. Since 1997, he has been holding a full professor position in HIT. In 2005, he was selected to be Cheung Kong Scholar by Ministry of Education, China. His research interests include light-weight composite materials, nanocomposite, thermal protection system and self-propagating High-temperature Synthesis Technology. He has authored or co-authored more than 200 journal papers, 30 conference papers, 20 patents, and one textbook. His other responsibilities include: Director Center for Composite Materials, Vice Dean of School of Astronautics in HIT, member of advanced material field of the National High Technology Research and Development Program in China, member of aerospace and astronautics field of the National High Technology Research and Development Program and member of demonstrator group of Moon Adventure Engineering in China. He serves as a Member of the Editorial Board of Nanoscience and Nanotechnology Letters since April controlling the process parameters This is superior to the template assistant method in which the nanotube geometry is determined and limited by the original template, as shown in Figure The nanotube arrays as the photoanodes eliminate the particle-to-particle contacts necessary in the randomly packed configuration for electron transportation. This 8 33 largely reduces electron scattering at the joining points. In nanotubes, usually the grains are stretched in the tube growth direction. This further decreases possible electron losses at grain boundaries. Other obvious benefit in nanotube photoanode includes vectorial transport of electrons to minimize trapping/detrapping events before collection In the case of anodized nanotube array, a (d) Fig. 1. Lateral, bottom, top (d) FESEM images of TiO 2 nanotube arrays prepared by electrochemical anodization. Reprinted with permission from [19], K. Shankar et al., Nanotechnology 18, (2007). 2007, IOP Publishing. J. Nanosci. Nanotechnol. 10, ,

4 Sun et al. Fig. 2. TEM images at different magnifications of TiO 2 nanotubes prepared by sol gel method. Reprinted with permission from [8], I. C. Flores et al., J. Photochem. Photobiol. A: Chem. 189, 153 (2007). 2007, Elsevier. barrier layer exists between the photoanode and the collecting electrode. Electrons collected are thus separated from the redox couple in the electrolyte, impossible for further recombination. However, performance of the anodized nanotube array based DSSCs is greatly affected by the nanotube geometry. This paper dissects this geometry to look into the details of the influence of wall thickness, length, pore diameter and intertube spacing to cast light into better design for high efficiency. Nanyang Technological University making it Fig. 3. Top and lateral images of anodic alumina template used for preparation of TiO 2 nanotubes and the corresponding SEM image of obtained TiO 2 nanotubes. Reprinted with permission from [32], N. N. Bwana, Curr. Appl. Phys. 9, 104 (2009). 2009, Elsevier. 2. EFFECT OF WALL THICKNESS The initial chemical potential of electrons in a semiconductor and an electrolyte is determined by Fermi level of the semiconductor and redox potential of the redox couples in the electrolyte, respectively. When the semiconductor is submerged in the electrolyte, and the initial chemical potential of the electrons is different for the two phases, a space charge layer (also called a depletion layer) develops in the semiconductor adjacent to the electrolyte, and a Helmholtz layer forms in the electrolyte in contact with the semiconductor As illustrated below, in the case of TiO 2 nanoparticles, the size (usually in the range of nm) of the particle is too small to support the space charge layer. The width of the space charge layer in the semiconductor can be determined by 35 w = 2 0 V B qn Where, is the dielectric constant of the semiconductor, 0 the permittivity of free space, q the electronic charge, N the charge carrier concentration in the semiconductor, and V B the amount of band bending in the depletion layer. The value of V B is given by 35 (1) V B = U U fb (2) Where, U is the electrode potential (Fermi level) of the semiconductor, and U fb is the flat-band potential of the semiconductor. For titanium dioxide with anatase structure which is widely employed in the conventional DSSCs, the width of space charge layer is about nm, using = N = cm U = 4 2 ev (quasi-fermi level of anatase TiO 2 under illumination), and U fb (usually depends on ph value of the electrolyte) Consequently, the size of a TiO 2 particle must be at least nm to support space charge layer in the particle. Now consider the photoanode of highly oriented TiO 2 nanotube array. If the wall thickness of the nanotube exceeds the critical value discussed above, the space charge layer forms. As a result, the conduction and valence band edges are upwardly bent 40 so that potential barriers are created near both inner and outer surfaces of the nanotubes against further electron transfer into the redox couples in the electrolyte, as shown in Figure 4. Once the electrons are injected into this high speed lane by dyes, they are confined in because of the potential barrier and move away from the source of the electron generation to form the drift current. This results in a high shortcircuit current density (J sc and a high open-circuit voltage (V oc. Figure 4 depicts the illumination process at DSSC with a photoanode of nanotube array, where the light illuminates through a Transparent Conducting Oxide (TCO) 4554 J. Nanosci. Nanotechnol. 10, , 2010

5 Sun et al. CE Conduction band E F TiO 2 NTA TCO Valence band e e Conduction band E F Valence Band CE TiO 2 NTA TCO anion CE cation electron line of electric force TiO 2 NTA CE electron percolation direction counter electrode TCO TCO transparent conducting oxide TiO 2 NTA TiO 2 nanotube array Light Fig. 4. Schematic diagram of electron percolation in a thick-walled and thin-walled nanotube, and its corresponding band structure as well as line of electric force in the cell. Instruction of the symbols and presentation of the 3D structure of the cell as well as the circuit under illumination. layer to be absorbed in the nanotube array, where photon absorption/electron generation takes place. In the event that the nanotube wall becomes too thin to support the space charge layer, negligibly small or even no band bending occurs, 40 the high speed lane will not form. In this case, cations in the electrolyte are adsorbed on the surfaces of the nanotubes (i.e., Helmholtz layer) leading to increased recombination (Fig. 4) between the electrons in the channel and the triiodide in the electrolyte. Consequently, the electron transport degrades to that in the photoanode based on nanoparticles. The electron transport is thus a diffusion process (as in contrast to the drift process in the case of existence of space charge layer), which can be explained by random walk model. Zhu and co-workers were surprised that the electron transport in the photoanode of highly oriented anodic TiO 2 nanotube array was comparable with that in the randomly packed TiO 2 nanoparticles, as shown in Figure In fact, the wall thickness of their nanotubes is about 8 nm and the size of the nanoparticles is around 24 nm. As discussed afore, the existence of the space charge layer needs a particle or wall thickness of at least 30 to 40 nm. Obviously, the tube wall thickness and particle size are both below this critical size. As such, no high speed lane J. Nanosci. Nanotechnol. 10, ,

6 Sun et al. Fig. 5. Comparison of transport time constants for nanotube and nanoparticle based DSSCs as a function of the incident photon flux. Transport (IMPS, solid symbols) and recombination (IMVS, open symbols) time constants for dye-sensitized nanotube films of various thicknesses as a function of the incident photon flux. Reprinted with permission from [54], K. Zhu et al., Nano Lett. 7, 69 (2007). 2007, American Chemical Society. forms, electron transport in both cases is dominated by the same diffusion process, resulting in comparable electron transport time. Similar phenomena were reported in TiO 2 nanotubes prepared by sol gel (10 nm in diameter), 7 randomly oriented nanowire (diameter of about 20 nm), 55 and nanoparticle-fused nanorod (effective diameter less than 5 nm) 15 structures. All these cells did not show advantage of electron transport over nanoparticles, most likely because of nonexistence of the space charge layer due to small size. Even without the high speed lane thicker nanotube walls give rise to higher open-circuit voltage, as illustrated in Table I. Table I puts together the geometries of various TiO 2 nanotube array based DSSCs and their performances. The following observations are highlighted: (1) There seems no direct relationship between the opencircuit voltage and the length of the nanotubes (V oc vs. L). (2) The pore diameters varied from about 30 to 130 nm while the open-circuit voltage only varied from 0.6 to 0.8 V. Thus the pore diameter does not affect V oc noticeably. (3) The influence of the wall thickness can be extracted from separating the cells into two groups: group I with the same wall thickness of 8 nm (cell # 2, 3, 4 and 5), and Group II with thickness more than its double, i.e., 17 to 24 nm (cell # 1, 6, 7 and 8). Though both groups wall thickness is below the critical thickness (30 nm) for high speed lane formation, thicker walled nanotubes still result in higher open-circuit voltage (0.8 V vs. 0.6 V), as a result of lower chances of electrons interfacing with redox electrolyte thus lower chances of recombination. (4) Cell # 11 achieved very high current density (16 ma/cm 2. However, it only achieved a conversion efficiency of 4.8%, likely due to its thin wall thickness (20 nm) which restricted the open-circuit voltage (only 0.43 V). Table I. method). Performance of DSSCs with TiO 2 nanotube array photoanodes (Cell # 1 10 by electrochemical anodization, Cell # 11 by template assistant Cell No. L ( m) T (nm) D (nm) J sc (ma/cm 2 ) V oc (V) FF (%) Refs. Back-side illumination ± 2 46± [33] ± 1 30± [54] ± 1 30± [54] ± 1 30± [54] ± 1 30± [54] 6 6± [34] ± ± ± [33] [19] Front-side illumination ± 2 46± [56] [33] [32] L: Length of nanotube array; T : wall thickness; D: pore diameter; J sc : short-circuit current density; V oc : open-circuit voltage; FF: fill factor; : overall conversion efficiency J. Nanosci. Nanotechnol. 10, , 2010

7 Sun et al. To allow fast and uninterrupted transportation of electrons through the nanotube, it is crucial that the tube wall is thick enough for the space charge layer to form. 3. EFFECT OF LENGTH Table I also shows that the short-circuit current density (from 2.4 to about 13 ma/cm 2 and the overall conversion efficiency increase (from 1.3% to about 7%) as the length of the nanotube array increases from about 0.4 to 20 m. This is a direct result of increased dye loading capacity. To obtain high conversion efficiency, the electrons injected into the conduction band of the TiO 2 photoanode should be collected as many as possible. A key parameter determining this process is the electron diffusion length presented as 2 57 L n = D n n (3) Where, D n and n are the electron diffusion coefficient and electron lifetime, respectively. Effective collection of electrons can be realized only if the electron diffusion length is larger than the film thickness (or nanotube length, L), i.e., 2 57 L n >L (4) That means, shorter nanotube length would result in more electrons being collected. On the other hand, however, the film must be thick enough to allow more photon/sensitizer reactions to generate more electrons. In a typical TiO 2 nanoparticle photoanode, the electron diffusion length is about 10 m, 58 i.e., L NP = 10 m (5) Figure 5 displays virtually no change in electron transport time as the length of the nanotube array changes from 1.9 to 5.7 m. As discussed before, under thin-walled condition, the electron transport time ( c is about the same in nanoparticle-based DSSCs (c.f., Fig. 5), which can be expressed as 4 c L 2 /2 35D n (6) Therefore, the electron diffusion coefficient in nanotubes is to some extent similar to that in nanoparticles. Based on the results of Zhu and co-workers, 54 the recombination time in nanotubes is an order of magnitude (10 times) greater than that in nanoparticles (similar result was also obtained in nanorod structure 15. If so, Eq. (3) thus gives rise to the electron diffusion length in nanotubes being 10 times that in a typical nanoparticle photoanode. As such, using Eqs. (3) and (5), the optimal length of TiO 2 nanotubes (L NT should be around Table II. Performance of DSSCs with different nanotube lengths. L ( m) J sc (ma/cm 2 ) V oc (V) FF (%) NT NT NT NGF-NT NGF-NT NGF-NT L: length of nanotube array; J sc : short-circuit current density; V oc : open-circuit voltage; FF: fill factor; : overall conversion efficiency. Source: Reprinted with permission from [59], D. Kim et al., Electrochem. Commun. 10, 1835 (2008). 2008, Elsevier. and co-workers assembled nanotube array based DSSCs with different lengths (Table II). 59 Maximum conversion efficiency seems to be with nanotube length of close to 30 m. An evaluation of the photocurrent for dyesensitized TiO 2 nanotube arrays displayed in Figure also indicates that the maximum photocurrent occurs at close to nanotube length of 30 m. Depending on the orientation of the illumination with respect to the electron collecting electrode, a DSSC can be illuminated from the front-side or back-side Electrons start their journey towards the load from the collecting electrode, thus this side is considered front. Consequently, the counter electrode (CE) side is back side, where the electrons have to travel all the way through the whole photoanode pathway to be collected at the front. Usually, transparent conducting oxide is used as collecting electrode in front-side illuminated DSSC because of its light transmission and electrical conduction properties, as shown in Figure 4. When Ti foil is employed as collecting electrode, the front side is no longer transparent (the foil is usually 100 or 250 m), illumination then L NT = 10L NP = m 30 m Shankar and co-workers suggested the optimal nanotube length is nominally 20 m (cell #8inTable I). 19 Kim Fig. 6. Photocurrent evaluation of the dye-sensitized nanotube layers grown to different tube length based on the photocurrent transients at 650 nm. Reprinted with permission from [60], J. M. Macak et al., Curr. Opin. Solid State Mater. Sci. 11, 3 (2007). 2007, Elsevier. J. Nanosci. Nanotechnol. 10, ,

8 comes from the transparent platinized counter electrode (the back-side ), thus the term back-side illuminated. It is noteworthy that the optimal nanotube length of about 30 m discussed afore is based on back-side illuminated DSSCs. Usually back-side illuminated DSSCs have much lower efficiency than the front-side illuminated counterpart. For example, cell # 1 and 9 in Table I have the same geometry, but # 1 is back-side illuminated and # 9 is front-side illuminated. Cell # 9 has much higher shortcircuit current density (7.87 ma/cm2 vs. 2.4 ma/cm2 ) and much higher conversion efficiency (2.9% vs. 1.3%), which is more than 2-fold increase in efficiency. Sun et al. The lower efficiency of the back-side illumination type comes because of the following reasons: (1) Energy loss due to reflection by the platinized counter electrode; (2) Absorption of the photons by the iodine in the electrolyte33 34 before reaching the nanotube array; (3) After the electrons are generated, they have to travel all the way through the photoanode to reach the collecting electrode. The increased distance of travel drastically increased the chances of recombination. In contrast, front-side illumination has the advantage of not having the above problems. (d) REVIEW (e) (f) Fig. 7. SEM images of anodized TiO2 nanotube array with bundle structure 1.1 m, 2.8 m, 6.1 m, and bundle-free structure (d) 1.1 m, (e) 2.8 m, (f) 6.1 m. Reprinted with permission from [65], K. Zhu et al., Nano Lett. 7, 3739 (2007). 2007, American Chemical Society J. Nanosci. Nanotechnol. 10, , 2010

9 Sun et al. In addition, since there is no such loss of photon energy, the electron density in the front-side illuminated photoanode is much higher under light soaking. As such, some of the electrons fall into the trapping states in the bandgap of the oxide, paving way for other electrons to move more freely. Since the traps that participate in the electron motion affect the value of the diffusion coefficient, 2 the reduction of quantity of traps results in increase in the electron diffusion coefficient D n. As illustrated in Eq. (3), the electron diffusion length L n is also related to electron lifetime c. According to Paulose and co-workers, 33 the electron lifetime for both illumination types is comparable. Consequently, the electron diffusion length in front-side illuminated photoanode should be larger than that in back-side illuminated architecture, which in turn, gives rise to longer optimal tube length. One recent estimation of the electron diffusion length suggests that the optimal tube length is in the order of 100 m intio 2 nanotube array based DSSCs, 64 using a method that takes into consideration the fact that the occupancies of electron traps are substantially different under open-circuit and short-circuit conditions. Table III. Performance of DSSCs with bundle and bundle-free nanotube structure. L ( m) J sc (ma/cm 2 ) V oc (V) FF (%) Bundle structure Bundle-free structure L: length of nanotube array; J sc : short-circuit current density; V oc : open-circuit voltage; FF: fill factor; : overall conversion efficiency. Source: Reprinted with permission from [65], K. Zhu et al., Nano Lett. 7, 3739 (2007). 2007, American Chemical Society. Roughness of the outer surface of the nanotubes also affects the efficiency in DSSCs. The bamboo-type nanotubes prepared by modulating electrochemical anodization process, 66 as illustrated in Figure 8, results in increased short-circuit current density (8.76 ma/cm 2 vs ma/cm 2 and overall conversion efficiency (2.96% vs. 1.90%) as compared to smooth nanotube outer surface, 4. EFFECT OF PORE DIAMETER AND INTERTUBE SPACING The pore diameter (or inner tube diameter) and intertube spacing have significant influence on surface roughness factor and porosity of nanotube arrays, thus affecting the performance of dye-sensitized solar cells. Nanotube arrays prepared by anodization have a unique close-end structure in which air may be trapped in during infiltration of the dye solution and the redox electrolyte. This is more serious in the case of nanotubes with small pore diameters. Conventional titanium dioxide nanoparticles used in DSSCs are of nm in diameter, which gives rise to very large surface area for possible generation of electrons. In nanotube arrays, however, the surface area is drastically reduced due to the tube structure and it becomes worse when large pore diameter tubes are used. Compared to other one-dimensional nanostructures, nanotubes have inner tube surface available for dye loading thus capable of producing more electrons to inject into the photoanode. The outer surface, however, may not be completely available for dye loading. This happens when the tubes are touching one another to form a strand or bundle of tubes (c.f., Fig. 7). 65 In this case, the dye molecules and redox electrolyte will not be able to wet each of the nanotube, thus drastically reduce the effective dye loading area. The result is reduced overall conversion efficiency, as illustrated in Table III. Ideally, the each individual tube should stand alone leaving enough space to facilitate infiltration of dyes and electrolyte as well as faster electron transportation. Fig. 8. SEM images of anodized TiO 2 nanotube array with smooth wall, and bamboo-type wall and. Reprinted with permission from [66], D. Kim et al., J. Am. Chem. Soc. 130, (2008). 2008, American Chemical Society. J. Nanosci. Nanotechnol. 10, ,

10 Sun et al. Table IV. Performance of DSSCs with Bamboo-Type nanotubes. L ( m) J sc (ma/cm 2 ) V oc (V) FF (%) L dye (au) NT B-NT B-NT L: length of nanotube array; J sc : short-circuit current density; V oc : open-circuit voltage; FF: fill factor; : overall conversion efficiency; L dye : dye loading per unit volume. Source: Reprinted with permission from [66], D. Kim et al., J. Am. Chem. Soc. 130, (2008). 2008, American Chemical Society. as shown in Table IV. This is because of the increased dye loading per unit volume. The additional area provided by the bamboo rings may allow both inner and outer surfaces to be covered by dye molecules as well SUMMARY This paper reviewed the effect of geometry of TiO 2 nanotube arrays synthesized by electrochemical anodization on the performance of the DSSCs using these nanotube arrays as the photoanodes. (1) There seems exist a critical wall thickness for fast electron transportation. The critical thickness comes from the formation of the space charge layer. If the wall is too thin to support this layer, the electron transport will be degraded to that of the nanoparticle case. The estimated critical wall thickness is about 30 to 40 nm. (2) There also exists an optimal nanotube length for optimal electron collection in consideration of dye loading capacity and electron diffusion length. In back-side illuminated DSSCs, the optimal length is close to 30 m. Under front-side illumination, the value could be a lot bigger. (3) The pore diameter should be as small as possible to increase surface area and the nanotubes should ideally be free of bundling to facilitate dye and electrolyte infiltration. References and Notes 1. B. O Regan and M. Grätzel, Nature 353, 737 (1991). 2. M. Grätzel, Inorg. Chem. 44, 6841 (2005). 3. M. Grätzel, J. Photochem. Photobiol. C: Photochem. Rev. 4, 145 (2003). 4. J. van de Lagemaat and A. J. Frank, J. Phys. Chem. B 105, (2001). 5. M. Law, L. 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