Ordered Semiconductor Photoanode Films for Dye-Sensitized Solar Cells Based on Zinc Oxide-Titanium Oxide Hybrid Nanostructures

Dye-sensitized solar cell (DSC) is a new type solar cell based on the photoelectric conversion occurred at the organic dye-semiconductor nanoparticle interface. DSC has attracted great research interests due to its high efficiency for energy conversion (11%) and low production cost compared with the traditional Si based photovoltaic cell (O'Regan et al., 1991; Gratzel et al., 2001). In confront of the huge difficulties in propelling the conversion efficiency of DSC up to 15% or higher, very recently, significant research efforts have been redirected to the optimization of photoanode, while others are stick to seeking better sensitizer or solid electrolytes (Mor et al., 2006a; Gratzel et al., 2006; Li et al., 2006; Zhang et al., 2011). The photoanode is the kernel component of DSC assuming two major functions, supporting dye molecules, and transporting photo-induced electrons to the bottom electrode. Traditionally, the photoanode is the nanoporous thick film (15-20 um in thickness) consisted of TiO2 nanoparticles, characterized by the disordered nature (Figure 1a). The electron transport in this disordered network of TiO2 film is rather difficult due to two factors: (1) The presence of a large quantity of electron traps at the nanostructured TiO2/electrolyte interface, such as the intrinsic defect sites (e.g. oxygen defects or surface states) and the grain boundaries, which may influence the interfacial charge-transfer kinetics. (2) The tortuosity of the electron path in the photoanode formed via the calcinations at high temperature, which may hinder the free transport of electron and increase the electron-hole recombination rate. In contrast, the ideal photoanode film (Figure 1b) should be built up by very thin (10-20 nm) and long (10-20 um) semiconductor nanowire array, possessing no surface states, which should exhibit the total photoelectric conversion efficiency of 24% based on the widely used black dye (N719) and I2/I3electrolyte system. However, limited by the current fabrication technology level, it is extremely difficult to realize this photoanode film in a short term. Therefore, the development of some practical measures to optimize the photoanode is fundamentally important. The photoanode film can be optimized further from two aspects, to enhance electron gathering and transporting efficiency and to inhabit the charge recombination at the same time. Figure 1c illustrates the schematic of the hybrid photoanode structure, using 1D ZnO nanowire as the major path for the electron transportation, and using 0D TiO2 nanoparticles as the supporting framework of dye molecules. By using two kinds of nanostructures assuming two different functions of


Introduction
Dye-sensitized solar cell (DSC) is a new type solar cell based on the photoelectric conversion occurred at the organic dye-semiconductor nanoparticle interface. DSC has attracted great research interests due to its high efficiency for energy conversion (11%) and low production cost compared with the traditional Si based photovoltaic cell (O'Regan et al., 1991;Grätzel et al., 2001). In confront of the huge difficulties in propelling the conversion efficiency of DSC up to 15% or higher, very recently, significant research efforts have been redirected to the optimization of photoanode, while others are stick to seeking better sensitizer or solid electrolytes (Mor et al., 2006a;Grätzel et al., 2006;Li et al., 2006;Zhang et al., 2011). The photoanode is the kernel component of DSC assuming two major functions, supporting dye molecules, and transporting photo-induced electrons to the bottom electrode. Traditionally, the photoanode is the nanoporous thick film (15-20 um in thickness) consisted of TiO 2 nanoparticles, characterized by the disordered nature ( Figure 1a). The electron transport in this disordered network of TiO 2 film is rather difficult due to two factors: (1) The presence of a large quantity of electron traps at the nanostructured TiO 2 /electrolyte interface, such as the intrinsic defect sites (e.g. oxygen defects or surface states) and the grain boundaries, which may influence the interfacial charge-transfer kinetics. (2) The tortuosity of the electron path in the photoanode formed via the calcinations at high temperature, which may hinder the free transport of electron and increase the electron-hole recombination rate. In contrast, the ideal photoanode film (Figure 1b) should be built up by very thin (10-20 nm) and long (10-20 um) semiconductor nanowire array, possessing no surface states, which should exhibit the total photoelectric conversion efficiency of 24% based on the widely used black dye (N719) and I 2 /I 3-electrolyte system. However, limited by the current fabrication technology level, it is extremely difficult to realize this photoanode film in a short term. Therefore, the development of some practical measures to optimize the photoanode is fundamentally important. The photoanode film can be optimized further from two aspects, to enhance electron gathering and transporting efficiency and to inhabit the charge recombination at the same time. Figure 1c illustrates the schematic of the hybrid photoanode structure, using 1D ZnO nanowire as the major path for the electron transportation, and using 0D TiO 2 nanoparticles as the supporting framework of dye molecules. By using two kinds of nanostructures assuming two different functions of the photoanode, the efficiency of the hybrid photoanode is expected to improve compared with the traditional nanoparticle-based photoanode.  (Law et al., 2005). The direct electrical pathways provided by the nanowires ensured the rapid collection of carriers generated throughout the device, and a full Sun efficiency of 1.5% was demonstrated, limited primarily by the surface area of the nanowire array. G. K. Mor et al. reported highly ordered transparent TiO 2 nanotube arrays (6-nm pore diameter, 17-nm wall thickness, and 360-nm length) as the photoanode, which were grown perpendicular to a fluorine-doped tin oxide-coated glass substrate by anodic oxidation of a titanium thin film (Mor et al., 2006b). Although the electrode was only 360nm-thick, the generated photocurrent was 7.87 mA/cm 2 , with a photocurrent efficiency of 2.9%, indicating the great potential of the ordered photoanode. K. Zhu et al. investigated the dynamics of electron transport and recombination in DSCs incorporating oriented TiO 2 nanotube arrays from electrochemically anodized Ti foils, and proved that the nanotubebased DSCs had significantly higher charge-collection efficiencies than the nanoparticlebased counterparts (Zhu et al., 2007). S H Kang et al. developed chemically synthesized TiO 2 nanorod based photoanodes (Kang et al., 2008). These nanorod-based DSCs showed improved photovoltaic properties (6.2 % versus 4.3 % for nanoparticle-based DSCs) owing to the characteristics of slightly enhanced electron transport and predominantly degraded charge recombination. W. Zhang et al. used mesoporous TiO 2 nanofibers with a high surface area of 112 m 2 /g prepared by electrospinning technique as the photoanode, and demonstrated the conversion efficiency of 1.82% in solid-state DSCs . Recently, C. S. Rustomji et al. designed and constructed photoanodes based on new 3-D configurations of TiO 2 nanotubes, which, unlike prior nanotube-based cells where tubes were grown vertically in a 2-D array, consisted of tubes that extended radially in a 3-D array from a grid of fine titania wires (Rustomji et al., 2010). Its overall efficiency reached 5.0%, and the incident photon-to-current efficiency exceeds 60% over a broad part of the visible spectrum. S. Guldin et al. presented a material assembly route toward double-layer photoanode of DSCs, by coupling a high-surface mesoporous underlayer with an optically and electrically active three-dimensionally periodic TiO 2 photonic crystal overlayer (Guldin et al., 2010). In contrast to earlier studies, the double layer structure exhibited porosity at the mesoporous and the microporous length scales as well as pore and electronic connectivity at all levels. This construct enabled effective dye sensitization, electrolyte infiltration and charge collection from both the mesoporous and the photonic crystal layers. Apart from various ordered TiO 2 nanostructures, ZnO has also been widely investigated as an alternative of TiO 2 to construct the photoanode of DSCs. For example, E. Hosono et al. pyrolyzed upright-standing sheets of layered hydroxide zinc carbonate, and obtained upstanding nanocrystalline ZnO films with the c-axis parallel to the substrate (Hosono et al., 2005). The corresponding DSCs possessed the conversion efficiency of 3.9%, much higher than the regular efficiency level of ~2% at that time (Rensmo et al., 1997). C. K. Xu et al. reported the photoanode using ultralong ZnO nanowires with up to 33 μm thick sensitizing films while maintaining high electron collection efficiency, which resulted in greatly improved performance (from 1.1% to 2.1%) compared to previously reported ZnO nanowire-based DSCs (Xu et al., 2010). J. X. Wang et al. used porous hierarchical disklike ZnO nanostructure fabricated via a simple low-temperature hydrothermal method as the photoanodes, and achieved improved photovoltaic performance (2.49%) compared with that of the ZnO nanowire arrays owing to the enlarged surface area and natural electron collection routes of ZnO hierarchical nanostructures (Wang et al., 2010). A. B. F. Martinson et al. introduced high surface area ZnO nanotube photoanodes templated by anodic aluminum oxide in DSCs via atomic layer deposition technique, providing a direct path for charge collection over tens of micrometers thickness (Martinson et al., 2007). Compared to similar ZnO-based devices, ZnO nanotube cells show exceptional photovoltage and fill factors, in addition to power efficiencies up to 1.6%. G Z Cao's group took the hierarchically popcorn-ball structured ZnO film as the photoanode, and attained the efficiency improvement from 0.6% to 3.5%, and finally 5.4%, by inducing light scattering within the photoelectrode films via the aggregation of ZnO nanocrystallites, thus demonstrating the huge potential of ZnO nanostructures in realizing DSCs with high efficiency (Chou et al., 2007;Zhang et al., 2008). In parallel with photoanode films consisted by merely one type of material (ZnO, TiO2, nanoparticles, nanowires, or nanotubes), the hybrid photoanode represents another important research trend, which is usually built by two or more materials or morphologies. Due to the obvious advantages of the hybrid photoanode including the improved interfacial structure/charge separation, the direct electron transporting path, and the vast possibilities to tune the microstructure and photoelectrochemical properties of the photoanode, this research trend has been gaining more and more interests. For example, K. M. Lee et al. incorporated multi-wall carbon nanotube in low-temperature fabricated TiO 2 photoanode, with the efficiency improvement of 20% (0.1% CNT), which may be resulted from retarded charge recombination between injected electrons and electron acceptors in the redox electrolyte (Lee et al., 2008). P. Brown et al. studied the single-wall carbon nanotube (SWCNT)/TiO 2 hybrid system, and proved that SWCNT support networks can be incorporated into mesoscopic TiO 2 films to improve the charge transport in DSCs (Brown et al., 2008). While no net increase in power conversion efficiency was obtained, an increase in photon-to-current efficiency (IPCE) represented the beneficial role of the SWCNT as a conducting scaffold to facilitate charge separation and charge transport in nanostructured semiconductor films. S. Pang et al. incorporated ZnO nanorods with different sizes into TiO 2 nanoparticle-based photoanode, and improved the conversion efficiency by 15% (Pang et al., 2007). They attributed the improved efficiency to the faster charge carrier transport rate, the decreased recombination, and the higher V oc . B. Tan et al. used the composites of anatase TiO 2 nanoparticles and single-crystalline anatase TiO 2 nanowires as electrodes to fabricate DSCs, which possessed the advantages of both building blocks, i.e., the high surface area of nanoparticle aggregates and the rapid electron transport rate and the light scattering effect of single-crystalline nanowires (Tan et al., 2006). An enhancement of power efficiency from 6.7% for pure nanoparticle cells to 8.6% for the composite cell with 20 wt% nanowires was achieved, showing that employing nanoparticle/nanowire composites represented a promising approach for further improving the efficiencies of DSCs. C. H. Ku et al. reported ZnO nanowire/nanoparticle composite photoanodes with different nanoparticle-occupying extents (Ku et al., 2008). Aligned ZnO nanowires were grown on the seeded FTO substrate using an aqueous chemical bath deposition (CBD) first, and then, growth of nanoparticles among ZnO NWs by another base-free CBD was preceded further for different periods. The corresponding DSCs showed an efficiency of 2.37%, indicating the good potential of the hybrid nanostructures in ordered photoanodes. Apart from the direct blending of two different semiconductor components as mentioned above, the coating technique has also been applied widely to create the hybrid photoanodes. M. Law et al. developed photoanodes constructed by ZnO nanowires arrays coated with thin shells of amorphous Al 2 O 3 or anatase TiO 2 by atomic layer deposition (Law et al., 2006). They found that, while alumina shells of all thicknesses acted as insulating barriers that improve cell open-circuit voltage only at the expense of a larger decrease in short-circuit current density, titania shells in thickness of 10-25 nm can cause a dramatic increase in V OC and fill factor with little current falloff, resulting in a substantial improvement in overall conversion efficiency (2.25%). They attributed the improved performance to the radial surface field within each nanowire that decreases the rate of recombination. K. Park et al. described a ZnO-TiO 2 hybrid photoanode by coating ultrathin TiO 2 layer by atomic layer deposition on submicrometer-sized aggregates of ZnO nanocrystallites (Park et al., 2010). The introduction of the TiO 2 ultrathin layer increased both the open circuit voltage and the fill factor as a result of the suppressed surface charge recombination without impairing the photocurrent density, thus realizing more than 20% enhancement in the conversion efficiency from 5.2% to 6.3%. S. H. Kang et al. examined effects of ZnO coating on the anodic TiO 2 nanotube array film on the conversion efficiency (Kang et al., 2007). Compared with the solid-sate cells consisted of an anodic TiO 2 film as the working electrode under backside illumination, an almost 20% improvement from the ZnO coating was achieved (from 0.578% to 0.704%), which can be attributed to the suppressed electron flow to the back-direction and the enhanced open-circuit voltage. Despite considerable effects in this area, however, the record efficiency of 11% for DSCs is not surpassed by these new type cells, due to the complexity of both the nanoporous photoanode and the cell structure of DSCs. Much comprehensive and in-depth work related to this topic is required. In this chapter, we focused on the ordered photoanode film built up by two semiconductor materials, zinc oxide (ZnO) and titanium oxide (TiO 2 ). Three type of ZnO nanostructures were selected, including the nanowire array (grown by the hydrothermal method), the nanoporous disk array grown on FTO substrate, and the nanoporous disk powder (transformed from the solution-synthesized zinc-based compound ZnCl 2 .[Zn(OH) 2 ] 4 .H 2 O). Different types of TiO 2 nanoparticles were used, including commercial nanoparticles P25 & P90 (Degauss Co., Germany), and home-made hydrothermal TiO 2 nanoparticles, which have been widely used in producing traditional high-efficiency DSCs. Two kinds of preparation technique of ZnO-TiO 2 hybrid film were used according to the status of ZnO nanostructures (array or powder). The microstructure, optical and electrical properties of the hybrid film were investigated, and the performance of corresponding DSCs was measured and compared with results of traditional cell. In s p e c i a l , t h e e m p h a s i s w a s p l a c e d o n t h e controlling method of the microstructure of ZnO-TiO 2 hybrid films, and on the electron transporting mechanism in the hybrid films.

ZnO nanowire array/TiO 2 NPs hybrid photoanodes
In this section, two types of ZnO nanowire (NW) array were selected, i.e., dense and sparse NW array, with an aim to examine the effects of the distribution density of NW on the microstructure and photoelectrochemical properties of the hybrid cells. For the dense NW array, the ultrasonic irradition was used to promote the penetration of TiO 2 nanoparticles in the interstice of ZnO NWs.

Hybrid photoanodes based on dense ZnO NW array
ZnO nanowire (NW) arrays were grown on ZnO-seeded fluorinated tin oxide (FTO, 20 Ω/□) substrates by chemical bath deposition method. ZnO seed layer was prepared by sol-gel technique. ZnO NW arrays were obtained by immersing the seeded substrates upside-down in an aqueous solution of 0.025 mol/L zinc nitrate hydrate and 0.025 mol/L hexamethylenetetramine (HMT) in a sealed beaker at 90 °C for 12 h. After the deposition of ZnO NW, TiO 2 nanoparticles (NPs) were coated on ZnO NW by dipping the substrate into a well-dispersed TiO 2 suspension containing 0.5 g TiO 2 NPs (P25), 20 μL acetyl acetone, 100 μL Triton X-100 in 10 mL distilled water and 10 mL ethanol with 20 μL acetic acid. To facilitate the attachment and the gap filling of TiO 2 NPs into the interstices of ZnO NWs, the ultrasonic irradiation generated from a high-density ultrasonic probe (Zhi-sun, JYD-250, Ti alloy-horn, 20-25 kHz) was applied to TiO 2 suspension. The working mode was adjusted to work for 2 seconds and idle for 2 seconds, with the repetition of 99 cycles. The electrodes were then withdrawn at a speed of 3 cm per minute, dried, and sintered at 450 °C for 30 min in air. Figure 2 gave the schematic for the fabrication of the hybrid ZnO NW array/TiO 2 photoanopde. For DSCs fabrication, ZnO NW based electrodes were immersed in a 0.5 mmol/L ethanol solution of N719 for 1 h for dye loading. The sensitized electrode was sandwiched with platinum coated FTO counter electrode separated by a hot-melt spacer (100 μm in thickness, Dupont, Surlyn 1702). The internal space of the cell was filled with an electrolyte containing 0.5 mol/L LiI, 0.05 mol/L I 2 , 0.5mo/L 4-tertbutylpyridine, and 0.6 mol/L 1-hexyl-3methylimidazolium iodide in 3-methoxypropionitrile solvent. The active cell area was typically 0.25 cm 2 .   Figure 3 showed the top and side-view SEM images of ZnO NWs grown on FTO substrate and ZnO-TiO 2 hybrid photoanode film with/without ultrasonic treatment. Results indicate that, for ZnO nanowire array with a density of ∼3.3×10 9 cm −2 and an average diameter of 80 nm and length of 3 μm, TiO 2 slurry with relatively high viscosity is difficult to penetreate into the inner pore of ZnO nanowires. As can be seen from Fig.3 c and d, only a small www.intechopen.com amount of TiO 2 NPs were covered on the side surface of NWs and most of the NPs sit on the top of NWs without filling in the inner gaps. When the ultrasonic irradiation was applied, the coverage of NPs on the side surface of NWs was significantly improved (Fig. 3 e-h), and TiO 2 NPs were uniformly infiltrated into the interstices of NWs rather than stuck to the top of NWs. The cavitation in liquid-solid systems induced by the ultrasonic irradiation bears intensive physical effects, which can promote the transfer of TiO 2 NPs and drive them infiltrating into the gaps of NPs. Figure 4 (left) showed the absorption spectra of the N719-sensitized ZnO NW, and hybrid ZnO NW/TiO 2 NP electrodes prepared with and without ultrasonic treatment, respectively. The absorption peak at around 515 nm, which corresponded to metal to ligand charge transfer (MLCT) in N719 dye (Nazeeruddin et al., 1993), significantly increased for the hybrid electrodes as compared to that of the pure ZnO NW electrode, proving that the dyeloading content is apparently increased upon the combination of ZnO NW with TiO 2 NPs. Besides, the hybrid electrode prepared with ultrasonic treatment showed an increase in the absorption in the wavelength range of 400-800 nm compared with that without ultrasonic treatment, indicating the higher surface area and the enriched light harvesting property by filling more TiO 2 NPs into the interstices between ZnO NWs with the assistance of ultrasonic irradiation. Figure 4 (right) illustrated I-V characteristics of DSCs based on pure ZnO NWs and ZnO/TiO 2 hybrid photoanodes. Results show that the short-circuit current density (I sc ) and the conversion efficiency (η) of ZnO NWs based cell can be dramatically improved by incorporating TiO 2 NPs, which can be ascribed to the increase in the surface area and the dye loading quantity. However, the open-circuit voltage (V oc ) and the fill-factor (FF) of the hybrid DSCs decreased compared to those of pure ZnO NW DSC, which may be resulted from the increased interfaces and surface traps in the hybrid photoanode which may act as the recombination center under illumination. For the hybrid photoanode prepared with ultrasonic treatment, its I sc , V oc , FF, and η was 3.54 mA/cm 2 , 0.60 V, 0.37, and 0.79%, respectively, indicating an approximately 35% improvement of the overall conversion efficiency compared with the photoanode without ultrasonic treatment. This improvement may originate from the enhanced light harvesting and the better attachment of TiO 2 NPs to ZnO NWs resulted from the efficient pore filling induced by the ultrasonic irradiation treatment. Fig. 4. The absorption spectra (left) of N719-sensitized ZnO NW arrays, and hybrid ZnO NW/TiO 2 NP photoanodes prepared without and with ultrasonic treatment, and I-V characteristics of corresponding DSCs (right). (Reproduced from Ref.  In summary, these results indicate that, for the hybrid films combining dense ZnO NW array and TiO 2 NPs, the crucial aspect is to make TiO 2 NPs contained in the slurry penetrate into the deep interstice of ZnO NWs. The application of ultrasonic irradiation or other external fields may be helpful for the penetration of TiO 2 NPs, which usually result in the increase of the photoelectrochemical performance of the hybrid cells. However, it seems that the full filling of TiO 2 in the dense NW array is very difficult based on the current technique. So it is meaningful to develop the sparse nanowire array or other forms of TiO 2 NPs, to realize the good combination of ZnO NW array and TiO 2 NPs.

Hybrid photoanodes based on sparse ZnO NW array
In this section, ZnO NW array with sparse density was integrated with TiO 2 NPs, to form the hybrid photoanode. The growth of sparse ZnO NW array was realized by reducing the pH value of the precursor via the chemical bath deposition (CBD) method. The substrates and the experimental parameters were similar to those of dense one except the concentration of Zn 2+ and HMT (both 0.02 mol/L), and the pH value (2.0-3.0). TiO 2 slurry was prepared following the method in Ref (Ito et al., 2008), and the mass ratio of TiO 2 , ethyl cellulose, and terpineol was 18 : 9 : 73. Due to the acid-dissolute nature of ZnO materials, the pH value of TiO 2 slurry should be controlled neutral or weak alkaline. The preparation of the hybrid film based on sparse ZnO NW array was similar to that of dense array, as described in Section 2.1. The sensitization of the film was carried out in N719 dye solution dissolved in a mixture of acetonitrile and tertbutyl alcohol (volume ratio, 1:1) for 20-24 hours at room temperature. The fabrication of the cells was similar to the procedure described in Section 2.1, with the electrolyte composition of 0.6 M BMII, 0.03 M I 2 , 0.10 M guanidinium thiocyanate and 0.5 M 4-tertbutylpyridine in a mixture of acetonitrile and valeronitrile (volume ratio, 85:15). Figure 5 gave SEM images of sparse ZnO NW on the surface and cross section. It can be seen that the density of ZnO nanowire on FTO substrate is much sparser than the dense ZnO NW (Figure 3 a&b). But with the decrease of the density, the diameter of ZnO NW increases greatly, up to several micrometers. The hybrid cell based on the sparse ZnO NW array exhibited the conversion efficiency of 2.16%, lower than the TiO 2 NPs-based cell (2.54%) as illustrated in Figure 6. The decreased efficiency of the hybrid cell is mainly resulted from the reduced photocurrent density compared with the TiO 2 cell, while the open voltage keeps unchanged and the fill factor improved from 0.06 to 0.078. The open-circuit voltage decay (OCVD) analysis ( Figure 6) indicated that the hybrid film exhibits longer decay time when the illumination is turned off, indicating lower recombination rate between photo-induced electrons and holes. We believe that the obviously reduced photocurrent density may be related to the reduced surface area induced by the incorporation of large size ZnO nanowires, which may resulted in the reduced dye loading content. So the improvement in the efficiency of DSCs via the integration of sparse ZnO NW array and TiO 2 NPs is possible, as long as the size of ZnO nanowire can be reduced to tens of nanometers. However, limited by the current technology level of ZnO nanowire array, it is not an easy task to grow ZnO NW array both sparse and thin enough for the application in the hybrid photoanodes of DSCs. In summary, we have successfully prepared the hybrid photoanode film using sparse ZnO www.intechopen.com  NW array and TiO 2 NPs. Although the total efficiency of the hybrid cell was lower than the TiO 2 NPs-based cell, the obvious improvement in the fill factor and the reduction in the recombination rate were observed. The reduced efficiency was mainly related to the decreased photocurrent density originated from the large-size ZnO NW. The further chance to improve the efficiency of ZnO NW based hybrid cell may reside in the realization of ZnO NWs with both sparse density and thin diameter.

ZnO nanoporous disk array/TiO 2 NPs hybrid photoanodes
In this section, an alternative ZnO nanostructure was used to prepare hybrid photoanode film, i.e., ZnO disk array possessing nanoporous feature. Compared with the traditional ZnO NW, the thickness of ZnO disk is lower and the surface area is higher. Thus higher effect in improving the conversion efficiency of DSCs can be expected. ZnO nanoporous disk array was transformed from the disk of a layered zinc-based compound -simonkollite (ZnCl 2 .[Zn(OH) 2 ] 4 .H 2 O, brief as ZHC) via calcinations. Conductive FTO glass coated by a thin TiO 2 layer (deposited by the hydrolysis of 40 mM TiCl 4 aqueous solution at 70 o C) was used as the substrate. Typically, ZHC disk was prepared by CBD method. Aqueous solutions of 20 ml ZnCl 2 (0.2 mol/l), 20 ml hexamethylenetetramine (HMT) (0.2 mol/l), and 40 ml ethanol were mixed in a beaker and heated to 70 o C in oven for 2 hours. After washing with H 2 O and ethanol carefully, ZHC nanodisk array deposited on TiO 2 /FTO substrate was sintered in air at 500 o C for 4 hours, to convert ZHC to ZnO nanoporous disk. TiO 2 NPs slurry was prepared by grinning TiO 2 commercial nanoparticles (P90, Degauss Co.) 0.5 g, H 2 O 2.5g, PEG 20000 0.25 g in porcelain mortar. The ZnO-TiO 2 hybrid film was prepared by the doctor blade method, and the ZnO nanoporous disk array grown on TiO 2 /FTO substrate was used. To achieve a specific thickness of the film, two layers of TiO 2 slurry were applied. The dried hybrid cell was sintered at 450 o C in air for 30 minutes. The sensitization of photoanode films and the fabrication of the cells were similar to those described in Section 2.1, except that the sensitizing time was prolonged to 24 hours. Figure. 7(a) illustrated SEM images of ZHC nanodisk array deposited on TiO 2 /FTO substrate. It can be seen that as-deposited ZHC exhibit rather regular hexagonal disk shape, with the size of ~ 10 um. The distribution of ZHC disks on substrate is sparse, satisfying the "low-content" requirement of ZnO in the hybrid photoanode film. After annealing at 500 o C, ZHC disks were transformed into ZnO with typical nanoporous structure (as shown in Figure 7(b)), while the sheet structure (~100 nm in thickness) was maintained. Figure 7 (c) and (d) showed SEM images of the hybrid films based on this sparse nanoporous ZnO disk array. We can see that the morphology of the hybrid film on the surface and the cross section was rather smooth and uniform, with little difference from the traditional pure TiO 2 film . In addition, ZnO sheet like structures can not be found in either the surface or the cross section due to the low content of ZnO in the hybrid film.  gave the optical transmittance spectra of FTO substrate, pure TiO 2 film and the hybrid film. Results indicate that in the wavelength rage of 470-800 nm, the hybrid film possesses relatively lower transmittance than the pure TiO 2 film, while in the wavelength band of 300-470 nm, the transmittance of the hybrid film is higher. The reduced transmittance in the higher wavelength band of the hybrid film may be related to the scattering effects of the large ZnO disk in the film. In view of the maximum absorption of N719 dye molecules located at ~ 525 nm (Figure 4), the scattering of ZnO nanoporous disks to the incident light has positive influence on the performance of the hybrid cells. The reduced transmittance in the lower band of the pure TiO 2 film may be related to the increased agglomeration of TiO 2 NPs, which can induce the larger secondary particles and the higher scattering effects in the lower wavelength range. In contrast, the presence of large-size ZnO sheet may reduce the agglomeration phenomena to some extent, thus exhibiting higher transmittance. Figure 8 (right) gave I-V curves of pure TiO 2 NPs cell and ZnO nanodisk array -TiO 2 NPs hybrid cell under AM 1.5 illumination (100 mW/cm 2 ). It can be seen that the cell based on the hybrid film possesses much higher photocurrent density than TiO 2 NPs cell, increasing from 7.84 mA/cm 2 to 11.70 mA/cm 2 . Also the improvements in the photovoltage and the fill factor of the hybrid cell are observed. As a result, the total conversion efficiency changes from 3.07% to 5.19%, increased by up to 60%.  Figure 9 (left) showed Nyquist plots of the hybrid and pure photoanode, and the lower table gave the simulation results according to the physical model given in the inset. Two arcs can be clearly identified in the Nyquist plot for each sample. The left (high frequency) arc corresponds to the charge transfer process at the Pt counter electrode (R ct1 ). The right large arc arises from the charge transport at the TiO 2 /dye/electrolyte interface (R ct2 ). The right small arc is related to the Warburg diffusion process of I -/I 3 -in the electrolyte, which is not discussed in this work. The overall series resistance of the cell (R s ) is the resistance measured when electrons are transported through the device in the high-frequency range exceeding10 5 www.intechopen.com Hz. By simulated calculation following the equivalent circuit, we can obtain the calculated value of R s , R ct1 , and R ct2 for each sample. Results show that the hybrid film exhibits obviously lower R s , R ct1 , and R ct2 than the pure TiO 2 film, indicating that the overall series resistance, the resistance at the Pt/electrolyte interface and at the TiO 2 /dye/electrolyte interface in the hybrid cell is lower than the traditional TiO 2 NPs cell. Figure 9 (right) showed OCVD curves of the hybrid and pure photoanode. While the pure TiO 2 cell exhibits rapid voltage decrease after the turning off of the illumination, the hybrid cell has much slower decay behavior, indicating that the photo-induced electron-hole recombination rate in the hybrid film is lower than the pure TiO 2 cell. We believe the reduced overall resistance, the interfacial resistance and the electron-hole recombination rate is responsible for the obvious improvement in the total conversion efficiency in the hybrid cell. In brief, we prepared ZnO-TiO 2 hybrid photoanode film based on sparse ZnO nanoporous disk array grown on TiO 2 /FTO substrate. Though the obvious change in the microstructure of the film could not be observed, the hybrid film possessed increasing scattering effects in the wavelength range of 470-800 nm, which was beneficial to the light absorption of the dye molecules. Also the integration of ZnO nanoporous disk into TiO 2 NPs film resulted in the decrease of the overall series resistance and the resistance at the Pt/electrolyte interface and at the TiO 2 /dye/electrolyte interface. As a result, the conversion efficiency was improved by 60%, indicating the great potential of the sparse ZnO nanoporous disk array in the field of hybrid DSC photoanodes.

ZnO nanoporous disk powder/TiO 2 NPs hybrid photoanodes
The disadvantage for the hybrid photoanode between ZnO array (both the nanoporous disk array and the nanowire array) and TiO 2 NPs lies in the difficulties in controlling the precise content of ZnO in the hybrid film, which is a crucial parameter for any composite material. Also, the distribution of ZnO array in the hybrid film may be not uniform, and difficult to control. Therefore, in this section, we attempted to blend ZnO nanoporous disk in the powder form into TiO 2 slurry, and prepared a uniform hybrid film via the doctor-blade technique. By this method, we can examine the effects of ZnO content in the hybrid film on the microstructure and properties of photoanode, and find an optimal composition for ZnO-TiO 2 hybrid photoanodes. The powder of ZnO nanoporous disks was synthesized by the pyrolysis of chemical bath deposited ZHC nanodisks. Two types of TiO 2 NPs were selected, i.e., the commercial P25 TiO 2 and the hydrothermal TiO 2 NPs following the procedure described in Ref (Ito et al., 2008).

Hybrid photoanodes based on P25 TiO 2 NPs
Layered ZHC was prepared by the chemical bath deposition method. Typically, aqueous solutions of 20 ml ZnCl 2 (0.2 mol/l), 20 ml hexamethylenetetramine (HMT) (0.2 mol/l), and 40 ml ethanol were mixed together and heated to 70 o C in oven for 2 hours, resulting in the white precipitation of ZHC. After centrifuging and drying, the powders were annealed at 500 o C in a tube furnace in air for 18 hours to convert ZHC into ZnO. Film electrodes for DSCs were deposited onto FTO substrate via TiO 2 or TiO 2 /ZnO slurry by the doctor blade technique. For P25 TiO 2 NPs, the TiO 2 slurry was prepared using the mixed suspension of TiO 2 NPs (P25) (0.5 g) and PEG-1000 (0.25 g). The TiO 2 /ZnO hybrid electrodes were made by using the mixed suspension of P25, PEG and ZnO disk powder, with the weight percentages of ZnO being 0 (S1), 0.5% (S3) and 1% (S2). The electrodes were sintered at 450 o C for 120 min. The detail preparation process of the hybrid film was illustrated in Figure 10.  The annealed electrodes were stained by N719 dye by soaking them in a 0.5 mmol/l solution of N719 for 12 hours. FTO glass substrates were coated by Pt catalyst layer by decomposing H 2 PtCl 6 . 6(H 2 O) at 400 o C, and were used as the counter electrode. The working electrode and the counter electrode were cohered together by surlyn 1702 hot melt foil. The electrolyte, consisted of 0.05 M I 2 , 0.1 M LiI and 0.5 M tertbutylpyridine in acetonitrile, was filled into the cell from the hole in the counter electrode. Figure 11 gave SEM images of as-prepared ZHC disks and nanoporous ZnO disks obtained by sintering. As-prepared ZHC possesses obvious disk-like feature, with the side length of 500-1000 nm and the thickness of 100-300 nm in average. After annealing, ZnO disks with the nanoporous feature were obtained, with the pore size ranging from 50-200 nm. In addition, the linking between neighboring ZnO particles in each disk can be clearly observed, which is expected to provide good electron transport in DSCs. Figure 12 showed the cross-section morphologies of the pure TiO 2 film and 1%ZnO/TiO 2 hybrid film. While the pure TiO 2 film shows a uniform surface morphology and typical nanoporous structure with a thickness of ~20 m, the hybrid film possesses a rough surface with large humps of ~10 m (Fig.12 c) and lower thickness (~6 m). The results indicate that even a small amount of ZnO powder blended in TiO 2 slurry can change the microstructure and thickness of the film electrodes significantly, which may be related to the change of the slurry viscosity during the preparation process. The presence of large ZnO particles (several m in size) in the hybrid slurries may hamper the free flow of the TiO 2 slurry, thus resulting in the formation of large humps on the surface and the higher internal roughness The optical transmittance spectra (Figure 13 (left)) of the TiO 2 and ZnO/TiO 2 hybrid films shows that both the pure and hybrid films exhibit strong scattering effects on the incident light in the visible and near infrared band. Compared with the pure TiO 2 film electrode, the hybrid film electrodes show much lower transmittance in the wavelength range of 500-1100 nm, indicating that a very small amount of ZnO disks can exert significant effects on the optical properties of the photoanode. Fig. 13. Optical transmittance (left) of TiO 2 film (S1) and ZnO/TiO 2 hybrid film on FTO glass substrate, with ZnO percentage of 1% (S2) and 0.5% (S3), and electrochemical impedance spectra (right) of the DSCs based on TiO 2 electrode (S1) and ZnO-TiO 2 hybrid electrodes (S2 and S3). The attached table illustrates EIS parameters calculated from the given equivalent circuit. (Reproduced from Ref (Gao et al., 2009)) Figure 13 (right) showed the impedance spectra of DSCs using TiO 2 and ZnO/TiO 2 hybrid photoanodes. Results show that, both hybrid cells exhibit lower R s , R ct1 , and R ct2 than those of the pure TiO 2 cell (S1), indicating that the incorporation of ZnO in the photoanode can decrease the overall series resistance of device significantly and facilitate the interfacial charge transport in both Pt/electrolyte and TiO 2 /dye/electrolyte interface. The reason for this improvement may be the combination of the high electron transport nature of onedimensional ZnO materials (Martinson et al., 2006), the large particle size and the network structure of ZnO disk. Figure 14 revealed I-V curves of DSCs with TiO 2 and ZnO-TiO 2 hybrid films. The overall efficiencies of three cells are in the order of S3>S2>S1. While the cell using pure TiO 2 electrode (S1) exhibits the lowest efficiency of 1.1%, the cells with 0.5% and 1% ZnO-TiO 2 hybrid electrodes show higher efficiency of 2.7% and 2.3%, improved by 145% and 109%, respectively. Two hybrid cells exhibit similar V oc but significantly higher I sc and higher fill factor than pure TiO 2 cell, indicating that the improvement in the photocurrent density and fill factor is the main reason for the efficiency improvement. Also, the concentration of ZnO in the hybrid film should be no higher than 1% in this case, which is consistent with our previous observation in the hybrid film based on the sparse ZnO nanoporous disk array (Section 3) and results of other researchers on the DSCs with ZnO nanorod-TiO 2 hybrid electrode (Kang et al., 2007).
Fig. 14. The I-V characteristic curves of DSCs based on TiO 2 electrode (S1) and ZnO/TiO 2 hybrid electrode with ZnO percentage of 1% (S2) and 0.5% (S3). (Reproduced from Ref (Gao et al., 2009)) In summary, we have successfully demonstrated that ZnO nanoporous disk prepared from layered zinc based compound ZHC can be used to improve the efficiency of TiO 2 photoanodes effectively. The direct incorporation of ZnO nanodisk powder into TiO 2 slurry combined with the doctor-blade technique was proved an effective way to prepare the hybrid films. Results showed that even a small amount of ZnO incorporation in the TiO 2 film (1%) can significantly influence the microstructure, optical and electrical properties. The rougher inner microstructure, the enhanced light-scattering effect on the visible and www.intechopen.com infrared light region, and the higher interfacial charge-transport rate were responsible for the improved efficiency in the hybrid photoanodes when compared with the pure TiO 2 film.

Hybrid photoanodes based on hydrothermal TiO 2 NPs
In this section, TiO 2 hydrothermal NPs were used as the source of TiO 2 for the preparation of the hybrid film. ZnO-TiO 2 hybrid slurry was prepared by adding specific amount of ZnO nanoporous disk powder into TiO 2 NPs slurry. TiO 2 NPs were prepared via the hydrothermal method and the slurry containing TiO 2 (18% by weight), ethyl cellulose (9%) and terpineol (73%) was prepared following Ref (Ito et al., 2008). ZnO nanoporous disk powder with the weight percentage of 0.5%-2% (compared with TiO 2 ) was blended into the slurry before the evaporation of ethanol via rotate-evaporator. Due to the acid-dissolute nature of ZnO materials, the pH value of TiO 2 slurry should be controlled at neutral or weak alkaline range. ZnO-TiO 2 hybrid photoanodes were prepared by the doctor-blade technique using the hybrid slurry. Conductive FTO glass coated by a thin TiO 2 layer (deposited by the hydrolysis of 40 mM TiCl 4 aqueous solution at 70 o C) was used as the substrate. The sensitization of the photoanode film and the fabrication of the cells were similar to the procedure described in Section 2.2. SEM analysis (not shown here) indicates that the microstructure of the hybrid film was similar to that of pure TiO 2 film, different from the results in P25 slurry (Section 4.1). We think this difference may be resulted from the solvent of the slurry. In P25 based hybrid slurry, water is the dispersant, and the presence of large-size ZnO disk may exert significant influences on the viscosity of the slurry, thus modifying the microstructure of the hybrid film. In the hydrothermal TiO 2 hybrid slurry, organic terpineol was used as the dispersant, which possessed much higher viscosity than water. So the presence of a very small percentage of ZnO disks in the slurry had only minor influences on the viscosity and other physical properties of the hybrid slurry. Correspondingly, the microstructure of the hybrid film differed little from the pure one. Figure 15 (left) showed optical transmittance spectra of ZnO-TiO 2 hybrid film deposited on FTO substrate. Different from microstructure results, the hybrid films exhibit obviously lower transmittance than the pure TiO 2 film in the visible-near infrared wavelength, which can be ascribed to the scattering effects of ZnO disks dispersed uniformly in the film. As mentioned above, this scattering behavior is beneficial to the absorption of N719 dye molecules. Also the hybrid film with 2% ZnO exhibits higher scattering effects than the one with 0.5% ZnO. I-V curves given in Figure 15 (right) indicate that the photoanode with 1% ZnO possesses the highest efficiency compared with 0.5% ZnO and 2% ZnO hybrid films. While the cell parameters of the cells based on 0.5% and 1%ZnO are similar, the cell with 2%ZnO shows much less photocurrent density, which may account for the main fall in the efficiency. Also we were trying to coat a thin TiO 2 layer on ZnO nanoporous disk before its incorporation into TiO 2 slurry via the sol-gel method. Results show that this coating is not helpful in this case, maybe resulted from the reduced surface area in ZnO nanoporous disk due to TiO 2 coating. Further and in-depth investigation in this direction is required. Fig. 16. Nyquist plots of ZnO nanodisk -TiO 2 NPs hybrid cells with ZnO content in hybrid slurry by weight of (a) 0.5%, (b) 1.0%, (c) 2.0%), and (d) 0.5% (ZnO disks were coated by a thin TiO 2 layer via sol-gel technique). The table illustrates EIS parameters calculated from the given equivalent circuit.  From the results of simulation, we can see that the hybrid cell with 1%ZnO content possesses the lowest resistance at Pt/electrolyte interface and at TiO 2 /dye/electrolyte interface. The resistance at two interfaces increases when the content of ZnO in hybrid film is either lower or higher than 1%. In special, for the hybrid film with 2% ZnO content, the resistance at both interfaces is higher than that of film with 1% or 0.5% ZnO content. Also for the film with 0.5% ZnO coated with a thin TiO 2 layer, the resistance at the TiO 2 /dye/electrolyte interface and the overall series resistance were increased, thus resulted in the reduction in the total conversion efficiency. In general, we can derive a linear relationship between the interfacial resistance obtained from EIS analysis and the overall conversion efficiency of the hybrid cell. Figure 17 showed OCVD plots of the hybrid cell with different ZnO content. Results show that 1%ZnO-TiO 2 hybrid cell exhibits the slowest voltage decay rate among all four cells, indicating the lower recombination rate, which is consistent with its high conversion efficiency. Also the coating of a thin TiO 2 layer on ZnO nanoporous disk via sol-gel technique can slightly improve the recombination properties of the hybrid cell, which is beneficial to the improvement of the efficiency. However, other consequences induced by the coating, possibly the blocking of the nanopore and the decrease of the surface area, resulted in the reduction of the overall efficiency at last. For ZnO-TiO 2 hybrid cell, it is important to discuss the effects of the treatment of the photoanode film in TiCl 4 aqueous solution. ZnO is intrinsically an amphoteric oxide, which can be dissolved in TiCl 4 aqueous solution with the strong acid nature. So it should be very careful in treating ZnO-TiO 2 hybrid films in TiCl 4 solutions. Figure 18 illustrated I-V curves of TiO 2 and ZnO-TiO 2 film with and without TiCl 4 treatment. Results show that the incorporation of ZnO into TiO 2 can improve the efficiency from 3.5% to 3.98% before the TiCl 4 treatment. However, after the treatment, the efficiency of the hybrid film decreases to 3.2%, while the efficiency of the pure TiO 2 film increases to 6.38%. The dissolution of ZnO dispersed uniformly in the hybrid film may be underlying reason for the efficiency reduction. On the other hand, the TiCl 4 treatment is a very powerful method to improve the efficiency of the pure TiO 2 cell. So it is necessary to develop an alternative treatment method in a neutral aqueous solution, which has minor influence on the structure of ZnO. Other methods may be also helpful including the protective coating technique on ZnO nanoporous disks by thin TiO 2 layer which can resist the corrosion of acid. However, when the protective coating technique is adopted, one has to be careful in avoiding the blocking of the nanopore present in ZnO nanostructures. In summary, this section discussed the preparation of ZnO-TiO 2 hybrid photoanode films based on ZnO nanoporous disk powder and hydrothermal TiO 2 NPs. An optimal ZnO content of ~1% in the hybrid film was observed, with the total conversion efficiency of ~2.94%. While the efficiency improvement in the hybrid cell was realized when compared with the pure TiO 2 film when no TiCl 4 treatment was performed, the decrease in the efficiency in the hybrid film after the TiCl 4 treatment indicated the sensitivity of the hybrid system. The development of other treatment methods suitable for ZnO-TiO 2 hybrid system or other protective means may afford the further direction in this field.

Conclusion
In summary, the chapter started with a general review on the ordered TiO 2 and ZnO photoanode and the hybrid photoanode, outlining a brief picture on the status of the ordered photoanode in the field of DSCs. Then focusing on the ZnO-TiO 2 hybrid photoanodes, four type of ZnO nanostructure (including dense and sparse nanowire array, sparse nanoporous disk array, and nanoporous disk powder) and three type of TiO 2 nanoparticles (including P25, P90, and home-made hydrothermal nanoparticles) were used to prepare various hybrid films. Results show that, in general, the integration of ZnO with TiO 2 is a powerful means to improve the efficiency of the photoanode of DSCs, with the improvement up to 150%. However, one has to take great care in realizing the ideal hybrid structures. The content of ZnO in the hybrid film has to be maintained at a low level (e.g. ~1% by weight) in order to obtain a positive effect, which means that sparse and thin nanowire array or nanodisk array instead of the dense array is preferred. Also great care has to be taken during the TiCl 4 treatment of the ZnO-TiO 2 hybrid photoanode or the protective coating of ZnO nanostructures, preventing the destruction to the microstructure of ZnO nanostructures. Though at the current stage the overall conversion efficiency of the hybrid cell has not overpassed the highest level of the pure TiO 2 cell, it represents a very powerful and important technical route to optimize the microstructure and the performance of the ordered photoanode. We believe, by the continuous efforts of the forthcoming researchers, the hybrid semiconductor photoanodes with ordered structures will eventually shed the light to the DSCs with the breakthrough efficiency.