Dye-Sensitized Solar Cells with Anatase TiO2 Nanorods Prepared by Hydrothermal Method

The hydrothermal method provides an effective reaction environment for the synthesis of nanocrystalline materials with high purity and well-controlled crystallinity. In this work, we started with various sizes of commercial TiO 2 powders and used the hydrothermal method to prepare TiO 2 thin films. We found that the synthesized TiO 2 nanorods were thin and long when smaller TiO 2 particleswereused,whilelargerTiO 2 particlesproducedthickerandshorternanorods.WealsofoundthatTiO 2 filmsprepared by TiO 2 nanorods exhibited larger surface roughness than those prepared by the commercial TiO 2 particles. It was found that a pure anatase phase of TiO 2 nanorods can be obtained from the hydrothermal method. The dye-sensitized solar cells fabricated with TiO 2 nanorods exhibited a higher solar efficiency than those fabricated with commercial TiO 2 nanoparticles directly. Further, triple-layer structures of TiO 2 thin films with different particle sizes were investigated to improve the solar efficiency.


Introduction
Dye-sensitized solar cells (DSSCs) have attracted much attention as possible candidates for low cost, high stability, and high efficient solar cells [1,2]. There are many innovations in this emerging technology such as new dyes which are absorbed at a wider range of wavelengths and the introduction of nanostructure titanium oxides (TiO 2 ) to increase the surface area [3][4][5]. The DSSCs with the nanostructure titanium oxide/Porphyrins dye thin films on transparent conducting oxide-(TCO-) coated glass can achieve a solar efficiency as high as 13% [6]. The major improvements of the research are made not only by introducing highly absorbing dyes as light harvesters, but also by using the nanostructure layer to improve the absorption and collection efficiency. In principle, fast electron transport and slow recombination will be needed to obtain a high solar conversion efficiency. For conventional DSSC, the mesoporous film consisted of nanocrystalline TiO 2 particles, enjoying the advantages of a large surface for greater dye adsorption and facilitating electrolyte diffusion within their pores [7][8][9][10][11][12]. The hydrothermal method provides an effective reaction environment for the synthesis of nanocrystalline TiO 2 with high purity and well-controlled crystallinity [13][14][15]. Therefore, we use the hydrothermal method to prepare TiO 2 thin films in this work. The Taguchi method [16][17][18][19][20] is used to find the optimal parameters for the formation of high-quality TiO 2 films. The Taguchi method [16] is a process optimization technique that investigates how multiparameters affect the performance of a process. It can minimize the variation in a process through robust design of experiments. The Taguchi method uses orthogonal arrays [17] to organize the parameters affecting the process and the levels at which they should be varied. It allows for the determination of factors mostly affecting a process performance characteristic with a minimum amount of experimentation. Generally, it employs a generic signalto-noise ( / ) ratio to quantify the variation. These / ratios are used as measures of the effect of noise factors on performance characteristics. There are several / ratio types of characteristics: larger is better, nominal is best, smaller is better, and so forth [16,18].
In addition, it is known that the strong back-scattering light due to the large particles near the conducting glass results in a light loss. To reduce light loss due to this strong back-scattering light, multiple-layer structure of TiO 2 with 2 International Journal of Photoenergy different particle sizes has been proposed in the past [21][22][23][24][25][26][27].
Here triple TiO 2 layer structure with small particle sizes is at the bottom, medium sizes in the middle, and large particle sizes on top which are also investigated to improve the solar performance of DSSCs.

Experiments
The 2 cm × 1.5 cm fluorine-doped SnO 2 -(FTO-) coated glass electrodes (sheet resistance 8 Ω/◻) were cleaned by acetone, isopropanol, and deionized water sequentially. In the hydrothermal procedure, 3 g TiO 2 powders were placed into a Teflon lined autoclave of 100 mL capacity. The autoclave was filled with 8 M, 10 M, or 12 M NaOH aqueous solution and sealed into a stainless steel tank and maintained at 180 ∘ C for 24 hrs. It was cooled down naturally to room temperature. The obtained sodium titanate was put into 200 mL of 1 N HCl aqueous solution at pH = 2 and stirred for 24 h. This HCl treatment was repeated many times in order to exchange Na + ions completely by H + ions leading to the formation of hydrogen titanate nanorods. Then these hydrogen titanate nanorods were washed with distilled water until the pH reached 7 and filtered to obtain the precipitated hydrogen titanate nanorods. These nanorods were dehydrated and recrystallized into the anatase TiO 2 nanorods. Table 1 shows the four factors and three levels used in our experiment according to the Taguchi method [16][17][18][19][20]. If three levels were assigned to each of these factors, then conventional method would require 3 4 or 81 experiments to find the optimal condition. Using the Taguchi method, we can reduce the number of experiments to nine. The orthogonal array of L9 type [17] is used and shown in Table 2. This design requires nine experiments with four parameters at three levels of each. The interactions of these four parameters were neglected. TiO 2 solutions are prepared by mixing 3 g of TiO 2 powders, 1 mL of titanium tetraisopropoxide (TTIP), 0.5 g of Polyethylene glycol (PEG), and 0.5 mL of triton X-100 in 50 mL of isopropanol (IPA). The mixture was then grinded and stirred by zirconia ball for 8 hours. It is known that the addition of TTIP in the solution can reduce the surface crack and the PEG can make a porous thin film after annealing. The TiO 2 thin films were formed by spin-coating TiO 2 solutions on FTO-coated glass and annealed at 500 ∘ C for one hour.   FTO-coated glass, which was further coated with H 2 PtCl 6 precursor and annealed at 450 ∘ C for 30 min. The cell was fabricated by applying a surlyn spacer, which is a hot-melting film with a thickness of 60 m, between two electrodes. Two FTO-coated glasses were made with the surlyn heated at 100 ∘ C. The electrolyte was injected into the space between the electrodes by capillary action. Finally, these two FTO-coated glasses were sealed completely. The active area of cells is 1 cm 2 . The photocurrent-voltage ( -) characteristic curves were measured using Keithley 2420 under AM1.5G illumination.

Results and Discussions
Nine different hydrothermal experiments were performed using the design parameter combinations shown in Table 2. Three specimens were fabricated for each of the parameter combinations. The factor effects on the solar efficiency and / ratio for each experiment are listed in Table 3. The higher solar efficiency is the indication of better performance. Therefore, the larger-is-better criterion was selected for the solar efficiency to obtain the optimal solar performance. The following / ratios for the larger-is-better case can be calculated [16,18]: where ( / ) LB stands for the larger-is-better signal-to-noise ratio, is the individually measured solar efficiency, and is the number of solar cell samples measured. Figure 1 shows the factor effects on the / ratio. The larger slope means that the factor has a stronger effect on solar efficiency. It indicates that NaOH concentration (factor A) has a stronger effect on solar efficiency. The annealing temperature (factor D) is the next most significant factor. The objective is to maximize the / ratio. This implies that one can obtain high solar efficiency by using the factor with higher / ratio. It is clear from Figure 1 that the highest / ratio values in each factor are Order Factor A NaOH concentration (M) B TiO 2 particle size (nm) C Autoclave temperature ( ∘ C) D Annealing temperature ( ∘ C) 1 10  , which correspond to the factor A1, B2, C1, and D1, respectively. Therefore, the best parameters of hydrothermal methods are (A1) NaOH concentration of 10 M, (B2) commercial TiO 2 particle size of 21 nm, (C1) the temperature of 180 ∘ C, and (D1) the annealing temperature of 450 ∘ C. Thus, these best parameters were used to prepare our TiO 2 nanorods. Figures 2(a) and 2(b) show the surface morphology of TiO 2 films prepared by commercial TiO 2 particles and TiO 2 nanorods which we prepared using hydrothermal methods, respectively. Clearly, a particle-like surface in the film is prepared using commercial particles versus a nanorodshape surface in the film prepared by our TiO 2 nanorods. From atomic force microscopy (AFM) measurement, it is observed that the mean roughness (∼63 nm) of the TiO 2 thin films prepared by the TiO 2 nanorods is larger than that (∼41.5 nm) prepared by the commercial TiO 2 particles. The large surface roughness in TiO 2 nanorods is beneficial for dye adsorption. In addition, a very pure anatase structure of TiO 2 nanorods is obtained by the hydrothermal method, as shown in Figure 3. There are no characteristic peaks of other impurity phases such as sodium titanium oxide or rutile TiO 2 , except pure anatase TiO 2 nanorods. This pure anatase structure of TiO 2 is extremely important to achieve high performance for electrons transport and dye adsorption in TiO 2 -based dye-sensitized solar cells [28,29]. Figure 4 compares the -characteristics of dye-sensitized solar cell prepared with hydrothermally grown TiO 2 nanorods and the DSSC prepared with commercial TiO 2 particles. The dyesensitized solar cells prepared with the hydrothermally grown TiO 2 nanorods clearly exhibit higher solar efficiency than that prepared with the commercial TiO 2 particles. This is due to the fact that TiO 2 nanorods have large surface area and pure anatase structure, which can absorb more dye and therefore better photoresponse.
It is also noted that the size of TiO 2 nanorods synthesized by hydrothermal method depends on the initial TiO 2 particle size. The nanorods are thin and long when small-size TiO 2 particles (size ∼14 nm) are used; however the nanorods become thick and short when large-size TiO 2 particles, are used (size, ∼100 nm), as shown in Figures 5(a) and 5(b). One can control the shape of TiO 2 nanorods by suitably choosing the initial TiO 2 particle sizes used in the hydrothermal process. Next we will examine the effect of TiO 2 thin film thickness on the solar efficiency of the fabricated DSSCs. In Figure 6, the cross-section scanning electron microscopy (SEM) images of different thickness of TiO 2 thin films are shown. We can see that a nanorod-like morphology is observed when 21 nm TiO 2 powder is used in the hydrothermal reaction. The optical absorption and   -characteristic curve of dye-sensitized solar cells with different TiO 2 thicknesses are shown in Figures 7(a) and 7(b), respectively. The optical absorption initially increases with increasing TiO 2 thickness and reaches a maximum at 12 m. For further increase in the TiO 2 film thickness, the light absorption begins to drop. The same behavior is observed in the photocurrent, as shown in Figure 7(b). The solar performance parameters of DSSCs with different TiO 2 thicknesses are listed in Table 4. The efficiencies of DSSCs with the TiO 2 thicknesses of 3.5, 5, 9.5, 12, and 15 m are 2.12, 2.44, 2.63, 2.85, and 2.67%, respectively. The DSSC with the TiO 2 thicknesses of 12 m exhibits the highest efficiency. It is known that dye in the film will build up with increasing TiO 2 thickness and hence increase the photocurrent. However, thicker TiO 2 layers will result in a decrease in the transmittance of light through these TiO 2 layers and thus reduce the incident light absorbed by the dyes. In addition, the charge recombination between electrons from the excited dye to the conduction band of TiO 2 and the I 3− ions in the electrolyte will become more difficult in thicker TiO 2 layers. Thus, there exists an optimal TiO 2 thickness to achieve higher solar efficiency for each particle size. In this work, the optimal TiO 2 thickness is 12 m for particle size of 21 nm used in the hydrothermal reaction.
It is known that large-size TiO 2 particles have the advantage of strong light scattering ability, while small size TiO 2 particles have the advantages of large contact area and low contact resistance [18][19][20][21][22][23][24]. In order to take the advantages of both the strong light scattering and the large contact area/low contact resistance, we constructed a triple-layer TiO 2 DSSC 6 International Journal of Photoenergy    Figure 8(a) shows the cross-sectional scanning electron microscopy (SEM) images of TiO 2 thin films with triple-layer structures.
The -curves of dye-sensitized solar cells with triple-layer structures are shown in Figure 8(b). The solar performance parameters of DSSCs with triple-layer structures are listed in Table 5. The efficiencies of DSSCs with the scattering layer prepared by 50, 100, and 200 nm particles are 3.62, 5.68, and 6.54%, respectively. The TiO 2 layers with larger particle sizes on the top layer exhibit higher solar efficiency than that with smaller particle sizes due to the strong back-scattering effect. It is known that smaller particles of TiO 2 layers have large surface area and adsorb more dyes. Hence, it has low contact resistance and high photocurrent. The strong backscattering light due to large particle size will also increase the reabsorption in the small particle size of TiO 2 layer. This smaller particle size on the bottom is beneficial to recapture the scattering light from the top scattering layer. The larger particle sizes of TiO 2 layers on the top can enhance the backscattering light effectively and result in higher photocurrent. Thus, the combination of larger particle sizes of TiO 2 on the top and smaller particle sizes of TiO 2 at the bottom will be better for achieving higher solar efficiency.

Conclusions
The dye-sensitized solar cells with the TiO 2 prepared by the hydrothermal method have demonstrated good solar performance. A high surface roughness and pure anatase structure are achieved by this method. The dye-sensitized solar cells with the TiO 2 nanorods exhibit higher solar efficiency than that with the commercial TiO 2 particles. The optimal TiO 2 thickness depends on the nanorod sizes of TiO 2 layer for achieving the maximum efficiency. The TiO 2 nanorod size formed through the hydrothermal method will depend on the initial TiO 2 particle size.