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International Journal of Photoenergy
Volume 2013 (2013), Article ID 563897, 9 pages
http://dx.doi.org/10.1155/2013/563897
Research Article

Particle Size Effects of TiO2 Layers on the Solar Efficiency of Dye-Sensitized Solar Cells

1Department of Electronic Engineering and Green Technology Research Center, Chang-Gung University, Taoyuan 333, Taiwan
2Department of Physics, University of Central Florida, Orlando, FL 32816, USA

Received 11 October 2012; Accepted 22 March 2013

Academic Editor: Tianxin Wei

Copyright © 2013 Ming-Jer Jeng et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Large particle sizes having a strong light scattering lead to a significantly decreased surface area and small particle sizes having large surface area lack light-scattering effect. How to combine large and small particle sizes together is an interesting work for achieving higher solar efficiency. In this work, we investigate the solar performance influence of the dye-sensitized solar cells (DSSCs) by the multiple titanium oxide (TiO2) layers with different particle sizes. It was found that the optimal TiO2 thickness depends on the particle sizes of TiO2 layers for achieving the maximum efficiency. The solar efficiency of DSSCs prepared by triple TiO2 layers with different particle sizes is higher than that by double TiO2 layers for the same TiO2 thickness. The choice of particle size in the bottom layer is more important than that in the top layer for achieving higher solar efficiency. The choice of the particle sizes in the middle layer depends on the particle sizes in the bottom and top layers. The mixing of the particle sizes in the middle layer is a good choice for achieving higher solar efficiency.

1. Introduction

Dye-sensitized solar cells (DSSCs) have attracted much attention as good candidates for low-cost, good-stability, and high-efficiency solar cells [1, 2]. There are many innovations in this emerging technology like the new dyes which absorb a longer range of wavelengths and the proposed nanostructure titanium oxides (TiO2) for increasing surface area and so forth [36]. The DSSCs with the nanostructure titanium oxide/porphyrins dye thin films on TCO glass can achieve a solar efficiency as high as with 13% [7]. The major improvement of the research is not only by introducing artificial synthesized dye as light harvesters instead of TiO2 itself, but also by using the nanostructure layer to improve the absorption and collection efficiency. In principle, rapid electron transport and slow recombination will be better for obtaining high solar conversion efficiency. For conventional DSSCs, the mesoporous photoelectrode films composed of small-sized TiO2 nanocrystalline particles have the advantages of providing a large surface for greater dye adsorption and facilitating electrolyte diffusion within their pores [8]. Therefore, optimizing the microstructure of the photoanode is vital for developing high solar efficiency of DSSCs. Recently, the light-scattering effects of TiO2 electrode have been proposed by using different particles in the TiO2 layers, which can significantly improve the light harvest efficiency [9]. Ferber and Luther [10] and Rothenberger et al. [11] confirmed the light-scattering effect with the transport theory and the many-flux model, respectively. Except for the scattering abilities of TiO2 layers, it is also important that the TiO2 electrode has a larger surface area, which can increase the dye adsorption and then high photocurrent generation. Recently, many efforts have been focused on the development of multifunctional TiO2 nanostructures for application in the DSSCs [1219]. However, they lack a systematic study for particle size effects. It is known that the large particle size of TiO2 layers can result in a strong light scattering ability and increase an optical absorption path. It indicates that it has higher short circuit current. However, the small nanoparticle size of TiO2 layers has large contact area and low contact resistance. It means that it has higher open circuit voltage. How to combine large and small nanoparticle sizes together is an interesting work for achieving higher solar efficiency. In this work, we investigate the performance influence of DSSCs prepared by multiple TiO2 layers with different particle sizes systematically.

2. Experiments

The 2 cm × 1.5 cm fluorine-doped SnO2 (FTO) electrodes (sheet resistance 8 Ω/square) were cleaned by acetone, isopropanol, and deionized water sequentially. TiO2 solutions are prepared by mixing 3 g of TiO2 powders with different particle sizes, 1 mL of TTIA, 0.5 g of polyethylene glycol (PEG), and 0.5 mL of Triton X-100 in 50 mL of isopropanol (IPA), and then ground 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 that the PEG can make a porous thin film by annealing. The TiO2 thin films were formed by spin-coating TiO2 solutions on FTO glass and annealed at 500°C for one hour. Three structures of single, double, and triple TiO2 layers with different particle sizes were prepared. The commercial TiO2 powders with different particle sizes used in the experiments are listed in Table 1. For double or triple layers, their total thicknesses are controlled to be the same of 12 μm (not an optimal thickness). It is noted that the thickness of each layer of TiO2 film can influence the results of solar performance parameters and EIS of DSSCs. We did our best to control the thickness of each layer of TiO2 film to be about 6 and 4 μm, respectively, for double and triple layers. TiO2 photoelectrodes were immersed for 24 hrs in anhydrous ethanol solution containing  M N719 dye. The liquid electrolyte consisted of 1 M lithium iodide (LiI), 0.1 M iodine (I2), 0.5 M 4-tert-butylpyridine (TBP), and 0.6 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII) in acetonitrile. The cathode electrode was made by FTO, which is coated by H2PtCl6 precursor and annealed at 450°C for 30 min. The cell was fabricated by applying Surlyn, with a thickness of 60 μm between the two electrodes, and the gap was fixed at 60 μm. Two FTO glasses were made with the Surlyn heated at 100°C. The electrolyte was injected into the space between the electrodes by capillarity. Finally, these two FTO glasses were sealed completely. The active area of cells is 0.25 cm2. The photocurrent-voltage (I-V) characteristic curves were measured by Keithley 2420 under AM1.5G illumination. Light absorption was measured by a UV-vis spectrophotometer. In addition, electrochemical impedance spectroscopy (EIS) is used to analyze the charge transport resistance [2024].

tab1
Table 1: The commercial TiO2 particle sizes used in the experiments.

3. Results and Discussion

The strong back-scattering light due to the large particles near the conducting glass unavoidably results in a light loss. To reduce light loss due to the strong back-scattering light, multiple-layer structure of TiO2 has been proposed to deposit the small particles first on the conducting glass for increasing contact area and then followed by the large particle layers for back-light scattering. To examine the particle size effect on the solar performance of DSSCs systemically, three structures of single, double, and triple TiO2 layers with different particle sizes are investigated. For single TiO2 layer, different particle sizes and thicknesses of TiO2 are studied. Different particle sizes in the top, middle, and bottom layers are examined, respectively, for the double or triple TiO2 layers.

Figure 1 shows the cross section and surface-view field emission scanning electron microscopy (FESEM) images of single TiO2 layer with three different particle sizes of 21 nm, 100 nm, and 200 nm. The solar efficiency dependence on the thickness of TiO2 layer with different particle sizes is shown in Figure 2. Clearly, the DSSCs with the large particle size of TiO2 layer have lower solar efficiency than those with small particle size of TiO2 layer under the same TiO2 thickness. The lower solar efficiency in large particle sizes of TiO2 layer can be attributed to a strong back-scattering light and can result in lower solar efficiency. It is known that the large particle size of TiO2 layer has smaller surface area than the small one. The number of adsorbed dyes in large particle size of TiO2 layer is less than that in small particle size of TiO2 layer due to small surface area. Therefore, the photocurrent of the DSSCs with large particle size of TiO2 layer is smaller than that with small particle size of TiO2 layer and results in lower solar efficiency. It is also observed that the optimal TiO2 thickness depends on the particle sizes of TiO2 layer for achieving the maximum efficiency. The larger the particle sizes of TiO2 layers, the shorter the optimal TiO2 thickness due to a strong back-scattering light. The larger thickness of TiO2 layers is expected to adsorb more dyes. But the generated electron in dyes cannot be injected to electrode effectively due to long distance when the thickness of TiO2 layers is long enough. Thus, the optimal thickness of TiO2 layers is different for each particle size due to different back-scattering lights and charge transport properties in the DSSCs. Dyes in the TiO2 layers will build up with increasing thickness and hence increase the photocurrent. However, thicker TiO2 layers will result in a decrease in the transmittance of TiO2 layers and thus reduce the incident light intensity to dyes. In addition, the charge transfer resistance increases with the increasing thickness of TiO2 layers. The charge recombination between electrons injected from the excited dye to the conduction band of TiO2 and the ions in the electrolyte will become more serious in thicker TiO2 layers.

563897.fig.001
Figure 1: The cross section and surface-view field emission scanning electron microscopy (FESEM) images of a single TiO2 layer with three different particle sizes of 21 nm, 100 nm, and 200 nm.
563897.fig.002
Figure 2: The solar efficiency dependence on the thickness of TiO2 layers with different particle sizes.

Figure 3 shows the schematic diagrams of double-layer structures with the different particle sizes in the top and bottom layers. The cross section FESEM images of double TiO2 layers with (a) 400 nm/5 nm/FTO, (b) 400 nm/100 nm/FTO, (c) 400 nm/200 nm/FTO, (d) 100 nm/10 nm/FTO, (e) 200 nm/10 nm/FTO, and (f) 400 nm/10 nm/FTO are shown in Figure 4. It is clear to demonstrate the different particle sizes in the top and bottom layers. Figures 5(a) and 5(b) show the photocurrent-voltage curves of the DSSCs with double-layer structures of different TiO2 particle sizes in the bottom and top layers, respectively. Clearly, the DSSCs with smaller particle size of 5 nm in the bottom have larger short-circuit current and efficiency than those with larger particle sizes of 100 and 200 nm, as shown in Figure 5(a). It is known that smaller particles of TiO2 layers have large surface areas and adsorb more dyes. Hence, they have low contact resistance and high photocurrent. In addition, the strong back-scattering light due to large particle size of 400 nm will also increase the reabsorption in the small particle size of TiO2 layer. These smaller particle sizes in the bottom are beneficial to recapture the scattering light from the top scattering layer. The larger particle sizes of TiO2 layers in the top can enhance the back-scattering light effectively and result in higher photocurrent, as shown in Figure 5(b). Thus, the combination of larger particle sizes of TiO2 in the top and smaller particle sizes of TiO2 in the bottom will be better for achieving higher solar efficiency. The solar performance parameters of the DSSCs with double-layer structure of different TiO2 particle sizes in the bottom and top layers are listed in Table 2. Interestingly, the solar efficiency of DSSCs with 400 nm/10 nm/FTO, 200 nm/10 nm/FTO, or 100 nm/10 nm/FTO structure is much higher than that with 400 nm/100 nm/FTO or 400 nm/200 nm/FTO structure. It means that the choice of particle size in the bottom layer is more important than that in the top layer for achieving higher solar efficiency. Figures 6(a) and 6(b) show the light absorption of double-layer structures with different TiO2 particle sizes in the bottom and top layers, respectively. The TiO2 layers with smaller particle sizes in the bottom exhibit higher light absorption than those with larger particle sizes, as can be seen in Figure 6(a). The TiO2 layers with larger particle sizes on the top layer exhibit higher light absorption than those with smaller particle sizes, as can be seen in Figure 6(b) [25]. The absorption behaviors in Figures 6(a) and 6(b) are consistent with the observation of photocurrent in Figures 5(a) and 5(b), respectively.

tab2
Table 2: Solar performance parameters of DSSCs with double-layer structure of different TiO2 particle sizes in the bottom and top layers.
fig3
Figure 3: The schematic diagrams of double-layer structures with the different particle sizes in the top and bottom layers.
fig4
Figure 4: The cross section FESEM images of double TiO2 layers with (a) 400 nm/5 nm/FTO, (b) 400 nm/100 nm/FTO, (c) 400 nm/200 nm/FTO, (d) 100 nm/10 nm/FTO, (e) 200 nm/10 nm/FTO, and (f) 400 nm/10 nm/FTO.
fig5
Figure 5: The photocurrent-voltage curves of DSSCs with double-layer structures of different TiO2 particle sizes in the (a) bottom and (b) top layers, respectively.
fig6
Figure 6: The light absorption of double-layer structures with different TiO2 particle sizes in the (a) bottom and (b) top layers, respectively.

To study the charge transfer effects, electrochemical impedance spectroscopy (EIS) is a useful method for analysis of charge transport process [2024]. The schematic diagram of the internal resistance related to the charge transfer kinetics in the DSSCs is shown in Figure 7. In general, the Nyquist plots exhibit three semicircles. The three semicircles are attributed to the redox reaction at the platinum counter electrode (Z1) in the high-frequency region, the electron transfer at the TiO2/dye/electrolyte interface (Z2) in the middle-frequency region, and carrier transport by ions within the electrolytes (Z3) in the low-frequency region. The resistance elements R1, R2, and R3 are described as the real parts of Z1, Z2, and Z3, respectively [26]. Figures 8(a) and 8(b) show the EIS curves of the DSSCs prepared by double-layer structures with different TiO2 particle sizes in the bottom and top, respectively, in the form of the Nyquist plots. Two semicircles are observed in these two figures. The semicircle at low-frequency region merged with the semicircle at the middle-frequency region due to the weak resistance of ions transport in the electrolytes. The charge transfer resistance of the large semicircle in the middle-frequency region may include the extent of electron transport in the photoanode [27]. It was explained that the back-transport of electrons from the FTO electrode to the was suppressed by the introduction of small particle sizes in TiO2 layers. During the deposition of Pt as a counter electrode, there were some slight differences such as the Pt crystallites contacting and the thickness of Pt layer between the counter electrodes. These differences caused the resistance for these two cells in the high frequency to be inconsistent [28]. The charge transport resistance parameters of EIS measurement for double TiO2 layers are listed in Table 3. The charge transport resistance R2 is 13.40, 18.10, and 44.33 Ω for double layers with 400/5/FTO, 400/100/FTO, and 400/200/FTO structures, respectively. The DSSCs with smaller particle size of TiO2 layers in the bottom exhibit the lower charge transport resistance. The charge transport resistance R2 is 18.67, 12.98, and 12.28 Ω for the double layers with 100/10/FTO, 200/10/FTO, and 400/10/FTO structures, respectively. The DSSCs with larger particle size of TiO2 on the top exhibit the lower charge transport resistance. It is noted that the charge transport resistance in the DSSCs with 400/10/FTO structures is lower than that with 400/5/FTO structures. It means that the interface between layers with different particle sizes will be an important factor to affect the solar performance except for considering back-scattering light and surface area.

tab3
Table 3: The parameters of charge transfer resistance in double-layer structures by EIS measurements.
563897.fig.007
Figure 7: The schematic diagram of the internal resistance related to the charge transfer kinetics in the DSSCs.
fig8
Figure 8: The EIS curves of the DSSCs prepared by double-layer structures with different TiO2 particle sizes in the (a) bottom and (b) top layers, respectively, in the form of the Nyquist plots.

It is known that the path-depth length of light increases with wavelength. If a better solar performance is obtained, the back-scattering particles will be gradually increased. Therefore, a triple-layer structure with gradually increasing the particle size was examined. Figure 9 shows the schematic diagrams of triple-layer structures with the different particle sizes in the first (bottom), second (middle), and third (top) layers. When the light is gradually scattered with wavelength, the smaller particle underlayer will recapture the scattering light gradually from the above scattering layer. Figure 10 shows the photocurrent-voltage curves of DSSCs with triple-layer structure of TiO2 layers. The solar performance parameters of these DSSCs are listed in Table 4. The DSSCs with 40 nm particle size in second layer exhibit higher solar efficiency than the other two of 10 and 100 nm. The fill factor degrades with the increasing particle size in the second layer due to small particle size having larger surface contact area. However, the DSSCs with 40 nm particle size in second layer present a maximal photocurrent. It possibly indicates that if the underlayer particle wants to effectively recapture the scattering light from the above scattering layer, the particle size ratio of layer by layer should not vary too much. There exists discontinuity interface between layer and layer. This discontinuity interface cannot be neglected for multiple-layer structures. The choice of the particle sizes in the middle layer will depend on the particle sizes in the bottom and top layers. In addition, the DSSCs with larger particle sizes in the top layer exhibit higher solar efficiency due to strong back-scattering light. Figure 11 shows the photocurrent-voltage curves of DSSCs prepared by triple layers with different particle sizes and the mixing of particle sizes in TiO2 layers. The solar performance parameters of these DSSCs are listed in Table 5. It is noted that the solar efficiency of the DSSCs in Figure 11 is higher than that in Figure 10 due to different TiO2 thicknesses in these DSSCs. The DSSCs with smaller particle sizes of TiO2 in the bottom layer exhibit higher solar efficiency than those with larger particle sizes of TiO2. The mixing of the particle sizes in the middle layers can achieve higher solar efficiency. Figure 12 shows the EIS curves of DSSCs with the triple-layer structures with different particle sizes of TiO2 layers. The charge transport resistance parameters of EIS measurement are listed in Table 6. The DSSCs with larger particle sizes in the top layer (400 nm/200 nm/10 nm) exhibit smaller charge transfer resistance than the other two of 400 nm/100 nm/10 nm and 200 nm/100 nm/10 nm. It indicates that the choice of the particle sizes in the middle layer will affect the charge transfer resistance of TiO2 layer. This behavior is consistent with the observation in Figure 10.

tab4
Table 4: Solar performance parameters of DSSCs with triple-layer structures of different TiO2 particle sizes in the first (bottom), second (middle), and third (top) layer.
tab5
Table 5: Solar performance parameters of the DSSCs with triple-layer structures of different TiO2 particle sizes in the first (bottom), second (middle), and third (top) layer.
tab6
Table 6: The parameters of charge transfer resistance in triple-layer structures by EIS measurements.
563897.fig.009
Figure 9: The schematic diagrams of triple-layer structures with the different particle sizes in the first (bottom), second (middle), and third (top) layers.
563897.fig.0010
Figure 10: The photocurrent-voltage curves of DSSCs with triple-layer structure of TiO2 layers.
563897.fig.0011
Figure 11: The photocurrent-voltage curves of DSSCs prepared by triple-layers with different particle sizes and the mixing of particle size in TiO2 layers.
563897.fig.0012
Figure 12: The EIS curves of DSSCs with the triple layer structures with different particle sizes of TiO2 layers.

4. Conclusions

The solar performance of DSSCs with different particle sizes in single, double, and triple TiO2 layers has been investigated. For single layer, the DSSCs with larger particle size of TiO2 layers have lower solar efficiency than those with smaller particle size of TiO2. The lower solar efficiency in larger particle sizes of TiO2 can be attributed to a strong back-scattering light and less dye adsorption. In addition, the optimal TiO2 thickness for achieving the maximum efficiency depends on the particle sizes of TiO2. The larger the particle sizes of TiO2, the shorter the optimal TiO2 thickness. For double layers, the DSSCs with larger particle sizes in the top layer and smaller particle sizes in the bottom layer can obtain higher solar efficiency due to stronger back-scattering light and large surface area, respectively. The experimental results indicate that the choice of particle size in the bottom layer is more important than that in the top layer for achieving higher solar efficiency. For triple layers, the DSSCs with larger particle sizes in the top layer and smaller particle sizes on the bottom layer can also obtain higher solar efficiency. However, the choice of the particle sizes in the middle layer will depend on the particle sizes in the bottom and top layers. The mixing of the particle sizes in the middle layer is a good choice for achieving higher solar efficiency.

Acknowledgment

This work was supported by the National Science Council of Taiwan (Project no. NSC100-2221-E-182-037).

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