Enhancement of the Photoelectric Performance of Dye-sensitized Solar Cells by Sol-gel Modified TiO 2 Films

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1 J. Mater. Sci. Technol., 2011, 27(8), Enhancement of the Photoelectric Performance of Dye-sensitized Solar Cells by Sol-gel Modified TiO 2 Films Yunfeng Zhao 1), Xiaojie Li 3), Qiuping Li 2) and Changsheng Deng 1) 1) Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing , China 2) PetroChina Pipeline &D Center, Langfang , China 3) ENN Science Technology Development Co. Ltd., Langfang , China [Manuscript received November 4, 2010, in revised form January 4, 2011] Modified TiO 2 films have been prepared by combining commercial titania powders (Degussa P25) with sol-gel made by titanium chloride (Ti-sol). The result shows that clusters are formed by nanoparticles and large pores can be seen on the surface of the TiO 2 films. The short circuit photocurrent density and photoelectric conversion efficiency of the solar cells are obviously enhanced compared with those without modification. The relationship between the photoelectric conversion efficiency and the amount of Ti-sol was investigated. With the addition of 30 wt% Ti-sol, the photoelectric conversion efficiency as high as 9.75% is achieved, increasing by 28.3% compared with the solar cells without modification. KEY WODS: Sol-gel; Titanium dioxide; Dye-sensitized solar cells 1. Introduction Dye-sensitized solar cells (DSCs) are currently attracting academic and commercial interests with their low cost, easy fabrication, environmentally benign and relatively high energy conversion efficiency [1,2]. Generally, DSC comprises a dye-sensitized nanocrystalline TiO 2 film, liquid electrolyte containing an I /I 3 redox couple, and a platinized TCO (transparent conducting oxide) glass substrate as the counterelectrode. In order to obtain high photovoltaic performance, the TiO 2 solar cells generally should have relatively large film thickness (8 17 µm), high surface area to adsorb large amount of dye for good solar light harvesting, lower ionic resistance within the TiO 2 electrode and good interconnected crystalline network for effective electron transfer from dye molecules [3 5]. Therefore, the design and development of TiO 2 films with such structure are an important approach to improve the performance of dye-sensitized solar cells. Ngamsinlapasathian et al. [6] synthesized Corresponding author. Ph.D.; address: zhaoyf04@yahoo.com.cn (Y.F. Zhao). nanocrystalline mesoporous titania (MP-TiO 2 ). The result showed that the photocurrent of the cell made from thin MP-TiO 2 film was higher than that of the cell made from P25 titania. Since the thickness of MP-TiO 2 films was too thin to obtain high cell performance, P25 was added into titania gel for increasing the film thickness and improving light harvesting efficiency. The IPCE (monochromatic incident photon-to-electron conversion efficiency) spectra can be improved by using the cell made from MP- TiO 2 +P25. The cell performance was also improved with double-layered titania cells by increasing light scattering and amount of adsorbed dye. The solar energy conversion efficiency up to 8.1% was obtained by using the double-layered titania cell sintered at 450 for 2 h. To obtain better performance of the solar cells, thicker TiO 2 films are preferable to support the large amount of dye. However, when the film thickness is increased, it tends to crack because of considerable stress in the TiO 2 film due to the film shrinkage in the heating process with the evaporation and decomposition of the organic substances and densification of the TiO 2 film [7]. In order to reduce the ionic diffusion resistance, Hu et al. [8] incorporated

2 Y.F. Zhao et al.: J. Mater. Sci. Technol., 2011, 27(8), nm polymethyl methacrylate (PMM) particles in the TiO 2 paste to make the electrode. With the optimal weight ratio of PMM/TiO 2 (w/w=3.75), the TiO 2 electrode exhibited larger pores ( 350 nm) uniformly distributed after sintering at 500 C, and the ionic diffusion resistance within the TiO 2 film could significantly be reduced. The cell conversion efficiency increased from 3.61% to 5.81% under illumination of 100 mw cm 2, an improvement of 55%. Chen et al. [4] loaded two types of commercial titania powders (Degussa P25 powders and Ishihara ST-01 powders) with high loading (74 wt%) in the Ti-sol, and successfully incorporated the commercial titania powders into a mesoporous structured TiO 2 crystalline network. The TiO 2 matrix has been well crystallized into anatase crystalline phase. The results of photoelectrochemical characterization of the films in a dyesensitized solar cell showed that both films presented high conversion efficiency (about 6%) when a quasisolid state electrolyte was employed. In this study, titania sol (Ti-sol) was prepared with titanic chloride and was added into the Degussa P25 powders to make TiO 2 paste. The purpose of this study was to make the kind of structure of DSCs with thicker as well as well interconnected and porous TiO 2 films in order to enhance the performance of the solar cells. 2. Experimental 2.1 Materials Degussa P25 powders were provided by Degussa Company (80% anatase and 20% rutile). Ethyl cellulose was dissolved beforehand in anhydrous ethanol to form 10 wt% solution. Cis-dithiocyanato-bis (4,4 - dicarboxy-2,2 bypyridine) ruthenium (II) (Bu 4 N) 2 (N719) was used as sensitizer. The electrolyte was composed of 0.6 mol/l butylmethylimidazolium iodide (BMII), 0.03 mol/l I 2, 0.1 mol/l guanidinium thiocyanate (GuSCN) and 0.5 mol/l 4-tert-butylpyridine (TBP) in a mixture solvent of acetonitrile and valeronitrile (volume ratio, 85:15) [3]. ll the materials were reagent grade and used as received. 2.2 Preparation of the TiO 2 films and dye-sensitized solar cells For preparing the Ti-sol, the mixture of 2.5 mol/l TiCl 4 and 0.65 mol/l HCl aqueous solutions were slowly titrated with 12.5% NH 4 OH under continuous mechanical stirring until the final ph of 5 was reached. The precipitate was then centrifuged, washed out with distilled water, and exposed to ultrasound (22 khz). Before the sonication, HNO 3 was added as the stabilizer in an amount corresponding to the TiO 2 :HNO 3 mole ratio of 5:1 [9]. Then the prepared Ti-sol was added to Degussa P25 powders, and the mixture was ground in a mortar with the addition of ethanoic acid, water and ethanol step by step. fter that, the terpineol and the 10 wt% ethyl cellulose solutions were added to the mixture, and then the mixture was sonicated and finally concentrated by vacuum distillation to yield a paste. The modified pastes were deposited onto a cleaned conductive FTO glass substrate (Nippon Sheet Glass Group, TEC 15 Ω sq 1, 2.2 mm thickness) by screenprinting. The 0.25 cm 2 films ( 10 µm) were preheated at 120 C for 6 min and then heated at 500 C for 1 h. Finally the films were immersed in 0.3 mmol/l N719 ethanol solution for 24 h [3]. The counter electrode was prepared with a Pt coated conducting glass, which was deposited from 10 mmol/l H 2 PtCl 6 of isopropyl alcohol solution. The dye-covered TiO 2 film electrode and Pt-counter electrode were separated by a surlyn 1702 spacer (25 µm thickness, DuPont). The space between the two electrodes was filled with a drop of the electrolyte. The sandwich-type DSC with an active area of 0.25 cm 2 was employed to measure the performance of the solar cells. 2.3 Measurements and analysis methods The information of the crystalline structure of Tisol modified TiO 2 (30 wt% Ti-sol modified Degussa P25) were determined by X-ray diffraction (XD) using a igaku D/max-2500 diffractometer with CuKα radiation. The powder samples were scratched from TiO 2 film on FTO conductive glass. The surface morphology of the TiO 2 films was observed by scanning electron microscopy (SEM, HITCHI, S-4800). The current-voltage (I-V ) curves were measured by a digital source meter (Keithley-2400, Keithley Co. Ltd., US) under Oriel solar simulator illumination (M1.5, 100 mw cm 2 ). 3. esults and Discussion Figure 1 shows the results of XD spectra of P25 powders and 30% Ti-sol modified Degussa P25. Distinctive peaks at 2θ angle of 25.2 and 27.4 deg. can be observed in Fig. 1(a), which suggests that it is Degussa P25 powders. Figure 1(b) also shows an obvious peak at 2θ angle of 25.2 and 27.4 deg., and the intensity is higher than that in Fig. 1(a), which suggests that there is larger amount of titania. It also can be concluded that the TiO 2 matrix has been well crystallized into anatase crystalline phase. Figure 2 shows morphology of TiO 2 films with different content of Ti-sol. The surface of TiO 2 film without Ti-sol shows porous structure. The clusters (i.e., about 110 nm) are formed by small size of nanoparticles (i.e., about 50 nm) and small intercluster pores can be found on the film (i.e., about 100 nm) (Fig. 2(a)). With the addition of Ti-sol (Fig. 2(b) (f)), more compact clusters (i.e., nm) and inter-cluster pores (i.e., nm) can be seen on the surface of the TiO 2 films. For

3 766 Y.F. Zhao et al.: J. Mater. Sci. Technol., 2011, 27(8), Intensity / a.u. (b) (a) (a) TiO 2 +Ti-Sol (b) TiO 2 : anatase peak, (101) : rutile peak, (110) / deg. Fig. 1 XD patterns of P25 powders (a) and 30 wt% Tisol modified P25 powders (b) annealed at 500 C for 1 h example, when 15 wt% Ti-sol is added (Fig. 2(b)), larger clusters about 100 nm and the inter-cluster pores about 150 nm are formed. When 30 wt% Tisol is added (Fig. 2(c)), the size of the clusters and the inter-cluster pores seems change little, while the amount of the inter-cluster pores is the most on the film. With increasing content of Ti-sol to 65 wt%, more compact clusters are formed and the size of the inter-cluster pores decreases to some extent and the amount of the inter-cluster pores reduces visibly. In addition, it can be seen that all nanopaticles are well-connected to each other although the clusters and pores are formed on the surface of the TiO 2 film. Based on XD images in Fig. 1(b), the crystalline on the particles suggest a good crystallization of the films. Considering the fact that interconnected crystalline network is an important structural property to obtain an effective electron transfer from the dye molecules in dye-sensitized solar cells [6], we can conclude that the Ti-sol modified TiO 2 films possess porous and well-interconnected crystalline network structure. Fig. 2 SEM photographs of TiO 2 films modified with different content of Ti-sol: (a) 0, (b) 15 wt%, (c) 30 wt%, (d) 50 wt%, (e) 65 wt%, (f) 80 wt%

4 Y.F. Zhao et al.: J. Mater. Sci. Technol., 2011, 27(8), Table 1 Photovoltaic characteristics of the DSCs based on the TiO 2 films modified with different content of Ti-sol Ti sol-gel/wt% I sc /m cm 2 V oc /V F F η/% Note: V oc open-circuit voltage Current density / m cm wt% Ti-sol 15 wt% Ti-sol 30 wt% Ti-sol 50 wt% Ti-sol 65 wt% Ti-sol 80 wt% Ti-sol Voltage / V Fig. 3 I-V curves of DSCs based on TiO 2 films modified with different content of Ti-sol The I-V curves of dye-sensitized solar cells based on TiO 2 films with different content of Ti-sol are shown in Fig. 3. The corresponding photovoltaic characteristics of DSCs are summarized in Table 1. It can be seen from Fig. 3 and Table 1 that the content of Ti-sol has important effects on the short circuit photocurrent density (I sc ), fill factor (F F ) and the overall conversion efficiency (η). The short circuit photocurrent density (I sc ) increases obviously as a function of Ti-sol content, and a maximum value m cm 2 is achieved when 30 wt% Ti-sol is added. Further increasing the amount of Ti-sol leads to the decrease in I sc. The overall conversion efficiency (η) shows the same trend with the short circuit photocurrent. The overall conversion efficiency (η) increases as a function of Ti-sol content at first and decreases then. The best performance of the solar cell is obtained with TiO 2 film modified with 30 wt% of Ti-sol. The overall conversion efficiency is 9.75%, an increase of 28.3% compared with the DSCs without modification. When the Ti-sol is added, it essentially functions as a catalytic active binder to immobilize TiO 2 nanoparticles on the substrate [4]. The TiO 2 nanoparticles can be well glued by the presence of sol-gel [5]. By considering the fact that interconnected crystalline network is an important structural property to obtain an effective electron transfer from the dye molecules in dye-sensitized solar cells [6], this kind of structure can enhance good electron transport properties of the films. Furthermore, as Ti-sol is added, titania clusters are formed and the clusters form large pores on the surface of the TiO 2 film. It has been reported that the scattering centers in TiO 2 films can enhance the overall conversion efficiency [3]. The presence of larger nanoparticles are beneficial to the enhancement of light reflection, which can lead to enhancement in red light harvesting, while smaller nanoparticles can provide large surface area for the adsorption of large amount of dye molecules [8]. The clusters and intercluster pores on the surface of the TiO 2 films may just act as the larger nanoparticles and the scattering centers. Meanwhile, the P25 particles may act as the smaller nanoparticles which can provide large BET surface area. Such structure can enhance solar light harvesting and accelerate interfacial charge transfer, which are two important factors affecting the efficiency of dye-sensitized solar cells. ccording to the analysis above, when the Tisol content is added, the connection between TiO 2 nanoparticles is enhanced, and more light is captured because of clusters and pores on the surface of the TiO 2 films. Therefore, it leads to the result of higher short circuit photocurrent density and overall conversion efficiency. When the Ti-sol content is 30 wt%, it reaches the optimum ratio, in which the amount of the inter-cluster pores is the most, leading to the best overall conversion efficiency of the solar cells. When the content increases, more compact clusters are formed on the TiO 2 films and the inter-cluster pores become less and smaller. s a result, the effect of light capture enhancement is less significant, while the less amount of dye is adsorbed in the TiO 2 films because of the compact structure, the fill factor and the overall conversion efficiency decreases. 4. Conclusion Clusters and pores are formed on the surface of the TiO 2 films when Ti-sol is added, which supply the light scattering centers that enhance the light capture of the solar cells. Meanwhile, it can enhance the connection of the TiO 2 nanoparticles and thus promote the effective electron transfer compared with the TiO 2 films without Ti-sol. The solar cells with Ti-sol show an improvement of short circuit photocurrent and overall conversion efficiency. The highest overall conversion efficiency is obtained with the TiO 2 films of adding 30 wt% Ti-sol, increasing by 28.3% compared with the solar cells without modification.

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