J. Phys. Chem. C 2010, 114, 3185–3189
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Effect of the Compact TiO2 Layer on Charge Transfer between N3 Dyes and TiO2 Investigated by Raman Spectroscopy Hsuan-Fu Wang,† Liang-Yih Chen,§ Wei-Nien Su,§ Jen-Chieh Chung,‡ and Bing-Joe Hwang*,§,| Graduate Institute of Materials Science and Technology, National Taiwan UniVersity of Science and Technology, Taipei, 106, Taiwan, Chemical Engineering DiVision, Institute of Nuclear Energy Research Atomic Energy Council, Lungtan, Taoyuan, 325, Taiwan, Department of Chemical Engineering, National Taiwan UniVersity of Science and Technology, Taipei, 106, Taiwan, and National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan ReceiVed: August 26, 2009; ReVised Manuscript ReceiVed: NoVember 19, 2009
The charge transfer between N3 dyes and TiO2 electrodes has been investigated by Raman spectroscopy. The TiO2 nanoparticles are synthesized at different temperatures to form the compact layer in the photoelectrode. The red shift of the Eg(3) mode of the TiO2 electrode increases with an increase in the synthesis temperature of TiO2 nanoparticles, which is also associated with its vibration mode and conversion efficiency. This indicates that the charge transfer between N3 dyes and the TiO2 electrode reaches optimal when TiO2 nanoparticles are synthesized at 190 °C. In electrochemical impedance spectroscopy measurements of the dye-sensitized solar cells (DSSC), the internal resistance of the cell decreases with an increase in the synthesis temperature, suggesting the improvement of the electron transfer from N3 dyes into the TiO2 electrode at higher synthesis temperature. Herein, we have established a relationship between the red shift of the Eg(3) mode of the TiO2 electrode and the energy conversion efficiency. This demonstrates that the Raman spectroscopic technique is a convenient and useful tool to investigate the charge transfer between N3 dyes into TiO2 electrode and its impact on the performance of DSSC. Introduction A dye-sensitized solar cell (DSSC) is a promising device to make use of solar energy because of its fabrication without a vacuum process and with relatively low production cost compared to those of conventional semiconductor solar cells. A DSSC comprises a dye-adsorbed nanocrystalline TiO2 layer fabricated on a transparent conducting oxide (TCO) as the working electrode, platinum (Pt) as the counter electrode, and an electrolyte solution with iodide/triiodide redox reagents.1 It has been shown that the charge recombination processes at interfaces among TCO, TiO2, dyes, and electrolytes in a DSSC are important in enhancing the photon-to-electron conversion efficiency. In our previous investigation of DSSC properties, the cell is deliberately designed with a two-layer structure of the TiO2 electrode,2 in which a PTFE-framed structure layer is planned to increase the capacity of adsorbing dyes and offer sufficient structural stability, while a TiO2 compact layer acts like a barrier to retard the charge recombination between TCO and electrolytes.3 The compact layer has great impact on the performance of DSSC, since the TiO2 compact layer is the first material encountering the transmitted light through the TCO glass. In addition, the dye plays multifunctional roles in a number of tasks: absorbing the incident light, injecting electrons into the semiconductor’s conduction band, reacting with the redox couple, etc.4 The interfacial bonding of the Ru(4,4′* To whom correspondence should be addressed. E-mail: bjh@ mail.ntust.edu.tw. † Graduate Institute of Materials Science and Technology, National Taiwan University of Science and Technology. § Department of Chemical Engineering, National Taiwan University of Science and Technology. ‡ Chemical Engineering Division, Institute of Nuclear Energy Research Atomic Energy Council. | National Synchrotron Radiation Research Center.
dicarboxylic acid-2,2′-bipyradine)2(NCS)2 (commonly known as “N3” dye) on the TiO2 electrode is therefore very critical. Dye adsorption depends strongly on the morphology and properties of TiO2. Only when the Ru(II) complex is well bound to the TiO2 surface can DSSC show good performance.4 The impact of the preparation temperature of TiO2 nanoparticles has not been well addressed. Recently, resonance Raman spectroscopy has been widely used to investigate the surface interactions of Ru(II) complexes with the semiconductor TiO2 nanoparticles.4-7 When the laser wavelength overlaps with singlet metal-to-ligand charge transfer (1MLCT) of N3, it transfers the Ru(II) 3d electron to the ligand π* state and the excited electron is essentially localized on one of the ligand bipyridine groups.8,9 Surface enhanced Raman scattering combined with resonance Raman spectrum (SERRS)10 can provide detailed information about the structural changes associated with the electron transfer. However, these research works mainly emphasize the effect of dyes on the TiO2 photoelectrode. Herein, a 10 µm compact layer was adopted to avoid the charge recombination. The main purposes of this study are to investigate the influence of the compact layer and its interaction with adsorbed N3 dyes. At the same time, we took a new approach to establish the relationship among complex interactions of various factors such as charge transfer, internal resistance, etc. by introducing Raman spectroscopy analysis. Experimental Section Synthesis of the Compact Layer Materials. TiO2 nanoparticles were fabricated by the typical hydrothermal method.11 A 9.25 mL sample of tetrabutyl titanate (Ti(OC4H9)4) was mixed with 2.5 mL of 2-propanol, and the mixture was slowly added into a solution consisted of 62 mL of deionized water and 20 mL of acetic acid in an ice bath with stirring. The solution was
10.1021/jp908233h 2010 American Chemical Society Published on Web 01/29/2010
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then heated to 80 °C and kept stirring vigorously for 8 h. TiO2 nanoparticles grew under hydrothermal conditions in a titanium autoclave for 12 h. Autoclave temperatures were set at 140, 170, 190, and 210 °C, respectively. Fabrication of Porous TiO2 Electrodes. An antimony-doped tin oxide overcoated ITO (ATO/ITO) glass plate (Geomatec, sheet resistance 5 Ω/cm2, 1.1 mm thick) was first cleaned by 3.5% detergent solution (Merck) in ultrasonic bath for 10 min. It was then rinsed with ethanol and dried in air. The compact layer was deposited by directly coating synthesized TiO2 nanoparticles on a clean TCO sheet glass without any binder and surfactant with use of a simple doctor-blade technique. The deposited layer was dried in an oven at 80 °C for 15 min and sintered at 500 °C for 30 min to form a TiO2 compact layer. A TiO2 structure layer was built by spreading a paste of appropriate proportion of PTFE (DuPont) and TiO2 P25 (Degussa) mixed with some ethanol (95% Merck) on top of the sintered compact layer. After initial trials, the optimal mixing proportion of TiO2 and PTFE was determined as 50/50 wt %, and the ratio applies to all PTFE-framed TiO2 samples unless otherwise stated. The coated plate was dried in an oven at 80 °C for 15 min, and then sintered at 500 °C for 60 min to form a PTFE-framed TiO2 film. The actual film thickness was calculated by subtracting the glass substrate thickness from the electrode thickness measured with a Dial Snap Meter (Mitutoyo). Solar Cell Assembly and Characterizations. In the solar cell, a platinum-coated ATO/ITO conducting glass was used as a counter electrode. The electrolyte was prepared as a mixture of 15.96 g of dimethylpropylimidazolium iodide (0.6 M), 1.34 g of lithium iodide (0.1 M), 1.26 g of iodine (0.05 M), and 6.76 g of tert-butylpryidine (0.5 M) in 100 mL of 3-methoxypropionitrile solution. The working electrode with the active area of 0.7 cm2 (10 mm × 7 mm) was immersed into the N3 solution (0.3 mM in 99% ethanol) and kept at room temperature for 16 h to form a dye-sensitized TiO2 electrode. All electrodes contained two-layer films with a compact layer and a structure layer, where the former was 10 µm thick and the latter was 40 µm thick.2 The total thickness of films on the electrode was kept constant at 50 µm for easy evaluation and comparison of the properties of various compact layers. Many authors have attempted to measure and control the dye loading by desorpting dyes with caustic KOH.12 However, it would be rather difficult to apply this method to specifically control the dye loading in each compact layer or separate it from the total loading value in this study. Since the structure layer and the total film thickness remained the same, it was presumed the variation of dye loading in the studied electrodes was insignificant. The electrode structure was examined with use of a fieldemission scanning electron microscopy (FE-SEM, JEOL JEM6500F). The microstructure of the TiO2 nanoparticles was characterized with a high-resolution analytical electron microscope (HR-AEM, Philips Tecnai G2 F20, 200KV). Electrochemical impedance spectra (EIS) were measured with an impedance analyzer (S1-1260, Solartron) connected with a potentiostat (S1-1286, Solartron) under AM1.5 illumination (100 mW/cm2, Model YSS-80A, Yamashita). EIS spectra were recorded over a frequency range of 10-1 to 106 Hz at 298 K. The applied bias voltage and ac amplitude were set at the open circuit voltage (Voc) of the DSSCs and 10 mV, respectively. The electrochemical impedance spectra were characterized with use of Z-View software (Solartron Analytical). Photocurrentvoltage was measured with a digital source meter (Keithley, Model 2420) under AM1.5 illumination.
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Figure 1. HR-AEM images of various TiO2 nanoparticles synthesized at different temperatures.
Microscopic Raman scattering was carried out with a Kaiser optical system, using an argon ion laser with 100 mW power at room temperature. The excitation wavelength of the laser is 532 nm and the resolution is 2 cm-1. The Rayleigh scattering light was filtered by a notch filter and analyzed by a double monochromator equipped with a charge-coupled device (CCD) detector. Results and Discussion The HR-AEM images of the compact layer TiO2 nanoparticle powders obtained at different synthesis temperatures of 140, 170, 190, and 210 °C are shown in Figure 1. The size of the TiO2 nanoparticles increased with increasing synthesis temperatures. The minimum size of the particle based on the synthesis process is about 5 nm, which was prepared at 140 °C. A schematic representation of the TiO2 electrode consisting of two layers, a compact layer and a PTFE-framed structure layer, is shown in Figure 2a. Compared with the compact layer, the structure layer is highly porous to enable more dye adsorption on the surfaces of TiO2 nanoparticles.2 The crosssectional image of the TiO2 electrode is shown in Figure 2b. Electrochemical impedance spectroscopy (EIS) has been applied to characterize DSSC for years. Han et al. observed three semicircles for their DSSC and revealed that the first semicircle (high-frequency 103-106 Hz) is the impedance related to charge transport at the Pt counter electrode, and the second semicircle (middle frequency, 1-103 Hz) represents resistance for the TiO2/dye/electrolyte interface and it exhibits behavior like the resistance of a diode.13,14 The third semicircle (low frequency, 10-1-1 Hz) is the Nernstian diffusion within the electrolyte. The internal resistance of a dye-sensitized solar cell is understood as the sum of resistances measured in different frequency ranges (10-1-106 Hz) and the sheet resistance of TCO.13 The AC impedances of various DSSC are illustrated in Figure 3, where the internal resistance of the cell decreased when TiO2 nanoparticles in the compact layer were synthesized at higher temperature. The third semicircles were hardly recognizable in measurements, indicating the resistance of Nernstian diffusion in electrolyte is insignificant in this DSSC. Moreover, it is to address that some authors related the first semicircle of
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Figure 2. (a) Schematic representation of the TiO2 electrode; (b) cross-sectional SEM micrograph of a TiO2 electrode.
TABLE 1: Comparative Performances of DSSC with Various Synthesis Temperatures temp conversion efficiency fill factor short-circuit current open-circuit (°C) (%) (%) density (mA/cm2) voltage (mV) 140 170 190 210
Figure 3. AC impedance spectra for TiO2 electrodes with different synthesis temperature of nanoparticles in the compact layer.
3.21 4.70 6.18 5.98
29.8 44.0 54.0 54.5
16.67 16.66 17.74 17.22
646 641 645 637
DSSC are summarized in Table 1. The optimal conversion efficiency of 6.18% is obtained when the TiO2 nanoparticles in the compact layer were synthesized at 190 °C, and then the value reduces to 5.98% at 210 °C. It is noticeable that the opencircuit voltages (Voc) were relatively similar. Theoretically, Voc is the difference between the Fermi energy level of the TiO2 nanoparticle and the reduction energy level of the electrolyte. When the dye loading in the structure layer is similar, Voc also will be similar. In addition, more significant variation in current density was also observed, indicating that more charge transfer took place as the synthesis temperature of TiO2 nanoparticles of the compact layer increased. The factor reached the maximum at 190 °C. To understand how the performance of a DSSC is influenced by the compact TiO2 layer, a resonance Raman technique was
Figure 4. I-V curves of DSSC with TiO2 compact layer synthesized at different temperatures.
AC impedance spectra only to the Pt counter electrode.13 Thus, the influence of different synthesis temperatures of TiO2 nanoparticles in the compact layer on the working electrode is first demonstrated and it also affects the first semicircle. From samples with constant compact layer thickness and varying structure layer thicknesses in our previous study, no evident change was observed in the first semicircle of impedance spectra.2 Herein, the effects of the synthesis temperature of TiO2 nanoparticles in the compact layer of the photoelectrode, from 140 to 210 °C, are clearly seen in the first and second semicircles. Figure 4 showed the I-V curves of DSSC with the compact TiO2 layers made of TiO2 nanoparticles synthesized at different temperatures. The overall conversion efficiency, filling factor, short-circuit current density, and open-circuit voltage of the
Figure 5. Raman spectra for N3 dye and dye-adsorbed TiO2 electrodes with different synthesis temperature of nanoparticles in the compact layer.
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Figure 6. Raman spectra of TiO2 electrode with different synthesis temperature of nanoparticles in the compact layer: (a) before adsorbing N3 dyes and (b) after adsorbing N3 dyes.
employed. The Raman spectra of free N3 and N3 dyes adsorbed on the two-layer TiO2 electrode, shown in Figure 5, were examined to study the interaction between dye molecules and the TiO2 electrodes. Comparing with the free N3 spectrum, it is found that the peak at the 1021 cm-1 peak corresponding to the bipyridine (bpy) ring breathing mode disappeared after the dye adsorption.8 The intensities of three bpy ring stretch modes located at 1469, 1541, and 1610 cm-1 also decreased after the dye adsorption. The shifts observed for the C-C inter-ring and C-O stretching modes at 1264 and 1307 cm-1 indicated the N3 dye is bound to TiO2 via the carboxyl group on the ligands. The carboxylate substituents on the bipyridine ligands are covalently bound to the surface Ti(IV) ion of TiO2, and the possible overlap of the accepting metal d orbital of the TiO2 conduction band and the N3 carboxylate π* orbital may promote electron transfer from the radical anion of bipyridine.8 Furthermore, the Raman spectra of the two-layer TiO2 electrodes, shown in Figure 6, were also taken at the same time. It is worth noting that strong Raman peaks corresponding to TiO2 electrodes were observed. The anatase TiO2 nanocrystals have six Raman active modes (A1g + 2B1g + 3Eg), which are located at 145 [Eg(1)], 197 [Eg(2)], 399 [B1g(1)], 513 [A1g], 519 [B1g(2)], and 639 cm-1 [Eg(3)].15 Before absorbing N3 dyes, the Raman spectra of TiO2 electrodes with different synthesis temperature of nanoparticles in the compact layer were almost identical in Figure 6a. The results show that the prepared TiO2 electrodes were characterized in the anatase phase and the crystallographies of different samples were pretty similar. After N3 dye adsorbing on the two-layer TiO2 electrodes, a low intensity of the vibration mode of the TiO2 phase was generally observed. This proves that the chemical adsorption of dye complexes on the surface of TiO2 occurs via the formation of ester-like linkage.16 Moreover, the Eg(3) mode of the TiO2 phase has significantly down-shifted from 634 cm-1 to 604 cm-1 when the synthesis temperature of TiO2 nanoparticles in the compact layer increased from 140 to 190 °C and then up-shifted back to 606 cm-1 when the temperature was further raised to 210 °C. Such shifts can be seen as the interaction between the dye and the anatase titania material through the carbonxylic groups. We also found that A1g and B1g(2) disappeared after N3 dye adsorption. When no dye is adsorbed on TiO2, no morphological distortion or other laser-induced effect was observed under irradiation, even for very high powers. However, in the case of the dye-sensitized TiO2 electrode, vibrational modes of TiO2 are influenced. The most pronounced deformation of the strong peaks in the 400-800 cm-1 region shows a strong electronic
Figure 7. Raman spectra of the structure layer before and after dye adsorption.
coupling effect between N3 and TiO2 and can be further interpreted as an indication of charge transfer, as seen in Figure 6b. Under resonant irradiation, the dye is very efficiently excited, and a large number of electrons are injected and then promoted to the TiO2 conduction band. Hence, the surface charge increases strongly, and TiO2 electrode loses, at least to some degree, its semiconducting nature. Additionally, we also found a large shift of the Eg(3) mode in Raman analysis. To understand the cause of the shift, the TiO2 structure layer was coated on the TCO glass and then immersed into N3 solution to absorb N3 dyes for Raman analysis. When N3 dyes adsorbed on the TiO2 structure layer only, intensities of TiO2 Raman peaks reduced, but no obvious Raman peaks shift was observed, as shown in Figure 7. The above result confirms that the large Raman peak shift of TiO2 with N3 dye was mainly enhanced by the existing compact layer in the electrode. Comparing the overall efficiency with the Eg(3) mode shift, shown in Figure 8, a correlation can be observed. When the synthesis temperature increased to 190 °C, the dye-sensitized TiO2 electrode had the largest shift of the Eg(3) mode and the DSSC had the highest overall conversion efficiency among these samples. On the basis of EIS measurements, we proposed that the TiO2 nanoparticles in the compact layer synthesized at higher temperature have lower resistance to improve electron transfer from N3 dye into the conduction band of the TiO2 layer continuously and collected by TCO to increase the current density and efficiency. To account for the influence of the electronic behavior of TiO2 nanoparticles comprehensively on the Raman spectrum, this topic requires further investigation in the future.
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J. Phys. Chem. C, Vol. 114, No. 7, 2010 3189 Eg(3) mode shift occurs under the circumstance of the highest efficiency of DSSC. As a result, the Raman technique is proposed as a powerful tool to investigate the interactions and charge transfer between N3 dyes on TiO2 electrodes. Acknowledgment. The authors gratefully acknowledge the financial support from National Science Council of Taiwan (NSC-97-2120-M-011-001 and NSC-97-2221-E-011-075-MY3) and the National Taiwan University of Science (NTUST). References and Notes
Figure 8. Correlation between the Eg(3) mode of the dye-sensitized TiO2 electrode and overall conversion efficiency at different synthesis temperatures.
Conclusions TiO2 nanoparticles synthesized at different temperatures, from 140 to 210 °C, were employed in forming the compact layer of a DSSC. The EIS measurements suggest that the preparation of the TiO2 nanoparticles plays an important role in the electrical behavior of the compact layer and the performance of the DSSC. It was found that the DSSC with TiO2 nanoparticles in the compact layer synthesized at higher temperature shows lower internal resistance. The DSSC demonstrated the highest efficiency and current density, when the TiO2 nanoparticles in the compact layer were synthesized at 190 °C. Additionally, evident red shift of the Eg(3) mode of the TiO2 electrode with adsorbing N3 dye appeared from 634 to 604 cm-1 and increased with increasing the synthesis temperature of TiO2 nanoparticles to 190 °C. Such strong electronic coupling by chemical adsorption indicated the overlapping of conduction bands and efficient charge transfer between the N3 dye and the TiO2 substrate. A strong correlation between the efficiency of DSSC and the Eg(3) mode shift is clearly obtained, where the greatest
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