A Combined Effect of Plasmon Energy Transfer and Recombination Barrier in a Novel TiO2/MgO/Ag Working Electrode for Dye-Sensitized Solar Cells
Novel TiO2/MgO/Ag composite electrodes were applied as working electrodes of dye-sensitized solar cells (DSSCs). The TiO2/MgO/Ag composite films were prepared by dip coating method for MgO thin films and photoreduction method for Ag nanoparticles. The MgO film thicknesses and the Ag nanoparticle sizes were in ranges of 0.08–0.46 nm and 4.4–38.6 nm, respectively. The TiO2/MgO/Ag composite films were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The TiO2/MgO/Ag composite electrodes were sensitized by immersing in a 0.3 mM of N719 dye solution and fabricated for conventional DSSCs. - characteristics of the TiO2/MgO/Ag DSSCs showed that the MgO film thickness of 0.1 nm and the Ag nanoparticle size of 4.4 nm resulted in maximum short circuit current density and efficiency of 8.6 mA/cm2 and 5.2%, respectively. Electrochemical Impedance Spectroscopy showed that such values of short circuit current density and efficiency were optimal values obtained from plasmon energy transfer by 4.4 nm Ag nanoparticles and recombination barrier by the ultrathin MgO film.
For more than 20 years, the first dye-sensitized solar cell (DSSC) has been published by O’Regan and Grätzel . The DSSCs have been extensively studied because of their high performance, simple fabrication processes, low-cost materials, and manufacturing processes. The DSSCs consist of transparent conducting oxide (TCO) coated glass, TiO2 photoelectrode, Ru complex photosensitizer such as N719 dye molecules, redox electrolyte such as (iodide/triiodide), and Pt counter electrode . High performance dye-sensitized solar cells require the nanocrystalline TiO2 electrode to have a large surface area, high crystallinity without cracks, and good electrical contact with the conducting glass substrate so that a high amount of dye molecules can be adsorbed and the electrons can be quickly transferred . However, a problem of the DSSC is the low energy conversion efficiency when compared with silicon solar cells. Main reasons are charge recombination loss arising at the semiconductor/dye/electrolyte interface and low dye absorption towards the infrared region.
Recombination with the dye cations and the electrolyte species () can drastically affect the open circuit voltage (). Improving the efficiency of DSSCs can be achieved by coating a thin film of oxide layers on the TiO2 electrode such as MgO, ZnO, Al2O3, Nb2O5, CaCO3, and SrTiO3 [3, 7, 8]. The oxide film has a wide band gap that delays the electrons back transfer to the electrolyte and minimizes charge recombination. In addition, the coating layer can increase the dye adsorption on the porous electrode and, hence, increase the photocurrent .
In addition, the surface plasmon resonance induced by silver (Ag) nanoparticles leads to an increase in absorption coefficient of dye in dye-sensitized solar cells (DSCs) [9–14]. The effect has been theoretically described as an increase of local electromagnetic field nearby metal surfaces which is found when wavelengths of irradiation sources are correlated with the optical absorption of the surface plasmon resonance [15–19]. The modification of the surfaces for an enhancement of optical absorption, hence, provides a good method to improve efficiency of an optoelectronic device involving photon absorption .
This research will improve the working electrode of the DSSCs by using both concepts of decreasing of recombination process between the electron on conduction band of TiO2 and triiodide ion in electrolyte () by MgO thin film  and increasing of light absorption coefficient of dye molecules by silver nanoparticles [4, 5]. These are known as effects of recombination barrier and plasmon energy transfer, respectively. Therefore, the TiO2/MgO/Ag composite film was demonstrated to be an efficient working electrode for DSSCs.
2. Materials and Methods
2.1. The Preparation of Mesoporous TiO2 Electrodes
The TiO2 electrodes were screen-printed from a TiO2 paste (Dyesol) 3 times on a fluorine-doped-tin-oxide (FTO) glass substrate (2 × 3 cm2 in size). A 200-mesh was used to obtain a TiO2 layer with area of 0.5 × 1.2 cm2 and a thickness of approximately 13.8 μm. In order to avoid contamination on the fresh film, screen printing was performed in a clean-room environment. After drying at 55°C for 30 minutes, the electrodes were sintered at 450°C for 30 minutes and then cooled down to room temperature. The electrodes were immersed in a 3 × 10−4 M of N719 dye solution, namely, cis-diisothiocyanato-bis(2,2-bipyridyl-4,4-dicarboxylato)ruthenium(II) bis(tetrabutylammonium) in absolute ethanol for 24 hours. The excess dye was removed from the electrode by rinsing in ethanol.
2.2. The Preparation of Mesoporous TiO2/MgO/Ag Composite Films
The TiO2 film was prepared by screen printing with 3 layers and calcined at 450°C for 30 minutes. Then the TiO2 film was dipped in a magnesium acetate solution with concentrations of 1 × 10−4, 1 × 10−3, and 1 × 10−1 M, respectively, at 40°C for 30 seconds. An excess solution in the TiO2/MgO films was washed with ethanol and calcined at 450°C for 30 minutes. Then, the TiO2/MgO composite films were immersed in the 0.1 M AgNO3 solution for 5 seconds, then rinsed with DI water, and dried in a N2 stream. The films were then exposed to UV irradiation at nm using a Spectroline CM-10 Fluorescence Analysis Cabinet at an intensity of ~0.31 mW/cm2. We designed the experiment to vary exposure times for 5, 120, and 240 minutes for the photocatalytic reduction of Ag+ to the metallic Ag nanoparticles , to form the Ag nanoparticles size of approximately 4.4, 19.2, and 38.6 nm, respectively. Finally, the TiO2/MgO/Ag composite electrodes were immersed in dye solution for 48 hours at room temperature and prepared as working electrodes of the DSSCs.
2.3. The Preparation of Pt Counter Electrodes
The counter electrodes were prepared by screen printing a thin layer of platinum (Pt) with a size of 0.5 × 1.2 cm2 using a platinum paste (Dyesol), on a FTO glass substrate (2 × 3 cm2), and then sintered at 450°C for 30 minutes.
2.4. DSC Fabrication
A sandwich-type cell  was fabricated by assembling a sensitized TiO2 electrode using Surlyn-based polymer sheet (80 μm thick) and sealed by a hot gun for a few seconds. The liquid electrolyte contained 0.5 M LiI, 0.05 M , and 0.5 M 4-tert-butylpyridine in 90 : 10 v/v of acetonitrile : 3-methyl-2-oxazolidinone. Electrolyte injecting holes, made on the counter-electrode side, were sealed with Surlyn and glass cover.
Optical absorption spectra of the film electrode samples were measured using a UV-Visible Spectrophotometer (Jasco model: V-530). In order to observe the microstructure and elemental analysis of the obtained Ag nanoparticles, the Ag nanoparticles were prepared on carbon-coated copper grids for observations by transmission electron microscopy (TEM JEOL model: JSM-2010). The X-ray diffraction (XRD JEOL- 300) patterns were obtained by analyzing the Ag/TiO2 films on the glass substrates. Scanning electron microscope (SEM JEOL model: JSM-6301F with attached energy dispersive X-Ray Spectrometer (EDX)) was employed to record cross-sectional micrographs of the Ag/TiO2 films. - measurements were performed under a 450 W xenon light source which is able to provide 1000 W·m−2 sunlight equivalent irradiation (AM 1.5), using Keithley digital source meter (model 2400) under the illuminated condition.
3. Results and Discussion
3.1. The Morphology of the TiO2/MgO/Ag Composite Films
Results show that the TiO2/Ag composite films are brown and become darker with the longer UV exposure time due to a prolonged photocatalytic reduction of Ag+ to Ag , while the TiO2/MgO composite films are white and translucent. Therefore, the colors of the TiO2/MgO/Ag composite films come from the Ag nanoparticles.
Figures 4(a), 4(b), and 4(c) show surface images of bare-TiO2, TiO2/MgO (0.10 nm)/Ag (4.4 nm), and TiO2/MgO (0.46 nm)/Ag (4.4 nm) composited films by SEM, respectively. The MgO thin film and Ag nanoparticles cannot be observed because of their ultrathin and ultrasmall nature [6, 23]. Figure 4(d) shows cross section image of the TiO2/MgO (0.10 nm)/Ag (4.4 nm) composite film showing the film thickness of approximately 11.15 μm.
The EDX technique of SEM was used to analyze the elements of Ti, Mg, and Ag, resulting in the peaks of Ti and Ag that were found on the surface image of the TiO2/MgO (0.10 nm)/Ag (4.4 nm) composite film but the Mg peak was not found suggesting that it was rinsed off from the film surface as shown in Figure 5(a). However, Mg peak was present in the cross section of the TiO2/MgO (0.10 nm) Ag (4.4 nm) composite film (see Figure 4(d)) indicating the presence of Mg element as shown in Figure 5(b).
Figure 6 shows a TEM image of the TiO2/MgO (0.10 nm) composite film. However, we can not observe the magnesium oxide thin film on surfaces of the TiO2 nanoparticles because of its ultrathin nature. The MgO content of the film was estimated by the extraction of MgO with HCl and atomic adsorption spectrophotometric estimation. The thickness of the MgO film was calculated as = (weight of MgO)/, where = surface area of TiO2 ( determined by the desorption of the dye into an alcoholic alkaline solution and spectrophotometric estimation = 560 times geometrical cross section of the film = 1 cm2) and = density of MgO . The concentrations of magnesium acetate solution of 1 × 10−4, 1 × 10−3, and 1 × 10−1 M were calculated to give the thicknesses of the magnesium oxide film as 0.08, 0.10, and 0.46 nm, respectively.
3.2. The Effect of the MgO Thin Film and Ag Nanoparticle Size on Optical Absorption Spectra
Figure 7 shows a comparison of the optical absorption spectra in the range of 370–800 nm between the bare-TiO2, TiO2/Ag, TiO2/MgO, and TiO2/MgO/Ag composite films. The optical absorption spectra of the bare-TiO2 film and TiO2/MgO composite film were found to be similar and lower than those of the TiO2/Ag (19.20 nm) and TiO2/MgO (0.10 nm)/Ag (4.40 nm) composite films. This agrees with research results of Bandara et al., reporting that a coating of MgO on TiO2 does not change the absorption property of TiO2 as MgO electron excitation energy falls above the excitation energy of TiO2 .
However, for the cases of TiO2/Ag (19.20 nm) and TiO2/MgO (0.10 nm)/Ag (4.40 nm) composite films, the optical absorption spectra have much higher absorption than the previous two cases at a wavelength range of around 400–600 nm. The optical absorption of the TiO2/MgO (0.10 nm)/Ag (4.40 nm) composite film is slightly higher than the TiO2/Ag (19.20 nm) film in the wavelength range of 400–600 nm, whereas in the wavelength range of 600–800 nm we found that the optical absorption spectrum of the TiO2/Ag (19.2 nm) composite film is higher than that of the TiO2/MgO (0.10 nm)/Ag (4.40 nm) composite film. Therefore, the Ag nanoparticles have been shown clearly to improve optical adsorption due to plasmon energy transfer effect [4, 9, 12, 13]. In addition, the light scattering effect of Ag nanoparticles enhances the absorption of photoanode .
When we consider the optical absorption spectrum of TiO2/MgO/Ag with the fixed MgO film thickness of 0.10 nm and varied Ag nanoparticles size in a range of 4.40–38.60 nm, the optical absorption of the bare-TiO2 has the lowest value, while the optical absorption of TiO2/MgO (0.10 nm)/Ag (38.60 nm) has the highest value as shown in Figure 8. The red shifted nature of the spectrum indicates surface plasmon effect [4, 25].
Although, for the 0.1 nm thick MgO thin films, the Ag nanoparticles are more influential than the MgO thin film on the TiO2 film for optical absorption (Figure 7), the optical absorption can be increased with increasing thickness of the MgO thin film as shown in Figure 9.
3.3. The Effect of the MgO Ultrathin Film and Ag Nanoparticles on Efficiency of the DSSCs
The efficiency of DSSCs with TiO2/MgO (0.10 nm)/Ag (4.40 nm) composite electrode has the highest efficiency among all conditions prepared including the bare-TiO2 electrode as shown in Table 1.
The results showed that the maximum efficiency of the DSSCs was obtained with TiO2/MgO (0.10 nm)/Ag (4.4 nm) composite electrode (Figure 10). The DSSCs with TiO2/MgO (0.10 nm)/Ag (4.4 nm) have the maximum efficiency, although this condition did not give the highest optical absorption (see Figure 9). The maximum efficiency obtained suggests that the MgO layer coated on the TiO2 film acts as a recombination barrier at the TiO2/MgO/Ag contacts and becomes a dominant effect [9, 11, 26, 27].
In addition, the efficiency of the DSSCs depends on the MgO film thickness, the maximum efficiency of DSSCs with TiO2/MgO (0.10 nm)/Ag (4.4 nm) working electrode. When the MgO film thickness increased, the efficiency of DSSCs is decreased, as shown in Figure 11.
Using the same materials and structure and improving the TiO2 working electrode by coating the Ag nanoparticles alone on the TiO2 film for increase in light absorption coefficient of the dye molecule, the efficiency up to 4.8% was obtained . The coating of the MgO thin films on the TiO2 films to reduce the recombination process between the electron on conduction band of TiO2 and triiodide in the electrolyte was reported to increase the efficiency up to 5.0% . While the conventional DSSCs fabricated have efficiency about 3.8%, both methods can improve the efficiency of DSSCs and lead to the coating TiO2 working electrode by MgO thin film and Ag nanoparticles for a novel TiO2/MgO/Ag working electrode in this study. The DSSCs using the TiO2/MgO (0.10 nm)/Ag (4.40 nm) composite film as working electrode have the maximum efficiency of approximately 5.2%, when compared with the DSSCs using other working electrodes such as the bare-TiO2, TiO2/Ag (19.20 nm) , and TiO2/MgO (0.10 nm)  as shown in Figure 12. When the Ag nanoparticles bigger than 4.4 nm were coated on TiO2/MgO (0.10 nm) composite film, the efficiency decreases, supposedly because the Schottky barriers were formed at the TiO2/MgO/Ag contacts, became dominant, and retarded electron transport in the conduction bands [9, 11].
Electrochemical impedance spectra (EIS) of the DSSCs can describe the internal resistances of the DSSCs. is related to the carrier transport resistance at the surface of Pt counter electrode, is related to carrier transport resistance at the TiO2/dye/electrolyte interface, and is related to the diffusion of iodide and triiodide within the electrolyte.
Figure 13 shows EIS results of the DSSCs with the bare-TiO2 and the TiO2/MgO/Ag (4.4 nm) electrodes with varied MgO film thicknesses. We found that the EIS of the DSSCs with the TiO2/MgO (0.10 nm)/Ag (4.4 nm) composite electrode have the smallest curve of . Thus the carrier transport resistance in TiO2/dye/electrolyte interface is the lowest at the MgO film thickness of 0.10 nm, which is consistent with the shunt resistance () of the DSSCs with the TiO2/MgO (0.10 nm)/Ag (4.40 nm) composite electrode with lowest value of 2.70 kΩ cm2 (see Table 2). This is due to the high value of indicating a slow back electron transfer rate from the TiO2 to the electrolytes at the TiO2/dye/electrolyte interface . Increasing the MgO film thickness to exceed 0.10 nm, decreases because electron injection from the excited dye molecules to the CB of TiO2 is hindered by the MgO film. An optimum of the MgO film thickness acts as the energy barrier to hinder the recombination process [6, 8]. However, as the MgO film thickness increases, will increase accordingly (Table 2) due to a negative shift of flat band potential after coating the thin MgO layer on the semiconductor particles. is determined by the difference between the quasi-Fermi level of electron in the oxide film and the energy of the redox couple in the electrolyte .
Electrochemical impedance spectra (EIS) of the DSSCs with varying Ag nanoparticle size ranging from 4.40 to 38.6 nm in the TiO2/MgO (0.10 nm)/Ag composite electrodes are shown in Figure 14. The EIS of DSSCs with the TiO2/MgO (0.10 nm)/Ag (4.40 nm) composite electrode were found to have the smallest curve, indicating that the carrier transport resistance at the TiO2/dye/electrolyte interface is the lowest. When the Ag nanoparticle size increases more than 4.4 nm, the carrier transport resistance in TiO2/dye/electrolyte interface was found to increase because of the Schottky barriers formed at the TiO2/MgO/Ag contacts [9, 11].
The TiO2/MgO/Ag composite electrode was prepared with the MgO thin film and the Ag nanoparticles by dipping and photoreduction method, respectively. The efficiency enhancement of the DSSCs is based on the principle of ultrathin outer shell of insulator and surface plasmon resonance. The optimum condition obtained was TiO2/MgO/Ag composite film consisting of the MgO film thickness of 0.10 nm and Ag nanoparticles size of 4.40 nm to give a maximum efficiency of 5.2%. The EIS of the DSSCs with the TiO2/MgO (0.10 nm)/Ag (4.4 nm) composite electrode showed the lowest carrier transport resistance at the TiO2/dye/electrolyte interface. The Ag nanoparticles influenced the optical absorption of the TiO2 film because of the surface plasmon resonance induced by the silver nanoparticles enhancing Raman scattering and optical absorption of the dye. A coating of MgO on the TiO2 acted as the energy barrier to hinder the recombination process and was found to significantly improve cell efficiency.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors thank the National Metal and Materials Technology Center (MTEC) and Suan Dusit University, Thailand, for financial support.
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