A Combined Effect of Plasmon Energy Transfer and Recombination Barrier in a Novel TiO<sub>2</sub>/MgO/Ag Working Electrode for Dye-Sensitized Solar Cells

Photiphitak, Chanu; Rakkwamsuk, Pattana; Muthitamongkol, Pennapa; Thanachayanont, Chanchana

doi:https://doi.org/10.1155/2015/795138

International Journal of Photoenergy

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Abstract Introduction Materials and Methods Results and Discussion Conclusion Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2015 | Article ID 795138 | https://doi.org/10.1155/2015/795138

A Combined Effect of Plasmon Energy Transfer and Recombination Barrier in a Novel TiO₂/MgO/Ag Working Electrode for Dye-Sensitized Solar Cells

Chanu Photiphitak,¹Pattana Rakkwamsuk,²Pennapa Muthitamongkol,³and Chanchana Thanachayanont³

Academic Editor: Leonardo Palmisano

Received15 Jun 2015

Revised09 Aug 2015

Accepted13 Aug 2015

Published06 Sept 2015

Abstract

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.

1. Introduction

For more than 20 years, the first dye-sensitized solar cell (DSSC) has been published by O’Regan and Grätzel [1]. 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 [2]. 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 [3]. 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 [3].

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 [20].

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 [6] 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 × 10⁻⁴M 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 × 10⁻¹M, 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 [4], 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 [21] 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.

2.5. Measurements

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

Figures 1–3 show appearances of the TiO₂/Ag, TiO₂/MgO, and TiO₂/MgO/Ag composite films, respectively.

(a)

(b)

(c)

(d)

(e)

(f)

(a)

(b)

(c)

(d)

(e)

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

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 [22], 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.

(a)

(b)

(c)

(d)

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).

(a)

(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 [23]. 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 [21].

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 [24].

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 [4]. 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% [6]. 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) [4], and TiO₂/MgO (0.10 nm) [6] 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 [28]. 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 [29].

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].

4. Conclusion

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.

Acknowledgments

The authors thank the National Metal and Materials Technology Center (MTEC) and Suan Dusit University, Thailand, for financial support.

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Copyright © 2015 Chanu Photiphitak 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.

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International Journal of Photoenergy

A Combined Effect of Plasmon Energy Transfer and Recombination Barrier in a Novel TiO2/MgO/Ag Working Electrode for Dye-Sensitized Solar Cells

Abstract

1. Introduction

2. Materials and Methods

2.1. The Preparation of Mesoporous TiO2 Electrodes

2.2. The Preparation of Mesoporous TiO2/MgO/Ag Composite Films

2.3. The Preparation of Pt Counter Electrodes

2.4. DSC Fabrication

2.5. Measurements

3. Results and Discussion

3.1. The Morphology of the TiO2/MgO/Ag Composite Films

3.2. The Effect of the MgO Thin Film and Ag Nanoparticle Size on Optical Absorption Spectra

3.3. The Effect of the MgO Ultrathin Film and Ag Nanoparticles on Efficiency of the DSSCs

4. Conclusion

Conflict of Interests

Acknowledgments

References

Copyright

A Combined Effect of Plasmon Energy Transfer and Recombination Barrier in a Novel TiO₂/MgO/Ag Working Electrode for Dye-Sensitized Solar Cells

2.1. The Preparation of Mesoporous TiO₂ Electrodes

2.2. The Preparation of Mesoporous TiO₂/MgO/Ag Composite Films

3.1. The Morphology of the TiO₂/MgO/Ag Composite Films