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.
For more than 20 years, the first dye-sensitized solar cell (DSSC) has been published by O’Regan and Grätzel [
Recombination with the dye cations and the electrolyte species (
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) [
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 (
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
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
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.
A sandwich-type cell [
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.
Figures
The TiO2/Ag composite films with varied UV exposure time as 5, 30, 60, 120, 180, and 240 min, corresponding to the silver nanoparticle sizes of (a) 4.4 nm, (b) 7.2 nm, (c) 11 nm, (d) 19.2 nm, (e) 27.5 nm, and (f) 38.6 nm, respectively [
The TiO2/MgO composite films with varied magnesium acetate solution concentrations of 1 × 10−4 M, 1 × 10−3 M, 1 × 10−2 M, and 1 × 10−1 M. These correspond to the MgO film thicknesses of (a) bare-TiO2, (b) 0.08 nm, (c) 0.10 nm, (d) 0.16 nm, and (e) 0.46 nm, respectively [
The TiO2/MgO/Ag composite films with varied magnesium acetate solution concentrations and silver nanoparticles sizes: (a) TiO2/MgO (0.08 nm)/Ag (4.4 nm), (b) TiO2/MgO (0.10 nm)/Ag (4.4 nm), (c) TiO2/MgO (0.46 nm)/Ag (4.4 nm), (d) TiO2/MgO (0.08 nm)/Ag (19.2 nm), (e) TiO2/MgO (0.10 nm)/Ag (19.2 nm), (f) TiO2/MgO (0.46 nm)/Ag (19.2 nm), (g) TiO2/MgO (0.08 nm)/Ag (38.6 nm), (h) TiO2/MgO (0.10 nm)/Ag (38.6 nm), and (i) TiO2/MgO (0.46 nm)/Ag (38.6 nm).
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 [
Figures
SEM images of TiO2/MgO/Ag composite films: (a) bare-TiO2, (b) TiO2/MgO (0.10 nm)/Ag (4.4 nm), (c) TiO2/MgO (0.46 nm)/Ag (4.4 nm), and (d) cross section of TiO2/MgO (0.10 nm)/Ag (4.4 nm).
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
EDX spectra taken from (a) surface and (b) cross section of TiO2/MgO (0.10 nm)/Ag (4.4 nm) composite film.
Figure
A cross-sectional TEM micrograph of the TiO2/MgO (0.10 nm) composite films prepared from 1 × 10−3 M of magnesium acetate solution (a) ×20,000, (b) ×200,000.
Figure
Optical absorption spectra of the bare-TiO2 film compared with the TiO2/Ag (19.2 nm), TiO2/MgO (0.10 nm), and TiO2/MgO/Ag composite films.
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 [
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
Optical absorption spectra of the bare-TiO2 compared with the TiO2/MgO/Ag composite films with varied Ag nanoparticle size in range of 4.40–38.60 nm.
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
Optical absorption spectra of the TiO2/MgO/Ag composite films with varied film thicknesses of magnesium oxide in range of 0.08–0.46 nm.
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
Optimization of the TiO2/MgO/Ag composite films by varying the MgO thin film thickness and the Ag nanoparticles size for enhanced efficiency of the DSSCs.
Silver nanoparticles size (nm) | Efficiency of DSSCs (%) | Efficiency of bare-TiO2 DSSCs (%) | ||
---|---|---|---|---|
Magnesium oxide film thickness (nm) | ||||
0.08 | 0.10 | 0.46 | ||
4.40 | 5.1 ± 0.1 | 5.2 ± 0.1 | 2.6 ± 0.2 | 3.8 ± 0.1 |
19.20 | 4.1 ± 0.2 | 4.1 ± 0.1 | 2.0 ± 0.3 | |
38.60 | 3.7 ± 0.1 | 3.8 ± 0.2 | 1.1 ± 0.6 |
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
The efficiency of DSSCs with the bare-TiO2 film and the TiO2/MgO (0.10 nm)/Ag composite films with various Ag nanoparticles sizes in range of 4.40–38.60 nm.
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
The efficiency of DSSCs with the bare-TiO2 film and the TiO2/MgO/Ag composite films with various MgO film thicknesses in range of 0.08–0.46 nm.
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 [
Comparison of the efficiencies of DSSCs with different types of electrodes, that is, the bare-TiO2, TiO2/Ag (19.2 nm), TiO2/MgO (0.10 nm), and TiO2/MgO (0.10 nm)/Ag (4.4 nm) electrodes.
Electrochemical impedance spectra (EIS) of the DSSCs can describe the internal resistances of the DSSCs.
Figure
The short circuit current densities (
Condition of working electrodes |
|
|
FF | Efficiency (%) |
|
|
---|---|---|---|---|---|---|
Bare-TiO2 | 6.60 | 0.71 | 0.81 | 3.80 | 10.02 | 2.87 |
TiO2/MgO (0.08 nm)/Ag (4.4 nm) | 8.37 | 0.78 | 0.74 | 5.10 | 10.39 | 2.69 |
TiO2/MgO (0.10 nm)/Ag (4.4 nm) | 8.63 | 0.79 | 0.76 | 5.20 | 15.14 | 2.70 |
TiO2/MgO (0.46 nm)/Ag (4.4 nm) | 4.42 | 0.80 | 0.72 | 2.60 | 36.38 | 4.97 |
TiO2/MgO (0.08 nm)/Ag (19.2 nm) | 6.70 | 0.77 | 0.79 | 4.10 | 15.39 | 3.70 |
TiO2/MgO (0.10 nm)/Ag (19.2 nm) | 6.94 | 0.78 | 0.75 | 4.10 | 18.67 | 4.35 |
TiO2/MgO (0.46 nm)/Ag (19.2 nm) | 3.46 | 0.79 | 0.72 | 2.0 | 41.63 | 4.62 |
TiO2/MgO (0.08 nm)/Ag (38.6 nm) | 6.25 | 0.76 | 0.78 | 3.70 | 15.75 | 4.35 |
TiO2/MgO (0.10 nm)/Ag (38.6 nm) | 6.29 | 0.77 | 0.78 | 3.80 | 20.40 | 3.92 |
TiO2/MgO (0.46 nm)/Ag (38.6 nm) | 2.00 | 0.79 | 0.68 | 1.10 | 61.87 | 6.22 |
Electrochemical impedance spectra (EIS) of DSSCs with the TiO2/MgO/Ag (4.4 nm) electrodes with varied thickness of MgO.
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
Electrochemical impedance spectra (EIS) of DSSCs with the TiO2/MgO (0.10 nm)/Ag electrodes with varied Ag nanoparticles size.
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.
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.