Synthesis and Characterization of Silver-Doped TiO2 Nanotubes on Dye-Sensitized Solar Cells

1. Introduction

Dye-sensitized solar cells (DSSCs) based on nanocrystalline semiconductor oxides are a low-cost alternative to conventional solid-state photovoltaic devices [1-6]. However, as one of the major way of enhancing light absorption, light scattering plays an important role in improving DSSC efficiency. Both theoretical and experimental studies have demonstrated that a light-scatting structure can increase the optical length within a film and enhance the light absorption of porous film, resulting in improved conversion efficiency of DSSC [3,4]. Metal nanoparticles can contribute to the effective light absorption of solar cells, both by local field enhancement through the localized surface plasmon resonance and by light scattering leading to prolonged optical-path lengths [5]. Many researchers demonstrated that the size of the metal nanoparticles is a key factor for determining the plasmonic phenomena, and especially, light scattering occurs more dominantly than light absorption as the size of the metal nanoparticles increases [6]. In this study, the synthesis of anatase TiO₂ nanotube  (TNT) arrays with Ag nanoparticles by anodic oxidation has been proposed. The influence of the length of the TNT arrays on the photovoltaic characteristics of the prepared devices was also examined. DSSCs based on TiO₂ nanotube arrays with Ag nanoparticles showed significantly enhanced performance than DSSCs consisting of bare TiO₂nanotube arrays.

2. Experimental

Ti foils (0.25 mm thickness, 99.7 wt% purity) were degreased by sonication in acetone, deionized (DI) water, and ethanol for 20 min, to remove surface impurities, and finally to dried in a nitrogen stream. Electrochemical anodic oxidation of the Ti foils was carried out in a two-electrode cell, with platinum (Pt) as the counter electrode at room temperature. The electrolyte for anodic oxidation was prepared with anhydrous ethylene glycol (EG) with NH₄F (0.25 wt%) and H₂O (3 vol%) mixture solution was stirred for 1h. The voltage was supplied by a DC power supply with a condition of 30/40/50 V at RT for 1h. After anodic oxidation, the samples were rinsed in ethanol and dried in air, followed by heat treatment at 450°C in N₂ for 2h to produce TiO₂ nanotube arrays. Ag nanoparticles was carried out by immersing the as-prepared TiO₂ nanotube in an aqueous electrolyte composed of 0.2 M Silver nitrate (AgNO₃) solution and 0.15 M Sodium borohydride (NaBH₄) for 5 min at room temperature. The TiO₂ nanotube arrays with Ag nanoparticles were then rinsed with DI water and dried in vacuum at 50°C overnight. To fabricate DSSCs, TiO₂ nanotube arrays with or without Ag modifications were soaked with N719 dye was added tert-butanol and acetonitrile solution, such mixing ratio 1:1 to prepare a concentration of 5 × 10^-4 M dye solution. The DSSCs were finally packed by sealing the dye-coated electrode with a thermally platinized FTO counter electrode through a thin thermal plastic film (Surlyn, 25 ƒÝm, Solaronix). An electrolyte composed of 0.1 M N-methyl benzimidiazoium, 0.6 M 1-propyl-3-methylimidiazolium iodide, 0.05 M I₂, 0.1 M guanidinium thiocyanate, and 0.2 M NaI in 3-methoxypropionitrile was introduced into the cell through a pre-drilled hole in the counter electrode. The hole was subsequently sealed with a microscope slip using Surlyn film.

3. Results and Discussion

Fig. 1 XRD patterns of TNT arrays by anodic oxidation at 30 V (a) without Ag nanoparticles, (b) with Ag nanoparticles, (c) TNTs peak, and (d) Ag peak.

Fig. 2         I-V characteristic curves of DSSCs with and without Ag nanoparticles by anodic oxidation at different voltage.

4. Conclusions

In conclusion, anatase TiO₂ nanotube arrays with Ag nanoparticles were synthesized by anodic oxidation. At the deposition voltage of 30 V and Ag-coated TiO₂ nanotube arrays, the η, Jsc, and FF all reached the maximum. Compared with the conventional DSSCs Jsc, Voc, and η were increased from 7.28 to 8.45 mA/cm², 0.7 to 0.72 V, and 4.03 to 4.81%. It is expected that the proposed approach for the synthesis of high-quality TiO₂nanotube arrays with Ag nanoparticles might open up potential applications for solid or liquid DSSCs and nanostructure devices in the near future.

 

References

[1]C. K. Lee et al., J. Hazard. Mater., 150, pp.494-503, 2008.

[2]K. S. Lin et al., Adsorption, 16, pp.47–56, 2010.

[3]B. Liu et al., J. Am. Chem. Soc.,131, pp.3985-3990, 2009.

[4]H. E. Wang et al., Appl. Phys. Lett., 96, 263104, 2010.

[5]H. A. Atwater et al., Nature Mater., 9, pp.205-213, 2010.

[6]D. D. Evanoff et al., J. Phys. Chem. B., 108, pp.13957-13962, 2014.