No.2

Effect of reactive gas in an atmospheric plasma

for dye adsorption on ZnO nanorods

De PHAM-CONG, Kyun AHN, Jong-Man KIM,

Se-Young JEONG and Chae-Ryong CHO^*

College of Nanoscience and Nanotechnology, Pusan National University, Busan 609-735

Jong Pil KIM and Hyun Gyu KIM

Busan Center, Korea Basic Science Institute, Busan 609-735

Hyung Soo AHN and Hong Seung KIM

Department of Applied Science, Korea Maritime University, Busan 606-791

In this study, ZnO nanorods (NRs) were grown on F-doped SnO₂ (FTO)/glass substrate by using a chemical bath deposition (CBD) method. The NRs were crystallized well. The surfaces of the NRs were modified using an atmospheric-pressure (AP) plasma containing reactive gases such as O₂, H₂, and N₂. In the case of the Ar/O₂ and Ar/N₂ plasma-treated ZnO NRs, chemical bonding states and the dye adsorption on the surface of the ZnO layer increased because of –O and –OH radicals. We present the efficiencies of ZnO-NR-based dye-sensitized solar cells (DSSCs) treated with AP plasmas containing reactive gases.

PACS number: 68.35.Md, 68.43.Mn, 52.75.-d, 79.60.-i

 Keywords: Surface energy, Plasma, Hydroxyl bonds, Dye adsorption

^*Corresponding author; E-mail: crcho@pusan.ac.kr

I. INTRODUCTION

Photovoltaic devices generate clean energy that can reduce the world’s dependence on conventional energy sources. Dye-sensitized solar cell (DSSCs) might be the most efficient and stable excitonic photocells [1, 2]. DSSCs with a metal oxide nanostructure have been studied as a low-cost alternative to silicon because of the simple manufacturing process, large-scale production capability, and potential applications as semiconductor-based p-n junction solar cells [3, 4]. Recently, ZnO was used as a candidate material for replacing TiO₂ because the energy-band structure of ZnO with a wide band gap (3.37eV) and its physical properties are similar to the energy-band structure and physical properties of TiO₂. Furthermore, ZnO has higher electronic mobility, which would be remarkable for electron transport (a high exciton binding energy of 60 meV), with possible reduced recombination loss [5].

In particular, the synthesis of one-dimensional nanostructures of ZnO, replacing the existing nanoparticle structure, is attracting a lot of interest owing to their unique properties and integration into DSSCs [6]. On the other hand, the increase in efficiency achieved using ZnO-based DSSCs has been limited because of the generation of the Zn²⁺/dye complex at the interface between ZnO and dye. ZnO is immersed in a solution of sensitizer dyes, and a small amount of Zn²⁺ ions from the ZnO surface are dissolved in the solution. During the process, a Zn²⁺/dye complex is formed on the ZnO surface. Among several dyes, N719 dye with two carboxyl groups having very low acidity is used to reduce the formation of a Zn²⁺/N719 dye complex on the surface of ZnO [7]. To increase dye adsorption, recent research has focused on the production of –O and –OH radicals on the surface of the ZnO layer by using physical and chemical treatment [8-10]. Among the various techniques for surface modification, plasma treatment is a simple, low-cost, and mass-production method, however, the effect of an atmospheric-pressure (AP)  plasma on solar cell efficiency has not been reported.

In this article, we report the growth of ZnO nanorods (NRs) achieved using a chemical bath deposition (CBD) method. For surface modification, the grown ZnO NRs were annealed and treated with an AP plasma containing the reactive gases Ar/O₂, Ar/N₂, and Ar/H₂. The crystal structures, morphologies and surface chemical bonding states of the ZnO NRs were investigated. In this paper, the results for the dye adsorption and the efficiency of the ZnO NRs-based DSSCs are discussed for the first time.

                                                                            

II. EXPERIMENTS AND DISCUSSION

ZnO thin films were deposited on a F-doped SnO₂ (FTO)/glass substrates by RF magnetron sputtering. For growing ZnO NRs, 400 ml of an aqueous solution containing 0.03-M zinc acetate dehydrate (ZnAc) [Zn(O₂CCH₃)₂(H₂O)₂, 98% purity] and 0.03-M hexamethylenetetramine (HMT)  [(CH₂)₆N₄, 99% purity] as a precursor was prepared at room temperature. The substrate was immersed at the bottom of the beaker containing the prepared aqueous solution, and the beaker was maintained at 92°C for 2.5 h for the growth of ZnO on the substrate. The substrate was reversed during the growth process. After the completion of the growth process, the NRs were rinsed with deionized water and dried in air to remove any residual salt and organic materials (U-ZnO).  After the NRs had been grown for 10 h, they were annealed at 400°C for 2 h (A-ZnO) and were treated with an AP plasma (FemtoScience, Plasmaflux^TM) containing O₂, N₂ and H₂ gases at a RF power of 100 W for 30 min (O-ZnO, N-ZnO and H-ZnO). The flow rate of Ar was 40 sccm, and that of each of the reactive gases, i.e., O₂, N₂, and H₂ was 5 sccm. To examine their photovoltaic behavior, we immersed the ZnO NRs in a solution of cis-bis(isothiocyanato)bis(2,2’-bipyridyl-4,4’dicarboxylato)-ruthenium(II)bis tetrabuthylammonium dye (N719, Solaronix) for 24 h at room temperature. A dye-adsorbed ZnO electrode was assembled with a Pt counter electrode to form a DSSC. One drop of an electrolyte solution (Iodolyte AG-50, Solaronix) was infiltrated between the two electrodes. The physicochemical properties of the ZnO NRs were investigated using field emission scanning electron microscopy (FESEM) (S-4700, Hitachi), X-ray diffractometry (XRD) (X’pert pro, Phillips), and X-ray photoelectron spectroscopy (XPS) (ESCALAB250, VG Scientific). The dye adsorption of the NRs was measured using a UV-VIS-NIR spectrophotometer (Cary 5000, Varian). The current density-voltage (J-V) characteristics of cells with active areas of 0.25 cm² were measured using an AM 1.5 solar simulator fitted with a Xe lamp.

Figure 1 shows SEM images of the ZnO NRs grown on ZnO/FTO/glass. The ZnO NRs had a hexagonal shape and were arranged in a vertical array in the [001] direction on the ZnO seed layer. After a few minutes, the NRs nucleated on the seeds in the ZnO thin layer. The diameters of these NRs were significantly larger than those of the ZnO nanoparticle seeds in a thin film. In the initial reaction step, after 2.5 h, the growth of ZnO NRs stopped, and their average length and diameter were approximately 1.75 μm and 76 nm, respectively (Fig. 1(a)). The length and the diameter of the NRs increased with increasing duration of the repeated reaction (5, 7.5, and 10 h) at the same precursor concentration, as shown in Figs. 1(b-d). The average length and diameter of the NRs after 10 h were approximately 3.8 μm and 150 nm, respectively. Moreover, the NR diameter increased steadily during the growth period, but at a much slower rate than the NR length. The growth rate of the ZnO NRs in the vertical direction was very fast during the initial reaction; however, the rate reduced with increasing reaction time because of an increase in NR growth in the radial direction. Figure 1(e) shows the morphology of A-ZnO NRs that were annealed to remove any residual organics material and to increase the crystallinity. The shapes of the AP-plasma-treated O-ZnO, N-ZnO and H-ZnO NR samples did not show any significant changes (Figs. 1(f-h)).

Structural analyses of the NR samples grown on ZnO/FTO/glass for various reaction times and annealed and treated with Ar/O₂, Ar/N₂ and Ar/H₂ AP plasma (shown in Fig. 2) were carried out using XRD. For the ZnO NRs with a hexagonal wurtzite structure, the (002) peak was dominant, and the peak intensity increased with increasing duration of the repeated reaction because of NR growth in the [002] direction, as shown in Figs. 2(b-d). The (002)/(101) peak height ratio increased with time because only those NRs that had a c-axis almost normal to the substrate, NRs that contributed to the (002) diffraction peak, continued to grow. The peak intensity corresponding to A-ZnO NR was much higher than that corresponding to U-ZnO NRs (Fig. 2(e)). This phenomenon was attributed to the removal of residual organic molecules and an increase in crystallinity, resulting from the arrangement of interior lattices. On the other hand, there were no significant changes in the peak intensities corresponding to the O-ZnO, N-ZnO and H-ZnO NRs samples.

   XPS was used to obtain more information about the surfaces of the ZnO NRs. Figure 3 shows the surface chemical bonding states in the Zn 2p and O 1s XPS spectra. Figure 3(a) shows the O 1s XPS spectra deconvoluted using a Gaussian-Lorentz fit. Herein, the peak at the binding energy of 530.1 eV corresponds to the Zn-O bond in the wurtzite structure. The peak at 531.5 eV was assigned to the -OH bond in zinc oxyhydroxide species with an oxygen-deficient region in the ZnO matrix. The binding energy of 532.3 eV was attributed to weakly bound oxygen, such as the oxygen in adsorbed H₂O and O₂ [11]. The Zn 2p spectra show a doublet peak at binding energies of 1021.2 eV and 1044.3 eV corresponding to the Zn 2p_3/2 and Zn 2p_1/2 core levels, respectively (Fig. 3(b)) [12]. The increase in the Zn 2p peak area of the treated ZnO NRs was greater than that for the U-ZnO NRs but there was no change in the peak area ratios of Zn 2p_3/2 to Zn 2p_1/2.

Figure 4 shows the absorption spectra and the J-V curves for the N719-sensitized ZnO-NR-based DSSCs. A broad absorption band peaking at 520 nm can be observed, which was assigned to the metal-to-ligand charge-transfer transition (MLCT) of N719 [13-15]. The dyes was adsorbed on the ZnO NRs after the formation of H₂O in chemical reactions involving -O, -OH radicals on the surface of the ZnO NRs and -COOH radical in the dye molecules [16, 17]. The absorption intensity for A-ZnO NRs was similar to that for U-ZnO NRs because of a decrease in the concentration of the -OH radical and an increase in the concentration of the -O radical. Furthermore, dye adsorption of O-ZnO and N-ZnO NRs increased when -OH radicals were added during the Ar/O₂, and the Ar/N₂ AP-plasma treatments. Therefore, the intensity of this band, which is a characteristic of dye adsorption, increased with increasing concentration of A-ZnO, O-ZnO and N-ZnO NRs. In particular, O-ZnO NRs showed the highest dye adsorption because the increase in the concentration of the -OH radical was more than that for N-ZnO NRs. On the other hand, H-ZnO NRs showed less dye adsorption than A-ZnO NRs because of a significantly decreased concentration of -O radicals on the NR surface.

The open-circuit voltage (V_oc) of the U-ZnO was 0.63 V, but those for the A-ZnO, O-ZnO, N-ZnO, and H-ZnO NRs were 0.71, 0.81, 0.80, and 0.80 V, respectively. The value of V_oc for U-ZnO NRs was less than those for A-ZnO NRs and plasma-treated ZnO NRs because of the presence of a relatively large amount of residual organic molecules on the surfaces of the ZnO NRs. The short-circuit current density (J_sc) of the U-ZnO NRs (1.76 mA/cm²) was smaller than those for the A-ZnO NRs (2.41 mA/cm²), O-ZnO NRs (3.37 mA/cm²), N-ZnO NRs (2.72 mA/cm²) and H-ZnO NRs (2.18 mA/cm²). In particular, the value of J_sc was highest for O-ZnO NRs and was relatively low for H-ZnO NRs as compared to N-ZnO and A-ZnO NRs. Consequently, the PV efficiency of O-ZnO NRs (1.21%) was higher than those of U-ZnO NRs (0.51%), A-ZnO NRs (0.83%), N-ZnO NRs (0.99%), and H-ZnO NRs (0.79%). The significant change in the cell efficiencies of O-ZnO NRs and N-ZnO NRs based DSSCs might be due to the analysis and measurement conditions. The XPS data were generally measured for a few top NR layers instead of the sidewalls of the NRs. On the other hand, the plasma affects the entire sample. The effect of annealing and O₂-AP-plasma treatment in series increased the PV efficiency to 62% and 137% more than that of the untreated samples, respectively.

III. CONCLUSION

The grown ZnO NRs were oriented along the [001] direction and had a hexagonal wurtzite structure. The shape of the NRs was determined and the length of the NRs were observed to increase with increasing reaction time. There were no significant changes in the morphologies and structures of A-ZnO, O-ZnO, N-ZnO and H-ZnO NRs after the annealing and plasma-treatment. The concentrations of -O and -OH radicals in A-ZnO, O-ZnO, and N-ZnO NRs increased because of the supply of these polar groups (-O and -OH) produced in AP plasmas containing reactive gases. O-ZnO NRs showed the highest level of dye absorption because of the largest polar components. Consequently, O-ZnO-NR-based DSSCs showed higher efficiency (1.21%) than the other DSSC samples because of the increased crystallinity and dye adsorption. Our results support the fundamental insights for enhancing the efficiency of nanostructure based DSSCs by treating them with AP plasmas containing reactive gases.

ACKNOWLEDGMENT

This study was supported by a Korea Research Foundation Grant (KRF-2008-313-D00607).

REFERENCES

[1]    M. K. Nazeeruddin, P. Pechy and T. J. Renouard, J. Am. Chem. Soc. 123, 1613 (2001).

[2]    P. Wang, M. S. Zakeeruddin, J. E. Moser, K. M. Nazeeruddin, S. Takashi and M. Graetzel, Nature Mater. 2, 402 (2003).

[3]    M. K. Nazeeruddin, F. D. Angelis, S. Fantacci, A. Selloni, G. Viscardi, P. Liska, S. Ito, B. Takeru and M. Grätzel, J. Am. Chem. Soc. 127, 16835 (2005).

[4]    G. Oskam, Z. S. Hu, R. L. Penn, N. Pesika and P. C. Searson, Phys. Rev. E 66, 011403 (2002).

[5]    Q. Zhang, C. S. Dandeneau, X. Zhou, and G. Cao, Adv. Mater. 21, 4087 (2009).

[6]    H. Horiuchi, R. Katoh, K. Hara, M. Yanagida, S. Murata, H. Arakawa and M. Tachiya, J. Phys. Chem. B 107, 2570 (2003).

[7]    F. Yan, L. Huang, J. Zheng, J. Huang, Z. Lin, F. Huang and M. Wei, Langmuir 26, 7153 (2010).

[8]    J. Chung, J. Lee and S. Lim, Physica B 405, 2593 (2010).

[9]    S. K. Park, C. Kim, J. H. Kim, J. Y. Bae and Y. S. Han, Curr. App. Phys. 11, 131 (2011).

[10] C. Kim, J. T. Kim, H. Kim, S. H. Park, K. C. Son and Y. S. Han, Curr. App. Phys. 10, 176 (2010).

[11] X. G. Han, H. Z. He, Q. Kuang, X. Zhou, X. H. Zhang, T. Xu, Z. X. Xie and L. S.

Zheng, J. Phys. Chem. C 113, 584 (2009).

[12] N. S. Ramgir, D. J. Late, A. B. Bhise, M. A. More, I. S. Mulla, D. S. Joag and K.

Vijayamohanan, J. Phys. Chem. B 110, 18236 (2006).

[13] N. Chantarat, Y. W. Chen, S. Y. Chen and C. C. Lin, Nanotechnology 20, 395201

(2009).

[14] G. Benkö, J. Kallioinen, J. E. I. Korppi-Tommola, A. P. Yartsev and V. Sundström, J.

Am. Chem. Soc. 124, 489 (2002).

[15] Y. Zhang, Z. B. Xie and J. Wang, Nanotechnology 20, 505602 (2009).

[16] K. Ahn, Y. S. Jeong, H. U. Lee, S. Y. Jeong, H. S. Ahn, H. S. Kim, S. G. Yoon and C.

R. Cho, Thin Solid Films 518, 4066 (2010).

[17] F. M. Fowkes, J. Phys. Chem. 382 (1962).