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ScienceDirect Energy Procedia 79 (2015) 360 – 365

2015 International Conference on Alternative Energy in Developing Countries and Emerging Economies

Efficiency Enhancement of ZnO Dye-sensitized Solar Cells by Modifying Photoelectrode and Counterelectrode Kritsada Hongsitha,b, Niyom Hongsithb,c, Duangmanee Wongratanaphisana,b, Atcharawon Gardchareona,b, Surachet Phadungdhitidhadaa,b, Supab Choopuna*,b a

Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand b Thailand Center of Excellence in Physics (ThEP center), CHE, Ratchathewi, Bangkok, 10400, Thailand c School of Science, University of Phayao, Phayao, 56000, Thailand

Abstract In this research, ZnO photoelectrode and Platinum counterelectrode in dye-sensitized solar cells (DSSCs) were modified by sparking technique and investigated photoconversion properties. The ZnO photoelectrode was adjusted by using double-layered technique. Here, the DSSC structures were FTO/double-layered ZnO/N719/electrolyte/Pt counterelectrode. The efficiency characteristics for DSSCs were measured under illumination of simulated sunlight with the radiant power of 100 mW/cm2. It was found that the best results of DSSCs were observed with power conversion efficiency of 2.53% at 250 sparking cycles for ZnO nanopowder over-layered which was significantly higher than 1.83% of the reference cell. The efficiency enhancement can be explained by the arising of short-circuit photocurrent due to increasing of light scattering and dye adsorption for double-layered photoelectrode and the increasing of the active surface area of Platinum nanoparticles in counterelectrode. © 2015 Published by Elsevier Ltd. This © 2015The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Organizing Committee of 2015 AEDCEE . Peer-review under responsibility of the Organizing Committee of 2015 AEDCEE

Keywords: Dye-sensitized solar cells, ZnO, Platinum, Nanoparticles, Sparking technique

1. Introduction Recently, ZnO semiconductors were widely used as photoelectrode in DSSCs [1-4]. The basic structure of ZnO DSSC was transparent conducting oxide (TCO) glass / ZnO / dye / electrolyte / Platinum / TCO. However, the energy conversion efficiency of DSSCs based on ZnO is relatively too low because it is affected by many factors depending on the structure of DSSCs especially, the limit of short circuit

* Corresponding author. Tel.: +66 53 943 375; fax: +66 53 357 511. E-mail address: [email protected].

1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Organizing Committee of 2015 AEDCEE doi:10.1016/j.egypro.2015.11.503

Kritsada Hongsith et al. / Energy Procedia 79 (2015) 360 – 365

current density on the morphologies of photo- and counterelectrode. Thus, there are many techniques to improve the efficiency by improvement of photoelectrode and counterelectrode by mainly increasing of short circuit current density. The double-layered (DL) technique of semiconductor [5,6] was used for enhancement the light harvesting in photoelectode. From our previous report [7], the photoelectrodes layer had been modified by using double layers of ZnO nanoparticles by sparking process/ZnO nanopowder and the results showed that the improvement of efficiency by increasing of short circuit current density. The enhanced performance of ZnO double-layered composite film for solar cells can be attributed to the combined effect of following factors. Firstly, the light scattering of over-layer enhances harvesting light of the DSSCs and the under-layer ZnO nanoparticle layer ensures good electronic contact between film electrode and the F-doped tin oxide (FTO) glass substrate. Furthermore, the high surface areas and the pore volume of ZnO nanoparticles are beneficial to adsorption of dye molecules. Moreover, the efficiency of DSSC is mainly affected by morphology or structure of Pt counterelectrode. There are several studies on modifying Platinum counterelectrode in order to improve the efficiency by increase surface area of Platinum for increase catalytic ability of electrolyte to improve short circuit current density [8-10]. From our previous report [11], the platinum layer in counterelectrode had been modified by using sparking process and the power conversion efficiency (PCE) as high as 2.13% compared with 1.81% of the cells fabricated with the thermal deposited platinum counterelectrodes. The enhanced efficiency is found to be attributed to the higher short circuit current arising from the increases in the active surface area. Thus, to highly improve the efficiency of DSSC, photoelectrode and counterelectrode should be modified. In this research, the nanoparticles of ZnO and platinum were prepared by high voltage sparking process for photoelectrode and counterelectrode in dye-sensitized solar cells (DSSCs) [7,11]. The doublelayered of ZnO in photoelectrode was formed for increasing of short-circuit photocurrent due to the increasing of light scattering and quantity of dye adsorption. Also, the high surface area of platinum nanoparticles in counterelectrode was modified and investigated for increasing of redox reaction due to the increasing of active surface area. 2. Methodology The ZnO nanoparticle films as photoelectrode were obtained by sparking the zinc wire (0.38 mm of diameter, 99.97% of purity) and platinum wire (0.40 mm of diameter, 99.99% of purity) on the fluorinedoped tin oxide glass (FTO, 8 :/cm2) at a constant DC power supply of 4.5 kV, Respectively. The distance between wire and substrate was set to be about 1 mm. The thickness of the ZnO nanoparticles films was controlled by varying the number of cycles in the sparking process and fixed the sparking area of was fixed on a 0.5x2 cm2. The experiment was repeatedly done at 100, 150, 200, 250 and 300 cycles in ambient air under atmospheric pressure. Then, as-deposited films were annealed at 400°C for 1 h in order to completely transform Zn particles to ZnO particles. Platinum nanoparticles as counterelectrode, the thickness of Platinum nanoparticles films was fixed the sparking cycles at 15 cycles and annealed at 550°C for 1 h. The surface morphology and film thickness were characterized by field emission scanning electron microscopy (FE-SEM). A DL of photoelectrode, the first layer of ZnO nanoparticle was prepared by sparking technique and the second layer of ZnO nanopowder (Nano Materials Technology Co.Ltd.) were prepared by screening technique. ZnO nanopowder was mixed with polyethylene glycol solution to form the ZnO paste. Then, FTO was covered by the block screen with area of 0.5 x 2 cm2. Next, the ZnO paste was added in the block screen and then brush it by cutter blade. The active area of the resulting cell exposed in light was approximately 1 cm2 and the thickness of all cells was fixed. The obtained double layer film was then heated at 400°C for 1 h in air ambient.

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To complete the sensitizer loading, after heating, the obtained ZnO photoelectrodes were soaked in a 1:1 (volume ratio) acetonitrile and tertbutanol mixture of ruthenium (II) dye (N719, 0.5 mM, Solaronix) for 1 h. The counterelectrodes reference were then fabricated by coating with a drop of Pt solution containing hydrogen hexachloroplatinate (IV) Hydrate (Cl6H2Pt) 0.012 g and Terpineol anhydrous 10% in acetone 1 ml on a FTO glass followed by heating at 550°C for 1 h. The dye-loading ZnO photoelectrode and counterelectrode were assembled into a sealed cell with parafilm spacer, schematic diagram of Photoand Counter electrode showed in Fig. 1. In the sealed cell, and I−/I3− acetonitrile electrolyte containing 0.2 M lithium iodide (LiI), 0.02 M iodine (I2), 0.5 M DMPII and 0.5 M 4-tert-butylpyridine (TBP) was injected from the edges into the open cell and the cell was tested immediately by using simulated AM 1.5 sunlight illumination with 100 mW/cm2 light output. The photocurrent–voltage measurement and the electrochemical impedance spectroscopy (EIS: 1 Hz to 10 kHz) of all DSSCs were performed.

Fig. 1. Schematic diagram of Photoelectrode and Counterelectrode.

3. Results and Discussion The surface morphologies of ZnO and Platinum films in photoelectrode and counterelectrode were showed in Fig. 2. Fig. 2. (a) and (b) showed surface morphology of the ZnO nanoparticles by sparking process and screening ZnO nanopowder which have average diameter about of 20 – 40 and 60-80 nm, respectively. It was found that the surface morphology of the sparking films ZnO showed higher porosity than that of ZnO nanopowder by screening methods. The cross-sectional SEM image of double-layer ZnO film was fabricated by sequentially depositing ZnO nanoparticles by sparking process and nanopowder using the screening technique onto FTO glass as shown in Fig. 2. (c). Fig. 2. (d) and (e) show surface morphology of the Platinum films by sparking process and Platinum films by thermal deposited. It was found that the surface of Platinum nanoparticles by sparking process showed higher roughness which have average diameter of about 40-60 nm. It was confirmed that the sparking films showed higher surface-to-volume ratio than that of Pt reference film.


Fig. 2. Surface morphology of the as-deposited films of (a) ZnO nanoparticles by sparking process, (b) ZnO nanopowder, (c) crosssection of a double layered of ZnO photoelectrode film, (d) Platinum films by sparking process and (e) Platinum films by thermal deposited.

The photovoltaic performances of DSSCs with the different films ZnO of single layer (SL) and DL based on reference platinum and sparking platinum was shown in Fig. 3, with the measured and calculated values obtained from the I-V curve shown in the inset. It can be seen that the short circuit current density (Jsc) and PCE of DL and SL ZnO film with sparking platinum counterelectrode were obviously higher than that of DL and SL ZnO film using reference platinum counterelectrode, however, the Jsc value of the fabricated DL ZnO with sparking platinum counterelectrode DSSC increased to 8.571 mA/cm2 and PCE enhanced up to 2.53%.

Fig. 3. Photovoltaic performance of the DSSCs fabricated with different film of ZnO single layer (SL) and double layer (DL) based on reference platinum and sparking platinum and Photovoltaic performance parameters of DSSCs (inset).

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The photovoltaic performances of DSSCs with the different films thickness of ZnO nanoparticle by sparking with ZnO nanopowder as double layer was shown in Fig. 4 (a). When the number of sparking cycles increases, the photoconversion efficiency increases from 1.83% to 2.04%, 2.09%, 2.15% and 2.53% under 100, 150, 200 and 250 sparking cycles, respectively. The result showed that the best results of DSSCs were observed with Jsc of 8.57 mA/cm2 and power conversion efficiency of 2.53% at 250 sparking cycles for ZnO nanopowder over-layered which was significantly higher than 1.83% of the reference cell. Here, the conversion efficiency slightly decreases at the 300 sparking cycles and the results were summarized in Table 1. The improvement of photoconversion efficiency is mainly due to an increase of the short circuit current density from increasing of dye absorption of ZnO nanoparticles, light scattering from double layer of ZnO photoelectrode [7,12] and the catalytic ability of electrolyte was increased from Platinum nanoparticles [11]. Thus, it was suggested that different morphology of ZnO double layer films in photoelectrode and platinum nanoparticles films in counterelectrode was strongly effect on the short circuit current density parameter. For investigation the electrical properties, the kinetics of electron transport in cell was characterized by using an electrochemical impedance spectroscopy (EIS). The EIS of DSSCs was measured and the Nyquist plots were showed in Fig. 4 (b). The semicircle in the Nyquist plot indicated that the charge transfer resistance (Rct) of the DSSCs. The values of Rct for each sample was calculated and summarized in Table 1. The lowest value of Rct of 7.90 Ω corresponding to the smallest semi-circle of Nyquist plot was observed on a ZnO photoelectrode double layer with sparking cycles of 250. The smallest semicircle in the Nyquist plot indicated the best charge transfer properties through the photoelectrode layer. It was suggested that the DSSC with 250 sparking cycles had the smallest value of charge transfer resistance and thus, resulted in the highest power conversion efficiency (2.53%).

Fig. 4. (a) Photovoltaic performance and (b) Nyquist plots of the DSSCs fabricated with different film thickness of ZnO nanoparticle by sparking to double-layered. Table 1. Photovoltaic performance parameters of DSSCs fabricated with different number of sparking cycle of ZnO nanoparticle by sparking process. Sparking cycles (cycle) Voc(V) Jsc(mA) FF PCE (%) Rct(Ω) 0 0.57 6.873 0.47 1.83 16.26 DL100 0.58 7.179 0.49 2.04 15.92 DL150 0.57 7.793 0.47 2.09 11.92 DL200 0.57 8.576 0.44 2.15 11.37 DL250 0.59 8.571 0.50 2.53 7.90 DL300 0.57 8.646 0.47 2.29 8.96


4. Conclusion We have successfully designed and fabricated a double-layer photoelectrode and Pt nanoparticles counterelectrode by simple and cost-effective sparking technique. The double-layer in photoelectrode and Platinum nanoparticles films in counterelectrode has strongly effect on the short circuit current density. The increasing of short circuit current density is caused by the increasing of the active surface area. The efficiency enhancement could be explained by effect of light scattering and dye adsorption. The double layer photoelectrode with 250 sparking cycles was regarded as the optimum condition to achieve the highest power conversion efficiency of 2.53%. Therefore, ZnO and Platinum nanoparticles prepared by sparking process can be an alternative photoelectrode and counterelectrode for ZnO DSSCs. Acknowledgements Kritsada Hongsith would like to acknowledge a financial support from the Graduate School, Chiang Mai University and 50th CMU Anniversary-Ph.D. Scholarship. References [1] K. Keis, E. Magnusson, H. Lindstrom, S. Lindquist, A. Hagfeldt. A 5% efficient photoelectrochemical solar cell based on nanostructured ZnO electrodes. Sol. Energy Mater. Sol. Cells 2002;73:51–58. [2] Y. Zheng, X. Tao, L.Wang, H. Xu, Q. Hou,W. Zhou. J.Chen, Novel ZnO-Based Film with Double Light-Scattering Layers as Photoelectrodes for Enhanced Efficiency in Dye-Sensitized Solar Cells. Chem. Mater 2010;22:928–934. [3] L. Lin, M. Yeh, C. Lee, C. Chou, R. Vittal , K. Ho. Enhanced performance of a flexible dye-sensitized solar cell with a composite semiconductor film of ZnO nanorods and ZnO nanoparticles. Electrochimica Acta 2012;62:341– 347. [4] S. Sutthana, N. Hongsith, S. Choopun. AZO/Ag/AZO multilayer films prepared by DC magnetron sputtering for dye-sensitized solar cell application. Current Applied Physics 2010;10:813–816. [5] G. Dai, L. Zhao, J. Li, L. Wan, F. Hu, Z. Xu , B. Dong, H. Lu, S. Wang, J. Yu. A novel photoanode architecture of dyesensitized solar cells based on TiO2 hollow sphere/nanorod array double-layer film. J. Colloid Interface Sci. 2012;365:46–52. [6] J. Tae Park, D. Kyu Roh, W. Seok Chi, R. Patel, J. Hak Kim. Fabrication of double layer photoelectrodes using hierarchical TiO2 nanospheres for dye-sensitized solar cells. J. Ind. Eng. Chem. 2012;18:449–455. [7] K. Hongsith, N. Hongsith, D. Wongratanaphisan, A. Gardchareon, S. Phadungdhitidhada, P. Singjai, S. Choopun. Sparking deposited ZnO nanoparticles as double-layered photoelectrode in ZnO dye-sensitized solar cell. Thin Solid Films 2013;539:260–266. [8] C. H. Yoon, R. Vittal, J. Lee, W. S. Chae, K. J. Kim. Enhanced performance of a dye-sensitized solar cell with an electrodeposited-platinum counter electrode. Electrochimica Acta 2008;53:2890–2896. [9] M. Y. Song, K. N. Chaudhari, J. Park, D. S. Yang, J. H. Kim, M. S. Kim, K. Lim, J. Ko, J. S.Yu. High efficient Pt counter electrode prepared by homogeneous deposition method for dye-sensitized solar cell. Applied Energy 2012;100:132–137. [10] C. C. Yang, H. Q. Zhang, Y. R. Zheng. DSSC with a novel Pt counter electrodes using pulsed electroplating techniques. Current Applied Physics 2011;11:S147eS153. [11] K. Hongsith, N. Hongsith, D. Wongratanaphisan, A. Gardchareon, S. Phadungdhitidhada, S. Choopun. Efficiency Enhancement of ZnO Dye-Sensitized Solar Cell Using Platinum Nanoparticles Prepared by Sparking Process. J. Nanosci. Nanotechnol. 2015;15:7025–7029. [12] A.Usami. Theoretical study of application of multiple scattering of light to a dye-sensitized nanocrystalline photoelectrochemical cell. Chemical Physics Letters 1997;277:105-108.

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