Effect of Annealing on Characterization of TiO 2 -Based DSSC

Transcription

1 Journal of Materials Science and Engineering A 2 (2) (2011) D DAVID PUBLISHING Effect of Annealing on Characterization of TiO 2 -Based DSSC Rangga Winantyo 1, Anne Zulfia 2, Nji Raden Poespawati 1 and Djoko Hartanto 1 1. Department of Electrical Engineering, Faculty of Engineering, University of Indonesia, Depok Department of Metallurgy and Material, Faculty of Engineering, University of Indonesia, Depok Received: August 05, 2011 / Accepted: September 06, 2011 / Published: February 10, Abstract: One of the most important parameter that affects Dye-Sensitized Solar Cell (DSSC) characteristics is. There are many aspects that can influence the such as particles size of titanium-dioxide (TiO 2 ), annealing temperature and processing parameter or fabrication itself. In this paper, one of important parameter will be discussed, i.e., effect of annealing on characterization of TiO 2 thin layer. First the TiO 2 thin film was formed on indium tin-oxide (ITO) glass by stir casting process using doctor balde to make thin layer on ITO glass, second the thin film was annealed at different temperature start from 100 C, 150 C, 200 C, 300 C and 350 C for 30 min. The samples then were characterized both microstructural analysis and electrical propertie. It is found that annealing at 200 C was optimum condition due to content of 32% which has good in electrical properties such as fill factor, conductivity, open-circuit voltage (V oc ) and short-circuit current (I sc ), respectively. Key words: DSSC; annealing; ; electrical properties; titanium-dioxide. 1. Introduction One of the interesting developments of solar cell technology is solar cell developed by Grätzel in 1999 [1] and often called as Grätzel cells or DSSC. Fig. 1 shows a schematic view of DSSC structure including the energy band diagram. Grätzel cell has 3 main components. The top component is a transparent anode (left side in Fig. 1) usually made from fluoride-doped tin dioxide (SnO 2 :F) and stored on the back of the plate (usually glass). On the back of the conductive plate is a thin layer of TiO 2, which is formed into highly porous structure. The plates are then soaked in a mixture of ruthenium-polypyridine photosensitive dye (also called as molecular sensitizers) and solvent [2]. The morphology and particle size of TiO 2 used in the DSSC have a fairly important part in the efficiency. Besides transporting electron, s also supporting dye molecules absorbing process. Corresponding author: Rangga Winantyo, research field: solar cell materials. rangga-ftui09@hotmail.com. Particularly, of TiO2 particles will affects the number of dye that can be adsorbed [4-7]. At a certain level of, s can absorb the maximum amount of dye and then this will affect the performance of DSSC cells. In this work, we study the effect on TiO 2 -based DSSC using fabrication approach. The effect of annealing on characteristic of TiO 2 -based DSSC will be discussed in this paper. It is assumed that annealing process will improve electrical properties of this cell due to effect. Annealing process can remove the binding and solvent which can increase electrically-connected network of TiO 2 particles [8]. This condition will increase TiO2 performance as the electron transport. According to Chang-Ryul et al. [9], is related with annealing duration and temperature, which affecting photocurrent density and the amount of absorbed dye. Effect of on DSSC characteristic was also discussed by Chang-Ryul et al., such as Fill Factor, conductivity, I sc and V oc.

2 Effect of Annealing on Characterization of TiO 2 -Based DSSC 233 Transparent Nanoporous semiconductor Conducting thin film (TiO 2 ) Counter Electrode Oxide (TCO) e - CB e - Dye * Maximum Diffusion hv voltage e - e - Redox Dye/Dye + mediator Load e - Fig. 1 Schematic diagram of electron transport in DSSC [3]. 2. Experimental Procedure The materials used for making TiO 2 thin film on ITO glass was TiO 2 powders with particles size of 400 nm. There are two steps to make a thin layer on ITO glass. First, 3.5 gr TiO 2 powder was stirred into 15 ml ethanol for 15 minutes to make TiO 2 pasta, and then this pasta was vibrated in ultrasonic cleaner for 30 min in order to find the homogenous pasta. Second, TiO 2 pasta is deposited on ITO glass by slip-casting using doctor balde to make thin layer with 1x1 cm 2 dimension. After TiO 2 pasta has been deposited on ITO glass then all samples are heated on the hot plate under different annealing temperature (100 C, 150 C, 200 C, 300 C, 350 C) for 30 min. After annealing, all samples are characterized both microstructure analysis by Scanning Electron Microscope (SEM) to investigate the content on the samples using Image Analysis software. All annealing samples were also immersed into dye solution (ruthenium complex) for 24 h. The last step will be adding the electrolyte solution (iodine) before covering the substrate with other ITO glass. The structure of DSSC can be seen on Fig. 2. All samples were lied under the illumination of halogen bulb (30,000-35,000 lux) to determine V oc of cell. To obtain further electrical properties were calculated current (I) using formula (V = I.R), after V oc and I sc are defined, so the fill factor (FF) can be obtained. The value of I can be obtained by applying various value of resistance on the cell, while the conductivity was measured using 4-point probe for Fig. 2 DSSC structure [11].

3 234 Effect of Annealing on Characterization of TiO 2 -Based DSSC three times measurements. 3. Results and Discussion 3.1 Microstructures Analysis of TiO 2 Layer Microstructures of TiO 2 thin film is shown in Fig. 3. The surface of TiO 2 layer was seen roughness morphology due to formed on the surface. Effect of annealing on is seen in this figure, the increased with higher annealing temperature (Fig. 4). It is seen that microstructure was also slightly changed. The s seem agglomerate each other and accumulated in some area of ITO glass. Higher doesn t always mean worse, but it is required for dye to be absorbed on TiO 2 thin layer with 32% content in this work. When the inter-particle distance is too wide, so the is also high and it can affect the dye particles unattached to s. This occurred when the amount of absorbing dye is more than the amount of dye which is attached to s surface. This condition will affect low cell performance [12]. 3.2 Characteristic of TiO 2 layer The I-V curve of TiO 2 DSSC can be determined by its behavior in an electrical circuit. These two 100 C 150 C 200 C 250 C 300 C 350 C Fig. 3 Microstructures of TiO 2 thin film was analyzed by SEM (mag. X5000). Fig. 4 Porosity vs Annealing temperature.

4 Effect of Annealing on Characterization of TiO 2 -Based DSSC 235 parameters are used to characterize solar cell output. Other parameter that also important is fill factor which is explained in this paper. The graph of I-V curve from TiO 2 DSSC can be seen on Fig. 5. It is seen that sample which annealed at 200 C for 30 minutes gives the optimum value of I-V and obvious seen compared to other samples. In this case, content after annealing at 200 C generated maximum dye absorbed and connected to TiO 2. Then all samples are attempted under illumination of halogen bulb with 30,000-35,000 lux. Halogen bulb gives more stable in light intensity than the sun. The distance between the cell and the bulb is 20 cm. If it is too close, the cell will break because of the heat of the bulb. Also, considering that the optimum temperature for solar cell is C [12-14] Further electrical propertie that related to are conductivity which were also measured using 4-point probe, the result is shown in Fig. 6. The graphs showed that higher content generated increased conductivity. The measurement from each sample sometimes gives very wide deviation value. This can happen because the surface of TiO 2 thin-film is not homogeneous. Further correlation between annealing as well as and DSSC electrical properties, such as V oc, I sc and fill factor parameters are also calculated according to Martin [12] and the results are plotted in Fig. 7. He revealed that reasonable fill factor of solar cell has a value in the range of 0-7 to 0.8. While in this work the DSSC with annealed at 200 C has a 0.69 fill factor which is closed to his work. Fig. 7, Graphs showing the value of V oc, I sc and FF plotted against and annealing temperature. It can be seen that each graph has similar trend. Each graph increased and reached its maximum value at 31.3% content. The same condition for annealing at 200 C which was also reached maximum value of V oc, I sc and FF. It s seem that annealing temperature and will affect electrical Fig. 5 I-V curve from different annealing temperature. Fig. 6 Conductivity vs Porosity.

5 236 Effect of Annealing on Characterization of TiO 2 -Based DSSC (a) (b) (c) (d) (e) (f) Fig. 7 Graphs showing the effect of and annealing temperature on fill factor (a,b), V oc (c,d), I sc (e,f) respectively. properties of this TiO 2 DSSC. 4. Conclusions From the experimental data, it can be seen that annealing temperature has important role on DSSC performance. The value of electrical properties increases as the value of and annealing temperature increase. Both parameters reached their peak at 31% value (annealed at 200 C) due to maximum value of fill factor (0.69), I sc (0.0081mA), and V oc (0.25V). The annealing temperatures higher than 200 C caused the value of electrical properties decrease. These occurred because of TiO 2 thin layer completely distorted, since it undergoes thermal degradation on the samples. This condition can cause poor adhesion of TiO 2 deposition, which resulting low dye absorption and reducing electrical-network between the particles. Acknowledgments The authors would like to thank to Universitas Indonesia for financial support and DirectorateResearch and Public Services of Universitas Indonesia for management supporting under 1 Billion Rupiah Research Grant Project for academic year

6 Effect of Annealing on Characterization of TiO 2 -Based DSSC 237 References [1] M. Grätzel, Photoelectrochemical cells, Nature 414 (2001) [2] F. Gao, Y. Wang, J. Zhang, D. Shi, M. Wang, R. Humphry-Baker, et al., A new heteroleptic ruthenium sensitizer enhances the absorptivity of mesoporous titania film for a high efficiency dye-sensitized solar cel, Chemical communications 23 (2008) [3] N. Meng, K.H. Michael, Y.C. Dennis, K. Sumathy, An analytical study of the effect on dye-sensitized solar cell performance, Solar Energy Materials and Solar Cells 90 (2006) [4] T.H. Meen, W. Water, W.R. Chen, S.M. Chao, L.W. Ji, C.J. Huang, Application of TiO 2 nano-particles on the electrode of dye-sensitized solar cells, Journal of Physics and Chemistry of Solids 70 (2009) [5] M. Grätzel, Review dye-sensitized solar cells, Journal of Photochemistry and Photobiology C: Photochemistry Reviews 4 (2003) [6] S. Nakade, Y. Saito, W. Kubo, T. Kitamura, Y. Wada, S. Yanagida, Influenc of TiO 2 Nanoparticle Size on Electron Diffusion and Recombination in Dye-Sensitized TiO 2 Solar Cells, J. Phys. Chem. 107 (2003) [7] Y. Saito, S. Kambe, T. Kitamura, Y. Wada, S. Yanagida, Morphology control of mesoporous TiO 2 nanocrystalline films for performance of dye-sensitized solar cells, Sol. Energy Mater. Sol. Cell 83 (2004) 1. [8] A.F. Nogueira, C. Longo, M.A. Paoli, Polymers in dye sensitized solar cells: overview and perspectives, Coordination Chemistry Reviews 248 (2004) [9] C.R. Lee, H.S. Kim, N.G. Park, Dependence of, charge recombination kinetics and photovoltaic performance on annealing condition of TiO 2 films, Front. Optoelectron. China 4 (1) (2001) [10] J.H. Yum, S.S. Kim, D.Y. Kim, Y.E. Sung, Electrophoretically deposited TiO 2 photo-electrodes for use in flexible dye sensitized solar cells, Journal of Photochemistry and Photobiology 173 (2005) 1-6. [11] Z.A.A. Junior, Fabrication of dye sensitized solar cell eosin Y/TiO 2 -based with electrophoresis method, Bachelor Degree Thesis, University of Indonesia, [12] M. Gratzel, Dye-sensitized solar cells, Journal of Photochemistry and Photobiology C: Photochemistry Reviews 4 (2003) 145. [13] R.I. Nathalie, Solid hybrid dye-sensitized solar cells: new organic materials, charge recombination and stability, École Polytechnique Fédérale de Lausanne, [14] M.A. Green, Solar cell: operating principles, technology and system applications, N.J.: Prentice-Hall, Englewood Cliffs, 1998, pp