Transparent Conductive Oxide Less Flexible Dye

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ABSTRACT. Transparent conductive oxide less flexible dye-sensitized solar cells (TCO-less DSC) with flat and cylinder shapes are reported. The cell consists of ...
Mater. Res. Soc. Symp. Proc. Vol. 1435 © 2012 Materials Research Society DOI: 10.1557/opl.2012.1695

Transparent Conductive Oxide Less Flexible Dye-sensitized Solar Cells with Flat and Cylinder Shapes Jun Usagawa, Byung-wook Park, Yuhei Ogomi, Shyam S. Pandey, and Shuzi Hayase Kyushu Institute of Technology, 2-4, Hibikino, Wakamatsu-ku, Kitakyushu 808-0196 Japan ABSTRACT Transparent conductive oxide less flexible dye-sensitized solar cells (TCO-less DSC) with flat and cylinder shapes are reported. The cell consists of a plastic cover, a flexible titania/dye sheet back contacted with a metal mesh sheet, a gel electrolyte sheet, and Pt layer on a Ti sheet. How to increase the efficiency were discussed. We concluded that making a titania/dye layer on a metal mesh sheet thinner and using a thinner electrolyte layer were effective for increasing the efficiency. A flat TCO-less DSC with 6.1 % efficiency and a cylindrical TCO-less DSC with 5.1 % efficiency are reported. INTRODUCTION

Light

Porous Metal

TCO‐less DSC (back contact)

TCO

Ti sheet

Glass with TCO

Charge  generation  sheet Ti sheet

Light

Glass only

The certified efficiency for dye sensitized solar cell (DSCs) reached 11 % [1,2]. DSCs usually consist of transparent conductive oxide glasses (TCO-glass). TCO-glasses are still expensive now and restrict solar cell shapes. We focused on structures and fabrication process of transparent conductive oxide less DSCs (TCO-less DSCs), where porous metal electrodes are fabricated at the back side of titania/dye layers as back-contact structures as shown in Fig.1. [37]

Iodide/i odine

DSC with TCO

Fig. 1  TCO‐less DSC with back contact electrodes and conventional DSC with TCO‐ layers

We have already reported flat type, fiber type, and cylinder type dye sensitized solar cells [812]. In this report, how to optimize the structure for each cell is discussed.

EXPERIMENTAL SECTION Fabrication of flexible titania sheet (S-1): S-1 was fabricated by coating titania paste on Ticoated stainless metal mesh sheet (diameter: 25 micron), followed by heating the substrates at 450 °C. The sheet was dipped in a dye solution (N719). S-1 is a flexible charge generation sheet consisting of a titania/dye layer and a Ti-coated stainless metal mesh sheet. The surface of the Ti was oxidized to titania which blocks charge recombination between electrons in a Ti electrode and redox species in an electrolyte layer. Cross sectional view of S-1 sheet observed by SEM shows that contact between titaia layer and metal surface was good. Plastics

TiO sheet (S‐1) Gel electrolyte

Pt  on Ti sheet F‐1 Fig. 2.  Structure and fabrication of TCO‐less back contact flexible DSC (F‐1)   

Fabrication of flexible TCO-less flat DSC (F-1): F-1 was fabricated by piling up a plastic sheet (PET sheet acting as a cover), S-1 sheet, a porous insulator layer (PTFE) containing electrolyte, and a Ti sheet with Pt. The edge was sealed with photocurable resins. Fabrication of flexible TCO-less cylinder DSC (F-2) : F-2 was fabricated by inserting porous silicone tube surrounded by a Ti metal mesh with Pt, a round shaped porous PTFE layer

Shrinkable plastic tube

Titania/dye  Ti mesh sheet

S‐1 film

Porous PTFE film with electrolyte Ti mesh Porous Silicone tube F‐2

Fig.3. Structure of TCO‐less back contact flexible dye‐sensitized solar cells with  cylinder shape (F‐2)   

consisting of electrolyte, a round shaped S-1 into a glass tube, as shown in Fig.3. Electrolyte was injected from the porous silicone tube, followed by sealed with photocurable resins. Therefore, redox species between the gel electrolyte layer and the porous titania layer diffuse without difficulty. The cell area was 0.21 cm2 for a F-1 cell and 0.6 cm2 for a F-2 cell. In the case of F-2, projected area (outside diameter of solar cell x length) was used as the calculation of efficiencies, because this corresponds to area where solar cells are set on the ground.

Life time /s

DISCUSSION In order to increase the efficiency, we focused on gap between a working electrode and a counter electrode, namely, the thickness of the S-1 film and a porous insulator layer (PTFE film). Previously, the nanoporous TiO2 layer covered a Ti-coated stainless steel mesh sheet completely from the top to the bottom and the thickness was 40 µm. The thickness was reduced to 30 µm by fabricating the porous TiO2 layer only on the top of the Ti-coated stainless steel mesh sheet. In addition, 35µm PTFE film was replaced by a porous TiO2 layer with 10 µm thickness. Totally, the gap between the two electrodes was reduced from 75 to 40 µm. The efficiency increased from 4.5% to 6.1 % drastically. FF, open circuit voltage (Voc) and short circuit current density (Jsc) was improved from FF:0.65, Voc: 0.70V, Jsc: 10.1mA/cm2 to FF:0.70, Voc: 0.76V, Jsc: 11.4 mA/cm2. The increase in the Voc from 0.70 to 0.76 was attributed to the decrease in the porous TiO2 thickness, which was supported by longer electron life time of the cell with 40 µm thickness as shown in Fig. 4. It is well know that thick TiO2 layer decreased Voc due to an increase in the opportunity of charge recombination [13]. 1

F-1 with improved S-1 titania sheet (30 μm) 0.1

F-1 with S-1 titania sheet (40 μm) 0.01 10

100 1000 Current density /μA/cm2

10000

Fig. 4. Electron life time of F‐1 before and after improvement The electron life time was measured by IMVS (incident light  modulated photovoltage spectroscopy) The efficiency improvement of cylindrical solar cells was also tried by reducing the gap between a working electrode and a counter electrode. The wide gap between these electrodes disturbs efficient diffusion of redox species in the electrolyte layer as was shown in the above

discussion. In addition, there was space between a cover plastic tube and a working electrode, where electrolyte presented. This prevented light harvesting ratio by working electrode (porous TiO2/dye layer) because of light absorption of I3- at around 400 nm in the electrolyte. In order to reduce these space gaps, a thermally shrinkable tube was introduced. Before an electrolyte was injected, the cell was heated at 120-140 °C. The diameter of the tube was reduced from 5.5 to 4.2 mm. Before the heat treatment, the efficiency was 3.4 %. The efficiency increased to 5.1% after the heat treatment. The current distribution was measured by LBIC (laser beam induced current) method. Even before the thermal treatment, current distribution was flat when 633 nm laser light was exposed to the cell, suggesting that current generation occurred homogeneously at 633 nm where there was no absorption of the redox species. However, large decrease in the signals was observed when 405 nm laser light was exposed, where I3- absoption presents. The LBIC signal at 405 nm increased drastically after the thermal treatment, suggesting that I3- in space between a cover tube and a working electrode was removed after the thermally shrinkable tube compressed the space. CONCLUSIONS It was proved that various TCO-less back contact DSCs with various shapes were prepared easily, due to the long electron diffusion length of TiO2 of dye-sensitized solar cells. Flexible TCO-less flat DSCs with back contact structure, flexible TCO-less cylinder DSCs with back contact structure were reported. These efficiencies were improved by narrowing the space between a counter electrode and a working electrode. Cylinder type DSCs are expected to have an advantage over flat type DSCs in terms of easy encapsulation. We believe that TCO-less back contact DSCs with these various shapes open new markets for new solar cells. REFERENCES 1.M. A. Green, K. Emery, Y. Hishikawa, W. Warta, and E. D. Dunlop, Prog. Photovolt: Res. Appl., 20, 12 (2012). 2.A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, and H. Pettersson, Chem. Rev, 110, 6595 (2010). 3.J. M. Kroon, N. J. Bakker, H. J. P. Smit, P. Liska, K. R. Thampi, P. Wang, S. M. Zakeeruddin, M. Graetzel, A. Hinsch, S. Hore, U. Wurfel, R. Sastrawan, J. R. Durrant, E. Palomares, H. Pettersson, T. Gruszecki, J. Walter, K. Skupien, and G. E. Tulloch, Prog. Photovol., 15, 1 (2007). 4.Y. Kashiwa, Y. Yoshida, and S. Hayase, Appl. Phys. Lett. 92, 033308 (2008). 5. N. Fuke, A. Fukui, Y. Chiba, R. Komiya, R. Hamanaka, and L. Han, Jpn. J. Appl. Phys. 46, L420 (2007). 6. L. Han, N. Koide, Y. Chiba, A. Islam, R. Komiya, N. Fuke, A. Fukui, and R. Yamanaka, Appl. Phys. Lett., 86, 213501 (2005). 7. T. Beppu, Y. Kashiwa, S. Hayase, M. Kono, and Y. Yamaguchi, Jpn. J. Appl. Phys., 48, 061504 (2009). 8. X. Huang, P. Shen, B. Zhao, X. Feng, S. Jiang, H. C. Li, and S. Tan, Sol. Energy Mater. Sol. Cells, 94, 1005 (2010). 9. Y. Wang, H. Yang, Y. Liu, H. Wang, H. Shen, J. Yan, and H. Xu, Prog. Photovolt: Res. Appl., 8, 285 (2010). 10. X. Fan, F. Wang, Z. Chu, L. Chen, C. Zhang, D. Zou, Appl. Phys. Lett., 90, 073501 (2007). 11. K. Miettunen, J. Halme, M. Toivola, and P. Lund, J.Phys. Chem. C., 112, 4011 (2008). 12. Y. Yoshida, S. S. Pandey, K. Uzaki, S. Hayase, M. Kono, and Y. Yamaguchi, Appl. Phys. Lett., 94, 093301-1 (2009). 13. M. G. Kang, K. S. Tyu, S. H. Chang, N. G. Park, J. S. Hong, and K-J. Kim, Bull. Korean Chem. Soc., 25, 742 (2004).