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Influence of carbon nanotubes in gel electrolyte on photovoltaic performance of ZnO dye-sensitized solar cells

October 22, 2013.

 

        Our research group at the Applied Physics Research Laboratory, Chiang Mai University is actively involving to harvest maximum energy conversion efficiency of solar power into electricity of dye-sensitized solar cells (DSSCs). For years now, DSSCs are considered to be one of the promising alternatives to the existing silicon based-solar cells due to ease of fabrication and low-cost fabrication [1]. Recently, DSSC-productions have been already commercialized for light generation purpose, but they have relatively low efficiency, in which the performance depends on both current and voltage of DSSCs. In this regard, the DSSC performance can be improved in an effective manner through an increase in photogenerated current and photovoltage by suppressing electron back transport. In this respect, it depends particularly on metal-oxide types to well-defined DSSC photoelectrodes. Due to our knowledge gained through zinc oxide (ZnO) materials [2-3], we are now interested in the efficiency improvement of DSSCs using ZnO as photoelectrode-based layers. The structure and mechanism of DSSCs are shown in Figure 1:

 


Figure 1 Schematic representation of a dye-sensitized solar cell (DSSC) presenting the structure and mechanism.

 

        ZnO is also currently an interesting material due to its similarities with the most studied semiconductor oxide, TiO2, which facilitates the important for the DSSC application. ZnO presents wide direct band gap and electron affinity similar to those of TiO2. As well as ZnO has higher electronic mobility that would be favorable for electron transport, with reduced recombination loss when used in DSSCs. In particular, it can be synthesized in a wide variety of nanoforms. Today, record efficiencies on ZnO-nanostructure-based DSSCs above 7.14% have been presented (while the energy conversion efficiency exceeding 12.3% has been achieved in TiO2-nanostructure-based DSSCs) [4-5].

        The one of the most importance composition of DSSCs is liquid electrolyte solution that transfers charge from a counterelectrode to a photoelectrode. In this work, we have studied on the efficiency enhancement and durability of DSSCs focusing on a layer of liquid electrolyte (LiI/I2+Popylene carbonate). But major drawbacks of DSSCs that use liquid electrolyte are corrosion and evaporation of solvent electrolyte as well as photodegradation of DSSCs [6]. In order to improve durability of DSSCs, alternative materials, such as p type inorganic semiconductor and quasi-solid state electrolyte using polymer matrix chains, are employed in DSSCs instead traditional liquid electrolyte. Therefore, to solve the durability problem, we have been focusing on quasi solid state electrolyte, such as gel polymer electrolyte [7]. Although this approach would solve the problem, the ionic mobility is limited at room temperature. Therefore, higher ionic mobility is necessary to improve cell efficiency. To further improve the cell efficiency, many efforts so far have focused on the inorganic semiconductor acting as nanofiller such as TiO2 that can be applied to reduce the charge transfer resistance and develop the porous structure of the gel polymer electrolyte [8]

        To meet this need, we have studied the influence of multi-walled carbon nanotubes (MWCNTs) in gel electrolyte for making DSSCs that are not only are more efficient but longer lasting [9]. MWCNT was the material that we have used to fabricate an alternative gel electrolyte to improve cells efficiency because of its excellence electronic properties, unique electrochemical catalytic properties, and high thermal resistivity. The chemical bonding spectra and the physical surface of the gel electrolyte by FT-IR and FE-SEM, respectively, have been observed as shown in Figure 2:


Figure 2 FT-IR spectra of MWCNTs gel electrolyte and FE-SEM (in the inset) image of MWCNTs gel electrolyte.

 

        We have fabricated an alternative MWCNTs gel electrolyte and reported that added MWCNTs in presentably amount (5 wt%) can reduce charge transfer resistance which effects to an increase in photocurrent and also its fill factor resulting in enhancement of photovoltaic efficiency explained by following equation:


Figure 2 FT-IR spectra of MWCNTs gel electrolyte and FE-SEM (in the inset) image of MWCNTs gel electrolyte.

 

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Figure 2 FT-IR spectra of MWCNTs gel electrolyte and FE-SEM (in the inset) image of MWCNTs gel electrolyte.

 

         is a fill factor that is not affected by the and , respectively. and are normalized shunt and series resistances, respectively, where is a resistance at the maximum power and is a characteristic resistance. It clearly shows that the value of and both affected to FF. Thus, increasing and lowering can lead to higher FF. When the FF is increased it effects to higher photoconversion efficiency as shown in Eq.4. If the excessive amount of MWCNTs is used in MWCNTs gel electrolyte, it affects to decreasing efficiency of the cell as shown in Figure 3:


Figure 3 Photocurrent density-voltage (J-V) curves showing the efficiency of gel polymer electrolyte ZnO-DSSCs with different amount of MWCNTs.

        This is due to nonuniform distribution of the MWCNTs on electrolyte composite leading to an increase of charge transfer resistance. On the other hand, due to their excellent electrical and electrochemical properties, therefore, if the amount of the MWCNTs is too high, the recombination may be easily occurred between the excited electron of dye and electrolyte material in surface contact of photoelectrode as shown in Figure 4:


Figure 4 A schematic showing influence of MWCNTs on charge transport of ZnO DSSCs with addition of MWCNTs into gel electrolyte.

        The stability of MWCNTs gel polymer electrolyte ZnO DSSCs was measured at various ranges of time at room temperature. The overall stability performance for the 0-5 wt% MWCNTs gel polymer electrolyte ZnO DSSCs after 1,006 h remains 85% of its initial performance. However, the stability performance was reduce when added MWCNTs more that 5%. The major factor of degradation of high ratio MWCNTs in gel electrolyte can be explained in terms of recombination probability of the excited electron from dye to electrolyte material resulting from nonuniform distribution of MWCNTs and high surface contact with photoelectrode.

        In conclusion, the foundation of the photovoltaic efficiency enhancement and stability of ZnO DSSCs is a new, gel-based electrolyte based on presentably amount of MWCNTs. With this new electrolyte, the maximum efficiency of the gel polymer electrolyte ZnO DSSCs was increased from 0.24% (Jsc = 2.23 mAcm-2, Voc = 0.50 V, FF = 0.21 with no MWCNTs addition.) to 0.75% (Jsc =2.60 mAcm-2, Voc = 0.54V, FF = 0.65 at condition 5wt% of MWCNTs). The overall stability performance for the MWCNTs gel polymer electrolyte ZnO DSSCs after 1,006 h remains 85% of its initial performance. The detail experimental results have been published in Electrochimica Acta (IF: 3.777) [10].

 

References

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[4] W-C. Chang, Y-Y. Cheng, W-C. Yu, Y-C. Yao, C-H. Lee, H-H. Ko, Enhancing performance of ZnO dye-sensitized solar cells by incorporation of multiwalled carbon nanotubes, Nanoscale Research Letters, 7 (2012) 166.
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[7] S. Xiaolin, X. Weilin, L. Guiji, Y. Hongjun, and Y. Mu, Influence of molecular weight of PEO on performance of quasi solid state dye sensitized solar cells, Journal of Wuhan University of Technology- Mater. Sci. Ed. 25 (2010) 218.
[8] H. Usui, H. Matsui, N. Tanabe, and S. Yanagida, Improved dye sensitized solar cells using ionic nanocomposite gel electrolytes, Journal of Photochemistry and Photobiology A: Chemistry 164 (2004) 97.
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[10] N. Khongchareon, S. Choopan, N. Hongsith, A. Gardchareon, S. Phadungdhitidhada, and D. Wongratanaphaisan, Influence of carbon nanotube in gel electrolyte on photovoiltaic performance of ZnO dye-sensitized solar cells, Electrocimica Acta 106 (2013) 195-200.

 

Reported by
Natthorn Khongchareon, Supab Choopun, and Duangmanee Wongratanaphaisan
Applied Physics Research Laboratory (APRL), Dept. of Physics and Materials Science, Fac. of Science,
Chiang Mai University, Chiang Mai – 50200, Thailand
Tel: +66-53-943375, +66-53-942463 Ext. 11, Fax: +66-53-357511, Web page: www.aprlcmu.net