Elsevier

Solar Energy

Volume 81, Issue 1, January 2007, Pages 117-122
Solar Energy

Quasi-solid-state dye-sensitized solar cells based on a sol–gel organic–inorganic composite electrolyte containing an organic iodide salt

https://doi.org/10.1016/j.solener.2006.05.003Get rights and content

Abstract

An organic–inorganic composite gel electrolyte based on TiO2 gel, γ-butyrolactone (γ-BL) and N-methyl pyridine iodide was prepared by the sol–gel method. This gel electrolyte shows high ambient ionic conductivity of 7.63 mS cm−1, which is close to the data of liquid electrolyte with the same organic iodide salt and γ-butyrolactone. Based on the gel electrolyte, a quasi-solid-state dye-sensitized solar cell was fabricated and the highest overall energy conversion efficiency of light-to-electricity of 3.06% was achieved under irradiation of 60 mW cm−2.

Introduction

Dye-sensitized solar cells (DSSCs) are one of the promising candidates for the next generation of solar cells because of their simple structure with relatively high conversion efficiency (10%), inexpensive fabrication procedures in contrast to amorphous silicon (Hagffeldt and Grätzel, 1995, Goetzberger et al., 2002, Green, 2004). Although DSSCs based on liquid electrolytes have reached efficiency as high as 10% under AM 1.5 (Nazeeruddin et al., 1993), the use of liquid electrolytes have created a lot of difficulties in sealing and long-term photochemical stability of the device (Papageorgiou et al., 1997).

To overcome these problems, many efforts have been made to replace the liquid electrolytes with solid or quasi-solid-type charge transport materials (Bach et al., 1998, Nogueira et al., 2001, Cao et al., 1995). Compared with other kinds of charge transport materials, the gel electrolytes show relatively high ambient ionic conductivity (6–8 mS cm−1) and comparative stability. Therefore, several types of gel electrolytes have already been used in quasi-solid-state dye-sensitized solar cells (Stathatos and Lianos, 2003, Wang et al., 2004, Cao et al., 1995, Lan et al., in press). Especially those used nanocomposite organic–inorganic gel electrolytes such as using sol–gel silica to solidify liquid electrolytes, which have achieved high overall energy conversion efficiency (Stathatos et al., 2001, Stathatos et al., 2002, Stathatos et al., 2004a, Stathatos et al., 2004b, Stathatos and Lianos, 2003). These kinds of nanocomposite organic–inorganic gel electrolytes have some advantages. For example, (1) Since, at the initial stage the sol is fluid, it can penetrate into the titania nanoporous structure and ensure electrical conduct between titania nanocrystals and the electrolyte. (2) These gel electrolytes can give relatively similar environment for ionic movement with liquid electrolytes, so they can show high ambient ionic conductivity. (3) Liquid electrolytes can be held in the gel nets so some problems such as device instability caused by evaporation of liquid electrolytes can be overcome with these quasi-solid-state gel electrolytes.

In this paper, an organic iodide salt N-methyl pyridine iodide has been successfully synthesized and used as substitute of inorganic iodide salt to provide I source. Owing to N-methyl pyridine iodide is an organic iodide salt, it is easy to dissolve in organic solvents. On the other hand, N-methyl pyridine iodide has a large cation, I can easily separate from the molecule (Wakihara et al., 1998). Titanium isopropoxide is used as precursor of titanium gelator to solidify liquid electrolyte through sol–gel method. Glacial acetic acid is used as catalyzer for sol–gel route. γ-Butyrolactone (γ-BL) is used as low viscosity organic solvent. Further, a quasi-solid-state dye-sensitized solar cell was fabricated by sandwiching the gel electrolyte.

Section snippets

Materials

Titanium isopropoxide, Triton X-100, glacial acetic acid, pyridine, methyl iodide, iodine, γ-butyrolactone (γ-BL) were all AR grade and all purchased from Xi Long Chemicals. All reagents were used without further treating.

Conducting glass plates (FTO glass, fluorine doped tin oxide over-layer, sheet resistance 8 Ω cm−2, purchased from Hartford Glass Co., USA) were used as substrates for precipitating TiO2 porous films. The glass plates were cut into 2 × 1.5 cm2 sheets. Sensitizing dye cis-[(dcbH2)2

Influence of Triton X-100 concentration on the ionic conductivity of gel electrolyte

DSSCs cannot function without a sufficiently expanded organic subphase. This is due to two reasons: (1) ionic conductivity is made possible only through the organic subphase; (2) the organic subphase penetrates into the titania nanoporous structure and ensures electrical conduct with an optimal number of nanocrystallites. It has been found that among the most efficient protocols, the organic subphase is the mixture of appropriate solvent and polyether-chain-bearing non-ionic surfactants, like

Conclusions

A novel gel electrolyte was fabricated by using TiO2 gel to solidify liquid electrolyte which contained an organic iodide salt N-methyl pyridine iodide as I source. In order to fabricate high quality gel, appropriate amount of surfactant Triton X-100 was used and the influence of the amount of the surfactant on the ionic conductivity was also investigated.

A relatively high quality of gel can be obtained by adding 20 wt% Triton X-100 into the system. It can be concluded that the ionic

Acknowledgements

The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (No. 50082003, 50372022) and the Key Scientific Technology Program of Fujian Province, China (No. 2005HZ01-4).

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