Elsevier

Solar Energy

Volume 80, Issue 2, February 2006, Pages 209-214
Solar Energy

Natural dyes as photosensitizers for dye-sensitized solar cell

https://doi.org/10.1016/j.solener.2005.05.009Get rights and content

Abstract

The dye-sensitized solar cells (DSC) were assembled by using natural dyes extracted from black rice, capsicum, erythrina variegata flower, rosa xanthina, and kelp as sensitizers. The ISC from 1.142 mA to 0.225 mA, the VOC from 0.551 V to 0.412 V, the fill factor from 0.52 to 0.63, and Pmax from 58 μW to 327 μW were obtained from the DSC sensitized with natural dye extracts. In the extracts of natural fruit, leaves and flower chosen, the black rice extract performed the best photosensitized effect, which was due to the better interaction between the carbonyl and hydroxyl groups of anthocyanin molecule on black rice extract and the surface of TiO2 porous film. The blue-shift of absorption wavelength of the black rice extract in ethanol solution on TiO2 film and the blue-shift phenomenon from absorption spectrum to photoaction spectrum of DSC sensitized with black rice extract are discussed in the paper. Because of the simple preparation technique, widely available and low cheap cost natural dye as an alternative sensitizer for dye-sensitized solar cell is promising.

Introduction

Dye-sensitized TiO2 solar cell (DSC) has become an attractive and cheap device for the conversion of solar light into electrical energy since Grätzel and O’Regan firstly reported the prototype of this solar cell in 1991 (O’Regan and Gratzel, 1991). DSC is assembled with an anode of conductive glass coated with platinum, a cathode of TiO2 porous film on a conductive glass substrate anchored a monolayer of dyes, and an electrolyte of certain organic solvent containing a redox couple, such as iodide/triiodide. Many efforts have focused on sensitizer dyes, since dye plays a key role in harvesting sunlight and transferring solar energy into electric energy. So far, several organic dyes and organic metal complexes have been employed to sensitize nanocrystalline TiO2 semiconductors, and one of the most efficient sensitizer is transition metal coordination compound (ruthenium polypyridyl complex). This is because the complex has intense charge-transfer (CT) absorption in the whole visible range, long excited lifetime and highly efficient metal-to-ligand charge transfer (MLCT). However, other organic dyes, such as phthalocyanine, cyanine dyes, xanthene dyes, coumarin dyes and so on, usually perform poorly in DSC because of weak binding energy with TiO2 film and low charge-transfer absorption in the whole visible range, but these dyes are very cheap and prepared easily, comparing to ruthenium polypyridyl complexes.

In nature, the fruit, flower and leaf of plants show various color from red to purple and contain various natural dyes which can be extracted by simple procedure. It has been emphasized by many researches to obtain useful dyes as photosensitizers for DSC from natural products, some natural dyes as sensitizers of DSC have been reported (Tennakone et al., 1997, Grätzel, 2003, Smestad, 1998, Dai and Rabani, 2002), but the efficiency of the natural dye sensitizers is dissatisfied. In this paper, the natural dyes were extracted from black rice, capsicum, erythrina variegata flower, rosa xanthina, and kelp, the interaction between the dye molecule (the core molecular groups of natural dyes) and TiO2 porous film was investigated. It was found that the black rice extract performed the best photosensitized effect in the extracts of natural fruit, leaves and flower chosen. Because of the simple preparation technique, widely source, and cheap cost, natural dye as an alternative sensitizer for dye-sensitized solar cell is promising.

Section snippets

Materials

Conductive glass plates (FTO glass, fluorine-doped SnO2 over-layer, sheet resistance 8 Ω/cm2, made by Beijing Building Material Factory) were used as a substrate for precipitating TiO2 porous film on and were cut into 2 × 1.5 cm2 sheets. TiO2 powder (P-25) was purchased from Degussa AG. All other reagents (from Xilong Chemicals) were used without further purification.

Preparation of natural dye sensitizers

The alcohol extracts of black rice, erythrina variegata flower, rosa xanthina flower, capsicum and kelp were obtained according to

Absorption and molecular structure of natural dye

Fig. 1 shows the UV–VIS absorption spectra of black rice, capsicum, erythrina variegata flower, rosa xanthina and kelp extracts in alcohol. From Fig. 1, it can be seen that there is the same absorbent peak in about 560 nm for the extracts of black rice, erythrina variegata flower and rosa xanthina, which is ascribed to the same dye cores (anthocyanin) contained in the three fruits extracts. Anthocyanin is the core compositions of some natural dye and often is found in fruits, flowers and leaves

Conclusion

Natural dyes were extracted from black rice, capsicum, erythrina variegata flower, rosa xanthina, and kelp. Using the natural dyes extracts as photosensitizers, some dye-sensitized solar cells (DSC) were assembled, and the ISC from 1.142 mA to 0.225 mA, the VOC from 0.551 V to 0.412 V, the fill factor from 0.52 to 0.63, and Pmax from 58 μW to 327 μW were obtained from the DSCs. Based on investigation on the structure and properties of dye molecules, it was found that black rice extract possesses the

Acknowledgements

The authors thank for jointly supporting by the National Natural Science Foundation of China (No. 50082003, No. 50372022) and the Provincial Natural Science Foundation of Fujian, China (No. 2001I006, No. E0210023).

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