Performance and stability of TiO2/dye solar cells assembled with flexible electrodes and a polymer electrolyte

doi:10.1016/S1010-6030(03)00106-0

Journal of Photochemistry and Photobiology A: Chemistry

Volume 159, Issue 1, 30 June 2003, Pages 33-39

https://doi.org/10.1016/S1010-6030(03)00106-0 Get rights and content

Abstract

Solid-state, flexible TiO₂/dye solar cells were assembled using flexible electrodes, a polymer electrolyte with I⁻/I₃⁻ and a Pt coated counter-electrode. The efficiency of the cells was enhanced when the plastic electrodes coated with TiO₂ were exposed to UV radiation, followed by heating at 140 °C in dry conditions. For comparison, a similar cell was prepared by the same procedure but using glass electrodes. The performance of these cells was investigated during a period of 50 days by current–potential and electrochemical impedance spectroscopy measurements. The flexible, solid-state TiO₂/dye solar cells (1 cm²) presented an open circuit potential of 0.72 V, short-circuit current of 60 μA cm⁻² and an efficiency of 0.32% under a light intensity of 10 mW cm⁻². This efficiency was maintained until the fourth day after assembling and decayed to 0.17% on the 14th day, remaining constant until the 40th day and decreasing to 0.13% on the 50th day. Impedance spectroscopy revealed that the series resistance increased with time, lowering the cell efficiency. This effect was not so evident for cells assembled with glass electrodes. Therefore, the flexible electrode limits the preparation of the porous TiO₂ photoelectrode and creates a large series resistance in the solar cell. However, these results are very promising for developing solar cells with lower costs and broader applicabilities.

Introduction

The TiO₂/dye solar cell consists of a dye-sensitized nanoparticulated TiO₂ film, which is usually deposited onto a transparent glass electrode, an electrolyte containing the I⁻/I₃⁻ redox couple and a Pt coated counter electrode (Grätzel’s solar cell) [1]. The energy conversion results from the injection of electrons from the photoexcited state of the sensitizer into the conduction band of the semiconductor. The redox couple in the electrolyte, after rereducing the oxidized dye, can be renewed in the counter-electrode, making the photoelectrochemical cell regenerative [1], [2].

Several attempts have been made to replace the liquid electrolyte in these systems, since leakage or evaporation of the solvent limits the long-term stability of the cells. In most studies, solid-state TiO₂/dye solar cells were assembled using the I⁻/I₃⁻ redox couple dissolved in a polymer gel medium [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. In our Laboratory, we have been investigating solar cells assembled with a polymer electrolyte based on poly(ethylene oxide-co-epichlorohydrin), poly(EO-EPI). The best efficiency for energy conversion (global efficiency, η_global) that we have obtained for an all solid-state TiO₂/dye solar cell (with an active area of 1 cm²) was η_global=2.6% under 10 mW cm⁻². Thus, its efficiency was lower than that obtained for cells assembled with a liquid electrolyte, probably due to the low conductivity of the polymer electrolyte and the reduced mobility of I₃⁻ in such a media. However, this polymer electrolyte makes the assembly of the cell much easier, since it also acts as an adhesive between the electrodes and no sealing is required, providing a possible application of these cells for low illumination conditions [13], [14].

In parallel with the studies concerning the electrolyte, the use of plastic electrodes for assembling flexible TiO₂/dye solar cells is also under investigation [15], [16], [17], [18], [19], [20]. Flexible electrodes, like poly(ethylene terephthalate) coated with tin-doped indium oxide (PET-ITO), present lower costs and technological advantages relative to glass-ITO electrodes, e.g. lower weight, impact resistance and less form and shape limitations. However, deposition of nanoparticulated TiO₂ on PET-ITO is difficult, because the thermal treatment must be limited to 150 °C, decreasing adhesion, electrical contact between the particles and adsorption of the dye. This affects the efficiency of the cell, particularly when using a polymer electrolyte [19], [20].

In an earlier study, we investigated a flexible, solid-state TiO₂/dye solar cell assembled with the polymer electrolyte poly (EO-EPI) and PET-ITO electrodes [20]. Its efficiency was η_global=0.22% under 10 mW cm⁻² (η_global=0.12% under 100 mW cm⁻²), almost 10 times lower than that obtained for solid-state solar cells assembled with the same polymer electrolyte using glass electrodes. Also, electrochemical impedance spectroscopy (EIS), measurements revealed a larger series resistance and a smaller diffusion coefficient for the ionic species for the flexible cells [20]. Both effects could be caused, to some extent, by organic residues (from the TiO₂ suspension), which cannot be eliminated using low temperatures.

In this work we demonstrate that UV radiation is a suitable method for removing organic residues from the TiO₂ film, considering the well-known activity of TiO₂ toward photodegradation of organic compounds [21]. This study also reports on the stability of flexible and solid-state TiO₂/dye solar cells, investigated by current–potential and EIS measurements by irradiating for 50 days.

Section snippets

Experimental section

Photoelectrochemical cells were assembled using transparent flexible PET-ITO electrodes (Innovative Sputtering Technology, 60 Ω cm) and glass-ITO electrodes (Delta Technology, 20 Ω cm) as substrates for the photoelectrode and counter-electrode. All solutions were prepared using p.a. grade reagents. Counter electrodes (CE) were prepared by sputter depositing a thin Pt film on plastic or rigid electrodes. For preparation of photoelectrodes, a small aliquot of TiO₂ suspension (Ti nanoxide-T, Solaronix

Results and discussion

The suspension used to prepare the TiO₂ films contains different organic ingredients, which must be removed to improve the photoactivity. When using glass electrodes, these impurities are removed by firing the electrode at 400–500 °C. With PET substrates this is not possible. Thus, our strategy was to use the ability of anatase TiO₂ to degrade organic compounds to remove such contaminants. Exposition to UV radiation can act upon the properties of TiO₂ films deposited onto flexible PET-ITO

Conclusion

The performance and the stability of solid-state, flexible solar cells assembled with flexible TiO₂ photoelectrodes treated with UV radiation were investigated over a period of 50 days. UV radiation is a fast and low energy consuming method that aided the preparation of TiO₂ films onto flexible PET-ITO electrodes, allowing degradation of the surfactants commonly used to prepare the TiO₂ suspensions. This treatment, followed by heating at 140 °C in dry conditions (2 h), resulted in mechanically

Acknowledgements

Authors acknowledge financial support from FAPESP (fellowships 00/03086-3 and 01/02454-1), PRONEx/CNPq, and thank Daiso Co. Ltd. Osaka, Japan, for providing the polymer electrolyte.

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Performance and stability of TiO2/dye solar cells assembled with flexible electrodes and a polymer electrolyte

Abstract

Introduction

Section snippets

Experimental section

Results and discussion

Conclusion

Acknowledgements

J. Photochem. Photobiol. A: Chem.

Sol. Energy Mater. Sol. Cells

Sol. Energy Mater. Sol. Cells

J. Photochem. Photobiol. A: Chem.

J. Photochem. Photobiol. A: Chem.

Electrochim. Acta

Solid State Ion.

Nature

J. Am. Chem. Soc.

J. Phys. Chem.

Chem. Mater.

J. Appl. Electrochem.

Performance and stability of TiO₂/dye solar cells assembled with flexible electrodes and a polymer electrolyte