(1)Netherlands Energy Research Foundation ECN
Westerduinweg 3, NL 1755 LE Petten, Netherlands
(2)Freiburg Materials Research Center FMF
Stefan-Meier-Str. 21, D 79104 Freiburg
(3)Solaronix SA
Rue de L’Ouriette 129, CH 1170 Aubonne
(4)Institut für Angewandte Photovoltaik INAP
Munscheidstr. 14, D 45886 Gelsenkirchen
Introduction
In the course of an EU project, the long-term stability of dye-sensitized electrochemical TiO2 solar cells (DSC) is investigated. Accelerated tests are performed in order to determine the life-time of DSC and in order to identify possible degradation mechanisms.
Experimental
For sophisticated investigations and in order to get a sufficient amount of statistical representative data, DSC were processed on so-called masterplates: five individual cells of 5 x 0.8 cm2 on a glass substrate of 7.5 x10 cm2. This allowed to produce several relative identical cells: The deviations of the cell parameters (ISC, VOC, FF, h) within one masterplate were within ± 10 %.
Special test stands were developed which allow the simultanous and continuous operation of up to 16 masterplates, i.e. 80 individual DSC, and the in-situ characterisation of their electrical performances. Electrical data of the different cells are recorded weekly. There are test stands with a sulphur lamp, and others with a UV light source. The aim was to separate the influence of the UV light from the visible light: The light of the sulphur lamp is nearly free of UV light. Its intensity corresponds to one sun equivalent intensity. As the homogeneity and stability of the sulphur lamps is sufficient, the masterplates are operated and electrically characterized within that test stands for long time periods.
The influence of temperature is investigated by exposing the cells to the different temperatures in different ovens, i.e. in the dark. These cells are also electrically characterized periodically.
Thus, the influence of the different degradation mechanisms (continous operation under visible light, UV light, temperature) can be investigated separately.
Results and discussion
First results showed – as expected – a strong influence of high temperatures on degradation (Fig. 1). This indicates that applying higher temperatures might be a suitable method for an accelerated aging method in order to predict the life-time of a DSC. An Arrhenius-like behaviour is observed for the influence of the temperature exposure on the cell life-time. However, the strong degradation at 60 °C might be due to sealing problems. Using cells with better sealing and other chemical composition, degradation was minimal for 30 days of testing at 60 °C, which is a very encouraging result.
Further results also showed the importance of the chemical composition of a DSC on its long-term stability. Even water can be tolerated to some extend within a DSC, as long as the additive 4-tert-butylpyridine (TBP) is present [1], [2]. TBP is a molecule attaching to the surface of TiO2 and probably shields the TiO2-surface which is not covered with dye. Thus unwanted side-reactions might be suppressed.
According to our experiments, the dye seems to be quite stable. Cells with certain solvents, used up to now for the redox electrolyte, turned out not to be stable on a longer time scale (Fig. 2). Also the purity of the solvent seems to be important, as different life-times are obtained with solvents from different producers. With suitable components, life-times under continuous irradiation of up to several thousands of hours were already reached, corresponding to several years of incident solar energy. Further improvements should be possible.
Modelling is necessary in order to extrapolate short-term measurement results, obtained within several months, onto a longer time scale and to predict the life-time of a DSC. With different degradation factors, different effects are observed: linear-dose effects like the influence of UV light, Arrhenius behaviour with respect to the temperature, etc. Different aging models are discussed, which could constitute a basis for the extrapolation of short-term measurement in order to get information on the long-term stability of dye-sensitized photovoltaic cells.
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Fig. 1: Short-circuit current, normed at its initial value, for three different DSC (solvent methoxyacetonitrile) which were aged under different temperatures. The results shown are representative ones for measurements on 5 identical cells. |
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Fig. 2: Short-circuit current, normed at its initial value, for three different DSC which were continuously operated under sulphur light. One curve is a typical result for a series of cells with acetonitrile (ACN) as the solvent for its redox electrolyte, the others are typical for series of cells filled with methoxyacetonitrile (MACN) from two different producers. The operation temperature was about 45 °C. |
References
[1] E. Rijnberg, J.M. Kroon et al., "Long-term stability of Nanocrystalline Dye-sensitized Solar Cells", Proc. of the 2nd World Conf. and Exhibition on PV Sol. En. Conv., Vienna 1998, p. 47-52
[2] R. Kern, N. van der Burg, G. Chmiel, J. Ferber, G. Hasenhindl, A. Hinsch, R. Kinderman, J. Kroon, A. Meyer, T. Meyer, R. Niepmann, J. van Roosmalen, C. Schill, P. Sommeling, M. Späth, I. Uhlendorf, "Long-term stability of Dye-sensitized Solar Cells for large area power applications", Proc. Conf. Cracow, 1999
email: ferber@fmf.uni-freiburg.de