Parameter Study on UV-induced Degradation of Dye-sensitized Solar Cells
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Parameter Study on UV-induced Degradation of Dye-sensitized Solar Cells Katrine Flarup Jensen1, 2, Welmoed Veurman1, Henning Brandt1, Chan Im2, Jürgen Wilde3, Andreas Hinsch1,* 1 Fraunhofer Institute for Solar Energy Systems (ISE), Heidenhofstr. 2, 79110 Freiburg, Germany 2 Konkuk University-Fraunhofer Next Generation Solar Cell Research Center (KFnSC), 120 Neungdong-ro, Gwangjin-gu, Seoul 143-701, Korea 3 Department of Microsystems Engineering – IMTEK, University of Freiburg, Georges-KöhlerAllee 103, 79110 Freiburg, Germany ABSTRACT The present work investigates the UV stability of the dye-sensitized solar cell (DSC) by parametrical investigation of the material influence on UV stability. UV illumination has been observed to cause degradation by slow photocatalysis in the DSC. Photooxidized impurities represent an unwanted side reaction with the redox pair of the electrolyte as the released electron will deplete the triiodide concentration. A study on the DSC cell was carried out with intermediate electrical characterization by cyclic voltammetry (CV) and electrical impedance spectroscopy (EIS) to map the influence of UV illumination as a function of the H2O concentration in the electrolyte, the plate distance and the triiodide concentration. The results show that the H2O content has a detrimental influence on the DSC stability during UV illumination. A higher concentration of triiodide can buffer the reaction with impurities, so that a longer-term stability is achieved. A recovery of triiodide in UV aged cells with either no remaining triiodide or with such a low concentration that the cell current has been diffusionlimited, was seen during CV to -0.75 V under illumination. The reappearance of triiodide was accompanied with a production of hydrogen bubbles, which was related to the H2O content in the electrolyte and the exposure to UV. Our approach can be used to test the purity and the UV stability of various electrolytes. INTRODUCTION The dye-sensitized solar cell (DSC), figure 1, imitates the photosynthesis of nature by the principle of a light-absorbing dye chemically adsorbed on mesoscopic TiO2 [1,2]. The charge separation, as induced by the photoexcited electronic state, occurs at the interface between the dye and the mesoporous semiconducting TiO2. The electron is then transported in the TiO2 to the TCO from where it enters the external circuit. The photo-oxidized dye injects an electronic hole into the electrolyte which in the form of I3- diffuses to the counter electrode. I3- is reduced back to I- at the catalytically active counter electrode with electrons supplied from the external circuit. UV illumination has been observed to cause degradation by slow photocatalysis in the DSC which is caused by a reaction between triiodide and oxidized impurities by e.g. formation of iodate [3-6]. The photooxidized impurities represent an unwanted side reaction with the redox pair of the electrolyte as the released electron will deplete the triiodide concentration through chemical reduction.
The objective of our wor
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