Highly Efficient Dye-sensitized Solar Cells
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Highly Efficient Dye-sensitized Solar Cells Liyuan Han, Ashraful Islam Photovoltaic Materials Unit, National Institute for Materials Science 1-2-1 Sengen Tsukuba, Ibaraki, 305-0047, Japan
ABSTRACT The present paper discusses the principle of dye-sensitized solar cells (DSCs) in terms of equivalent circuit model and the key issues to improve the device efficiency. Equivalent circuit model is proposed following analysis by electrochemical impedance spectroscopy of the voltage dependence of the internal resistance elements of DSCs. The influence of these elements upon cell performance in areas such as short circuit current density (Jsc), open circuit voltage (Voc), and fill factor (FF) was examined based on the equivalent circuit. Efficient sensitization of nanocrystalline TiO2 film was observed across the whole visible range and into the near-IR region as far as 1000 nm with a new panchromatic substituted β-diketonato Ru(II)-terpyridine dye (HIG1). Introduction of bulky alkyl substituent group in a β-diketonato Ru(II)-terpyridine dye (A3) suppress aggregate formation result in an improved performance of DSCs and the performance is independent of the additive added during the dye adsorption process. The haze factor of TiO2 electrodes is a useful index when fabricating light-confined TiO2 electrodes to improve Jsc. It was demonstrated that blocking of bare TiO2 surface with small molecules is an effective way of suppress interfacial charge recombination at the TiO2-dye/electrolyte interface and of improving shunt resistance and Voc. FF was also improved by reduction of the internal series resistance, which is composed of the following three elements: the redox reaction resistance at the platinum counter electrode, the resistance of carrier transport by ions in the electrolyte, and resistance due to the sheet resistance of the transparent conducting oxide. Finally, the highest efficiency scores of 10.4% and 11.1% (aperture illumination area 1.004cm2 and 0.219cm2, respectively) were confirmed by a public test center. INTRODUCTION Dye-sensitized solar cells (DSCs) are promising next-generation alternatives to conventional silicon-based photovoltaic devices owing to DSCs’ low manufacturing cost [1-3]. As shown in Figure 1(a), DSCs generally comprise a dye-sensitized nanocrystalline titanium dioxide (TiO2) electrode fabricated on transparent conducting oxide (TCO) glass, a platinum (Pt) counter electrode, and an electrolyte solution containing dissolved iodide/tri-iodide ions as a redox couple between the electrodes. The mechanism is thought to be as follows: electrons are injected from the photoexcited dye into the conduction band of the semiconductor oxide, from where they pass through the nanoparticles to the transparent conducting oxide (TCO) current collector and into the external circuit. The sensitizer is regenerated by electron transfer from a donor, typically iodide ions, which are dissolved in the electrolyte that is present in the porous semiconductor. The triiodide ions formed during the reaction diffuse to the
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