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Controlling Adsorption Structure of Eosin Y Dye on Nanocrystalline TiO Films for Improved Photovoltaic Performances 2
Fan Zhang, Feng Shi, Wei Ma, Fei Gao, Yang Jiao, Hui Li, Jingchuan Wang, Xinyan Shan, Xinghua Lu, and Sheng Meng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp404439p • Publication Date (Web): 21 Jun 2013 Downloaded from http://pubs.acs.org on June 24, 2013
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The Journal of Physical Chemistry
Controlling Adsorption Structure of Eosin Y Dye on Nanocrystalline TiO2 Films for Improved Photovoltaic Performances Fan Zhang, Feng Shi, Wei Ma, Fei Gao, Yang Jiao, Hui Li, Jingchuan Wang, Xin-Yan Shan, Xinghua Lu, and Sheng Meng* Beijing National Laboratory for Condensed Matter Physics, and Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, P. R. China
* To whom correspondence should be addressed. Tel: + 86-10-82649396. E-mail:
[email protected].
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The Journal of Physical Chemistry
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ABSTRACT: Adsorption structure of Eosin Y dyes on nanocrystalline TiO2 can be manipulated by adding a small fraction of water into organic electrolyte. Binding mode switching from hydrogen bonded monodentate to bidentate bridging configuration has been observed and confirmed by Raman and infrared spectroscopy measurements, with vibration peaks assigned using density functional theory calculations. Photovoltaic measurements on the fabricated dye-sensitized solar cells indicate that energy conversion efficiency is enhanced by manipulating molecular adsorption configuration of Eosin Y dyes. This opens a new avenue to improving photovoltaic performance by suppressing unfavorable adsorption configurations in dye solar cell devices. Keywords: Dye adsorption, Molecular configuration, Raman spectroscopy, Dye-sensitized solar cells, Vibration analysis
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The Journal of Physical Chemistry
INTRODUCTION The dye-sensitized solar cell (DSC), invented by Prof. M. Grätzel at École Polytechnique Fédérale de Lausanne in 19911, is a promising route towards low-cost, environment friendly power generation, which uses dye molecules adsorbed on the nanocrystalline oxide semiconductors such as TiO2 to collect sunlight. In a DSC, light absorption (by dyes) and charge collection (by semiconductors) processes are separated, mimicking natural light harvest in photosynthesis. Only when dye molecules effectively bind to TiO2 surface can the following processes such as charge injection and transportation proceed with high efficiency. For the majority of dyes, a carboxyl group is employed as an effective anchor through which dyes bind onto TiO2 surfaces. It is well known that the adsorption geometry has a remarkable influence on photovoltaic parameters of DSC such as the open circuit voltage and short circuit current. De Angelis et al. investigated the adsorption configuration of most popular Ru-complex N-719 dyes on TiO2 and showed that dipole moment orientation of the sensitizers resulted from different binding configurations, can lead to as large as a 0.61 eV shift in TiO2 conduction band edge (CBE), introducing a larger open circuit voltage (Voc)2. Jiao et al. studied all organic cyanoacrylic dyes and concluded that the adsorption configuration with Ti-N binding is beneficial to electron injection, which improves short circuit current3. However, the precise interface structures upon dye adsorption on TiO2 have been under heavy debate for decades with controversial conclusions. Taking N-719 dyes as an example, three adsorption configurations were proposed: monodentate, bidentate chelating and bidentate bridging modes4. To find out which one dominate in real systems, Grätzel et al. investigated Ru complex dyes by Fourier Transform Infrared (FTIR) spectroscopy according to the Deacon-Philips rule5. This rule introduces an important parameter Δν, which is the frequency splitting of surface bound carboxylate stretching bands―asymmetric and symmetric vibrations, measured by infrared (IR) or Raman spectroscopy. Two cases are compared: Δν for dyes in solid state, Δν(solid), and Δν for adsorbed dyes, Δν(ads). If Δν(ads) > Δν(solid), the dye molecule takes a monodentate binding mode; if Δν(ads) < Δν(solid), the bidentate bridging mode is more preferred; if Δν(ads)
