Molecular Design of Unsymmetrical Squaraine Dyes for High Efficiency Conversion of Low Energy Photons into Electrons Using TiO2 Nanocrystalline Films

Authors

  • Thomas Geiger,

    Corresponding author
    1. Empa, Swiss Federal Laboratories for Materials Testing and Research Laboratory for Functional Polymers Überlandstrasse 129, 8600 Dübendorf (Switzerland)
    • Empa, Swiss Federal Laboratories for Materials Testing and Research Laboratory for Functional Polymers Überlandstrasse 129, 8600 Dübendorf (Switzerland).
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  • Simon Kuster,

    1. Empa, Swiss Federal Laboratories for Materials Testing and Research Laboratory for Functional Polymers Überlandstrasse 129, 8600 Dübendorf (Switzerland)
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  • Jun-Ho Yum,

    1. Laboratory for Photonics and Interfaces Institute of Chemical Sciences and Engineering School of Basic Sciences, Swiss Federal Institute of Technology 1015 Lausanne (Switzerland)
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  • Soo-Jin Moon,

    1. Laboratory for Photonics and Interfaces Institute of Chemical Sciences and Engineering School of Basic Sciences, Swiss Federal Institute of Technology 1015 Lausanne (Switzerland)
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  • Mohammad K. Nazeeruddin,

    1. Laboratory for Photonics and Interfaces Institute of Chemical Sciences and Engineering School of Basic Sciences, Swiss Federal Institute of Technology 1015 Lausanne (Switzerland)
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  • Michael Grätzel,

    1. Laboratory for Photonics and Interfaces Institute of Chemical Sciences and Engineering School of Basic Sciences, Swiss Federal Institute of Technology 1015 Lausanne (Switzerland)
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  • Frank Nüesch

    1. Empa, Swiss Federal Laboratories for Materials Testing and Research Laboratory for Functional Polymers Überlandstrasse 129, 8600 Dübendorf (Switzerland)
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Abstract

An optimized unsymmetrical squaraine dye 5-carboxy-2-[[3-[(2,3-dihydro-1, 1-dimethyl-3-ethyl-1H-benzo[e]indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-3,3-dimethyl-1-octyl-3H-indolium (SQ02) with carboxylic acid as anchoring group is synthesized for dye-sensitized solar cells (DSCs). Although the π-framework of SQ02 is insignificantly extended compared to its antecessor squaraine dye SQ01, photophysical measurements show that the new sensitizer has a much higher overall conversion efficiency η of 5.40% which is improved by 20% when compared to SQ01. UV-vis spectroscopy, cyclic voltammetry and time dependent density functional theory calculations are accomplished to rationalize the higher conversion efficiency of SQ02. A smaller optical band gap including a higher molar absorption coefficient leads to improved light harvesting of the solar cell and a broadened photocurrent spectrum. Furthermore, all excited state orbitals relevant for the ππ* transition in SQ02 are delocalized over the carboxylic acid anchoring group, ensuring a strong electronic coupling to the conduction band of TiO2 and hence a fast electron transfer.

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