Squaraine Dyes with Pyrylium and Thiopyrylium Components for Harvest of Near Infrared Light in Dye-Sensitized Solar Cells

Unsymmetrical squaraine dyes bearing pyrylium or thiopyrylium components were synthesized, and their light-absorbing properties and performance as sensitizer in dye-sensitized solar cells were investigated. Condensations of 2,6-di-tert-butyl-4-methylpyrylium and thiopyrylium salt with butyl squarate afforded corresponding semisquaryliums. The semisquaryliums were reacted with indolenium with carboxyl group to give target unsymmetrical dyes. They exhibited intense absorption in the far-red and near-infrared region in solution and on the TiO2 surface. Electrochemical measurements revealed that their oxidation potentials were moderately high although electron injection from the excited dyes to the TiO2 conduction band was thermodynamically permitted. Thus, pyrylium and thipyrylium components push absorbance into longer wavelength region as well as bring the highest occupied molecular orbitals (HOMOs) of these dyes in the upward level. A molecular-orbital calculation indicated that the electron distribution moved from the cyclobutene core to indolenine components bearing carboxy anchors upon photoexcitation. The dye-sensitized solar cells (DSSCs) based on these dyes exhibited moderate spectral response in the near-infrared region reflecting their absorption spectral features. However, their photovoltaic performance was far from state-of-the-art visible metal-free sensitizers owing to low open-circuit voltages and current density. These suppressed values might have originated from the high HOMO levels of these dyes, thus, pyrylium and thipyrylium components strongly affect not only the physical properties of the dyes but also the photovoltaic performances of DSSCs based on these dyes.