Molecular engineering of cyanine dyes to design a panchromatic response in co-sensitized dye-sensitized solar cells

Abstract

Cyanines are optically tunable dyes with high molar extinction coefficients, suitable for applications in co-sensitized dye-sensitized solar cells (DSCs); yet, barely thus applied. This might be due to the lack of a rational molecular design strategy that efficiently exploits cyanine properties. This study computationally re-designs these dyes, to broaden their optical absorption spectrum and create dye⋯TiO₂ binding and co-sensitization functionality. This is achieved via a stepwise molecular engineering approach. Firstly, the structural and optical properties of four parent dyes are experimentally and computationally investigated: 3,3′-diethyloxacarbocyanine iodide, 3,3′-diethylthiacarbocyanine iodide, 3,3′-diethylthiadicarbocyanine iodide and 3,3′-diethylthiatricarbocyanine iodide. Secondly, the molecules are theoretically modified and their energetics are analyzed and compared to the parent dyes. A dye⋯TiO₂ anchoring group (carboxylic or cyanoacrylic acid), absent from the parent dyes, is chemically substituted at different molecular positions to investigate changes in optical absorption. We find that cyanoacrylic acid substitution at the para-quinoidal position affects the absorption wavelength of all parent dyes, with an optimal bathochromic shift of ca. 40 nm. The theoretical lengthening of the polymethine chain is also shown to effect dye absorption. Two molecularly engineered dyes are proposed as promising co-sensitizers. Corresponding dye⋯TiO₂ adsorption energy calculations corroborate their applicability, demonstrating the potential of cyanine dyes in DSC research.