Donor-acceptor dyes incorporating a stable dibenzosilole π-conjugated spacer for dye-sensitized solar cells

Four novel organic dyes including three based on dibenzosilole (YS01-03) and one based on fluorene (YS04) were synthesized, and their photophysical properties and dye-sensitized solar cell (DSC) performances were characterized. The silicon-containing dibenzosilole-based dyes (YS01-03) were superior to the carbon analogue fluorene-based dye YS04 in incident-photon-to-current conversion efficiency (IPCE), and total solar-to-electric conversion efficiency (eta), with YS03, which has the bulkiest and most branched electron donor group, achieving the highest eta of 5.07% compared to 2.88% of YS04. To better understand how silicon influences the excited state oxidation potentials (S+/*) and absorption maxima (lambda max), the equilibrium molecular geometries of dyes YS01-04 were calculated using density functional theory (DFT) utilizing B3LYP energy functional and DGDZVP basis set. It was shown that the torsion angles (theta 1 and theta 2) across the biphenyl linkages of dyes containing silicon (YS01-03) were less twisted than that of the silicon-free dye (YS04), which enhanced the pi-pi* overlap, and that translated into photocurrent enhancements in the silicon-containing dyes YS01-03. Moreover, the vertical electronic excitations and S+/* of dyes YS01-04 were studied using different long-range corrected time-dependent DFT methods, including CAM-B3LYP, LC-BLYP, WB97XD, and LC-wPBE at the basis set level DGDZVP. Excellent agreement between the calculated, using CAM-B3LYP/DGDZVP, and experimental results was found.