Molecular engineering of largely π-extended metal-free sensitizers containing benzothiadiazole units: Approaching 10% efficiency dye-sensitized solar cells using iodine-based electrolytes

Based on the structural motif of a robust organic dye, new largely π-extended organic sensitizers for dye-sensitized solar cells (DSSCs) have been designed to optimize the interactions between the dye/TiO2 layer and the redox electrolyte. The molecular tailoring was performed by i) positioning an alkyl chain onto the π-spacer and ii) increasing the bulkiness of the electron-donor. A deep theoretical investigation reveals that the main electronic transitions of the three dyes own only a weak charge-transfer character. Notwithstanding these unusual photo-excitation dynamics experimentally supported by solvatochromism, impressive photovoltaic performances are obtained for the three sensitizers, reaching power conversion efficiencies up to 9.8% under 1.0 sun illumination, that were remarkably higher than those shown by N719 standard. This result represents one of the highest performances exhibited by fully organic sensitizers employing the I-/I3- redox shuttle.