Molecular Engineering of D–D−π–A-Based Organic Sensitizers for Enhanced Dye-Sensitized Solar Cell Performance

A series of molecularly engineered and novel dyes WS1, WS2, WS3, and WS4, based on the D35 donor, 1-(4-hexylphenyl)-2,5-di­(thiophen-2-yl)-1<i>H</i>-pyrrole and 4-(4-hexylphenyl)-4<i>H</i>-dithieno­[3,2-<i>b</i>:2′,3′-<i>d</i>]­pyrrole as π-conjugating linkers, were synthesized and compared to the well-known LEG4 dye. The performance of the dyes was investigated in combination with an electrolyte based on Co­(II/III) complexes as redox shuttles. The electron recombination between the redox mediators in the electrolyte and the TiO₂ interface decreases upon the introduction of 4-hexylybenzene entities on the 2,5-di­(thiophen-2-yl)-1<i>H</i>-pyrrole and 4<i>H</i>-dithieno­[3,2-<i>b</i>:2′,3′-<i>d</i>]­pyrrole linker units, probably because of steric hindrance. The open circuit photovoltage of WS1-, WS2-, WS3-, and WS4-based devices in combination with the Co­(II/III)-based electrolyte are consistently higher than those based on a I^–/I₃^– electrolyte by 105, 147, 167, and 75 mV, respectively. The WS3-based devices show the highest power conversion efficiency of 7.4% at AM 1.5 G 100 mW/cm² illumination mainly attributable to the high open-circuit voltage (<i>V</i>_OC)