Significant Improvement of Dye-Sensitized Solar Cell Performance by Small Structural Modification in π-Conjugated Donor-Acceptor Dyes

Two donor-p-acceptor (D-p-A) dyes are synthesized for application in dye-sensitized solar cells (DSSC). These D-p-A sensitizers use triphenylamine as donor, oligothiophene as both donor and p-bridge, and benzothiadiazole (BTDA)/cyanoacrylic acid as acceptor that can be anchored to the TiO2 surface. Tuning of the optical and electrochemical properties is observed by the insertion of a phenyl ring between the BTDA and cyanoacrylic acid acceptor units. Density functional theory (DFT) calculations of these sensitizers provide further insight into the molecular geometry and the impact of the additional phenyl group on the photophysical and photovoltaic performance. These dyes are investigated as sensitizers in liquid-electrolyte-based dye-sensitized solar cells. The insertion of an additional phenyl ring shows significant influence on the solar cells' performance leading to an over 6.5 times higher efficiency (? = 8.21%) in DSSCs compared to the sensitizer without phenyl unit (? = 1.24%). Photophysical investigations reveal that the insertion of the phenyl ring blocks the back electron transfer of the charge separated state, thus slowing down recombination processes by over 5 times, while maintaining efficient electron injection from the excited dye into the TiO2-photoanode.