Improving energy transfer efficiency of dye-sensitized solar cell by fine tuning of dye planarity

Two push–pull metal-free sensitizers with 5,11-dihydroindolo[3,2-b]carbazole derivatives as electron-donating groups and 4-(benzo[c][1,2,5]thiadiazol-4-ylethynyl)benzoic acid (BTZ) as electron-withdrawing unit, denoted by SK201 and SK202, were synthesized and used for fabrication of dye-sensitized solar cells (DSSCs). SK202 contains a thienyl group between the donor and acceptor, whereas in SK201 the donor and acceptor are connected directly by a single bond. Introduction of a thienyl group improved the planarity of the dye molecule, broadened the absorption spectrum, enhanced the molar extinction coefficient, increased the dye loading on TiO₂, and accelerated interface electron transfer on TiO₂. This fine tuning of dye structure improved the performances of DSSCs based on SK202 sensitizers and gave a power conversion efficiency (PCE) of 11.0% (J_sc 16.5 mA cm⁻², V_oc 932 mV, and fill factor 71.7%), compared with that of 7.2% for SK201, under standard AM1.5G solar irradiation (100 mW cm⁻²) with a Co(II/III) complex based redox couple.