Influence of Number of Benzodioxan-Stilbazole-based Ancillary Ligands on Dye Packing, Photovoltage and Photocurrent in Dye-Sensitized Solar Cells

Two novel heteroleptic Ru(II) bipyridyl complexes, HD-2 and HD-2-mono, were molecularly engineered, synthesized and characterized for dye-sensitized solar cells (DSCs). The influences of mono versus bis electron-donor benzodioxan ancillary ligands on optical, dye packing, electrochemical and photovoltaic properties were examined and compared to the benchmark N719. HD-2 and HD-2-mono achieved solar-to-power conversion efficiencies (%η) of 9.64 and 9.50, respectively, compared to 9.32 for N719 under the same experimental device conditions. Optical results showed that HD-2 and HD-2-mono have much higher molar extinction coefficients, longer excited state lifetimes and narrower HOMO-LUMO gaps compared to N719. Although the molar extinction coefficient of HD-2-mono was 27% less than that of HD-2, it outperformed HD-2 in photovoltaic performance when anchored on TiO2, owing to better dye packing and loading of the former. Charge recombination at the dye/TiO2 interface by impedance spectroscopy analysis showed that the recombination resistance and the lifetime of injected electron in TiO2 conduction band is directly proportional to the open-circuit voltage (Voc) observed. Furthermore, compared to HD-2 and HD-2-mono, the greater Voc of N719 can be attributed to the greater negative free energy for dye regeneration. Both HD-2 and HD-mono have almost the same negative free energy, which explains why they achieved almost the same Voc. Decay dynamic analysis for solar devices fabricated from the named dyes, by time correlated single photon counting (TCSPC), elucidated that the lowest excited state decay lifetime for HD-2-mono, HD-2 and N719 are 3, 10 and 20 ps, respectively. The shorter the decay lifetime, the less kinetic redundancy, which leads to better photocurrent, and that explanation is consistent with the measured photocurrent and total solar-to-power conversion efficiency of the named dyes in the order of HD-2-mono > HD-2 > N719.