Efficiency-Limiting Processes In Cyclopentadithiophene-Bridged Donor-Acceptor-Type Dyes For Solid-State Dye-Sensitized Solar Cells

The charge generation and recombination processes in three novel push-pull photosensitizers for dye-sensitized solar cells (DSSCs) are studied by ps-mu s transient absorption (TA) and quasi-steady-state photoinduced absorption (PIA) spectroscopy. The three cyclopentadithiophene-based photosensitizer dye molecules exhibit comparably low power conversion efficiencies ranging from 0.8% to 1.7% in solid-state DSSCs. We find that the photocurrents increase in the presence of Li-salt additives. Both TA and PIA measurements observe long-lived dye cations created by electron injection from the dyes' excited state for two dyes from the series. However, the third dye shows significantly lower performance as a consequence of the less efficient electron injection even after the addition of Li-salts and faster electron-hole recombination on the ns-mu s time scale. In essence, the prerequisites for this class of donor-pi bridge-acceptor photosensitizers to reach higher charge generation efficiencies are a combination of strong dipole moments and fine tuning of the electronic landscape at the titania-dye interface by Li-salt addition. Published by AIP Publishing.