Solvent-Acidity-Driven Change in Photophysics and Significant Efficiency Improvement in Dye-Sensitized Solar Cells of a Benzothiazole-Derived Organic Sensitizer

The photophysics and solar cell efficiency of organic sensitizers comprising a cyanoacrylic acid group are greatly influenced by an equilibrium between the neutral ("non-deprotonated", COOH) and anionic ("deprotonated", COO ) forms, whose ratio depends on the solvent polarity and its H-bonding properties, dye concentration, and temperature used. Herein, we report a detailed investigation on the relationship between the portions of COOH and COO- dye forms and the photophysical and solar cell properties of an organic dipolar sensitizer, BTZA-II, bearing triphenylamine electron-donating and benzothiazole electron-withdrawing moieties. The photophysics has been studied by stationary and timeresolved fluorescence spectroscopy in apolar and polar solvents with a dye concentration ranging from 5 x 10(-7) to 5 x 10(-5) M, also upon addition of small amounts of an external acid or base to change the solvent acidity, allowing us to distinguish the contribution and lifetime of the neutral and anionic forms. The fluorescence of BTZA-II in apolar toluene originates from the neutral form, which has a lifetime of 1.9 ns. Addition of a strong base (1,8-diazabicyclo[5.4.0]undec-7-ene, DBU) shifts the equilibrium toward the less fluorescent anionic COO- form with a lifetime of 1.1 ns. The situation is different in the polar acetonitrile, where the fluorescence of the anionic form dominates (with a lifetime of 2.0 ns). Adding small amounts of acetic acid (AcOH) protonates the COO- form of the BTZA-II dye and reveals significant quenching of the fluorescence because of the increased contribution of the neutral species with a lifetime of 0.4-0.5 ns. This quenching of the neutral species in acetonitrile has been also observed in concentrated solutions and is due to excited-state proton transfer. By contrast, the photophysics of a dye Btz-NPh2, similar to BTZA-II but without the cyanoacrylic acid group, is not affected upon adding acetic acid (H-donor) or DBU base (H-acceptor), which rules out the Bronsted-base role of the benzothiazole scaffold in the aforementioned observations. Finally, dyesensitized solar cells (DSSCs) with a solid-state electrolyte were prepared from toluene and toluene + acid solutions. A significant increase in the solar cell efficiency, eta, by 58% (eta reaching a value of 4.9%) has been achieved after addition of a small, appropriate amount of acetic acid into the initial BTZA-II dye solution.