Dye-Anchoring Functional Groups on the Performance of Dye-Sensitized Solar Cells: Comparison between Alkoxysilyl and Carboxyl Groups

We have compared two dye-anchoring functional groups, alkoxysilyl and carboxyl groups, to investigate their influence on the performance of dye-sensitized solar cells. (Dimethylamino)­azobenzene was selected as a chromophore possessing a donor–acceptor transition for the light absorption. Electrochemical and optical measurements were performed for 4-(dimethylamino)­azobenzene-4′-carboxylic acid and 4-(dimethylamino)­azobenzene-4′-triethoxysilane attached TiO₂ films. Electrochemical measurements and DFT calculations indicated almost identical potential energy levels and electron density of HOMO and LUMO states between these two dyes. Solar cell APCE spectra and charge recombination kinetics at the dye/TiO₂ interface revealed almost identical charge-transfer rates/yields from and to the dye. The difference was observed on improvement of an open circuit photovoltage (<i>V</i>_oc) by 60 mV and on the lifetimes of an electron in the TiO₂ conduction band for the dye with the alkoxysilyl functional group compared to the carboxyl group, suggesting that an alkoxysilyl functional group is more attractive to retard the charge recombination reaction between an electron in the TiO₂ conduction band and an oxidized form of an electrolyte redox couple. The highest solar energy conversion efficiency of 2.6% was achieved for dye-sensitized solar cells based on an azobenzene dye sensitizer under AM1.5<i>G</i>, one sun condition