Thiophene-Functionalized Coumarin Dye for Efficient Dye-Sensitized Solar Cells:  Electron Lifetime Improved by Coadsorption of Deoxycholic Acid

This paper reports a new coumarin dye, 2-cyano-3-(5-{2-[5-(1,1,6,6-tetramethyl-10-oxo-2,3,5,6-tetrahydro- 1H,4H,10H -11-oxa-3a-aza-benzo[ de ]anthracen -9-yl)-thiophen-2-yl]-vinyl}-thiophen-2-yl)-acrylic acid (NKX-2700), and its application in dye-sensitized solar cells (DSSCs). Under illumination of simulated AM1.5G solar light (100 mW cm -2 ) with an aperture black mask, 5.0% of power conversion efficiency [short-circuit photocurrent density ( J sc ) = 12.0 mA cm -2, open-circuit photovoltage ( V oc ) = 0.59 V, and fill factor (FF) = 0.71] was obtained for NKX-2700 based DSSC, which was significantly improved to 8.2% ( J sc = 15.9 mA cm -2, V oc = 0.69 V, FF = 0.75) upon addition of 120 mM deoxycholic acid (DCA) to the dye solution for TiO 2 sensitization. Coadsorption of DCA decreased dye coverage by ∼50% but significantly improved the J sc by 33%. The breakup of π-stacked aggregates might improve electron injection yield and thus J sc . Electrochemical impedance data indicate that the electron lifetime was improved by coadsorption of DCA, accounting for the significant improvement of V oc . These results suggest that interfacial engineering of the organic dye-sensitized TiO 2 electrodes is important for highly efficient photovoltaic performance of the solar cell.