Metal-Free and Fluorescent Diketopyrrolopyrrole Fluorophores for Dye-Sensitized Solar Cells

Novel metal-free and fluorescent dipyrrolopyrrole (DPP) dyes consisting of a central bis(lactam)-bearing pendent benzyl or branched hydrocarbon groups as solubilizing fragments and two orthogonal side arms (dimethylaminopropyne and benzoate anion) have been designed and synthesized. The UV/Vis absorption spectra recorded in THF are dominated by intense low-energy pp* absorptions centered at 488 nm or 602 nm, respectively, for the phenyl- or thiophene-based DPP dyes. The fluorescence spectra also display broad bands at 555 nm (FF=0.57 and tF=4.6 ns) for the phenyl- and at 624 nm (FF=0.31 and tF=3.5 ns) for the thiophene-based molecular structures. Under standard global AM 1.5G irradiation a maximum photon-to-electron conversion efficiency of 2.54?% was achieved with dye-sensitized solar cells based on nanocrystalline TiO2 (Jsc=7.5 mA?cm-2, Voc=0.49 V, and FF=0.70) for the phenyl-based DPP dye and 1.89?% (Jsc=7.1 mA?cm-2, Voc=0.41 V, and FF=0.65) for the thiophene-based DPP dye.