Conjugated linker correlated energetics and kinetics in dithienopyrrole dye-sensitized solar cells

We report two triarylamine-cyanoacrylic acid based push-pull dyes C252 and C253 featuring the π-conjugated linkers of 2,6-di(thiophen-2-yl)-4H- dithieno[3,2-b:2′,3′-d]pyrrole and 4H,4′H-2,2′- bidithieno[3,2-b:2′,3′-d]pyrrole, respectively. Benefitting from an improved coplanarity of the conjugated units, the C253 dye displays a red-shifted absorption peak and an enhanced maximum molar absorption coefficient in comparison with C252. However, this pattern of conjugated linker alternation is associated with an 80 mV negative shift of the ground-state oxidation potential, which dominates an almost 5 times reduced rate of hole injection from the oxidized state of C253 to the divalent tris(2,2′-bipyridine)cobalt (Co-bpy) cation in the redox electrolyte, resulting in a considerably poor net charge separation yield. On the other side, a dye-sensitized solar cell employing the C252 photosensitizer and the Co-bpy electrolyte exhibits a good power conversion efficiency of 9.5% measured under the 100 mW cm^-2 simulated AM1.5 sunlight. The dissimilarity of cell photovoltage is scrutinized by evaluating the shift of the titania conduction band edge and the variation of interfacial charge recombination kinetics, the latter of which presents a clear correlation with dye coating thickness on titania derived from X-ray photoelectron spectroscopy measurements. Our work has underlined the important energetic and kinetic interplays which should be seriously considered in the further optimization of active components in dye-sensitized solar cells.