An Energetic and Kinetic View on Cyclopentadithiophene Dye-Sensitized Solar Cells: The Influence of Fluorine vs Ethyl Substituent

A comprehensive understanding on the structure property relationship of metal-free organic chromophores in dye-sensitized solar cells will form the basis of further design of advanced materials and realization of considerable enhancement of device performance, boosting the widespread utilization of solar energy at an affordable cost. In this paper we report profound influences of the fluorine versus ethyl substituent of the cyclopentadithiophene conjugated spacer of push pull photosensitizers, upon the energetic and kinetic characteristics of dye-sensitized solar cells. Joint electrochemical and spectral measurements reveal that with respect to the fluorine-containing dye, the ethyl counterpart exhibits a 0.18 V more negative excited-state redox potential upon anchoring on titania, which overwhelms the 0.01 eV upward conduction band edge shift of the dye-coated titania film. The ethyl-correlated more favorable dye/titania interface energetics benefits a remarkably higher electron injection yield, which is verified with the transient emission and photocurrent action spectrum measurements. Furthermore, analysis on the electrical impedance data affords a notably smaller recombination reaction order for the cell made from the ethyl-substituting chromophore than that of the fluorine counterpart, accounting for a remarkably suppressed interfacial charge recombination with triiodide and thus explaining the observed higher open-circuit photovoltage.