Surface Photovoltage Spectroscopy Resolves Interfacial Charge Separation Efficiencies in ZnO Dye-Sensitized Solar Cells

摘要

Any optimization of dye-sensitized solar cells (DSSCs) must consider the energetics and charge transfer kinetics of the dye, substrate, and redox couple. Here, we use surface photovoltage spectroscopy to probe the energetics and photochemical charge transfer efficiency in fluorenyl-thiophene dye (OD-8)-sensitized ZnO films. Discrete photochemical charge transfer events at the dye-ZnO interface and at the dye- I-/I-3(-) or [Co(2,2'-bipyridyl)(3)](3+/2+) interfaces can be observed as negative photovoltage under dye excitation at 1.7 eV (460 nm). Without a redox couple, charge separation at the ZnO/dye interface is only 4% effective, likely due to the short electron hole separation distance. In the presence of the redox couples, charge separation approaches 26-54% of the theoretical limit, emphasizing the importance of the dye regeneration reaction via the redox couple. On the basis of the open circuit voltage, charge separation in fully assembled DSSCs is 100% efficient with iodide, but only 61% efficient with the cobalt redox couple. This suggests that device improvements are possible by optimizing the dye regeneration reaction with the cobalt redox couple.