(739d) Electrochemical Impedance Spectroscopy of ZnO Nanowire and Nanoparticle Dye Sensitized Solar Cells

Dye sensitized solar cells (DSSCs) are one of the most promising designs for low-cost solar cells. In DSSCs, a mesoporous, wide band gap semiconductor is coated with a monolayer of sensitizing dye and infiltrated with a liquid electrolyte containing I-/I3- redox couple. Photons are absorbed by the dye molecules, which inject photoexcited electrons into the semiconductor conduction band where they are then transported to the conducting substrate. The dye is regenerated by reduction by I-, and the resulting I3- diffuses to the counterelectrode where it is reduced to complete the circuit. Optimization of DSSCs for commercial use requires fundamental understanding of electronic and ionic transport within the individual materials and at the interfaces within the cell. Electrons are transported to the substrate by diffusion, and achieving high charge collection efficiencies requires that diffusion times be much less than recombination times. Electron transport in the ZnO electrode can be improved by using nanowires instead of nanoparticles because nanowires provide a direct path for electron diffusion, as opposed to nanoparticles where many interparticle hopping steps are required.

In this paper, we will compare electron transport and recombination in DSSCs built with ZnO electrodes employing nanoparticle and nanowire morphologies. Transport, accumulation, and recombination of charge were investigated using electrochemical impedance spectroscopy (EIS). In EIS, the frequency-dependent complex impedance is determined by measuring the current response to an AC voltage perturbation. EIS spectra are plotted in Bode or Nyquist form, and are used to determine an equivalent circuit model. The model describes electronic and ionic processes in different parts of the cell using appropriate circuit elements. For example, ionic diffusion in the electrolyte is modeled using a bounded Warburg impedance, charge transfer at the platinum counterelectrode by an RC circuit, and electron diffusion and recombination in the ZnO electrode by a Warburg impedance in parallel with a capacitance. We will discuss the differences in electron transport observed between nanoparticle and nanowire ZnO DSSCs determined by EIS; as well as how EIS data can be used in conjunction with I-V curve characteristics to determine which fundamental processes limit efficiencies in both cells.