Effects of Electrode Structure on Photoelectrochemical Properties of ZnO Electrodes Modified with Porphyrin-Fullerene Composite Layers with an Intervening Fullerene Monolayer

ZnO nanorods and nanoparticle electrodes have been applied to solar cells possessing characteristics of bulk heterojunction and dye-sensitized devices. First, the ZnO electrodes were covered with fullerene acid molecules to yield the monolayer on the electrodes. Then, zinc porphyrin and fullerene acid molecules were spin coated onto the modified surfaces to give porphyrin-fullerene-modified ZnO electrodes. The porphyrin-fullerene-modified ZnO nanorod devices with the intervening fullerene monolayer exhibited efficient photocurrent generation compared to that of the reference systems without the fullerene monolayer. The significant improvement of the photocurrent generation efficiency by the fullerene monolayer may be associated with efficient charge separation in the porphyrin-fullerene composite layer, followed by electron injection into a conduction band of the ZnO nanorod electrode together with the suppression of charge recombination between the separated charges by the fullerene monolayer. Cell performance was optimized by altering the length and diameter of the ZnO nanorods and density of the ZnO nanorod array. The effects of the ZnO electrode structures (i.e., nanorod versus nanoparticle) on the photoelectrochemical properties were also compared under the same conditions. Despite a larger surface area of the ZnO nanoparticle electrode by a factor of 3 compared with that of the ZnO nanorod electrode, we noted similar photocurrent generation efficiencies. This can be rationalized by the facts that only the top surface of the porous ZnO nanoparticle electrode is covered by the porphyrin-fullerene composite layers, whereas the entire surface of the ZnO nanorod electrode is wrapped in the composite layers. The photocurrent generation mechanism was also corroborated by steady-state fluorescence and fluorescence lifetime measurements. The results obtained from this study will provide basic and valuable information on the design of electrode structures and the donor-acceptor combination toward the improvement of cell performance in dye-sensitized bulk heterojunction solar cells.