Aggregation of ZnO nanocrystallites for high conversion efficiency in dye-sensitized solar cells.

As a relatively new class of photovoltaic devices with a photoelectrochemical system consisting of a dye-sensitized semiconductor film and an electrolyte, dye-sensitized solar cells (DSSCs) have been regarded as a promising alternative to conventional solid-state semiconductor solar cells. They are relatively cost-effective, are easy to manufacture, and can be readily shaped with flexible substrates to satisfy the demands of various applications. Avery important feature of DSSCs is the photoelectrode, which includes mesoporous wide-bandgap oxide semiconductor films with an enormous internal surface area, typically a thousand times larger than that of bulk films. To date, the highest solar-to-electric conversion efficiency of over 11% has been achieved with films that consist of 20-nm TiO2 nanocrystallites sensitized by ruthenium-based dyes. However, further improving the energy conversion efficiency of DSSCs remains a challenge. Competition between the generation and recombination of photoexcited carriers in DSSCs is a main bottleneck for developing higher conversion efficiency. One possible solution is to use one-dimensional nanostructures that are able to provide a direct pathway for the rapid collection of photogenerated electrons and, therefore, reduce the degree of charge recombination. However, such one-dimensional nanostructures seem to have insufficient internal surface area, which limits their energy conversion efficiency at a relatively low level, for example, 1.5% for ZnO nanowires and 4.7% for TiO2 nanotubes. [9] Another way to increase efficiency is to increase the light-harvesting capability of the photoelectrode film by utilizing optical enhancement effects, which can be achieved by means of light scattering by introducing scatterers into the photoelectrode film. Usami, Ferber and Luther, and Rothenberger et al. have demonstrated theoretically that optical absorption by TiO2 nanocrystalline films can be promoted by additionally admixing large TiO2 particles in an optimal volume ratio. This idea was verified experimentally when TiO2 nanocrystalline films were combined with large SiO2, Al2O3, or TiO2 particles. [14–17] By coupling a photonic crystal layer to conventional TiO2 nanocrystalline films as the light scatterer, Nishimura et al. and Halaoui et al. also succeeded in enhancing the light-harvesting capability of solar-cell photoelectrodes. However, the drawback is that the introduction of larger particles into nanocrystalline films will unavoidably lower the internal surface area of the photoelectrode film and, therefore, counteract the enhancement effect of light scattering on the optical absorption, whereas the incorporation of a layer of TiO2 photonic crystal may lead to an undesirable increase in the electron diffusion length and, consequently, increase the recombination rate of photogenerated carriers. Herein we report hierarchically structured ZnO films as the photoelectrodes in DSSCs for the enhancement of energy conversion efficiency. The films are comprised of polydisperse ZnO aggregates consisting of nanosized crystallites. The aggregates are submicrometer-sized and, thus, can function as efficient light scatterers, while the nanocrystallites provide the films with the necessary mesoporous structure and large internal surface area. An overall energy conversion efficiency up to 5.4% has been achieved from the film including polydisperse ZnO aggregates, much higher than 1.5–2.4% for ZnO nanocrystalline films, 0.5–1.5% for ZnO nanowire films, 23] and 2.7–3.5% for uniform ZnO aggregate films. Polydisperse ZnO aggregates were synthesized by the hydrolysis of zinc salt in polyol medium at 160 8C, similar to the method reported by Jezequel et al. Rapid heating at a rate of 10 8Cmin 1 was intentionally used to obtain polydisperse aggregates, that is, with a relatively wide size distribution. The resulting colloidal dispersion was drop-cast onto a fluorine-doped tin oxide (FTO) coated glass substrate to form a film of approximately 9 mm in thickness, and the film was subsequently annealed at 350 8C for 1 h in air to remove residual solvents and any organic compounds as well as to improve the contact between the film and the substrate and the connection between the nanocrystallites and between the aggregates. Figure 1 shows the scanning electron microscopy (SEM) images of ZnO film with polydisperse aggregates and a schematic illustration showing the structure of an aggregate. Figure 1a indicates that the film is well stacked with submicrometer-sized ZnO aggregates. Figure 1b presents the highly disordered structure of the film assembled by polydisperse ZnO aggregates with diameters ranging from several tens to several hundreds of nanometers. Figure 1c is a magnified SEM image of an individual ZnO aggregate, revealing that the ZnO aggregate is nearly spherical in [*] Dr. Q. F. Zhang, Dr. T. P. Chou, B. Russo, Prof. G. Z. Cao Department of Materials Science and Engineering University of Washington Seattle, WA 98195 (USA) Fax: (+1)206-543-3100 E-mail: gzcao@u.washington. edu Homepage: http://depts.washington. edu/solgel/