Effects of Lithium Ions on Dye-Sensitized ZnO Aggregate Solar Cells

We report on the synthesis of ZnO nanocrystallite aggregates in the presence of lithium ions and films consisting of these aggregates for dye-sensitized solar cell applications. A maximum overall conversion efficiency of 6.1% has been achieved with these films. This value is much higher than the 4.0% obtained for the films that are comprised of ZnO aggregates synthesized in the absence of lithium ions. The lithium ions were found to have an influence on the growth and assembly of ZnO nanocrystallites, leading to an increase in the nanocrystallite size and a polydisperse distribution in the size of the aggregates. The increase in the nanocrystallite size is due to a lithium-induced increase in the diffusivity of interstitial zinc atoms, which leads to an improvement in the crystallinity. This, in turn, yields an oxygen-enriched ZnO surface, which acts to suppress the dissolution of zinc atoms at the ZnO surface in the case of an acidic dye. As such, the formation of a Zn 2+ /dye complex is avoided. This collaborates with an increase in the pore size of the aggregates in view of the increase in the nanocrystallite size, allowing dye molecules to undergo a thorough infiltration into the photoelectrode film so as to be more adsorbed. The polydisperse size distribution of the aggregates is believed to favor light scattering so that the traveling distance of light within the photoelectrode film can be significantly extended. Both the improved dye adsorption and the enhanced light scattering serve to increase the light-harvesting efficiency of the photoelectrode and, thus, promote the overall conversion efficiency of solar cells.