Effect of Sb ions on the morphology of chemical bath-deposited ZnO films and application to nanoporous solar cells

Low concentrations (0.1-1% of the Zn concentration) of Sb ions in an alkaline ZnO chemical bath deposition solution were found to lead to pronounced changes in the ZnO film morphology. The tapered nanorods obtained in the absence of Sb become flat-topped and more closely packed when Sb is present, and the nanorod diameter changes, the direction of these changes depending on other bath parameters. An initial compact layer that was comprised of very small nanocrystals is formed. Sb was found to be present in the films, mostly in the compact initial layer. We postulate that initial Sb-rich nuclei promote ZnO nucleation but retard subsequent ZnO crystal growth. As the Sb concentration (relative to the Zn) in the bath drops, ZnO nanorods can then grow, but with altered morphology because of preferential adsorption of the low levels of Sb in solution on (002) ZnO crystal faces. These films were found to be very suitable for solid state semiconductor-sensitized solar cells. Such cells normally require a separate deposition of a blocking compact layer before the nanoporous layer. The initial in situ compact layer in our films is very efficient for this purpose, giving much more reproducible performance.