Publication date: 27th June 2014
ZnO, with a high band gap and tunable nanostructures, is a well known material for sensitized solar cell. There have been several examples, where ZnO nanorods thin film is used as electron conductor for sensitized solar cell. But it has often been seen that the ZnO nanorod solar cell has never crossed the efficiency produced by TiO2 solar cell inspite of having single crystal nanorod structure and higher surface area. Several reasons have been given for it eg. Non conformal deposition of absorber layer, shunt and series resistance due to hole conductor etc.
We propose that the fundamental reason for the lesser efficiency of ZnO nanorod based solar cell is the complete ignorance about the anomalous behavior of different crystal surfaces and the band gap of different crystal surfaces. The band gap of {0002} and {11-20} crystal surfaces is 3.2 eV (same as bulk ZnO), while for {10-10} surfaces is 2.85 eV-1 eV. The {10-10} crystal surface forms more than 80% of the ZnO nanorod crystal surface area. With lower band gap of {10-10} nanorod crystal surface, the conduction band (CB) edge goes down with respect to the absorber (CdS, Sb2S3) CB edge. This may cause a lower transition rate of the carriers (in accord with the Fermi’s golden rule), and thus a lower charge separation. Also the (0002), (000-2) and {10-10} surfaces have different schottky barrier heights for different semiconductor and metal junctions. These surfaces also differ in their mobility, defect density, surface energy and available density of states.
Due to these fundamental differences, we see that the ZnO nanorods have never performed as good as TiO2 mesoporous structures for solar cell applications. On depositing TiO2 on ZnO, we are able to increase the efficiency of solar cell as it forms a cascading conduction band structure, which helps in better charge separation. Also it is seen that, a flat layer of ZnO results in better solar cell as compared to nanorods, because we don’t have to fight with anomalous band gaps, electron mobility etc.
References:
1. N. L. Marana, V. M. Longo, E. Longo, J. B. L. Martins, and J. R. Sambran, “Electronic and Structural Properties of the (101j0) and (112j0) ZnO Surfaces” J. Phys. Chem. A 2008, 112, 8958–8963.
2. Qifeng Zhang, Christopher S. Dandeneau, Xiaoyuan Zhou, and Guozhong Cao, “ZnO Nanostructures for Dye-Sensitized Solar Cells” Adv. Mater. 2009, 21, 4087–4108.
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