Zinc oxide (ZnO) is attracting a lot of attention in the field of excitonic solar cells because it is a cheap and transparent wide band-gap semiconductor, with natural n-type conductivity and good electron mobility, which can be doped with a high concentration of trivalent atoms (e.g. Al, Ga or In) to increase the electrical conductivity and make it a TCO. But, above all, ZnO has also the peculiar property of being easily obtained in the form of nanostructures [1-2], allowing researchers to design more complex 3D structures for ZnO based TCOs with smarter light and more efficient short range charge collection in excitonic solar cells. Solution based growth of vertically aligned ZnO nano-wires (NWs) is generally the most suitable technique for cheap and large scale production, but it cannot always be matched easily with the deposition processes of other cell components or the intrinsic doping from solution inclusions may degrade the device performance. In contrast, vapour phase growth is generally less controllable and more expensive, especially when metal-organic precursors or metal catalysts are used in order to improve the synthesis reproducibility. In the present work authors have explored an alternative solution free and catalyst-free synthesis process that combines two different techniques, i.e. a pulsed electron deposition (PED) of ZnO:Al (AZO) films on commercial glass substrates and a self-catalysed vapour phase growth of vertically aligned ZnO-NWs at relatively low temperature.

 ZnO-NWs were grown by the optimized vapour phase technique described in [3] directly on the ZnO and AZO (2 wt% nominal Al content) films obtained by PED and sputtering. Commercial Zn was used as source material in a quasi-closed container and it was placed side-by-side with substrate in a tubular furnace in the downstream position. The growth temperature was 480 °C and Ar was used as an inert transporting gas. O₂ was introduced for oxidation (1:20 O₂: Ar ratio, 50 sccm) only when the maximum temperature was reached and then kept for 5 min only.

 As reported, small Zn nanoclusters on the polar surface of a (001)-oriented ZnO/AZO film can generate preferential, energetically favored and oriented nucleation sites for the growth of homogeneous arrays of aligned NWs. A growth setup that promotes the formation of such nanoclusters has been defined and 20-50 nm thick and 1-3 μm long aligned ZnO nanowires have been obtained on the AZO substrate obtained by PED. But, the nanowires growth on the sputtered AZO substrate has been different in size due to large epitaxial grain size compare to the AZO obtained from PED. 100-200 nm thick and 2-4 μm long aligned ZnO nanowires have been obtained on the sputtered AZO substrate. AZO resistivity after NWs growth was still higher than 1.0x10⁻³ cm and transparency still high. The obtained samples are homogeneous on the growth area (few cm²) and both the film deposition and the NWs growth can be easily patterned by mechanical masks.

Acknowledgements

This project has been partially supported by”Nanowiring” European FP7-People: ITN project.

References

[1] Z. L. Wang, Mater. Today 7, 26 (2006).

[2] M. Z. Zha et al , Nanotechnology 19, 325603 (2008).

[3] D. Calestani et al , CrystEngComm 13, 1707 (2011).

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