The modification of thin film surface structure via low temperature atmospheric pressure CVD post process treatment

Thomson, M

Abstract

In photovoltaic thin film cells, a transparent conducting oxide (TCO) layer is required to transport current. The most common TCOs used are F:SnO2 (fluorine doped tin oxide), ZnO (zinc oxide) and ITO (indium doped tin oxide). ZnO is normally deposited in a vacuum based process, sputtering or low pressure chemical vapour deposition (LPCVD). Atmospheric pressure chemical vapour deposition (APCVD) is an attractive alternative for ZnO deposition. A critical parameter for TCOs in photovoltaic thin films is the surface morphology which defines the optical scattering properties. The ability to control the spectral sensitivity and degree of scattering are both important process parameters for high performance cells. This thesis investigates APCVD for film growth of ZnO plus dopants (fluorine and aluminium), and the effects of atmospheric pressure plasma etching of ZnO and F:SnO2.
ZnO was deposited in multiple system geometries all based on thermal activated CVD. The oxidant source purity is shown to be critical for stable growth at higher temperatures required for dopant incorporation. A fundamental problem was encountered with fluorine doping, whereby the films would crack beyond a critical thickness. A solution was found with the development of a F:SnO2 and F:ZnO composite stack. Photovoltaic testing of this hybrid TCO was encouraging, showing the potential benefit of the composite structure.
Modification of the surface morphology was achieved by atmospheric pressure plasma, based on a dielectric barrier discharge configuration. This novel system enables the etching of TCO films without the introduction of hazardous wet chemistry. In this thesis the effects of etching regime and feedgas composition are studied and an etching mechanism is proposed. Isolation of the etching environment enabled investigation into the feed gas mixture, demonstrating which were critical for etching. Both materials showed a dependence on the feed gas mixture for etching, with F:SnO2 requiring HCl and O2 and H2O for ZnO.

Citation

Thomson, M. The modification of thin film surface structure via low temperature atmospheric pressure CVD post process treatment. (Thesis). University of Salford

• Thesis Type: Thesis
• Deposit Date: Mar 7, 2014
• Publicly Available Date: Apr 5, 2016
• Additional Information: Funders : Engineering and Physical Sciences Research Council (EPSRC)