Control of zinc oxide surface structure using combined atmospheric pressure-based CVD growth and plasma etching

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 the degree of scattering are both important process parameters for high performance cells.In this work, we employ APCVD for film growth and study the effects of atmospheric pressure plasma etching of ZnO. The ZnO films were deposited onto glass, with the surface morphology analyzed by AFM. Optical assessment was conducted using custom built equipment. Modification of the surface morphology was achieved by atmospheric pressure plasma, based on a dielectric barrier discharge configuration. In this study, we explored the effects of etching conditions, particularly the variation of feed gas composition (N2, O2 and H2O).By controlling the treatment environment, we were able to establish a relationship between the feed gas mixture and the etchant rate. Careful selection and control of processing parameters demonstrated the ability to modify the morphology with a high degree of uniformity and enhance the optical scatter properties. Atmospheric pressure-based processes are attractive for scalable production development, and future work will explore the potential of this process for large scale application.