Deposition and characterization of NiOx coatings by magnetron sputtering for application in dye-sensitized solar cells

Nickel oxide (NiOx) due to its p-type nature has considerable potential as a photocathodic material in energy conversion devices such as dye-sensitized solar cells (DSSCs). However, NiOx has not been extensively used for this application mainly because of low light harvesting efficiency due to limited dye loading on the coatings. In this study NiOx coatings were deposited using the dc-magnetron sputtering technique from a nickel target in an argon/oxygen plasma. One of the advantages of magnetron sputtering is the ability to control coating properties such as mechanical performance and chemical stoichiometry. It is anticipated that by enhancing the interconnectivity between NiOx particles and by optimizing surface roughness, it may be possible to enhance dye adsorption and increase its ability to absorb visible light. NiOx coatings were deposited onto both silicon wafer and indium tin oxide (ITO) covered glass substrates. The influence of deposition parameters such as pressure, nickel target current and substrate bias voltage were correlated with the coating properties of surface roughness, thickness, crystallographic structure and surface energy. This evaluation was carried out using optical profilometry, spectroscopic ellipsometry, XRD and contact angle measurements respectively. It was observed that deposited coating morphology and roughness were significantly influenced by the deposition parameters. For example increasing the deposition pressure from 0.20 to 0.40 Pa led to an increase in surface roughness (Ra) from 1.6 to 3 nm. Associated with this increase in roughness the surface energy increased from 36 to 61 mN/mm. The NiOx coatings were spectrally sensitized with Ru-complex dye containing -COOH groups as anchoring moieties. The dye adsorptions on NiOx coatings, deposited on ITO substrates, were investigated in transmission mode using UV-vis spectroscopy in the range of 400-800 nm. It was observed that for the coatings with the highest surface energy, there was an increase of up to 60% in the level of dye adsorption. The electroactivity of the NiOx thin films deposited on Ni substrate at 0.4 Pa has been verified through the occurrence of redox processes of reduction and lithium intercalation within the oxide film. (C) 2010 Elsevier B.V. All rights reserved.