Dendritic CuIn Films Grown by Electroless Deposition
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Dendritic CuIn Films Grown by Electroless Deposition David W Lane, Jonathan D Painter, Keith D Rogers, Ian Forbes*, Robert W Miles*, Kathleen M Hynes* Centre for Materials Science and Engineering, Cranfield University at Shrivenham, Swindon, Wiltshire, SN6 8LA, UK * Northumbria Photovoltaics Applications Centre, Northumbria University, Newcastle upon Tyne, NE1 8ST, UK ABSTRACT CuInS2 films are often grown in a two-step process with the deposition of a Cu-In alloy followed by high temperature sulphurisation in either S vapour or H2S. Numerous techniques exist for the deposition of Cu-In. In this work Cu-In films have been deposited on tin oxide coated glass using the electroless deposition technique, a low cost, low temperature approach. The films were found to consist of a two layered structure. Initially, Cu islands grew forming a layer on the substrate. The deposition process subsequently produced dendritic Cu-In alloys from the tops of the Cu islands. Various CuIn phases were observed, namely Cu9In4 and CuIn, the presence of which was related to the solution pH. None of the films were found to contain metallic In, indicating that this process may be well suited to subsequent high temperature sulphurisation.
INTRODUCTION The drive for low cost photovoltaic devices continues and Si, CdTe and CuInSe2 are among the materials actively used in commercial photovoltaic devices. There are however, environmental concerns regarding the production and use of these materials and consequently alternatives are continually being investigated. Relatively recently CuInS2 has re-emerged as a possible absorber material for photovoltaic solar cells, as its band gap of 1.52 eV is well matched to the solar spectrum. Thin film CuInS2 can be grown by several different processes, these include its direct growth using techniques such as spray pyrolysis and its growth by a two stage process in which a precursor layer of CuInX alloy, grown by processes such as sputtering or coevaporation, is converted to CuInS2 by a high temperature reaction with either S vapour or H2S. In this paper we present some results for precursor layers grown by the electroless method. This is a low-cost, convenient deposition method that has been previously reported to successfully deposit CuInX films [1,2,3]. The conditions for electroless deposition of CuInX require a conducting substrate (e.g. SnO2 coated glass) that is electrically connected to an easily oxidizable redox component (e.g. Al). Both electrodes are then immersed in the chemical solution containing Cu2+ and In3+ ions and an electrochemical potential difference is generated between the two electrodes. These ions are then reduced at the conducting substrate surface to form the Cu-In alloy film. The electroless deposition of specific phases of Cu-In alloy requires careful complexing of the ions because of the large difference between the standard potentials for the reduction of Cu2+ and In3+. In our work we have specifically aimed to use the electroless technique to produce precursor layers wi
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