Application of closed field magnetron sputtering deposition in thin film photovoltaics

Thin film solar cell technology is highly promising to enable clean and low cost generation of solar electricity for various applications. The high efficiency, flexibility and lightweight advantages of thin film solar cells, together with stable performance and potentially low production costs, further enhance their attractiveness for both terrestrial and space applications. A distinct manufacturing advantage of thin film solar cells is the use of fast vacuum deposition methods, providing the high throughput essential to reduce manufacturing costs. However, an essential pre-requisite is the development of deposition techniques which combine capability to deposit the solar cell thin film multilayer preferably within a single vacuum cycle, removing the requirement for certain process steps to be carried out using non-vacuum wet chemistry. Moreover, process development is also needed to provide low temperature processing and low stress multilayer thin film structures which enable photovoltaic devices to be deposited on to low cost flexible polymer or metal substrates. In this paper a new sputtering tool strategy is introduced, utilising high plasma densities (~10mA.cm-2) and low ion energies, thereby lowering process temperature and film stress for deposition onto both flexible and solid substrates. The technique uses magnetrons of opposing magnetic polarity to create a "closed field" in which the plasma density is enhanced without the need for high applied voltages. A prototype batch system has been designed which employs a rotating vertical drum as the substrate carrier and a symmetrical array of four linear magnetrons. The magnetrons are fitted with target materials for each of the thin films required in the PV stack including the CdTe absorber layer, CdS buffer layer and the back TCO contact. Details of the system design will be provided together with optical, electrical and metrology data already obtained from ITO thin films. The "closed field" sputtering technology allows scale up not only for larger batch system designs but it is also configurable for "in-line" or "roll to roll" formats for large scale production