Investigation of copper plated contacts for silicon solar cells

INVESTIGATION OF COPPER PLATED CONTACTS FOR SILICON SOLAR CELLS N.E. Posthuma, J.L. Hernandez, V. Lazov, H. Philipsen, V. Prajapati, T. Janssens, E. Van Kerschaver and J. Poortmans IMEC vzw, Kapeldreef 75, B-3001 Leuven, Belgium Phone: +32-16-28 1211; Fax: +32-16-28 1501 ABSTRACT: As an alternative method to printing techniques, the metal contacts of silicon solar cells can be realized by plating. Copper is an interesting choice of metal, especially taking into account its high conductivity and low price. However, barrier layers are needed to prevent copper diffusion into the silicon. In this paper we use TXRF, QSSPC and CDI to evaluate the potential diffusion of copper through the silicon nitride anti reflection coating after exposing the wafers to Cu plating solutions. Three plating solutions with varying purity are investigated. Furthermore first PERC solar cells have been realized with copper plated contacts having a Ti/Cu contact layer with a best large area cell efficiency of 18.3%. Keywords: Electrodeposition, Metallization, Solar Cell 1 INTRODUCTION The development of silicon solar cells is since several years focused on the reduction of substrate thickness (< 150 µm) and at the same time on improving the energy conversion efficiency. One of the many topics of research is the method of metallization for the emitter and base contacts. As a successor of the currently industrially used method of screen-printing, alternative printing techniques, like inkjet-printing and aerosol jetting, are being developed to optimize the aspect ratio of the contacts while maintaining a high throughput capability [1]. As an alternative method to printing techniques, the contacts can also be realized by plating. This is a well known technique from the CMOS industry and is also being used for solar cells [2, 3, 4]. Plating allows for an efficient deposition of a layer of pure metal and is suited for thin substrates since no substantial mechanical force is used during the process. Apart from the method of metallization, also the choice of metal is important. So far, in case of printing techniques, mainly silver and aluminum are used to contact the n-type emitter and p-type base, respectively. As an alternative a two step metallization process can be used. This process is based on the idea to apply first a thin metal layer and subsequently apply a thick highly conductive metal layer. In this way the amount of freedom to optimize both the properties of the solar cell contacts and the underlying emitter is enlarged. The thick metal layer needs to be highly conductive, so preferably it will consist of either silver or copper. Even though not many solar cell manufacturers have applied copper [5], it is an interesting choice of metal, especially taking into account the price predictions of both materials. Although the cost of both metals has increased over the years, copper is relatively cheap compared to silver [6]. The thin metal contact layer has several functions. It will serve as a barrier layer to prevent diffusion of the copper into the silicon. Furthermore, it should serve as a seed layer to enable electro-plating. The barrier layer will be optimized to contact a lowly doped emitter, with a surface dopant concentration of 1·10