Developing environmentally benign routes for semiconductor synthesis: Improved approaches to the solution deposition of cadmium sulfide for solar cell applications

Chemical Bath Deposition (CBD) is a useful method for the deposition of thin films of semiconductors such as CdS, which are important components of polycrystalline solar cells. Such devices potentially offer low cost solar energy conversion which are interesting sustainable energy sources. We are attempting to reduce the environmental impact on the fabrication of such devices by developing routes in which cadmium is used efficiently in the deposition of films. At present the fabrication of CdS window layers by CBD generates considerable quantities of cadmium-containing waste and the incorporation of cadmium into the films accounts for only a small fraction of the total used in the process. The use and disposal of cadmium-containing compounds is highly regulated in the EU and elsewhere. In CBD, the extent of the heterogeneous film formation process is limited by two major factors: homogeneous reaction in solution (precipitation) and the deposition of material on to the reactor walls. In this paper we describe the development of a novel high-efficiency CdS CBD system, using concentrations of cadmium orders of magnitude lower than in previously reported methods. The modelling of chemical equilibria have enabled us to develop this method which also offers the potential to eliminate ammonia (which is volatile and undesirable for large-scale CBD operations). Films have been characterized as good quality CdS by spectroscopic methods (UV-VIS, PL and XPS), microscopy (SEM and TEM) and powder XRD.