Screen printed layers of CdS for solar cells
It is generally accepted nowadays that a significant cost reduction in terrestrial solar cell application could be brought about by investigating alternative fabrication techniques for solar cells. It is believed that screen printing (or the so called thick film technique) is one such technique which promises a potentially low cost method for fabricating flexible, large area solar energy conversion cells. The active research on this technique started in 1976 in Japan. However, it was not until 1983, that wide interest developed when the Matsushita group in Japan reported an efficiency of 12.8% for their entirely screen printed CdS/CdTe solar cells. This was the highest reported efficiency for any thin film solar cell. However, the details of the fabrication processes of these cells were not reported and several scientific groups in the world started to explore this technique. The first published report was in 1985. In the last few years these groups have reported results on various aspects of this technique. Nevertheless there are still major parameters to be investigated. This thesis represents a concise reference for the application of the screen printing technique to solar cells. In the course of this study many new investigations have been made which supplement the previous work by other groups. Starting with a pure CdS powder with suitable grain size and distribution is a prerequisite for achieving the best morphological and electrical behaviour of screen printed layers of CdS. Careful paste mixing is of uppermost importance which can override any other parameters involved in the fabrication processes. It is essential to impose restricted sintering conditions for adequate utilization of the doping and fluxing function of the CdCl(_2) material. Standardization of the printing, preparation and sintering conditions involved in the fabrication processes were necessary to ensure reproducible CdS layers. Good quality screen printed layers were fabricated on soda lime substrates. The significance of other substrate materials for CdS preparation was also investigated and optimum substrate choice is suggested. The properties of the CdS screen printed layers were investigated by forming simple Schottky devices and more complicated heterojunction solar cells. Good rectification behaviour of the Schottky diodes was achieved. The CdS/CdTe solar cells revealed a wide spectral response. However, the photovoltaic behaviour was relatively poor largely due to the high resistivity of the CdTe part of the cell structure.