Structural and electrical characteristics of CdS-Cu2S thin film solar cells
A study has been made of a variety of factors influencing the efficiency and operational stability of front-wall CdS-Cu2S solar cells. In the course of this work -1 cm2 cells were fabricated with conversion efficiency of up to 8% without attempting to reduce reflection losses.The CdS films were produced by vacuum evaporation and the electrical and structural characteristics of these films were studied as a function of the rate and temperature of the deposition. Previously there had been some controversy concerning the nature of the CdS source material required for fabricating high performance CdS-based solar cells, but this work has shown that a variety of CdS sources can be employed successfully provided that the film deposition parameters are suitably chosen.A conventional chemical exchange technique was employed to convert the CdS film surface to Cu2SI with the thickness and stoichiometry of the resultant Cu2S layer being examined by means of electrochemical analysis.Changes in the electrical properties of the CdS-Cu2S cells due to post- fabrication anealing under a variety of different conditions were studied and correlated with structural changes monitored by means of Auger electron spectroscopy with the aid of argon ion etching. Depth profiles of the constituent element concentrations indicate that, for samples annealed in air, a deep penetration of copper into the CdS layer occurs together with a significant out-diffusion of cadmium from the CdS after only a few minutes at 1000C. In contrast, the copper penetration which results from vacuum or hydrogen annealing treatment is substantially less and no significant out-diffusion of cadmium is observed for annealing temperatures up to 4000C. Two different diffusion processes, one in the grain boundaries and one in the mid-grain regions, have been identified and their relative importance has been studied for annealing cycles performed under the same three different ambient atmospheres (air, vacuum or hydrogen). The normally rapid and undesirable grain boundary diffusion of copper was found to be significantly inhibited by the use of flowing hydrogen during annealing. A further technologically important observation concerns the effect of the deposition of a film of copper over the copper sulphide layer of a cell and subsequent annealing of it in air. The improved electrical stability which this treatment yields has been shown to be directly associated with reduced interdiffusion at the CdS-Cu2S interface. This interfacial diffusion has also been shown to be influenced by the CdS stoichiometry in the vicinity of the junction.Finally, a brief investigation was made into the use of the ion implantation technique as a means of doping the upper layer of the OdS film with copper without annealing the completed cell. The results have demonstrated the feasibility of this technique, with the best results being obtained using a copper ion fluence of 5.1014 ions cm-2 at 50 keV ion energy.