Effects of heat treatment in H2-N2 atmosphere on the performance of screen-printed CdS/CdTe solar cells

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Abstract
Heat treatments in a 50% H2+50% N2 atmosphere have been applied to the fabrication process of screen‐printed CdS/CdTe solar cells for improving cell performance. The heat treatment at 450 °C after the sintering of a C (carbon) electrode on a CdTe film was found to be most effective in improving conversion efficiency. The efficiencies increased from about 8% to about 10% because of a large increase in fill factor and a slight increase in open‐circuit voltage. Current versus voltage characteristics, short‐circuit current versus open‐circuit voltage characteristics, and spectral responses were analyzed. The above improvement was found to result from a decrease in CdS film resistivity, a small change in the diode characteristic of the junction under illumination, and the reduced voltage dependence of the light‐generated current arising from the increased minority carrier diffusion length in the CdTe. The heat treatment at 450 °C after the sintering of the CdTe film also increased fill factor, resulting in efficiencies of about 9%. The improvement was caused by a decrease in CdS film resistivity. A stability test under sunlight conditions showed that the efficiency of the cells heat treated after the C‐electrode sintering decreased gradually because of a degradation in the CdTe‐C contact. However, the CdS film resistivity and junction properties of the cells were stable. On the other hand, the cells heat treated after the CdTe film sintering were stable under the same conditions.