The degradation of the photovoltaic properties of CdS-Cu2S solar cells stored in the dark and under illumination has been investigated. Unencapsulated devices maintained in air at room temperature degraded much less rapidly in the dark than those maintained under constant illumination. Examination using the ESCA technique showed that the illuminated devices developed a higher concentration of surface oxygen. The degradation, which is attributed to the formation of surface oxide with consequent loss of copper from the Cu2S layer could be retarded significantly by replacing the ambient air with argon. However, if cells were stored in argon at 50 degrees C instead of 22 degrees C, copper diffused from the Cu2S into the CdS, leading to degradation by this process.
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