Degradation of the performance of Cu 2S/CdS solar cells due to a two-way solid state diffusion process

Abstract

A model of a mechanism of degradation in Cu 2S/CdS solar cells, arising out of the growth of a recombination layer in the skin region near the junction due to the two-way diffusion of copper and cadmium atoms, was proposed by Deb and Saha to explain the time variation in the performance of such cells using ceramic tablets. The analytical relations given by Deb and Saha were mainly guessed through intuitive reasoning. In the present paper an attempt is made to develop the model further and to give it an analytical basis through a solution of the relevant diffusion equation, subject to the assumption that the concentrations of excess copper and cadmium atoms at the skin region boundary and junction side boundary respectively of the recombination layer are constant and that the distributions of the diffusing species in the medium are linear. Apart from confirming the linear law for the normalized short-circuit current density versus the square root of time, the relation thus obtained leads to more reliable estimates of the rate constants mentioned earlier. The expressions derived are used to analyse the degradation in performance of vacuum-deposited thin film Cu 2S/CdS solar cells, as opposed to the ceramic cells studied earlier. As in the ceramic cells, a slowing down of the degradation with heat treatment carried out in air containing the dopant vapour was found with the thin film cells. This is also accounted for by the model. Some results obtained for the variations in the reverse saturation current, diode factor and open-circuit voltage with time are also discussed.