Damp heat induced degradation mechanisms occurring in coloured oxide/metal/oxide films for thin-film solar cells

Abstract

Oxide/metal/oxide (OMO) layer stacks are promising replacements for conventional transparent conductive oxides in thin-film photovoltaics. They can provide colouration of the solar cells by adjusting their thickness to optimised optical resonances. Green OMO multistacks were deposited on glass by subsequent sputtering of aluminium doped zinc oxide (ZnO:Al), silver (Ag) and ZnO:Al. Part of these samples were packaged. In order to study their degradation behaviour and identify possible degradation mechanisms, samples were exposed to either damp or dry heat. It was observed that the conductivity of non-packaged OMO films in damp heat increased during the first 30 h, while complete loss of conductivity occurred after ~50 h. Analysis of the optical properties showed that the green colour had disappeared after 600 h of exposure to damp heat. These degradation effects seemed to be caused by the formation of silver spheres, leading to stack rupturing. In contrast, dry heat samples experienced no visual changes or sudden loss of conductivity. Similarly, packaged samples only slightly degraded along the edges after 600 h of damp heat exposure. This indicates that moisture plays a key role in the degradation mechanism of the coloured OMO. It is likely that moisture diffusion resulted in the increased mobility and agglomeration of the silver layer. The OMO layers are thus likely stable under adequate packaging.