Thermodynamics of the photocorrosion of n-ZnSe and p-ZnTe

In this paper we review the thermodynamics of the stability of n-ZnSe and p-ZnTe in aqueous environments relevant to photoelectrochemical cells: polyiodide and polychalcogenides (polysulphides, polyselenides and polytellurides). The following photoelectrochemical reactions were considered: (i) bond cleavage photoreactions, (ii) direct photooxidation (n-ZnSe) and direct photoreduction (p-ZnTe), (iii) oxidative (n-ZnSe) and reductive, (p-ZnTe) photodecomposition, (iv) oxidative (n-ZnSe) and reductive (p-ZnTe) exchange photoreactions. Chemical chalcogen exchange reactions in the relevant electrolytes were also taken into account. It results that n-ZnSe can undergo the following degradation reactions: (i) bond cleavage photoreactions and direct photooxidation in both neutral and alkaline aqueous solutions, (ii) oxidative photodecomposition in polyiodide, polysulphide, polyselenide and polytelluride electrolytes and (iii) oxidative exchange photoreactions in polysulphide and polytelluride electrolytes. p-ZnTe can undergo the following degradation reactions: (i) reductive exchange photoreactions in polysulphide and polyselenide solutions and (ii) a scarcely probable bond cleavage photoreaction in polytelluride electrolytes. The formation of heterojunctions due to the formation of different semiconducting layers at the electrode/electrolyte interface due to decomposition reactions, and their predictable effects in terms of conversion efficiency were taken into account. It results that n-ZnS layers grown on n-ZnSe, even though they act as window layers as far as their bandgap is concerned, give rise to a barrier at the solid-solid junction hindering the flow of holes generated in n-ZnSe towards the electrolyte. p-ZnTe grown onto n-ZnSe has a smaller bandgap, it does not hinder the transport of holes generated in n-ZnSe to the electrolyte, but electrons are photogenerated in the p-type outer semiconductor layer. n-ZnS and n-ZnSe layers grown onto p-ZnTe have larger bandgaps, they do not tend to hinder the flow of electrons photogenerated in p-ZnTe towards the electrolyte, but give rise to the photogeneration of holes in the external semiconducting layer.