2014
In this study, we investigated which atmospheric species actually drive the degradation of aluminium-doped zinc oxide, ZnO:Al. This transparent conducting layer is used in thin-film solar cells and other optoelectronic devices because it is abundant, non-toxic, conductive and visibly transparent. Its stability matters: when the layer degrades, resistance rises and device performance can fall. We wanted to identify the chemistry behind that failure, so that protection can be designed around the cause rather than the symptom.
We exposed sputtered ZnO:Al films to carbon dioxide, oxygen, nitrogen and air, both as gases and dissolved in water. Electrical, optical, structural and compositional measurements let us separate the effect of each species from the effect of their combinations. Damp-heat tests can show that a problem exists, but they do not always reveal what is causing it.
Neither water nor carbon dioxide alone caused the severe degradation we saw. The critical condition was their combination. In water containing carbon dioxide, the ZnO:Al film degraded dramatically and, in one case, dissolved within hours. Water and air also produced a strong rise in sheet resistance, major mobility loss, infrared optical changes and local gaps close to the glass interface. Gaseous oxygen or carbon dioxide by themselves had little effect, while water purged with nitrogen or oxygen caused only slow changes.
Our compositional analysis explained why. Water and carbon dioxide together greatly increased hydroxyl species in the film, especially near its air and glass interfaces. We also detected carbon-containing species consistent with zinc hydroxide carbonate compounds forming along grain boundaries. These reactions disrupt the electrical pathways through the columnar ZnO:Al structure. Humidity, in other words, is not one undifferentiated risk; the chemical combination matters.
The practical implication is direct. Encapsulation is often designed around water vapour and oxygen barriers, but carbon-dioxide ingress needs attention too when ZnO:Al must survive humid conditions. This gives developers a more specific basis for choosing barrier materials and designing protective stacks that address the actual mechanism of degradation.