Enhancing the mechanical and chemical integrity of Ag/ZnO-based transparent and conductive thin film coatings: an atomistic modelling study

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The commercialisation of ZnO/Ag/ZnO based flexible TCEs (transparent conductive electrodes) is still hindered by the adhesion between Ag and ZnO that results in a weak Ag/ZnO interface. This weakness is further reflected by the observed dewetting of Ag on ZnO. Optical coatings containing these stacks also require them to survive a glass tempering process, during which Ag will rapidly diffuse into the adjacent ZnO layers. These phenomena are undesirable since they may compromise the multilayer’s integrity and a theoretical understanding has become necessary.

Using a first principles and Crystal Orbital Hamilton Population analysis, the Ag/ZnO interfacial bonding characteristics that subsequently determine the adhesion, have been investigated. We have predicted how substitutional doping in ZnO affects the interfacial adhesion in terms of charge transfer. An additional mechanism has been found involving the hybridisation of Ag-dopant electronic states and is enhanced if the dopant contains desired d-states. By carefully choosing the dopant type the Ag/ZnO interfacial adhesion can be improved.

A new strategy is proposed to limit Ag interdiffusion in ZnO by elemental doping. First principles nudged elastic band calculations indicate that doping ZnO with desired donors (e.g. Al, Sc, Sn, and Bi) increases the Ag migration barrier along [0001] ZnO. A charge density analysis shows that ionic size and electrostatic effects have a more significant impact on Ag migration than charge transfer. In particular, the binding between Ag, O, and the dopant is found to be the main effect responsible for inhibiting Ag diffusion.

A NiCr alloy of technological interest has been investigated as a barrier layer material for Ag diffusion. Combining first principles and classical molecular dynamics techniques, it is shown that alloying Ni with ~20 at.% Cr can effectively reduce the diffusion of Ag in Ni, despite the fact that this is limited to early-stage Ag diffusion when the Ag concentration is sufficiently low. The major effect of Cr is that it repels Ag atoms and prevents them from clustering with vacancies. Cr also increases the local migration barrier of Ag as a minor effect.

Bristowe, Paul
materials modelling, heterointerfaces, ionic diffusion, transparent conductive electrodes, defects
Doctor of Philosophy (PhD)
Awarding Institution
University of Cambridge
China Scholarship Council Cambridge Commonwealth, European & International Trust Seton Cavendish Fund, Downing College Cambridge University HPC Cluster UK National Supercomputing Service, ARCHER UKCP Consortium: under EPSRC grant EP/P022596/1 Materials Chemistry Consortium: under EPSRC grant EP/L000202/1