The origin of the high work function of chlorinated indium tin oxide

Abstract

The impact of halogenation, in particular Cl and F, on the work functions of indium tin oxide (ITO) surfaces was studied using density functional theory calculations. We found that a strong surface dipole layer induced by the halogen, rather than a change in the electrochemical potential (that is, Fermi level) of the ITO, led to a dramatic increase in the work function. However, the work function for F-coated ITO was lower than that of Cl-coated ITO. This result contradicts the well-known fact that F is much more electronegative than Cl. Detailed computations reveal that both electronegativity and atomic size collectively contribute to the extraordinarily high work function of Cl-ITO. Additionally, the work function increases linearly with increasing surface halogen coverage for both systems, which was consistent with experimental data. The transparent tin-doped indium oxide (ITO) is a particularly attractive material for use in the electrodes of organic electronic devices. However, ITO suffers from a low ‘work function’ — the energy required to remove an electron from the material's surface and an important characteristic of such devices, which rely on charge transport between layers of different materials. Coating the surface of ITO with halogen atoms, particularly chlorine, has recently been shown to increase the work function. Now, Yao He at Yunnan University and co-workers in China and Canada have used density functional theory calculations to explore why this occurs. The work function of ITO increases linearly as its surface becomes covered with either chlorine or fluorine — to notably higher values with chlorine — owing to charge transfer from the material to the halogen. The magnitude of the effect is influenced by the halogen's electronegativity and ionic radius; chlorine's superior efficiency arises because it strikes the correct balance between the two. The work functions of indium tin oxide terminated with Cl and F have been studied using DFT calculations. The results show that the work function of Cl-terminated ITO is much higher than that of F-terminated ITO despite the fact that F is more electronegative than Cl. Detailed analysis through visualization of the atomic-scale charge transfer at these adatom–oxide interfaces reveals that both high electronegativity and atomic size are crucial to increase the work function of ITO.