Orientation dependent ionization potential of In₂O₃: a natural source for inhomogeneous barrier formation at electrode interfaces in organic electronics

Abstract

The ionization potentials of In(2)O(3) films grown epitaxially by magnetron sputtering on Y-stabilized ZrO(2) substrates with (100) and (111) surface orientation are determined using photoelectron spectroscopy. Epitaxial growth is verified using x-ray diffraction. The observed ionization potentials, which directly affect the work functions, are in good agreement with ab initio calculations using density functional theory. While the (111) surface exhibits a stable surface termination with an ionization potential of ∼ 7.0 eV, the surface termination and the ionization potential of the (100) surface depend strongly on the oxygen chemical potential. With the given deposition conditions an ionization potential of ∼ 7.7 eV is obtained, which is attributed to a surface termination stabilized by oxygen dimers. This orientation dependence also explains the lower ionization potentials observed for In(2)O(3) compared to Sn-doped In(2)O(3) (ITO) (Klein et al 2009 Thin Solid Films 518 1197-203). Due to the orientation dependent ionization potential, a polycrystalline ITO film will exhibit a laterally varying work function, which results in an inhomogeneous charge injection into organic semiconductors when used as electrode material. The variation of work function will become even more pronounced when oxygen plasma or UV-ozone treatments are performed, as an oxidation of the surface is only possible for the (100) surface. The influence of the deposition technique on the formation of stable surface terminations is also discussed.

Transparent Conducting Oxides: Electronic Structure–Property Relationship from Photoelectron Spectroscopy with in situ Sample Preparation

“…, one might expect the following order of the ionization potentials of In 2 O 3 : cation terminated (100): mostly positive In ions at the surface <(110): neutral surface < (111): small negative surface dipole due to O ions being only slightly above the In plane < oxygen terminated (100): mostly negatively charged O ions in the topmost surface plane. This order of ionization potentials is well reproduced by DFT calculations …”

“…By postdeposition oxidizing treatments, the ionization potential of ITO can be increased to ~8.1 eV . More recently, the different ionization potentials could be attributed to different surface orientations by using density functional theory calculations and photoemission measurements of epitaxial In 2 O 3 films . An ionization potential of 7.1 eV corresponds well with the In 2 O 3 (111) surface, whereas the value of 7.7 eV is characteristic for the (100) surface orientation.…”

Reduction-induced Fermi level pinning at the interfaces between Pb(Zr,Ti)O3 and Pt, Cu and Ag metal electrodes

“…These values compare well with those given by Scott (χ = 3.5 ± 0.2 eV [1,17]). It is noted, however, that the ionization potential may depend strongly on surface orientation and surface termination [27,28] and is therefore a poor quantity for estimating Schottky barrier heights.…”

Influence of the indium tin oxide/organic interface on open-circuit voltage, recombination, and cell degradation in organic small-molecule solar cells

“…The ITO work function and ionization potential considerably depend on the specific crystallographic surface as has been calculated for low index surfaces 75 and measured for epitaxially grown In 2 O 3 . 76 We derive the ITO work function from the secondary electron onset in UPS spectra [see Fig. 4(a)].…”