Doping limits, band gaps, work functions and energy band alignments of undoped and donor-doped transparent conducting oxides ZnO, In2O3, and SnO2 as accessed by X-ray and ultraviolet photoelectron spectroscopy (XPS/UPS) are summarized and compared. The presented collection provides an extensive data set of technologically relevant electronic properties of photovoltaic transparent electrode materials and illustrates how these relate to the underlying defect chemistry, the dependence of surface dipoles on crystallographic orientation and/or surface termination, and Fermi level pinning.

No abstract

The growth of Al 2 O 3 onto Sn-doped In 2 O 3 (ITO) by atomic layer deposition (ALD) was studied in situ using X-ray photoelectron spectroscopy. Significant diffusion of oxygen from the substrate destroys the self-terminated monolayer adsorption of the metal precursor and results in a nominal initial growth per cycle of >1 nm. The observed mechanism precludes the preparation of monolayer thick Al 2 O 3 films on ITO substrates by ALD. The energy band alignment at the ITO/Al 2 O 3 interface is significantly different from that obtained when magnetron sputtering is used for the deposition of Al 2 O 3 onto ITO [Gassenbauer et al., Phys. Chem. Chem. Phys. 2009, 11, 3049]. The difference is attributed to a pinning of the Fermi level in the ALD-Al 2 O 3 layer close to midgap, which is attributed to the incorporation of hydrogen in the film during growth.

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.

The interface formation between transparent conducting Sn-doped indium oxide (ITO) and dielectric aluminium oxide has been studied by photoelectron spectroscopy using in situ sample preparation by magnetron sputtering. The electronic structure including band alignment, changes in Fermi level position and work function is determined. The changes of Fermi level are related to the deposition technique used.