Density-functional theory (DFT) calculations of intrinsic point defect properties in zinc oxide were performed in order to remedy the influence of finite-size effects and the improper description of the band structure. The generalized gradient approximation (GGA) with empirical self-interaction corrections $(\mathrm{GGA}+U)$ was applied to correct for the overestimation of covalency intrinsic to GGA-DFT calculations. Elastic as well as electrostatic image interactions were accounted for by application of extensive finite-size scaling and compensating charge corrections. Size-corrected formation enthalpies and volumes as well as their charge state dependence have been deduced. Our results partly confirm earlier calculations but reveal a larger number of transition levels: (1) For both the zinc interstitial as well as the oxygen vacancy, transition levels are close to the conduction band minimum. (2) The zinc vacancy shows a transition rather close to the valence band maximum and another one near the middle of the calculated band gap. (3) For the oxygen interstitials, transition levels occur both near the valence band maximum and the conduction band minimum.

Doping limits, band gaps, work functions and energy band alignments of undoped and donor-doped transparent conducting oxides Zn₀, 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.

Polycrystalline anatase thin films, (001)- and (101)-oriented anatase TiO ${}_2$ single crystals and (001)- and (110)-oriented rutile TiO ${}_2$ single crystals with various surface treatments were studied by photoelectron spectroscopy to obtain their surface potentials. Regardless of orientations and polymorph, a huge variation of the Fermi level and work function was achieved by varying the surface condition. The most strongly oxidized surfaces are obtained after oxygen plasma treatment with a Fermi level ∼2.6 eV above the valence band maximum and ionization potentials of up to 9.5 eV (work function 7.9 eV). All other treated anatase surfaces exhibit an ionization potential independent of surface condition of $7.96\pm 0.15$ eV. The Fermi level positions and the work functions vary by up to 1 eV. The ionization potential of rutile is ∼0.56 eV lower than that of anatase in good agreement with recent band alignment studies.

The nature of the band gap of indium oxide is still a matter of debate. Based on optical measurements the presence of an indirect band gap has been suggested, which is 0.9 to $1.1\mathrm{eV}$ smaller than the direct band gap at the $\Gamma$ point. This could be caused by strong mixing of $\mathrm{O}2p$ and $\mathrm{In}4d$ orbitals off $\Gamma$. We have performed extensive density functional theory calculations using the LDA+U and the GGA+U methods to elucidate the contribution of the $\mathrm{In}4d$ states and the effect of spin-orbit coupling on the valence band structure. Although an indirect band gap is obtained, the energy difference between the overall valence band maximum and the highest occupied level at the $\Gamma$ point is less than $50\mathrm{meV}$. It is concluded that the experimental observation cannot be related to the electronic structure of the defect free bulk material.

Surfaces of ${\mathrm{In}}_2{\mathrm{O}}_3$ and tin-doped ${\mathrm{In}}_2{\mathrm{O}}_3$ (ITO) were investigated using photoelectron spectroscopy. Parts of the measurements were carried out directly after thin film preparation by magnetron sputtering without breaking vacuum. In addition samples were measured during exposure to oxidizing and reducing gases at pressures of up to $100\mathrm{Pa}$ using synchrotron radiation from the BESSY II storage ring. Reproducible changes of binding energies with temperature and atmosphere are observed, which are attributed to changes of the surface Fermi level position. We present evidence that the Fermi edge emission observed at ITO surfaces is due to metallic surface states rather than to filled conduction band states. The observed variation of the Fermi level position at the ITO surface with experimental conditions is accompanied by a large apparent variation of the core level to valence band maximum binding energy difference as a result of core-hole screening by the free carriers in the surface states. In addition segregation of Sn to the surface is driven by the surface potential gradient. At elevated temperatures the surface Sn concentration reproducibly changes with exposure to different environments and shows a correlation with the Fermi level position.