Understanding Conductivity Anomalies in CuI-Based Delafossite Transparent Conducting Oxides: Theoretical Insights

The CuI-based delafossite structure, CuIMIIIO2, can accommodate a wide range of rare earth and transition metal cations on the MIII site. Substitutional doping of divalent ions for these trivalent metals is known to produce higher p-type conductivity than that occurring in the undoped materials. However, an explanation of the conductivity anomalies observed in these p-type materials, as the trivalent metal is varied, is still lacking. In this article, we examine the electronic structure of CuIMIIIO2 (MIII = Al, Cr, Sc, Y) using density functional theory corrected for on-site Coulomb interactions in strongly correlated systems (GGA+U) and discuss the unusual experimental trends. The importance of covalent interactions between the MIII cation and oxygen for improving conductivity in the delafossite structure is highlighted, with the covalency trends found to perfectly match the conductivity trends. We also show that calculating the natural band offsets and the effective masses of the valence band maxima is not an ideal method to classify the conduction properties of these ternary materials.

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Understanding Conductivity in SrCu2O2: Stability, Geometry, and Electronic Structure of Intrinsic Defects from First Principles

Density functional theory calculations have been performed on stoichiometric and intrinsically defective p-type transparent conducting oxide SrCu2O2, using GGA corrected for on-site Coulombic interactions (GGA+U). Analysis of the absorption spectrum of SrCu2O2 indicates that the fundamental direct band gap could be as much as 0.5 eV smaller than the optical band gap. Our results indicate that the defects that cause p-type conductivity are favoured under all conditions, with defects that cause n-type conductivity having significantly higher formation energies. We show conclusively that the most stable defects are copper and strontium vacancies. Copper vacancies introduce a distinct acceptor single particle level above the valence band maximum, consistent with the experimentally known activated hopping mechanism.

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