Cu₂O derived materials are currently leading candidates for p-type ransparent conducting oxides (TCOs). In the "Novel Advanced Transparent Conducting Oxides" (NATCO) project, we are using first principles density functional theory (DFT) calculations in collaboration with experiment to develop and characterise candidate p-type TCO materials. Currently leading p-type TCOs based on Cu₂O are SrCu₂O₂, CuAlO₂ and CuGaO₂, which are alloys of Cu₂O and a second oxide material. Simple substitutional doping of Cu₂O with nickel has recently been studied. In this presenation, we use DFT to study substitutional doping of Cu₂O, with particular emphasis on the effect of dopant ionic radius and electronic structure. We firstly show that in Cu₂O, the origin of the small band gap arises from the presence of Cu-Cu interactions. In terms of doping, ions with larger ionic radius that Cu⁺ (Sn²⁺, La³⁺, Ce⁴⁺) strongly distort the atomic structure around the dopant site, disrupting the Cu-Cu interactions, while doping with ions that show a smaller ionic radius (Mg²⁺, Al³⁺, Cr⁴⁺) has no effect on the local atomic structure around the dopnat site. For Sn and La, the distortions to theo Cu-Cu interactions open up the band gap, while for Al and Cr, the lack of structural distortion leads to a small change in band gap. This demonstrates explicitly the important, yet unexplored, role of Cu-Cu interactions in Cu₂O in determining the transparency of doped Cu₂O. However, the dopant electronic states also play a crucial role, as exemplified by Ce and In, both of which have larger ionic radii than Cu₊ and show strong distortions around the dopant site; however, the presence of the dopant electronic states that interact with the Cu₂O valence or conduction band edge, reduces the band gap and transparency. From these extensive calculations a set of rules for choosing potential dopants are presented.

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