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Effects of Electronic and Lattice Polarization on the Band Structure of Delafossite Transparent Conductive Oxides

Authors: Fabio Trani, Julien Vidal, Fabien Bruneval, Miguel A. L. Marques, and Silvana Botti

Ref.: TCM 2010 3nd International Symposium on Transparent Conductive Materials, 17 - 21 October, 2010 Analipsi / Hersonissos, Crete, Greece (2010)

Abstract: Copper delafossite transparent conducting oxides [Cu(Al,Ga,In)O2] are expected to play a very important role in the future of thin-film technology. In particular, they are chemically very similar to the CIS/CIGS [Cu(In,Ga)(Se,S)2] family of materials whose technology could challenge the current hegemony of silicon solar panels in photovoltaic applications. As known, thin film solar cells require transparent contacts. In practice, these contacts are built from insulating oxides that for a certain range of doping become conductive while retaining transparency in the visible spectrum. An appropriate oxidation of the surface of the CIS/CIGS absorbing layer could easily provide the transparent contact for the solar cell, forming a layer of copper delafossite oxides, that is as an extremely interesting candidate for the transparent layer in solar cell technology.

But copper delafossites have other exciting properties not found in other TCOs. In fact, while most of the TCOs (SnO2, TiO2, ZnO …) show an n-type doping, copper delafossites can be doped with holes (p-type doping). Furthermore, one of the members of the copper delafossite family (the In compound) exhibits bipolar doping, which is unique among TCOs, allowing the production of transparent junctions and electronics, and opening the way to science-fiction-like technologies, like windows that produce electricity, toward the so called transparent electronics. Also, delafossite materials exhibit all sorts of interesting phenomena, like huge excitonic and polaronic effects, making them a very curious playground for both experimental and theoretical physicists.

There has been a substantial experimental and theoretical effort to study the electronic properties of these materials. The results were, however, conflicting, without a clear picture emerging from the published data. To resolve this issue, we applied the most accurate first-principles method available (the quasi-particle self-consistent GW) to study delafossite Cu(Al, In)O2.

Our results point to a relatively complex situation: the purely electronic band-gap should be much larger than obtained from previous calculations. It is the effect of the lattice polarization (polarons), that decreases the gap by around 1 eV, restoring agreement with experiment. We can expect that this situation, of a large gap that is reduced substantially by polaronic effects, is quite general and is present in many more materials that previously expected. We propose a theoretical approach that includes polaron effects in a first-principles quasi-particle self-consistent GW scheme. The scheme is very general and innovative, and it allows for a calculation of polaron effects in the band-structure of materials. In order to have a clear picture of the phenomenon, we complemented our calculations with results obtained using the most accurate pure and hybrid exchange-correlation functionals (we performed calculations using LDA, GGA, LDA+U, B3LYP, HSE03, HSE06, PBE0).

This communication has been awarded the prize for the Best Oral presentation of TCM2010.