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Electronic excitations in thin-film materials for solar cells: beyond standard density functional theory

Authors: S. Botti

Ref.: Theory and computation of electronic structure: new frontiers Symposium @ DPG Spring Meeting, Dresden, Germany, 17/3/2011 (2011)

Abstract: During the past years, Cu(In,Ga)(Se,S)$_2$ (CIGS) thin-film solar cells emerged as a technology that could challenge the current hegemony of silicon solar panels. CIGS compounds conserve to a very high degree their electronic properties in a large non-stoichiometric range and are remarkably insensitive to radiation damage or impurities. CIGS are now challenged by the family of kesterites Cu$_2$ZnSe(S,Se)$_4$. Like CIGS, kesterites exhibit an optimal band gap of around 1.5 eV for efficient light absorption, but they have the clear advantage over the former of being composed of abundant, non-toxic, less expensive chemical elements. 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. Thin films of transparent conductive oxides of the delafossite family, namely Cu(Al,Ga,In)O$_2$, are particularly interesting as they show bipolar conductivity. The origin of the exceptional electronic properties of all these complex materials is still not completely understood, despite the large amount of experimental and theoretical work dedicated to that purpose. In particular, standard density functional theory yields band structures in quantitative and qualitative disagreement with experiments. This is a serious problem when it comes to designing new materials for more efficient photovoltaic energy conversion. In this context, can ab-initio calculations of electronic excitations beyond ground-state density functional theory give a crucial contribution? I will discuss which theoretical approaches are reliable at a reasonable computational cost and what is the physical insight that they allow to gain on electronic excitations in new materials for photovoltaics.