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Electronic excitations in materials for solar cells

Authors: S. Botti

Ref.: Seminar at Laboratory for Photovoltaics, Luxembourg, 6/3/2012 (2012)

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. The family of kesterites Cu$_2$ZnSe(S,Se)$_4$ exhibits very similar electronic properties. Moreover, kesterites have the clear advantage of being composed of abundant, non-toxic, less expensive chemical elements. The origin of the exceptional electronic properties and the defect physics of 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 often results 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? By presenting some examples of calculations, 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. Finally, I will present an example of application of material design, that represents at present the new frontier in the field of theoretical material science. Using the minima hopping method, we found and characterized new low-enthalpy phases of silicon with almost-direct band gaps and displaying strong absorption in the visible.