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Electronic properties of CIGS materials for photovoltaic conversion: self-consistent GW calculations

Authors: S. Botti

Ref.: Workshop ‚ÄúComputational Nanoscience for Renewable Energy Solutions‚ÄĚ, Helsinki, Finland, 14/9/2009 (2009)

Abstract: During the past years, Cu(In,Ga)(Se,S)$_2$ (CIGS) thin-film solar cells have 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. This appears as a consequence of self-healing mechanisms that compensate for the creation of defects. The origin of this unusual behavior is still not understood, but it is clear that its understanding would pave the way to the tuning of new materials for more efficient photovoltaic energy conversion. In this context, ab initio calculations that can predict and analyze the interplay between structural and electronic properties can give a crucial contribution to the interpretation of numerous experiments that, in this field, is often far from straightforward. I will discuss why density functional theory - the standard ab initio tool in modern condensed matter theory - is plagued by serious shortcomings when applied to electronic properties of CIGS systems. Moreover, also standard perturbative GW calculations fail in giving good electronic states. I will show how a state-of-the-art (restricted) self-consistent GW approach allows for the correct treatment of d-electrons in these materials. In particular, the self-consistency in the many-body treatment is critical to estimate both the quasi-particle band-gap and band offsets.