An Ab Initio Study of CuGaSe2(001) Surface


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The chalcopyrite material CuIn(1-x)GaxSe(2) makes one of the best performing devices for thin film solar cells. Despite this, there is still a lack of understanding of some of the fundamental properties of the material. CuGaSe2 based solar cells show only an efficiency of 9.5%, approximately half the efficiency of Cu(In,Ga)Se2 based solar cells. It is important to investigate surface and bulk properties of this material and its defect states to improve solar cell efficiency. In this work we investigated the CuGaSe2 (001) polar surface under a variety of growth conditions using ab initio theoretical studies. Experimental studies on this surface have already been done (Deniozou et al., 2005a, Deniozou et al., 2005b), and LEED characterization of the surface shows clear (4x1) reconstruction without faceting for CuGaSe2 grown under moderate Cu-excess conditions. The sample grown under low Cu-excess conditions exhibits steps/facets, and the sample at higher Cu-excess exhibits weaker reconstruction. Our study complemented the above experimental investigations by identifying the stable surface structures of the CuGaSe2 (001) reconstruction, and analyzing their surface states and resonance. Total energy electronic structure calculations based on the implementation of Density Functional Theory (DFT) in the CRYSTAL code.