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Development of materials for high-efficient solar energy technologies requires fundamental understanding of the material properties on an atomic level, like for instance alloying, interfaces, and defects.Although it is from growth and manufacture that devices will be realized, theoretical modeling has the enormous advantage to be able to explore new material compounds, as well as to analyze device structures with a minimal time and cost.First-principles calculations within the density functional theory (DFT) have been extremely successful over the last four decades to explain the ground-state properties of condensed matter.Todays progress in supercomputer capacity, code development, and improved description of the electronic interactions support an even more sophisticated analysis of various materials and complex material structures.In this talk, we discuss how one can benefit from the atomistic first-principles DFT modeling of materials to understand the underlying physics of semiconductors for solar cell technologies.With a detailed understanding of various materials and crystalline structures, we can design semiconductor alloys with improved material properties for better device performance.The talk will focus on controlling the electronic and optical properties materials by isovalent alloying of the group Ⅱ-Ⅵ semiconductors [e.g.Zn(O,S)], group Ⅰ-Ⅲ-Ⅵ2 [e.g.Cu(In,Ga)(S,Se)2], and group Ⅰ2-Ⅱ-Ⅳ-Ⅵ4 [e.g.kesteriteCu2ZnSn(S,Se)4], but also discuss the possibility to explore novel types of alloys,like for instance by alloying ZnO with GaN (Refs.1-4).[1] C.Persson, et al., Phys.Rev.Lett.97, 146403 (2006).[2] C.Persson, Appl.Phys.Lett.93, 072106 (2008).[3] C.Persson, J.Appl.Phys.107, 053710 (2010).[4] M.Dou, et al.Phys.Status Solidi A 209, 75 (2012).