This chapter describes the state of the art in computer simulations in the context of the development of high-efficiency solar cells. It discusses how one analyses by theoretical means the structural, electronic, and optical properties of emerging copper-based chalcogenides, employing atomistic first-principles computational methods within density functional theory. The fundamental material characteristics of the compounds are analysed, and the optoelectronic performances are improved by alloying with isovalent elements. In order to develop inorganic photovoltaics based on an ultrathin, photon-absorbing film (i.e., with thickness d < 100 nm), the material should exhibit an optimised band gap energy, Eg, as well as have a very high absorption coefficient α(ω), especially for photon energies in the lower energy region of the absorption spectrum: Eg ≤ E < (Eg + 2 eV). To develop high-efficiency solar cells, we therefore suggest tailor making the materials to form direct-gap, multi-valley band edges, and energy bands with rather flat dispersions. These properties can typically be achieved by considering alloys with heavy elements that have relatively localised sp-like orbitals. With such tailored materials, we demonstrate that it is possible to reach a theoretical maximum efficiency as high as ηmax ≈ 30% for film thickness of d ≈ 50–100 nm. Such an approach is useful to support the search for new materials to drive innovation in solar technology in the future.