We propose trigonal Cu2-II-Sn-VI4 (II = Ba, Sr and VI = S, Se) quaternary compounds for earth-abundant solar cell applications. Through density functional theory calculations, we show that these compounds exhibit similar electronic and optical properties to kesterite Cu2ZnSnS4 (CZTS): high optical absorption with band gaps suitable for efficient single-junction solar cell applications. However, the trigonal Cu2-II-Sn-VI4 compounds exhibit defect properties more suitable for photovoltaic applications than those of CZTS. In CZTS, the dominant defects are the deep acceptors, Cu substitutions on Zn sites, which cause non-radiative recombination and limit the open-circuit voltages of CZTS solar cells. On the contrary, the dominant defects in trigonal Cu2-II-Sn-VI4 are the shallow acceptors, Cu vacancies, similar to those in CuInSe2. Our results suggest that the trigonal Cu2-II-Sn-VI4 quaternary compounds could be promising candidates for efficient earth-abundant thin-film solar cell and photoeletrochemical water-splitting applications.

The crystal structures of Ru2C were extensively investigated using the structure searching method coupled with first-principles calculations. In contrast to the previously proposed P-3m1 phase, two energetically more favorable structures with space groups P-31m and P63/mmc were found, which are stable at pressure ranges of 0-32 GPa and 32-50 GPa, respectively. The dynamical stabilities of both phases at ambient and high pressures are confirmed by the phonon dispersions. Further calculations indicate that the two new predicted phases are ultra-incompressible metals due to a strong covalent Ru-C bond.