First-principles prediction of two hexagonal silicon crystals as potential absorbing layer materials for solar-cell application

Abstract

Based on first-principles calculations, the structures, stabilities, electronic properties, and mechanical properties as well as optical properties of two new hexagonal silicon allotropes (Hex-193 and Hex-194) were theoretically investigated. Both Hex-193 and Hex-194 are confirmed to be meta-stable phases with energetic stability exceeding the previously proposed Si-20 and the experimentally viable Si-24. They are also confirmed to be dynamically and elastically stable silicon phases according to their calculated phonon band structures and mechanical properties. The HSE06-based band structures of Hex-193 and Hex-194 indicate that they are quasi-direct bandgap semiconductors with fundamental bandgaps of 1.275 eV (direct bandgap is 1.277 eV) and 1.200 eV (direct bandgap is 1.302 eV), respectively. Further investigation of the optical properties shows that both Hex-193 and Hex-194 exhibit a stronger absorption coefficient in comparison to that of diamond silicon, which indicates that both are potential materials for solar-cell application.