First-principle calculations to investigate electronic and optical properties of carbon-doped silicon

Silicon (Si) is a highly promising candidate for the applications of photovoltaics. However, the bandgap constraints, low conductivity, and bounded photon energy absorption make the applications restricted. Herein, we investigated the optimized structural, electronics and optical characteristics of carbon (C) doped Si by first-principle calculations. The analysis of the various doping illustrates that doping of the C atom in Si adds extra stability by reducing the Si–Si bond length from 2.3 Å to 2.002 Å Si–C bond length which leads to the shrinkage of lattice parameters. In addition, the bandgap of Si is reduced from 1.13 eV to 0.2 eV if a 14.28 % C-doping is employed. As the C-doping is reduced to 6.66 %, the bandgap increases to 0.87 eV with a transition from indirect to direct bandgap. Further reduction in C-doping varies the bandgap from 0.87 eV to 1.13 eV. The density of states calculations show that for a low C-doping, the transitions are mainly due to Si-3p and Si-3s orbitals in the valence band which leads to a higher energy level, while for higher C-doping, the C atom 2p orbital contribution causes a reduction in bandgap and addition of stability in the doped structures. The conductivity of the Si can be increased by utilizing higher C-doping which reduces hole effective mass by 46 % and electron effective mass by 24 %. Optical absorption of Si can be enhanced at a higher spectral range while adopting a higher C-doping, which shifts the absorption peak shapes from spikes to flat and extends to 7 eV photon energy.