Wurtzite silicon as a potential absorber in photovoltaics: Tailoring the optical absorption by applying strain

We present ab initio calculations of the electronic structure and the optical properties of wurtzite Si (Si-IV). We find an indirect band gap of 0.95 eV (Gamma(5) -> M-1) and an optically forbidden direct gap of 1.63 eV (Gamma(5) -> Gamma(10)), which is due to a backfolding of the L-1 state of Si in the diamond structure (Si-I). Optical absorption spectra including excitonic and local-field effects are calculated. Further, the effects of hydrostatic pressure, uniaxial strain, and biaxial strain on the absorption properties are investigated. Biaxial tensile strains enhance the optical absorption of Si-IV in the spectral range which is relevant for photovoltaic applications. High biaxial tensile strains (>4%) even transform Si-IV into a direct semiconductor.