Towards Direct-Gap Silicon Phases by the Inverse Band Structure Design   Approach
  Diamond silicon (Si) is the leading material in current solar cell market. However, diamond Si is an indirect band gap semiconductor with a large energy difference (2.4 eV) between the direct gap and the indirect gap, which makes it an inefficient absorber of light. In this work, we develop a novel inverse-band structure design approach based on the particle swarming optimization algorithm to predict the metastable Si phases with better optical properties than diamond Si. Using our new method, we predict a cubic Si20 phase with quasi-direct gaps of 1.55 eV, which is a promising candidate for making thin-film solar cells. 
H. J. Xiang, Bing Huang, Erjun Kan, Su-Huai Wei, X. G. Gong
cond-mat.mtrl-sci