  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.
