Low-energy silicon allotropes with strong absorption in the visible for   photovoltaic applications
  We present state-of-the-art first-principle calculations of the electronic and optical properties of silicon allotropes with interesting characteristics for applications in thin-film solar cells. These new phases consist of distorted sp$^3$ silicon networks and have a lower formation energy than other experimentally produced silicon phases. Some of these structures turned out to have quasi-direct and dipole-allowed band gaps in the range 0.8--1.5\,eV, and to display absorption coefficients comparable with those of chalcopyrites used in thin-film record solar cells. 
Silvana Botti, Jos\'e A. Flores-Livas, Maximilian Amsler, Stefan   Goedecker, Miguel A. L. Marques
cond-mat.mtrl-sci