Low-energy silicon allotropes with strong absorption in the visible for photovoltaic applications

We present state-of-the-art first-principles 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 sp³ silicon networks and have a lower formation energy than other experimentally produced silicon phases. Some of these structures turned out to have quasidirect 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.