  In our present work, five previously proposed sp$^3$ carbon crystals were
suggested as silicon allotropes and their stabilities, electronic and optical
properties were investigated by first-principles method. We find that these
allotropes with direct or quasi-direct band gaps in range of 1.2-1.6 eV are
very suitable for applications in thin-film solar cells. They display strong
adsorption coefficients in the visible range of the sunlight in comparison with
diamond silicon. These five silicon allotropes are confirmed possessing
positive dynamical stability and remarkable themodynamical stability close to
that of diamond silicon. Especially, the direct band gap M585-silicon
possessing energy higher than diamond silicon only 25 meV per atom is expected
to be experimentally produced for thin-film solar cells.
