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 $s{p}^3$ 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.

Figure 1

Representation of the most important $s{p}^3$ Si structures found in this work. The superscript in Imma${}^{\left(2\right)}$ serves to distinguish this structure from the high-pressure phase Si-XI of the same symmetry.

Figure 2

Energy per atom as a function of volume per atom for the most important $s{p}^3$ silicon structures found in this work. The zero of energy is the total energy per atom of the cubic diamond structure in equilibrium. For comparison we also include some already known structures of silicon, e.g., cubic diamond, a clathrate,[51] BC8, and R8.

Figure 3

Absorption spectra of the most important $s{p}^3$ Si structures found in this work compared to the reference air mass 1.5 solar spectral irradiance,[59] given in arbitrary units.