Route leading to exotic silicon allotropes and compounds*

Over the past decades, diamond silicon has become the fundamental building block in solar cell market due to its high abundance and stability. However, the band gap of d-Si is indirect, therefore the absorbing crystal layer has to be thick and pure, so that the mean-free path of the carriers is comparable with the size of the layer. Due to these well-known limitations, materials with better absorption coefficients have been put forward in the past years. Emphasis has obviously been given to direct band-gap materials with absorption spectra that strongly overlap with the solar spectrum. This would allow for thinner, more flexible, and cheaper silicon solar cells. Using swarm-intelligence-based structure prediction methods, we predict a new Cmcm-SrSi₈ compound that can be synthesized under epitaxial strain conditions at high pressures. The stability of Cmcm-SrSi₈ down to zero pressure has been demonstrated using phonon spectrum calculations. After pressure release, we estimate that using the SrSi₈ clathrate-like structure as a precursor, the Sr degassing process will create a direct bandgap Si phase, designated Si₃₂. Due to the direct bandgap of 1.15 eV, we propose that Si₃₂ could be a potential solar energy absorber. These encouraging results shed fresh light on the developmental approach for direct bandgap semiconductor design. Therefore, there is potential to create a far more diverse landscape for silicon materials with advanced properties.