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.