Synthesis of Group IV Clathrates for Photovoltaics

Although Si dominates the photovoltaics market, only two forms of Si have been thoroughly considered: amorphous Si and Si in the diamond structure ( d-Si). Silicon can also form in other allotropes, including clathrate structures. Silicon clathrates are inclusion compounds, which consist of an Si framework surrounding templating guest atoms (e.g., Na). After formation of the type II Na ₂₄Si₁₃₆ clathrate, the guest atoms can be removed (Si₁₃₆), and the material transitions from degenerate to semiconducting behavior with a 1.9 eV direct band gap. This band gap is tunable in the range of 1.9-0.6 eV by alloying the host framework with Ge, enabling a variety of photovoltaic applications that include thin-film single-junction devices, Si₁₃₆ top cells on d-Si for all-Si tandem cells, and multijunction cells with varying Si/Ge ratios. In this study, we present electronic structure calculations that show the evolution of the direct transition as a function of Si/Ge ratio across the alloy range. We demonstrate the synthesis of type II Si/Ge clathrates spanning the whole alloy range. We also demonstrate a technique for forming Si clathrate films on d-Si wafers and sapphire substrates.