Development of ZnSiP$_{\mathbf 2}$ for Si-Based Tandem Solar Cells

A major technological challenge in photovoltaics is the implementation of a lattice matched optically efficient material to be used in conjunction with silicon for tandem photovoltaics. Detailed balance calculations predict an increase in efficiency of up to 12 percentage points for a tandem cell compared with single junction silicon. Given that the III-V materials currently hold world record efficiencies, both for single and multijunction cells, it would be transformative to develop a material that has similar properties to the III-V's which is also lattice matched to silicon. The II-IV-V2 chalcopyrites are a promising class of materials that could satisfy these criteria. ZnSiP₂ in particular is known to have a bandgap of ~2 eV, a lattice mismatch with silicon of 0.5%, and is earth abundant. Its direct bandgap is symmetry-forbidden. We have grown single crystals of ZnSiP2 by a flux growth technique. Structure and phase purity have been confirmed by X-ray diffraction and transmission electron microscopy. Optical measurements, along with a calculation of the absorption spectrum, confirm the ~2 eV bandgap. Because of its structural similarity to both crystalline silicon and the III-V's, ZnSiP₂ is expected to have good optoelectronic performance.