A dozen predicted SiGe alloys with low enthalpies and strong absorption of sunlight for photovoltaic applications

Abstract

Silicon germanium alloy materials have promising potential applications in the optoelectronic and photovoltaic industries due to their good electronic properties. However, due to the inherent brittleness of semiconductor materials, they are prone to rupturing under harsh working environments, such as high stress or high temperature. Here, we conducted a systematic search for silicon germanium alloy structures using a random sampling strategy, in combination with group theory and graph theory (RG²), and 12 stable SiGe structures in 2–8 stacking orders were predicted. All 12 stable SiGe crystals exhibit a popular bandwidth of 1.06–1.19 eV, approaching the optimal Shockley Queisser limit (≈1.4 eV). Among these, 6 structures showed quasi-direct band gaps. Considering their potential photovoltaic applications, we systematically studied the changes in their enthalpy, stability, mechanical stability (elastic moduli), lattice parameters, band structures, and light absorption under a stress load of up to 20 GPa. These new SiGe crystals featured relatively low enthalpies (even as low as 0.009 eV per atom), and good stability and mechanical properties. In addition, the absorption spectra of these materials demonstrated a high absorption intensity for the solar spectrum that was approximately 3 times higher than that of conventional diamond silicon, even under a 20 GPa stress. The present study uses the predicted 2–8H SiGe to provide new insights into the photovoltaic applications of SiGe alloy structures.