Sn_xGe_1–x Alloy Nanocrystals: A First Step toward Solution-Processed Group IV Photovoltaics

Nontoxic, sustainable, and cost-effective, Group IV materials are attractive for a broad range of electronic and optoelectronic applications, although the indirect principal band gaps of silicon and germanium (Ge) present complications that impact device design and cost. Previous studies have shown that the band structures of these materials can be modified by the influence of quantum confinement in nanostructures or by alloying with tin (Sn) in metastable thin films; to date, neither method has produced a material with a direct band gap of appropriate energy for application in, for example, efficient solar photovoltaics. We have developed a facile colloidal method for the synthesis of size-controlled, homogeneous Sn x Ge 1– x alloy nanocrystals (NCs) with remarkably high tin concentration ( x up to 0.42). We demonstrate that NCs of the same size exhibit a pronounced, systematic red-shift in the optical band gap, and a significant increase in molar absorptivity, with increasing Sn-content, and a measurable photoluminescence was observed from NCs with high contents. The indications of at least partial direct-gap character in these NCs, combined with their broad tunability throughout the infrared, suggest their promise for use in solution-processed solar cells.