SnxGe1–xAlloy Nanocrystals: A First Step toward Solution-ProcessedGroup IV Photovoltaics.

Nontoxic,sustainable, and cost-effective, Group IV materials areattractive 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. Previousstudies have shown that the band structures of these materials canbe modified by the influence of quantum confinement in nanostructuresor by alloying with tin (Sn) in metastable thin films; to date, neithermethod has produced a material with a direct band gap of appropriateenergy for application in, for example, efficient solar photovoltaics.We have developed a facile colloidal method for the synthesis of size-controlled,homogeneous SnxGe1–xalloy nanocrystals (NCs) with remarkably high tinconcentration (xup to 0.42). We demonstrate thatNCs of the same size exhibit a pronounced, systematic red-shift inthe optical band gap, and a significant increase in molar absorptivity,with increasing Sn-content, and a measurable photoluminescence wasobserved from NCs with high contents. The indications of at leastpartial direct-gap character in these NCs, combined with their broadtunability throughout the infrared, suggest their promise for usein solution-processed solar cells. [ABSTRACT FROM AUTHOR]