TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: Compositions for upper and lower subcells are selected from germanium (Ge), silicon (Si), silicon germinide (SiGe), carbon gemanium (CGe), germanium tin (GeSn), silicon germanium tin (SiGeSn), and carbon silicon germanium tin (CSiGeSn). Upper subcell comprises SiGeSn, and lower subcell comprises Ge. Group-IV solar cells comprise 1st layer containing solar cell emitter layer formed from group-IV layer and 2nd layer containing tunnel junctions formed from greater than or equal to 1 III-V semiconductor layers in which tunnel junction semiconductor layers are selected from gallium arsenide (GaAs), aluminum gallium arsenide (AlGaAs), gallium indium arsenide (GaInAs), aluminum gallium indium arsenide (AlGaInAs), indium phosphide (InP), gallium phosphide (GaP), gallium indium phosphide (GaInP), gallium indium phosphorus arsenide (GaInPAs), aluminum indium phosphide (AlInP), aluminum gallium indium phosphide (AlGaInP), indium arsenide (InAs), indium phosphorus arsenide (InPAs), aluminum indium arsenide (AlInAs), aluminum arsenide (AlAs), gallium antimonide (GaSb), gallium arsenic antimonide (GaAsSb), indium antimonide (InSb), gallium indium arsenic antimonide (GaInAsSb), gallium indium nitride arsenide (GaInNAs), gallium indium nitride arsenide antimonide (GaInNAsSb), gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN), gallium indium nitride (GaInN), aluminum gallium nitride (AlGaN), aluminum indium nitride (AlInN), and/or aluminum gallium indium nitride (AlGaInN). Tunnel junctions are selected from p+GaAs/n+GaAs tunnel junctions, p+AlGaAs/n+GaAs tunnel junctions, p+GaAs/n+AlGaAs tunnel junctions, p+AlGaAs/n+AlGaAs tunnel junctions, p+AlGaAs/n+GaInP tunnel junctions, p+AlGaAs/n+AlGaInP tunnel junctions, p+AlGaInP/n+AlGaInP tunnel junctions, strained or unstrained p+Ge/n+Ge tunnel junctions, strained or unstrained p+SiGe/n+SiGe tunnel junctions, strained or unstrained p+CGe/n+CGe tunnel junctions, strained or unstrained p+SiGeSn/n+SiGeSn tunnel junctions, strained or unstrained p+CSiGeSn/n+CSiGeSn tunnel junctions, strained p+SiGe/n+Ge tunnel junctions, p+Ge/strained n+SiGe tunnel junctions, strained p+CGe/n+Ge tunnel junctions, p+Ge/strained n+CGe tunnel junctions, p+SiGeSn/n+Ge tunnel junctions, p+Ge/n+SiGeSn tunnel junctions, p+CSiGeSn/n+Ge tunnel junctions, and p+Ge/n+CSiGeSn tunnel junctions.