A goal for concentrating photovoltaics is to realize efficiencies over 50%. Recent 4J bonded solar cells show a path to such high efficiency devices by separately growing the top and bottom solar cells. Present experimental devices use InP-based materials for the bottom junctions. III-Sb solar cells can be good candidates for bottom solar cells. Sb-containing III-V alloys have shown high electron mobility, wide band gap range including small band gaps, flexible band alignment, and small effective electron mass [1]. In addition, GaSb alloys can be grown with low defect densities on GaAs. This paper investigates GaSb-based solar cells. We show AlGaSb based solar cells grown directly on semi-insulator GaAs (001) substrates by Molecular Beam Epitaxy (MBE). Device and structural investigations have been performed to assess the electrical properties and material quality. Devices in the GaSb material system show Woc of 0.30, a very high value for a low band gap solar cell. To control the device properties, GaSb based solar cells grown on GaAs (100) substrates were compared to the devices grown on GaSb substrates.