AlGaInP　is　a　prospectively　valuable　material　for　high-efficiency　lattice-matched　tandem　AlGaInP/AlGaInAs/GaAs/GaInNAs/Ge　(2.05/1.7/1.4/1.0/0.7　eV)　five-junction　solar　cells.　We　examine　the　optical　properties　of　AlGaInP　materials　with　trimethylantimony　(TMSb)　incorporated　at　various　flow　speeds.　Then,　we　apply　Al0.1GaInP　with　TMSb　incorporated　at　15.6　mu　mol/min,　whose　band　gap　is　2.04　eV,　to　the　fabrication　of　an　AlGaInP/Ge　double-junction　(DJ)　solar　cell.　Moreover,　we　analyze　the　photovoltaic　current　density-voltage　(J-V)　characteristics,　external　quantum　efficiencies　(EQEs),　and　internal　quantum　efficiencies　(IQEs)　of　DJ　cells　under　a　1-sun　AM0　spectrum.　As　elsewhere,　Sb　incorporation　improves　the　crystal　quality　of　AlGaInP　and　the　fill　factor　of　the　DJ　solar　cell;　on　the　other　hand,　it　increases　the　band　gap　of　AlGaInP.　Al0.08GaInP　with　Sb　incorporation　shows　the　same　absorption　edge　of　EQE　as　Al0.1GaInP　without　Sb　incorporation,　and　J(sc)　increases　approximately　by　15.3%　owing　to　the　reduction　of　2%　Al　composition.　(c)　2017　The　Japan　Society　of　Applied　Physics