Robyn L. Woo 1 , William D. Hong 1 , Shoghig Mesropian 1 , Marina S. Leite 2 , Harry A. Atwater 2 , Daniel C. Law 1 ! "# $ Spectrolab has demonstrated the first lattice matched InAlAs/InGaAsP/InGaAs triple junction solar cell grown on InP substrate. X-ray diffraction characterization shows high quality solar cell materials. Preliminary 1-sun AM1.5D testing of the triple junction solar cell shows promising results with an open circuit voltage (Voc) of 1.8V, a short-circuit current density (Jsc) of 11.0 mA/cm 2 , a fill factor of 64.4 %, and a 1-sun AM1.5D efficiency of 13.8%. The same cell also passes 27-suns under concentration. Improvements in layer design and crystal quality of advanced features can further raise the 1-sun and concentrated AM1.5D conversion efficiency of the InP- based triple junction cell beyond 20%. High-efficiency III-V multijunction solar cells for terrestrial applications have traditionally been grown lattice matched to either bulk Ge or GaAs substrates. However, reaching even higher cell efficiency requires solar cell designs with optimal bandgap combination, which in turn, requires materials that are outside of Ge or GaAs lattice constants. One of the existing solutions in the III-V solar cell community is the use of highly metamorphic materials in the upright or inverted configuration [1-2]. An upright metamorphic approach requires the active cells be grown on top of metamorphic buffer grade; whereas, an inverted metamorphic approach requires the thick low bandgap cell to be grown last. Both cases require meticulous material engineering against defects induced by large mismatch and long thermal history. One alternative approach to existing solutions is to explore InP or near InP lattice space parameter, as low bandgap (~0.7-1.2-eV) component subcells at the InP or near InP lattice constant are becoming one of the key components in the ultra-high efficiency multijunction solar cell structure such as inverted metamorphic, upright metamorphic [1-4] and semiconductor-bonded multi-junction solar cell [5]. However, high bandgap materials available in InP or near InP lattice constant are scarce and have not been intensively studied. The ternary compound InxAl1-xAs stands out as a potential candidate for middle to high bandgap materials for those lattice space parameters. Recently, In0.52Al0.48As single junction solar cell lattice matched to InP lattice constant has been demonstrated [6]. In this paper, we take one step further and focus on the development of lattice matched monolithic InAlAs/InGaAsP/InGaAs triple junction solar cell on InP substrate as shown in Figure 1. Preliminary 1-sun AM1.5D testing of the first triple junction cell shows promising results. !" # $% Tunnel Junction &!& # Back surface field Window Window Back surface field &!& # $% !" # Tunnel Junction &!’ # Back surface field Window &!’ # $% !" # $% Tunnel Junction &!& # Back surface field Window Window Back surface field &!& # $% !" # Tunnel Junction &!’ # Back surface field Window &!’ # $% Figure 1: A schematic drawing of lattice matched monolithic InAlAs/InGaAsP/InGaAs triple junction solar cell design on InP substrate. ( The growth of InAlAs/InGaAsP/InGaAs triple junction solar cell was carried out in Spectrolab’s Veeco E-400 metalorganic vapor phase epitaxy reactor. For the first 3J cell test structure, advanced features such as a transparent tensile InAlAs window and back surface field layers of subcell 1 were omitted. In addition, subcell 2 and subcell 3 thicknesses were reduced to 1µm from their optimal thicknesses. The epitaxial materials were characterized by x-ray diffraction. The grown wafer was processed into 1x1