The authors report on the development of two compositions of a GaInAsP lattice-matched to GaAs for photovoltaic applications. Successful development of cascade solar cells necessitates the development of both high bandgap (1.5 to 1.9 eV) as well as low bandgap (0.7 to 1.4 eV) materials. The GaInAsP lattice-matched to GaAs is an excellent candidate for the high band gap material. Ga/sub 0.84/In/sub 0.16/As/sub 0.68/P/sub 0.32/ cells, with a band gap of 1.55 eV, have been developed that have demonstrated a V/sub oc/ of 1.047 volts, a J/sub sc/ of 22.5 mA/cm/sup 2/, a fill factor of 0.849, and an active area efficiency of 21.8 per cent under AM1.5 direct illumination. A Ga/sub 0.84/In/sub 0.16/As/sub 0.68/P/sub 0.32/ tunnel diode has also been developed with a peak current of 4.33/spl times/102/spl sim/mA/cm/sup 2/ at a voltage of 65 mV. Both the Ga/sub 0.84/In/sub 0.16/As/sub 0.68/P/sub 0.32/ cell and tunnel diode are being used in conjunction with a Ge cell to develop a monolithic Ga/sub 0.84/In/sub 0.16/As/sub 0.68/P/sub 0.32Ge cascade cell. Ga/sub 0.68/In/sub 0.32/As/sub 0.34/P/sub 0.66/ cells, with a band gap of 1.7 eV, have been developed that have demonstrated a V/sub oc/ of 1.161 volts, a J/sub sc/ of 28.9 mA/cm2 a fill factor of 0.86, and an active area efficiency of 21.4 per cent under AMO illumination. The Ga/sub 0.68/In/sub 0.32/As/sub 0.34/P/sub 0.66/ cells have also demonstrated resistance to radiation damage as well as a recovery of preirradiation performance after low temperature annealing. >