Future NASA missions will require significant improvements in photovoltaic energy conversion efficiency (>30%) and mass specific power (>600 W/kg), and improved radiation tolerance. We propose to develop a high efficiency monolithic three-junction, two-terminal solar cell using lattice matched HgCdZnTe and/or HgCdMgTe alloys, which will offer great advantages in terms of weight and interconnect simplicity as well as improvements in efficiency. The use of frequency down conversion by divalent Sm/Eu and trivalent Yb/Tb halides embedded in the protective coatings of the solar cells to increase the efficiency of photovoltaic conversion will be investigated. The predicted ideal efficiencies are greater than 50%. During Phase I we will determine the optimal layer thicknesses, doping profiles and current density matching requirements for two-terminal multiple-heterojunction solar cell designs. We will grow HgCdZnTe layers with Zn composition ≈ 0.5 on a Si substrate using a thin CdTe/ZnTe strained-layer superlattice to eliminate cracking and minimize strain, will optimize the growth of HgCdMgTe, and will measure mobilities and minority carrier lifetimes in the layers. We will fabricate and test a two-junction solar cell. Frequency down conversion will be demonstrated, progress toward its optimization will be made, and optical fusion of the solar cell and protective layers will be investigated.