High-efficiency multijunction solar cells are highly desirable for space and terrestrial applications. This paper proposes novel multijunction solar cell designs using lattice-matched II/VI (ZnCdMg)(SeTe) and III/V (InAlGa)(AsSb) direct bandgap materials that can be grown on GaSb or InAs substrates. Both material systems have been studied in detail separately for different applications. The combination of these lattice-matched materials uniquely offers very broad and continuous wavelength coverage, from UV to IR. The monolithic integration of the lattice-matched materials enables the growth of solar cells with many junctions without generating misfit dislocations. It is then possible to reach ultrahigh efficiencies of 43% under 1 sun AM0 condition and 52% under 240 suns for a 6-junction cell design. The availability of type-II interface configuration of many of the heterojunctions of the design will enable tunnel diodes with ultralow differential resistance. The use of wide bandgap lattice-matched materials such as ZnMgSeTe gives excellent surface and interface passivation.