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Enabled by a heteroepitaxial nucleation process that yields GaP-on-Si integration free of heterovalent-related defects, GaP/active-Si junctions were grown by metalorganic chemical vapor deposition. n-type Si emitter layers were grown on p-type (1 0 0)-oriented Si substrates, followed by the growth of n-type GaP window layers, to form fully active subcell structures compatible with integration into monolithic III-V/Si multijunction solar cells. Fabricated test devices yield good preliminary performance characteristics and demonstrate great promise for the epitaxial subcell approach. Comparison of different emitter layer thicknesses, combined with descriptive device modeling, reveals insight into recombination dynamics at the GaP/Si interface and provides design guidance for future device optimization. Additional test structures consisting of GaP/active-Si subcell substrates with subsequently grown GaAsyP1-y step-graded buffers and GaAs0.75P0.25 terminal layers were produced to simulate the optical response of the GaP/Si junction within a theoretically ideal dual-junction solar cell.