The fabrication of a high performance GaAs0.7P0.3solar cell on a transparent GaP substrate with an AM1.5 efficiency of 15.4% for a concentration ratio of 30× is reported for the first time. The measured transparency of the GaP substrate allows these cells to be mechanically stacked on silicon solar cells in a manner that should yield combined conversion efficiencies well over 25%. This mechanically stacked two‐band‐gap cell design is particularly attractive because it utilizes the already well‐developed Si solar cell and because the materials foundation for the GaAs0.7P0.3on GaP cell has been laid by work on light emitting diodes.