We have studied the effects of metalorganic chemical vapor deposition (MOCVD) growth conditions on the properties of GaAs solar cells grown upon Ge substrates, and in particular the GaAs/Ge interface. The interface properties were found to strongly depend on growth conditions. By small changes in the growth temperature, the GaAs/Ge interface was altered from active to passive. Only a narrow temperature window (600–630 °C) for the initial GaAs layer growth gave the passive‐Ge junction together with good surface morphology. Accordingly, a high efficiency (19%, AM0) GaAs solar cell was grown by atmospheric pressure MOCVD on a Ge substrate without any junction in the Ge.