Experimental study of the band structure at the heterojunction interfaces of GaInP solar cells was performed by admittance spectroscopy. Admittance measurements were analyzed using numerical simulations. A good agreement between simulation and experiment was obtained. A potential barrier of about 0.6 eV at the p-GaAs/p-AlInP interface formed due to the high valence band offset was observed by the experiment. This high barrier creates fundamental limitation for the usage of this interface in p–n GaInP solar cells. A way to reduce the effective barrier height to 0.257 0.02 eV using a double layer p-AlGaAs/p-AlGaInP window avoiding deterioration of I–V curves was demonstrated.