The GaP/Si heterojunctions fabricated by molecular beam epitaxy (MBE) and plasma enhanced atomic layer deposition (PE-ALD) were studied. The degradation of charge carrier lifetime in Si was observed during the growth of single-crystalline GaP layer on Si substrates by MBE at 500-600 °C. The study performed by PL and DLTS has demonstrated the presence of the defective layer in Si, which is located within ~30 nm near to the GaP/Si interface. This defective layer leads to significant reduction of solar cell performance for anisotype n-GaP/p-Si heterojunction due to strong recombination in the space charge region. The GaP/Si heterostructure with Si n-p homojunction exhibits better performance compared to the anisotype n-GaP/p-Si heterojunction because the defective layer is located in the n-Si emitter formed by intentional P diffusion. On the contrary, the deposition of amorphous GaP layer by PE-ALD at T < 380 °C does not lead to the degradation of Si wafer charge carrier lifetime.