We have previously reported that phosphine (PH3) surface treatment of a p-type crystalline germanium (c-Ge) substrate improves the c-Ge heterojunction solar cell performance. In this study, the effects of the heterointerface properties of the c-Ge heterojunction solar cell were investigated. We found that the deposition temperature of the heterojunction layer and O-2 surface treatment before the PH3 surface treatment influence the interface phase structure and band structure. Consequently, a conversion efficiency of 7.61% with a high open-circuit voltage of 0.270 V was obtained.