We located the electrical junction (EJ) of Cu(In, Ga)Se 2 (CIGS) and Cu 2 ZnSnSe 4 (CZTS) solar cells with ∼20-nm accuracy using a scanning capacitance spectroscopy (SCS) technique. A procedure was developed to prepare the cross-sectional samples and grow critical high-quality insulating layers for the SCS measurement. We found that CIGS has a buried homojunction with the EJ located at ∼40 nm inside the CIGS/CdS interface. An n-type CIGS was probed in the region 10–30 nm away from the interface. By contrast, the CZTS/CdS cells have a heterointerface junction with a shallower EJ (∼20 nm) than CIGS. The EJ is ∼20 nm from the CZTS/CdS interface, which is consistent with asymmetrical carrier concentrations of the p-CZTS and n-CdS in a heterojunction cell. The unambiguous determination of the junction locations helped explain the large open circuit voltage difference between the state-of-the-art devices of CIGS and CZTS.