Solar cell, multi-junction solar cell, solar cell module, and solar power generation system

A solar cell of an embodiment includes: a substrate; an n-electrode; an n-type layer; a p-type light absorption layer which is a semiconductor of a Cu-based oxide; and a p-electrode. The n-electrode is disposed between the substrate and the n-type layer. The n-type layer is disposed between the n-electrode and the p-type light absorption layer. The p-type light absorption layer is disposed between the n-type layer and the p-electrode. The n-type layer is disposed closer to a light incident side than the p-type light absorption layer. The substrate is a single substrate included in the solar cell.