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

A solar cell of an embodiment includes a p-electrode; an n-electrode; a p-type light-absorbing layer located between the p-electrode and the n-electrode and mainly containing a cuprous oxide; and an n-type layer located between the first n-type layer and the n-electrode, the n-type layer including a first n-type layer and a second n-type layer or a first n-type region and a second n-type region; wherein the first n-type layer and the first n-type region is located on the p-type light-absorbing layer side, the second n-type layer and the second n-type region is located on the n-electrode side, the first n-type layer and the first n-type region mainly contain a compound represented by Gax1M1x2Ox3, the M1 is one or more selected from the group consisting of Hf, Zr, In, Zn, Ti, Al, B, Sn, Si, and Ge, the x1, the x2, and the x3 are more than 0, and the x3 when a sum of the x1 and the x2 is 2 is 3.0 or more and 3.8 or less, the second n-type layer and the second n-type region mainly contain a compound represented by Gay1Zny2M2y3M3y4Oy5, the M2 is one or more selected from the group consisting of Hf, Zr, In, Ti, Al, B, Si, and Ge, the M3 is Sn or/and Mg, the y1, the y2, the y3, and the y4 are 0 or more, a sum of the y3 and the y4 is more than 0, and the y5 when a sum of the y1, the y2, the y3, and the y4 is 2 is 2.2 or more and 3.6 or less.