Single-junction solar cells have just one p-n junction, as against multi-junction solar cells, which have more than one p-n junction. p-n junction refers to the boundary between two types of semiconductor material. One type is called the p-type and the other is called the n-type. In a typical solar cell, one side of the cell is doped with boron (the p type), and the other side is doped with phosphorus, the n-type. The p-n junction generates current and voltage due to the “built-in” electric field at the junction of the p-type and n-type material.