Limiting Efficiency of Heterojunction Solar Cells

A heterojunction solar cell consists of a p-n junction between a high-bandgap and a low-bandgap semiconductor. As the cell is made up of two different semiconductors, the fundamental question that arises is whether the limiting efficiency of the heterojunction solar cell is determined by the low-bandgap semiconductor, the high-bandgap semiconductor, some value in-between or, perhaps, even some other value exceeding the limiting efficiency of single gap solar cells. In this respect, in this paper, we demonstrate that the limiting efficiency lies in-between, without the absolute limiting efficiency of single gap solar cells being exceeded. However, in spite of this result, we find that the open-circuit voltage of heterojunction solar cells can exceed the bandgap (divided by the electron charge) of the low-bandgap semiconductor, something that is not possible in single gap solar cells.