In contrast, in the three-terminal configuration the subcells are not electrically isolated; the bottom of each cell is electrically connected to the top of the cell beneath it. The fabrication of a monolithic three-terminal device is relatively straightforward, although more complex than the fabrication of a two-terminal device. The semiconductor structure must be designed to provide a layer for contact to the intermediate terminal, and to accommodate the processing steps necessary to put the intermediate terminal in place. With this intermediate terminal, the different subcells in the stack do not need to have the same photocurrents. Furthermore, in this three-terminal configuration, the different subcells in the stack may have different polarities, e. g. p/n for the top cell and n/p for the bottom cell.
Module-level interconnection of four – and three-terminal devices is discussed in detail by Gee [17].
