The two-terminal series-connected configuration provides the fewest possibilities for interconnection of the devices. This configuration requires that the subcells be of the same polarity and that the photocurrents of the subcells be closely matched, since in this series connection the subcell with the least photocurrent limits the current generated by the entire device. This current-matching constraint, about which more will be said shortly, puts relatively tight constraints on the selection of bandgaps for the various junctions in this structure. Against these disadvantages, however, are critical advantages. The existence of high-quality monolithic tunnel-junction subcell interconnects means that these stacks can be made as monolithic two-junction structures with metallization at the top and bottom of the stack only. This, in turn, means that such devices can be integrated into modules with the same simplicity afforded by single-junction devices. Figure 8.5 shows a schematic cross-section of a monolithic two-terminal series-connected three-junction solar cell, along with typical materials parameters for the realization of this device structure as a GaInP/GaAs/Ge cell. The two-terminal, series-connected configuration will be the focus of the following sections, where we analyze in detail the dependence of the cell performance on the cell design parameters.