The multijunction approach requires that an incident photon with a given energy be directed onto the correct subcell. Perhaps the conceptually simplest approach would be to use an optically dispersive element such as a prism to spatially distribute photons with different energies to different locations, where the appropriate cells would be placed to collect these photons. This approach is illustrated in Figure 8.4a. Although conceptually simple, in practice the mechanical and optical complexities of this scheme make it undesirable for most circumstances. A preferable approach is to arrange the cells in a stacked configuration, as illustrated in Figure 8.4b, arranged so that the sunlight strikes the highest bandgap first, and then strikes the progressively lower-bandgap junctions. This arrangement
Figure 8.4 Schematic comparison of (a) spatial – and (b) stacked-configuration approaches to distributing light to subcells of different bandgaps. (c) Illustration of two-, three-, and four-terminal connection to a two-junction cell. The figure shows the subcells as mechanically separate, but the two – and three-terminal devices can be monolithic makes use of the fact that junctions act as low-pass photon energy filters, transmitting only the sub-bandgap light. Thus, in Figure 8.4b, photons with hv > Eg1 are absorbed by that subcell, photons with Eg2 < hv < Egi are absorbed by the Eg2 subcell, and so on. In other words, the junctions themselves act as optical elements to distribute the spectrum to the appropriate junctions for multijunction photoconversion. The bandgaps must decrease from top to bottom of the stack. The stacked arrangement avoids the necessity for a separate optical element such as a prism to distribute the spectrum. Also, even if the junctions are physically separate from each other, they can be mechanically brought together into a relatively compact package, called a mechanical stack. The stacked configuration requires, of course, that all the junctions in the stack except the bottom one be transparent to light below their bandgaps, which, in practice, can set challenging constraints on the substrates and the back-contact metallizations of these junctions through which sub-bandgap light must pass. An elegant approach to this problem, which has several other advantages as well, is to fabricate all the junctions, each one atop the last, monolithically on a single substrate. This monolithic stack approach is the emphasis of this chapter.