Henry has calculated the limiting terrestrial one-sun efficiencies for conversion with 1, 2, 3, and 36 bandgaps; the respective efficiencies are 37, 50, 56, and 72% [16]. The improvement in efficiency from one to two bandgaps is considerable, but the returns diminish as more bandgaps are added. This is fortunate since the practicality of a device with five or more junctions is questionable. Note that the promise of the multijunction efficiency improvements will not be realized unless the bandgaps of the multiple junctions are correctly chosen; this choice will be discussed below in detail. Theoretical efficiency limits for multijunction devices based on thermodynamic fundamentals are presented in Chapter 4.
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