Solar cell efficiency is maximized through multijunction architectures that minimize carrier thermalization and increase absorption. Previous proposals suggest that the maximum efficiency for a finite number of subcells is achieved for designs that optimize for light trapping over radiative coupling. We instead show that structures with radiative coupling and back reflectors for light trapping, e.g. spectrum-splitting cells, can achieve higher conversion efficiencies. We model a compatible geometry, the polyhedral specular reflector. We analyze and experimentally verify the effects of spectral window and radiative coupling on voltage and power. Our results indicate that radiative coupling with back reflectors leads to higher efficiencies than previously studied architectures for practical multijunction architectures (i.e., #20 subcells).The photovoltaic community is closer than ever to achieving ultra-high multijunction solar cell efficiencies (>50%).1-8 Subcells from III-V compound semiconductors are approaching ideal Shockley-Queisser behavior and emit signicant radiation of photons with energies equal to or above the optical bandgap because nonradiative recombination has been minimized with advanced growth processes.6,9 The optical environment of a solar cell controls where the radiated photons from a subcell are directed and this greatly affects its efficiency.2,3 Thus the optical design of multijunction architectures is crucial for maximizing performance. To date, (1) light trapping and (2) radiative coupling have been investigated as promising optical design strategies. Light trapping inhibits the radiative emission of a subcell in order to reduce the dark current and increase voltage.For example, this can be achieved by including a back reector on a cell.9 By contrast, radiative coupling directs radiative emission between neighboring subcells for reconversion.
2,8Cells that have a high degree of radiative coupling have higher currents and are more tolerant of spectral mismatch because photons can be redistributed and boost carrier generation in the current-limited subcells.10-15 Thus including both strong light trapping and radiative coupling could yield very high efficiencies. However, only geometries that optimize for either strong light trapping or strong radiative coupling have been considered in the previous literature.2 Until now, a proposed structure that only optimizes for light trapping and completely blocks radiative coupling using frequency selective reectors matched to the band gap emission of each subcell has been assumed to be the most efficient structure for discrete numbers of junctions. This 'selective reector' design has been shown to give the highest efficiencies for time symmetric structures comprised of a realistic number
Broader contextEven with the recent advances in photovoltaics research, 50% solar cell efficiencies have not yet been achieved. Previous designs have focused on a tandem stack structure where semiconductor layers are epitaxially grown or wafer bonded on top of e...