Up-conversion of sunlight by GaInP/GaAs/Ge cell stacks: Limiting efficiency, practical limitation and comparison with tandem cells

Recent work proposed an alternative approach for photon up-conversion in that two or more low-band-gap solar cells drive a large-band-gap light-emitting diode (LED), emitting high-energy photons to be absorbed by a large-band-gap solar cell on the top. Compared with the old tandem structure, this multijunction up-conversion system has superior spectral robustness valuable under varying spectral conditions in real world. An example of interest is the use of a GaAs/Ge solar cell stack to drive a GaInP LED, behind a GaInP bifacial solar cell. This work investigates its one-sun limiting efficiency as well as possible practical efficiency if it were fabricated using state-of-the-art technologies for a world-record GaInP/GaAs/Ge tandem cell, taking luminescent coupling between subcells into account. It is shown that, although the limiting efficiency for such an up-conversion system is close to the record efficiency of a tandem device, its practical performance may be much lower due to carrier recombination in low-band-gap cells reducing the LED's emissions significantly. To obtain more insight into practical designs, the theoretical efficiency is explored further for the lattice-mismatched case where the band-gap of GaInP cells is reduced, both when there are luminescent couplings between cells in the rear up-converter and there are not. With a maximum theoretical efficiency of 44%, it is shown that the top cell's band-gap is a trade-off and luminescent coupling between rear cells reduces the efficiency.