  Nearly perfect photon recycling helped GaAs cells achieve the highest
efficiency ever reported for a solar cell. Recent reports of photon recycling
in perovskite solar cells suggest that, once optimized, it may as well achieve
GaAs-like performance. In this paper, we show that GaAs and perovskite cells
recycle photons in different ways. First, although bare-perovskite has been
shown to have lifetimes (~1us) in the radiative limit, non-radiative
recombination at the transport layers restricts the solar cell operation far
below the "photon-recycling" regime. GaAs cells have no such limitation.
Second, even if the transport layers were optically and electrically perfect,
the poor mobility of the perovskite layer would still restrict the optimum
thickness ~1um. Thus, a very high quality mirror (reflectivity >96%) is
required to utilize photon-recycling. The mirror reflectivity restriction was
far more relaxed for the thicker (~2-3um) GaAs cells. Therefore, a nontrivial
co-optimization of device geometry, mirror reflectivity, and material choice is
necessary for achieving highest theoretical efficiency anticipated for
perovskite cells.
