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Here we use time-resolved and steady-state optical spectroscopy onstate-of-the-art low- and high-bandgap perovskite films for tandems to quantifyintrinsic recombination rates and absorption coefficients. We apply these datato calculate the limiting efficiency of perovskite-silicon and all-perovskitetwo-terminal tandems employing currently available bandgap materials as 42.0 %and 40.8 % respectively. By including luminescence coupling between sub-cells,i.e. the re-emission of photons from the high-bandgap sub-cell and theirabsorption in the low-bandgap sub-cell, we reveal the stringent need forcurrent matching is relaxed when the high-bandgap sub-cell is a luminescentperovskite compared to calculations that do not consider luminescence coupling.We show luminescence coupling becomes important in all-perovskite tandems whencharge carrier trapping rates are < 10$^{6}$ s$^{-1}$ (corresponding to carrierlifetimes longer than 1 ${\mu}$s at low excitation densities) in thehigh-bandgap sub-cell, which is lowered to 10$^{5}$ s$^{-1}$ in thebetter-bandgap-matched perovskite-silicon cells. We demonstrate luminescencecoupling endows greater flexibility in both sub-cell thicknesses, increasedtolerance to different spectral conditions and a reduction in the totalthickness of light absorbing layers. To maximally exploit luminescence couplingwe reveal a key design rule for luminescent perovskite-based tandems: thehigh-bandgap sub-cell should always have the higher short-circuit current.Importantly, this can be achieved by reducing the bandgap or increasing thethickness in the high-bandgap sub-cell with minimal reduction in efficiency,thus allowing for wider, unstable bandgap compositions (>1.7 eV) to be avoided.Finally, we experimentally visualise luminescence coupling in an all-perovskitetandem device stack through cross-section luminescence images.

Relaxed current matching requirements in highly luminescent perovskite tandem solar cells and their fundamental efficiency limits