  Here we use time-resolved and steady-state optical spectroscopy on
state-of-the-art low- and high-bandgap perovskite films for tandems to quantify
intrinsic recombination rates and absorption coefficients. We apply these data
to calculate the limiting efficiency of perovskite-silicon and all-perovskite
two-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 their
absorption in the low-bandgap sub-cell, we reveal the stringent need for
current matching is relaxed when the high-bandgap sub-cell is a luminescent
perovskite compared to calculations that do not consider luminescence coupling.
We show luminescence coupling becomes important in all-perovskite tandems when
charge carrier trapping rates are < 10$^{6}$ s$^{-1}$ (corresponding to carrier
lifetimes longer than 1 ${\mu}$s at low excitation densities) in the
high-bandgap sub-cell, which is lowered to 10$^{5}$ s$^{-1}$ in the
better-bandgap-matched perovskite-silicon cells. We demonstrate luminescence
coupling endows greater flexibility in both sub-cell thicknesses, increased
tolerance to different spectral conditions and a reduction in the total
thickness of light absorbing layers. To maximally exploit luminescence coupling
we reveal a key design rule for luminescent perovskite-based tandems: the
high-bandgap sub-cell should always have the higher short-circuit current.
Importantly, this can be achieved by reducing the bandgap or increasing the
thickness 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-perovskite
tandem device stack through cross-section luminescence images.
