  The power conversion efficiency of the market-dominating silicon
photovoltaics approaches its theoretical limit. Bifacial solar operation with
harvesting additional light impinging on the module back and the
perovskite/silicon tandem device architecture are among the most promising
approaches for further increasing the energy yield from a limited area. Here,
we calculate the energy output of perovskite/silicon tandem solar cells in
monofacial and bifacial operation considering, for the first time, luminescent
coupling between two sub-cells. For energy yield calculations we study
idealized solar cells at both, standard testing as well as realistic weather
conditions in combination with a detailed illumination model for periodic solar
panel arrays. Considering typical, experimental photoluminescent quantum yield
values we find that more than 50% of excess electron-hole pairs in the
perovskite top cell can be utilized by the silicon bottom cell by means of
luminescent coupling. As a result, luminescent coupling strongly relaxes the
constraints on the top-cell bandgap in monolithic tandem devices. In
combination with bifacial operation, the optimum perovskite bandgap shifts from
1.71 eV to the range 1.60-1.65 eV where already high-quality perovskite
materials exist. The results can hence change a paradigm in developing the
optimum perovskite material for tandem solar cells.
