Relaxed Current Matching Requirements in Highly Luminescent\nPerovskite Tandem Solar Cells and Their Fundamental Efficiency Limits

Perovskite-based\ntandem solar cells are of increasing interest\nas they approach commercialization. Here we use experimental parameters\nfrom optical spectroscopy measurements to calculate the limiting efficiency\nof perovskite–silicon and all-perovskite two-terminal tandems,\nemploying currently available bandgap materials, as 42.0% and 40.8%,\nrespectively. We show luminescence coupling between subcells (the\noptical transfer of photons from the high-bandgap to low-bandgap subcell)\nrelaxes current matching when the high-bandgap subcell is a luminescent\nperovskite. We calculate that luminescence coupling becomes important\nat charge trapping rates (≤10<sup>6</sup> s<sup>–1</sup>) already being achieved in relevant halide perovskites. Luminescence\ncoupling increases flexibility in subcell thicknesses and tolerance\nto different spectral conditions. For maximal benefit, the high-bandgap\nsubcell should have the higher short-circuit current under average\nspectral conditions. This can be achieved by reducing the bandgap\nof the high-bandgap subcell, allowing wider, unstable bandgap compositions\nto be avoided. Lastly, we visualize luminescence coupling in an all-perovskite\ntandem through cross-section luminescence imaging.