Highly Efficient Perovskite–Perovskite Tandem Solar Cells Reaching 80% of the Theoretical Limit in Photovoltage

Organic–inorganic hybrid perovskite multijunction solar cells have immense potential to realize power conversion efficiencies (PCEs) beyond the Shockley–Queisser limit of single-junction solar cells; however, they are limited by large nonideal photovoltage loss (V _oc,loss) in small- and large-bandgap subcells. Here, an integrated approach is utilized to improve the V _oc of subcells with optimized bandgaps and fabricate perovskite–perovskite tandem solar cells with small V _oc,loss. A fullerene variant, Indene-C60 bis-adduct, is used to achieve optimized interfacial contact in a small-bandgap (≈1.2 eV) subcell, which facilitates higher quasi-Fermi level splitting, reduces nonradiative recombination, alleviates hysteresis instabilities, and improves V_oc to 0.84 V. Compositional engineering of large-bandgap (≈1.8 eV) perovskite is employed to realize a subcell with a transparent top electrode and photostabilized V _oc of 1.22 V. The resultant monolithic perovskite–perovskite tandem solar cell shows a high V _oc of 1.98 V (approaching 80% of the theoretical limit) and a stabilized PCE of 18.5%. The significantly minimized nonideal V _oc,loss is better than state-of-the-art silicon–perovskite tandem solar cells, which highlights the prospects of using perovskite–perovskite tandems for solar-energy generation. It also unlocks opportunities for solar water splitting using hybrid perovskites with solar-to-hydrogen efficiencies beyond 15%.