Triple-cation low-bandgap perovskite thin-films for high-efficiency four-terminal all-perovskite tandem solar cells

All-perovskite multi-junction photovoltaics, comprised of a wide-bandgap (WBG) and a low-bandgap (LBG) perovskite solar cell (PSC), has opened a new window to cost-effective yet highly efficient solar cells (>33%). However, the poor operational stability of LBG PSCs is a major obstacle to the technological advance of all-perovskite tandem solar cells (all-PTSC). This study demonstrates that introducing minute quantities of Cs (1-10%) into the LBG FA0.8MA0.2Sn0.5Pb0.5I3 perovskite semiconductors (Eg = 1.26 eV) significantly improves the operational photo-stability of the corresponding LBG PSCs, due to a reduction of residual nanosized SnyPb(1-y)I2 aggregates, resulting in a beneficial stoichiometric composition. For an optimal concentration of Cs (2.5%) in the investigated range, the LBG PSCs attain remarkable power conversion efficiency (PCE) as high as 18.2% and maintain up to 92% of their initial power output after two hours under simulated one sun illumination. By mechanically stacking high-performance LBG bottom PSCs with semi-transparent top PSCs (Eg = 1.65 eV), four-terminal all-PTSCs with high PCE of 23.6% are attainable.