Efficient MA-free Pb-Sn alloyed low-bandgap perovskite solar cells via surface passivation

Lead-tin (Pb-Sn) alloyed low-bandgap perovskite solar cell (PSC) is the critical component of all-perovskite tandem solar cells. Although there is a clear trend of avoiding volatile methylammonium (MA) cations in the state-of-the-art lead-halide PSCs, the high-efficiency Pb-Sn mixed PSCs (with efficiencies over 20%) are still generally containing over 30 mol% MA cations making their thermal and operational stabilities very questionable. Here, our experiments show that the PSC based on MA-free FA 0.83 Cs 0.17 Pb 0.5 Sn 0.5 I 3 composition has the T 80 20 times longer than that of MA-containing FA 0.6 MA 0.4 Pb 0.6 Sn 0.4 I 3 based device, but a lower efficiency of 17.71% versus 20.45% ( T 80 refers as the efficiency decrease to 80% of the initial value upon continuous one-sun illumination). Thus, it is greatly necessary to develop high-efficiency MA-free Pb-Sn mixed PSCs. Herein, we developed a facile post-treated method to enhance the efficiency of MA-free FA 0.83 Cs 0.17 Pb 0.5 Sn 0.5 I 3 PSCs. By growing an ultrathin layer of phenethylammonium iodide (PEAI) on top of FA 0.83 Cs 0.17 Pb 0.5 Sn 0.5 I 3 film, it not only passivated the film surface detects but also suppressed the Sn 2+ oxidation evidenced by the improved charge carrier lifetime from 540 ns to 682 ns and a reduced amount of Sn 4+ species. In addition, the thin layer of PEAI also modified the junction quality at the perovkite/C 60 interface leading to more efficient electron transportation. Finally, the PEAI-treated PSCs delivered an improved open-circuit voltage ( V oc ) of 0.795 V and efficiency of 19.12% versus 0.766 V and 17.72% for the control device. We hope this work could shed more light on the further progress of MA-free Pb-Sn mixed PSCs. • MA-free FA 0.83 Cs 0.17 Pb 0.5 Sn 0.5 I 3 PSC presents stronger stability but lower PCE compared with MA-containing counterpart. • PEAI on MA-free FA 0.83 Cs 0.17 Pb 0.5 Sn 0.5 I 3 perovskite film can passivate the surface detects and mitigate the Sn 2+ oxidation. • The PSC based on FA 0.83 Cs 0.17 Pb 0.5 Sn 0.5 I 3 perovskite with PEAI treatment delivers V oc and efficiency of 0.795 V and 19.12%.