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

Abstract

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 FA0.83Cs0.17Pb0.5Sn0.5I3 composition has the T80 20 times longer than that of MA-containing FA0.6MA0.4Pb0.6Sn0.4I3 based device, but a lower efficiency of 17.71% versus 20.45% (T80 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 FA0.83Cs0.17Pb0.5Sn0.5I3 PSCs. By growing an ultrathin layer of phenethylammonium iodide (PEAI) on top of FA0.83Cs0.17Pb0.5Sn0.5I3 film, it not only passivated the film surface detects but also suppressed the Sn2+ oxidation evidenced by the improved charge carrier lifetime from 540 ns to 682 ns and a reduced amount of Sn4+ species. In addition, the thin layer of PEAI also modified the junction quality at the perovkite/C60 interface leading to more efficient electron transportation. Finally, the PEAI-treated PSCs delivered an improved open-circuit voltage (Voc) 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.