Reducing Hole Trap Density in Sn–Pb Perovskite Solar Cells via Molecular Phenylhydrazine

Narrow‐bandgap Sn–Pb‐alloyed perovskite is potential for a bottom cell in perovskite tandem solar cells. However, Sn‐contained perovskites tend to be readily oxidized in air atmosphere, which has ill influence on stability and photovoltaic performance. Herein, a method to suppress oxidation of divalent tin via post‐treatment of perovskite film with phenylhydrazine is reported. Phenylhydrazine‐treated FA 0.5 MA 0.5 Pb 0.5 Sn 0.5 I 3 perovskite films effectively reduce the hole traps caused by the oxidation of Sn 2+ on the perovskite surfaces. In addition, surface energetics are well aligned by post‐treatment, which is beneficial for voltage gain and charge transport. As a result, power conversion efficiency (PCE) is increased from 18.16% to 20.67% after post‐treatment due mainly to the significant improvement of open‐circuit voltage from 0.75 to 0.83 V. Furthermore, the device stability is improved, where 91.67% of initial PCE is maintained after 1000 h.