Band bending reversal and enhanced electron mobility at the film surface achieved through a selective polishing strategy in tin-lead perovskite solar cells

Tin-lead (Sn-Pb) perovskite with its ideal bandgap is a promising candidate for perovskite solar cells (PSCs). Nevertheless, the excessive aggregation of Sn and the presence of Sn vacancies (V_Sn) result in more pronounced p-type doping at the surface compared to the film interior. This in turn leads to a mismatch in band bending and a reduction in electron mobility at the electron transfer interface. In this study, a selective polishing strategy is introduced to optimize the Sn-to-Pb ratio and inhibit the oxidation of Sn²⁺. Pyrophosphate (PP) forms more soluble complexes with Sn than Pb, making the strategy feasible. This approach induces a transition from upward to downward surface band bending and increases electron mobility at the film surface in p-i-n structured PSCs, which boosts charge transfer and suppresses Auger recombination at the perovskite/electron transport layer interface. Hence, the power conversion efficiency of the Sn-Pb narrow-bandgap PSC rises from 20.96% to 23.85% with a notable increase in V_OC from 0.87 V to 0.91 V. A two-terminal monolithic all-perovskite tandem solar cell with a high PCE of 28.10% is achieved. Meanwhile, P₂O₇⁴⁻ also forms stable coordination with the perovskite surface, thereby improving the photothermal stability and air stability of the resulting devices.