High-performance tin-lead mixed perovskite solar cells with vertical compositional gradient

Sn-Pb mixed perovskites with bandgaps in the range of 1.1−1.4 eV are ideal candidates for single-junction solar cells to approach the Shockley-Queisser limit. However, the efficiency and stability of Sn-Pb mixed perovskite solar cells (PSC) still lag far behind those of Pb-based counterparts due to the easy oxidation of Sn2+. Here, a reducing agent 4-hydrazinobenzoic acid (HBA) is introduced as an additive along with SnF2 to suppress the oxidation of Sn2+. Meanwhile, vertical Pb/Sn compositional gradient is formed spontaneously after an antisolvent treatment due to different solubility and crystallization kinetics of Sn and Pb-based perovskites and can be finely tuned by controlling the antisolvent temperature. Because the band structure of a perovskite is dependent on its composition, graded vertical heterojunctions are constructed in the perovskite films with compositional gradient, which can enhance photocarrier separation and suppress carrier recombination in resultant PSCs. Under optimal fabrication conditions, the Sn-Pb mixed PSCs show power conversion efficiency up to 22% along with excellent stability during light soaking.