Importance of tin (II) acetate additives in sequential deposited fabrication of Sn-Pb-based perovskite solar cells

Sn-Pb-based perovskite is a promising solar cell material for its low toxicity and narrow band gap. However, the poor crystallinity of Sn-containing perovskite films and the facile oxidation of Sn2+ are obstacles to the efficiency and stability of Sn-Pb-based perovskite solar cells (PSCs). Herein, we developed a new additive strategy of tin acetate (SnAc2) to fabricate high-quality (HC(NH2)2)n(CH3NH3)1-nPb0.7Sn0.3IxBr3-x films by a two-step method and simultaneously restrict the oxidation of Sn2+ to Sn4+ by coordination between Sn2+ and Ac-. Moreover, SnAc2 built an Sn-rich environment to alleviate the intrinsic Sn2+ vacancies. All these effectively reduced the defect state densities and carrier nonradiative recombinations in the device, resulting in improvements in the performance of PSCs. The PSCs with the SnAc2 additive exhibited a increase in average power conversion efficiency (PCE) from 15.60% to 16.69%. Furthermore, the phenethylammoniumiodide (PEAI) post-treatment was introduced to passivate the SnAc2-containing perovskite films. Finally, the optimal cell with a PCE of 17.93% was obtained, which was second-best compared to the reported Sn-Pb-based PSCs by the two-step methods. Furthermore, the producibility and stability of devices are also improved due to the inhibited oxidation of Sn2+ by SnAc2 additives and the protection of two-dimensional (2D) (PEA)2PbI4 perovskites, the unencapsulated PSC maintained 80.4% of its initial efficiency when stored in air after 24 days.