Advancing Tin-Based Perovskite Solar Cells: Surface Recrystallization & PbS Capping for Efficiency and Stability

Tin-based perovskite solar cells (PSCs) are promising environmentally friendly alternatives to their lead-based counterparts, yet they currently suffer from much lower device performance. Due to variations in the chemical properties of lead (II) and tin (II) ions, similar treatments may yield distinct effects resulting from differences in underlying mechanisms. In the first work, we conducted a surface treatment on tin-based perovskite with a commonly employed ligand, iso-butylammonium iodide (iso-BAI). Unlike the passivation effects previously observed in lead-based perovskites, such treatment leads to the recrystallization of the surface, driven by the higher solubility of tin-based perovskite in common solvents. By carefully designing the solvent composition, we effectively modified the perovskite surface while preserving the integrity of the bulk. The treatment led to enhanced surface crystallinity, reduced surface strain and defects, and improved charge transport. Consequently, the best-performing power conversion efficiency of FASnI₃ PSCs increases from 11.8% to 14.2%. Despite the efficiency enhancement, the iso-BAI treatment does not show substantial improvement in stability. In the second work, we grow an ultra-thin layer of lead sulfide (PbS) on the tin-based perovskite with an in-situ reaction between lead (II) pyridinecarboxylic salt (Pb(PyA)₂) and bis(trimethylsilyl)sulfide ((TMS)₂S). By tuning the ratio between lead and sulfur, a Pb-rich PbS layer creates an n-type interface between the perovskite and the electron transport layer, facilitating charge transfer. In addition, a similar lattice constant between the perovskite and the PbS enables the bonding of lead and the dangling iodide at the interface, passivating the vulnerable surface. The open-circuit voltage boosts from 0.727V to 0.845V, resulting in a PCE increase from 12.0% to 14.4%. Benefitting from the air-stable PbS layer, the solar cell devices experience negligible performance decay after 2000h of storage in N₂ environment. In this project, we distinguish the mechanism of surface treatments in tin-based perovskites from that of lead-based counterparts and propose tailor-made surface strategies designing principles for fabricating efficient tin-based perovskite solar cells.