Materials and Interface Engineering for Tin Perovskite Solar Cells

Tin-based perovskites (TinPVKs) have become the most promising candidates for lead-free perovskite solar cells, owing to its low toxicity and improved photovoltaic performance. However, due to the absence of 4f shell and upshifted conduction and valence band edges, TinPVKs suffer from uncontrolled crystallization and energy-level mismatched charge transport materials, limiting the power conversion efficiency (PCE) of tin perovskite solar cells (TinPSCs). Here, I discussed our recent efforts on a systematical approach from TinPVKs to charge transport materials and interfaces with the goal to achieve highly efficient and stable TinPSCs. We utilized pseudo-halide anions for large monoammonium cations to make quasi-2D Ruddlesden-Popper (RP) TinPVKs for a target formula of PEA₂FA₄Sn₅I₁₆ with reduced small n-value phases and improved 2D crystal orientations. We further investigated the ligand regulated crystallization process of TinPVK to achieve a heterostructure of the 3D α-phase FASnI₃ over the 2D (n = 2, 4-FPEA₂FASn₂I₇) and 2D (n = 1, 4-FPEA₂SnI₄) phases (3D-over-2D) via a proposed diffusion-propagation mechanism. To amplify hole extraction characteristics of a commonly used hole transport material PEDOT:PSS, we post-treated the PEDOT:PSS layer with aromatic diammonium acetate salts dissolved in a highly volatile but interactive solvent. The comprehensive effects of the salts and solvent not only modified the surface morphology, energy level, and charge transport of PEDOT:PSS but also improved the crystallinity of TinPVKs. We also explored using mixed self-assembled monolayers (SAMs) as hole transport layers (HTLs) for TinPVKs. The controlled surface hydrophobicity and energy levels of the mixed SAMs plus the engineered compatible TinPVK thin films and an efficient electron transport layer led to a high efficiency of TinPSCs. To elucidate the fundamental material growth mechanisms and the resulted material properties, we applied a suite of in situ and ex situ techniques such as GIWAXS, fs-transient absorption spectroscopy (TAS), and static and time-resolved photoluminescence (PL). Furthermore, we also conducted device physics studies to understand charge generation, recombination, and transport in the devices. Finally, I discussed our perspective about designing effective HTLs in TinPSCs.