Thin Films and Solar Cells Based on Semiconducting\nTwo-Dimensional Ruddlesden–Popper (CH₃(CH₂)₃NH₃)₂(CH₃NH₃)_<i>n</i>−1Sn_<i>n</i>I_3<i>n</i>+1 Perovskites

Abstract

Low electrical resistivity (high\ndark carrier concentration) of\nCH₃NH₃SnI₃ often leads to short-circuiting\nin solar cells, and appropriate thin-film modifications are required\nto ensure functional devices. The long-term durability of organic–inorganic\nperovskite solar cells necessitates the protection of perovskite thin\nfilms from moisture to prevent material decomposition. Herein, we\nreport that the electrical resistivity and the moisture stability\nof two-dimensional (2D) Ruddlesden–Popper (CH₃(CH₂)₃NH₃)₂(CH₃NH₃)_<i>n</i>−1Sn_<i>n</i>I_3<i>n</i>+1 perovskites are considerably\nimproved compared to those of the three-dimensional (3D) CH₃NH₃SnI₃ perovskite and subsequently show the\nsolar cell fabrication using a simple one-step spin-coating method.\nThese 2D perovskites are semiconductors with optical band gaps progressively\ndecreasing from 1.83 eV (<i>n</i> = 1) to 1.20 eV (<i>n</i> = ∞). The <i>n</i> = 3 and <i>n</i> = 4 members with optimal band gaps of 1.50 and 1.42 eV for solar\ncells, respectively, were thus chosen for in-depth studies. We demonstrate\nthat thin films of 2D perovskites orient the {(CH₃NH₃)_<i>n</i>−1Sn_<i>n</i>I_3<i>n</i>+1}^2– slabs\nparallel to the substrate when dimethyl sulfoxide solvent is used\nfor deposition, and this orientation can be flipped to perpendicular\nwhen <i>N</i>,<i>N</i>-dimethylformamide solvent\nis used. We find that high-purity, single-phase films can be grown\nonly by using precursor solutions of “pre-synthesized”\nsingle-phase bulk perovskite materials. We introduce for the first\ntime the use of triethylphosphine as an effective antioxidant, which\nsuppresses the doping level of the 2D films and improves film morphology.\nThe resulting semiconducting 2D Sn-based iodide perovskite films were\nincorporated in solar cells yielding a power conversion efficiency\nof 2.5% from the Sn₄I₁₃ member. From the temporal\nstability standpoint, the 2D Sn perovskite solar cells outperform\ntheir 3D analogs.