Stable and Reproducible 2D/3D Formamidinium−Lead−Iodide Perovskite Solar Cells Abhishek Thote, †,∥ Il Jeon,* ,†,∥ Jin-Wook Lee, ‡ Seungju Seo, † Hao-Sheng Lin, † Yang Yang, ‡ Hirofumi Daiguji,* ,† Shigeo Maruyama,* ,†,§ and Yutaka Matsuo* ,† † Department of Mechanical Engineering, School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan ‡ Department of Materials Science and Engineering and California Nano Systems Institute, University of California, Los Angeles, California 90095, United States § Energy Nano Engineering Laboratory, National Institute of Advanced Industrial Science and Technology (AIST), Ibaraki 305-8564, Japan * S Supporting Information ABSTRACT: 2D perovskite-stabilized FACsPbI 3 (FA = formamidinium) perovskite solar cells were fabricated in both normal- type and inverted-type architectures. While the normal-type devices exhibited a high power conversion efficiency of 20.2%, their reproducibility was limited. On the other hand, the inverted-type devices exhibited an efficiency of 18.2% with a greater stability and higher reproducibility than those of the normal-type devices. The reduced reproducibility of the normal-type devices was associated with the crack formation on the perovskite films during a spin-coating process. The hardness of both the perovskite and the sublayer was directly linked to the crack formation. Inverted-type 2D/3D FACsPbI 3 with ozone-treated poly(triarylamine) as sublayer exhibited the highest phase stability owing to the hydrophobic nature of poly(triarylamine) and improved energy level alignment upon an ozone treatment. In addition, strong interaction between phenethylamine cations of the 2D perovskite and of the 3D FACsPbI 3 crystal at grain boundaries contributed to the high phase stability. KEYWORDS: perovskite solar cell, 2D perovskite, formamidinium perovskite, mechanical property of perovskite, phase stability ■ INTRODUCTION Organic−inorganic lead halide perovskite solar cells (PSCs) have attracted great attention owing to high absorption coefficient, low fabrication cost, and flexible applications. The high absorption arises from a suitable bandgap of perovskite materials, which is close to the Shockley−Queisser limit of 1.4 eV. 1 The bandgap of perovskite materials can also be tuned by substituting either cations and anions. 2−4 Substitution of iodide anions by a smaller halide increases the bandgaps. 4 Substitution of methylammonium (MA) cation by a bigger formamidinium (FA) cation decreases the bandgap while retaining the cubic structure. 5−7 FA lead iodide perovskite (FAPbI 3 ) has been reported to possess an ideal bandgap of 1.4 eV, which corresponds to sunlight absorption up to approximately 886 nm (Figure S1 in Supporting Informa- tion). 8−10 Thus, FAPbI 3 is potentially more suitable as an active material than MAPbI 3 for PSCs. However, pure FAPbI 3 has been reported to exhibit low stability because its trigonal α- phase is sensitive to humidity 11 and readily turns into a nonphotoactive hexagonal δ-phase at room temperature. 12,13 Although more thermally and structurally stable FAPbI 3 has been realized through partial substitution of FA by Cs (FACsPbI 3 ), 14−16 the stability should be improved further to surpass MAPbI 3 . Recently, 2D perovskite-added FACsPbI 3 (2D/3D FACsPbI 3 ), in which the quasi-structured 2D perovskite 17,18 protecting the 3D perovskite grains from humidity, was reported; 19 the 2D/3D FACsPbI 3 -based PSCs Received: November 12, 2018 Accepted: March 8, 2019 Published: March 8, 2019 Article www.acsaem.org Cite This: ACS Appl. Energy Mater. 2019, 2, 2486-2493 © 2019 American Chemical Society 2486 DOI: 10.1021/acsaem.8b01964 ACS Appl. Energy Mater. 2019, 2, 2486−2493 Downloaded via UNIV OF CALIFORNIA LOS ANGELES on April 26, 2019 at 23:13:18 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.