Ultimate 2D Perovskite-FA DJ 2D Perovskites with Maximum Symmetry

Two-dimensional halide perovskites (2D-HaP) have emerged as a class of highly durable solution-processed organic-inorganic (hybrid) low-dimensional semiconductors. They exhibit a combination of properties derived from four classes of materials - quantum wells, atomically thin 2D materials, organic semiconductors, and three-dimensional (3D)-HaP perovskites. The general formula of 2D-HaP is (A′)_m(A) _n−1M_nX_3n+1, where A′ is a bulky organic cation, A is a small organic cation, M is a divalent metal, and X is a halide, with m=2 in Ruddlesden-Popper (RP) phases and m=1 in Dion-Jacobson (DJ) phases. The number n determines the thickness of the perovskite layer, which consists of alternate organics (A’)m and hybrid (A)_n−1M_nX_3n+1 layers, making 2D-HaP a platform to engineer hybrid composites with attractive optoelectronic properties.<br/>Currently, similar to 3D-HaP, most of the 2D-HaP takes Methylammonium (MA) as the A site cation. While the MA⁺ cation has the most suitable effective radius of any monovalent cation (either organic or inorganic) for forming a stable perovskite structure in a strain-stress perspective, the chemical instability of MA had motivated the use of less suitably sized but more chemically stable cations, such as formamidinium (FA).¹ Different from 3D-HaP which already has some successful demonstrations using FA as A site cation,² FA based 2D-HaP remains underexplored, with only a few reports achieving an “n” equal to 1 or 2.³ We propose that the difficulty of synthesizing FA based 2D perovskite coming from the larger size of FA compared to MA, which will increase lattice mismatch between the organic parts and the inorganic parts in the 2D perovskites.⁴ Therefore, the key to synthesizing a multi-layer FA based 2D perovskite is to find the appropriate organic spacer which has a small lattice mismatch with the inorganic part.<br/>Here, we demonstrate a synthesis of FA based 2D DJ perovskites crystals, from n=1 to n=4 after screening a series of different organic spacers. This new series of 2D perovskites shows a surprisingly zero distortion along both in-plane and out-of-plane (Pb-I-Pb angle is equal to 180 degrees for both directions), which is never observed before in any multi-layer 2D perovskites. They are all tetragonal phase and with P4/mmm as the space group, which is the maximum symmetry a 2D perovskite can theoretically have.⁵ The interlayer distance (below 4 Å) of the FA DJ 2D perovskite is also one of the smallest among all reported 2D perovskite, and this could allow for better interlayer electronic coupling. As the consequence of zero distortion and short interlayer distance, this new 2D perovskite series exhibits systematically lower band gap, probably the smallest bandgap among all the 2D perovskite to our best knowledge. Preliminary stability test shows this new FA DJ maintained 75% of the perovskite phase under 1.2 suns plus 100% RH (relative humidity) over 2.5 hours.<br/><br/>1. Metcalf, I. <i>et al.</i> Synergy of 3D and 2D Perovskites for Durable, Efficient Solar Cells and Beyond. <i>Chemical Reviews</i> <b>123</b>, 9565–9652 (2023).<br/>2. Park, J. <i>et al.</i> Controlled growth of perovskite layers with volatile alkylammonium chlorides. <i>Nature</i> <b>616</b>, 724–730 (2023).<br/>3. Gélvez-Rueda, M. C. <i>et al.</i> Formamidinium-Based Dion-Jacobson Layered Hybrid Perovskites: Structural Complexity and Optoelectronic Properties. <i>Advanced Functional Materials</i> <b>30</b>, 2003428 (2020).<br/>4. Kepenekian, M. <i>et al.</i> Concept of lattice mismatch and emergence of surface states in two-dimensional hybrid perovskite quantum wells. <i>Nano letters</i> <b>18</b>, 5603–5609 (2018).<br/>5. Quarti, C., Katan, C. & Even, J. Physical properties of bulk, defective, 2D and 0D metal halide perovskite semiconductors from a symmetry perspective. <i>Journal of Physics: Materials</i> <b>3</b>, 042001 (2020).