Stable Ruddlesden-Popper Phase Inorganic-Cation Pseudohalide 2D Cs₂Pb(SCN)₂Br₂ Perovskite Single Crystal for Photodetector

3D metal halide perovskites (ABX₃ where A = Cs⁺, CH₃NH³⁺, CH(NH₂)²⁺; B = Sn²⁺, Pb²⁺; X = Cl⁻, Br⁻, I⁻) have outstanding optoelectronic properties but suffer from poor long term stability or durability. 2D or quasi-2D layered perovskites may be part of the solution by introducing a separation between [PbX₆] octahedron inorganic layers using long carbon chain spacers in the A-site or the X-site. As organic spacers are insulating, appropriate selection is important to ensure the resulting perovskites retain small enough exciton binding energy and sufficient conductivity at least in a vertical direction through each inorganic slab. Here we demonstrate the advantage of X-site substitution over A-site substitution for layered perovskite resulting in much smaller separation between the constituent perovskite layers and hence optoelectronic properties.<br/>This report is the 3rd one on X-site-substituted Ruddlesden-Popper (RP) phase perovskite single crystals since 2015 [1-3] and the first report on inorganic-cation pseudohalide 2D phase perovskite single crystal, Cs₂Pb(SCN)₂Br₂. We have been able to synthesize the crystal at room temperature (lower than those reported previously [1-2]) using the antisolvent vapor-assisted crystallization (AVC) method. Interestingly, the 2D single crystal undergoes first-order phase transformation to 3D cubic CsPbBr₃ above 450K but the transformation is reversible. Mechanisms of reversible transformations will be presented in the conference. A significant advantage of this Cs₂Pb(SCN)₂Br₂ single crystal is its small exciton binding energy of 160 meV, which is one of the lowest values reported for 2D perovskites (n = 1) and is comparable to the quasi-2D A-site substituted RP phase perovskite values. Finally, a Cs₂Pb(SCN)₂Br₂ single crystal photodetector is demonstrated with a respectable responsivity of 8.46 mA W⁻¹ and a detectivity of ≈1.2 × 10¹⁰ Jones at a low bias voltage of 0.5 V.<br/><br/><b>References</b><br/>[1] M. Daub, H. S. Hillebrecht. Synthesis, Single Crystal Structure and Characterization of (CH3NH3)2Pb(SCN)2I2. <i>Angew. Chem.</i>, <b>2015</b>, 127, 11168.<br/>[2] J. Li, Q. Yu, Y. He, C. C. Stoumpos, G. Niu, G. G. Trimarchi, H. Guo, G. Dong, D. Wang, L. Wang, M. G. Kanatzidis, Cs2PbI2Cl2, All-Inorganic Two-Dimensional Ruddlesden–Popper Mixed Halide Perovskite with Optoelectronic Response. <i>J. Am. Chem. Soc.</i>, <b>2018</b>, 140, 11085.<br/>[3] C. H. Liao, C. H. Chen, J. Bing, C. Bailey, Y. T. Lin, T. M. Pandit, L., Granados, J. Zheng, S. Tang, B. H. Lin, H. W. Yen, D. R. McCamey, B. J. Kennedy, C. C. Chueh, A. W. Ho-Baillie. Inorganic Cation Pseudohalide 2D Cs2Pb(SCN)2Br2 Perovskite Single Crystal. <i>Adv. Mater.,</i> <b>2022</b>, 34(7), 2104782.