Inorganic‐Cation Pseudohalide 2D Cs2Pb(SCN)2Br2 Perovskite Single Crystal

Abstract Most of the reported 2D Ruddlesden–Popper (RP) lead halide perovskites with the general formula of A n +1 B n X 3 n +1 ( n = 1, 2, …) comprise layered perovskites separated by A‐site‐substituted organic spacers. To date, only a small number of X‐site‐substituted RP perovskites have been reported. Herein, the first inorganic‐cation pseudohalide 2D phase perovskite single crystal, Cs 2 Pb(SCN) 2 Br 2 , is reported. It is synthesized by the antisolvent vapor‐assisted crystallization (AVC) method at room temperature. It exhibits a standard single‐layer ( n = 1) Ruddlesden–Popper structure described in space group of Pmmn (#59) and has a small separation ( d = 1.69 Å) between the perovskite layers. The SCN − anions are found to bend the 2D Pb(SCN) 2 Br 2 framework slightly into a kite‐shaped octahedron, limiting the formation of a quasi‐2D perovskite structure ( n > 1). This 2D single crystal exhibits a reversible first‐order phase transformation to 3D CsPbBr 3 ( Pm 3 m #221) at 450 K. It has a low exciton binding energy of 160 meV—one of the lowest for 2D perovskites ( n = 1). A Cs 2 Pb(SCN) 2 Br 2 ‐single‐crystal photodetector is demonstrated with respectable responsivity of 8.46 mA W −1 and detectivity of ≈1.2 × 10 10 Jones at a low bias voltage of 0.5 V.