LAUSR

Most of the reported 2D Ruddlesden–Popper (RP) lead halide perovskites with the general formula of An+1BnX3n+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, Cs2Pb(SCN)2Br2, 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)2Br2 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 CsPbBr3 (Pm3m #221) at 450 K. It has a low exciton binding energy of 160 meV—one of the lowest for 2D perovskites (n = 1). A Cs2Pb(SCN)2Br2‐single‐crystal photodetector is demonstrated with respectable responsivity of 8.46 mA W−1 and detectivity of ≈1.2 × 1010 Jones at a low bias voltage of 0.5 V.