Halide Perovskite–Chalcohalide Nanocrystal Heterostructures as a Platform for the Synthesis and Investigation of the CsPbCl ₃ –CsPbI ₃ Epitaxial Interface

Abstract

Abstract Halide exchange in lead‐based halide perovskites has been studied extensively. While mixed Cl/Br or Br/I alloy compositions can be formed with no miscibility gaps, this is precluded for mixed Cl/I compositions, due to the large difference in Cl − and I − ionic radii. Here, perovskite‐chalcohalide CsPbCl 3 –Pb 4 S 3 Cl 2 nanocrystal heterostructures are exploited to study the Cl→I exchange and to isolate new types of intermediate structures. The epitaxial interface between the Pb 4 S 3 Cl 2 chalcohalide and the CsPbCl 3 perovskite significantly influences the intermediate stages of halide exchange in the perovskite domain, leading to coexisting CsPbCl 3 and CsPbI 3 domains, thereby delivering segmented CsPbI 3 –CsPbCl 3 –Pb 4 S 3 Cl 2 , energetically favorable heterostructures, with partial I‐alloying of the CsPbCl 3 domain and at the perovskite–chalcohalide interface. The I:CsPbCl 3 domain between CsPbI 3 and Pb 4 S 3 Cl 2 enables a gradual lattice expansion across the heterostructure. This design accommodates interfacial strain, with a 5.6% mismatch at the CsPbCl 3 –CsPbI 3 interface and a 3.4% mismatch at the perovskite–chalcohalide interface. Full halide exchange leads to CsPbI 3 –Pb 4 S 3 Cl 2 heterostructures. Both in intermediate and fully exchanged heterostructures, the CsPbI 3 domain is emissive. In the intermediate structures, the band alignment between the two perovskite domains is type‐I, with the carriers photogenerated in the CsPbCl 3 domain quickly transferring to the CsPbI 3 domain, where they can recombine radiatively.