Air-Stable Direct Bandgap Perovskite Semiconductors: All-Inorganic Tin-Based Heteroleptic Halides A_xSnCl_yI_z (A = Cs, Rb)

Semiconducting halide perovskites are a class of materials with exciting photoelectronic properties. Compared to the widely studied hybrid organic-inorganic perovskites, the all-inorganic derivatives are less well understood even as they promise high inherent stability. Currently, such materials are limited due to the fact that there is a very narrow choice of inorganic cations that can stabilize the desirable perovskite structure. Herein we report on the synthesis and characterization of novel all-inorganic tin-based perovskites and perovskitoids that can be stabilized by the heteroleptic coordination of chloride and iodide anions, Cs₂SnCl₂I₂ (1) and Cs_2.38Rb_1.62Sn₃Cl₈I₂ (2), consist of two-dimensional (2D) layers of [SnCl₄I₂]^4- octahedra with different connectivity modes. Compound 1 is an n = 1 Ruddlesden-Popper type perovskite adopting the tetragonal archetype structure (I4/mmm space group; a = 5.5905(3) Å, c = 18.8982(13) Å), while compound 2 crystallizes as an orthorhombic modification (Cmcm space group; a = 5.6730(11) Å, b = 25.973(5) Å, c = 16.587(3) Å) with corrugated layers. The crystal chemistry changes drastically when Cs⁺ is replaced by the smaller Rb⁺ cation which leads to the isolation of the low dimensional compounds Rb₃SnCl₃I₂ (3a), Rb₃SnCl_2.33I_2.67 (3b) and Rb₇Sn_4.25Cl₁₂I_3.5 (4), thus illustrating the importance of the A-cation size in the formation of perovskites. The 2D perovskites show wide band gaps and relatively large resistivities, associated with their chemical stability against the oxidation of Sn²⁺. The chemical stability is coupled with remarkable electronic properties that derive from the perovskite structure. DFT calculations suggest that both compounds are direct band gap semiconductors with large bandwidths, consistently with the experimentally determined band gaps of E_g = 2.62 and 2.81 eV for 1 and 2, respectively. The combination of stability and favorable electronic structure in heteroleptic-halide perovskites presents a new direction toward the realization of functional devices made exclusively from inorganic perovskites.