1D, 2D, 3D Perovskites and perovskitoids: structure diversity and applications

Three-(3D) and two-dimensional (2D) layered halide perovskites are highly promising semiconductors for optoelectronic applications ranging from solar cells, light emitting diodes, soft radiation detector, hard radiation detectors, etc. The 3D versions of these compounds adopt the three-dimensional ABX₃ perovskite structure, which consists of a network of corner-sharing BX₆ octahedra, where the B atom is a divalent metal cation (typically Ge²⁺, Sn²⁺ or Pb²⁺) and X is a monovalent anion (typically Cl⁻, Br⁻, I⁻); the A cation is selected to balance the total charge and it can be a Cs⁺ or a small molecular species. Another class of materials gaining significance are the two-dimensional (2D) perovskites -a blend of perovskites with layered crystal structure- (Ruddlesden-Popper type) offer a greater synthetic versatility and allow for more specialized device implementation due to the directional nature of the crystal structure. A remarkable advantage of the 2D perovskites is the readily tunable functionality by incorporating a wide array of organic cations into the 2D framework and by controlling the slab thickness, in contrast to the 3D analogues which have limited scope for structural engineering. We present the new homologous series, (C(NH₂)₃)(CH₃NH₃)_nPb_nI_3n+1 (n = 1, 2, 3), of layered 2D perovskites which is different from Ruddlesden-Popper type. These compounds adopt an unprecedented structure type which is stabilized by the alternating ordering of the guanidinium and methylammonium cations in the interlayer space (ACI). The these 2D perovskites combine structural characteristics from both Dion-Jacobson (DJ) and Ruddlesden-Popper (RP) structure archetypes. We also report the first examples of hybrid DJ hybrid 2D lead iodide perovskites which consist of thick perovskite slabs (n>1) with layer number (n) ranging from 1 to 5. We describe two new DJ perovskite series based on bivalent (+2) spacer cations deriving from a piperidinium (C₅NH₁₂) organic backbone. The new DJ perovskites are built from 3AMP (3AMP = 3-(aminomethyl)piperidinium) and 4AMP (4AMP =  4-(aminomethyl)piperidinium) spacers and methylammonium (MA) perovskitizers cations to form A’(MA)_n-1Pb_nI_3n+1 (A’ = 3AMP or 4AMP, n = 1-4) homologous series acting as spacers. Surprisingly, a slight difference in the position of the-CH₂NH₃⁺ group on the piperidine chair (3- and 4- position with respect to the piperidine nitrogen) exerts a strong influence on the crystal structure, which is reflected on the distortion of the inorganic layers. This difference has a major impact on the optical and electronic properties.