Hybrid organic–inorganic perovskites (HOIPs) can have a diverse range of compositions including halides, azides, formates, dicyanamides, cyanides and dicyanometallates. These materials have several common features, including their classical ABX3 perovskite architecture and the presence of organic amine cations that occupy the A-sites. Current research in HOIPs tends to focus on metal halide HOIPs, which show promise for use in solar cells and optoelectronic devices; however, the other subclasses also exhibit a diverse range of physical properties. In this Review, we summarize the chemical variability and structural diversity of all known HOIP subclasses. We also present a comprehensive account of their intriguing physical properties, including photovoltaic and optoelectronic properties, dielectricity, magnetism, ferroelectricity, ferroelasticity and multiferroicity. Moreover, we discuss the current challenges and future opportunities in this exciting field. Hybrid organic–inorganic perovskites (HOIPs) comprise a diverse range of chemical compositions from halides and azides to formates, dicyanamides, cyanides and dicyanometallates. In this Review, advances in the synthesis, structures and properties of all HOIP subclasses are summarized and their future opportunities are discussed.

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Chemically diverse and multifunctional hybrid organic–inorganic perovskites

The performance of perovskite solar cells recently exceeded 15% solar-to-electricity conversion efficiency for small-area devices. The fundamental properties of the active absorber layers, hybrid organic-inorganic perovskites formed from mixing metal and organic halides [e.g., (NH4)PbI3 and (CH3NH3)PbI3], are largely unknown. The materials are semiconductors with direct band gaps at the boundary of the first Brillouin zone. The calculated dielectric constants and band gaps show an orientation dependence, with a low barrier for rotation of the organic cations. Due to the electric dipole of the methylammonium cation, a photoferroic effect may be accessible, which could enhance carrier collection.

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Structural and electronic properties of hybrid perovskites for high-efficiency thin-film photovoltaics from first-principles

Goldschmidt's concept of ionic Tolerance Factors was recently shown to be a valuable guideline for the preparation of new compounds within the field of organic–inorganic perovskites. Here, we extend this approach and calculate Tolerance Factors for over 2500 amine–metal–anion permutations of the periodic table. The results suggest the potential existence of more than 600 undiscovered hybrid perovskites including alkaline earth metal and lanthanide based materials.

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An extended Tolerance Factor approach for organic–inorganic perovskites

As our understanding and control of intra- and intermolecular interactions evolve, ever more complex molecular systems are synthesized and assembled that are capable of performing work or completing sophisticated tasks at the molecular scale. Commonly referred to as molecular machines, these dynamic systems comprise an astonishingly diverse class of motifs and are designed to respond to a plethora of actuation stimuli. In this Review, we outline the conditions that distinguish simple switches and rotors from machines and draw from a variety of fields to highlight some of the most exciting recent examples of opportunities for driven molecular mechanics. Emphasis is placed on the need for controllable and hierarchical assembly of these molecular components to display measurable effects at the micro-, meso-, and macroscales. As in Nature, this strategy will lead to dramatic amplification of the work performed via the collective action of many machines organized in linear chains, on functionalized surfaces, o...

http://pubs.acs.org/doi/pdf/10.1021/acsnano.5b03367

Controlling Motion at the Nanoscale: Rise of the Molecular Machines.

The underlying phase transitions of ferroelectric mechanisms in molecular crystals are mainly limited to order–disorder and displacive types that are not involved in breaking of the chemical bonds. Here, we show that the bond-switching transition under ambient pressure is designable in molecular crystals, and demonstrate how to utilize the weaker and switchable coordination bonds in a novel molecular perovskite, [(CH3)3NOH]2[KFe(CN)6] (TMC-1), to afford a scarce multiaxial ferroelectrics with a high Curie temperature of 402 K and 24 equivalent ferroelectric directions (more than BaTiO3). The high-quality thin films of TMC-1 can be easily fabricated by a simple solution process, and to reveal perfect ferroelectric properties at both macroscopic and microscopic scales, suggesting TMC-1 as a promising candidate for applications in next-generation flexible electronics. The presented molecular assembly strategy, together with the achieved bond-switching ferroelectric mechanism, opens a new avenue for designing...

A Molecular Perovskite with Switchable Coordination Bonds for High-Temperature Multiaxial Ferroelectrics.

The inclusion compound [(CH3)2NH2]2[KCo(CN)6] exhibits a marked temperature-dependent dielectric constant and can be considered as a model of tunable and switchable dielectric materials. Crystal structure and solid-state NMR studies reveal a switchable property between low and high dielectric states around 245 K. This originates from an order–disorder phase transition of the system, changing the dynamics of the polar dimethylammonium (DMA) cation. Furthermore, the tuning of the dielectric constant at temperatures below the phase transition point is related to increasing angular pretransitional fluctuations of the dipole moment of DMA.

Tunable and switchable dielectric constant in an amphidynamic crystal.

One novel organic-inorganic hybrid compound, (p-CH3OC6H4NH3)+(18-crown-6)·H2PO4–·2H3PO4 (1) ((p-CH3OC6H4NH3)+ = 4-methoxyphenylammonium; 18-crown-6 = 1, 4, 7, 10, 13, 16-hexaoxacyclooctadecane), has been synthesized and characterized by IR spectroscopy, thermogravimetric analysis and single crystal X-ray diffraction. In the structure, the organic (p-CH3OC6H4NH3)+(18-crown-6) supermolecular cation layer and inorganic phosphate anion layer arrange alternately. Temperature- and frequency-dependent dielectric constants of compound 1 show no distinct dielectric anomalies, suggesting no phase transition within the measured temperature range (100–380 K). A high dielectric constant of 442.3 at a low frequency of 200 Hz suggests slow molecular motion and/or space charge polarization rather than fast electronic motion.

Crystal Structure and Dielectric Property of (p‐CH3OC6H4NH3)+(18‐crown‐6)·H2PO4–·2H3PO4

Distinct dielectric transitions in 1-ethylimidazole-based cadmium(II) complexes

Reaction of Cu(ox)(H2O)0.5 (ox = oxalate dianion) with HIm (HIm = imidazole) in aqueous solution yields two CuII complexes, [Cu3(H2O)4(HIm)4(μ-Im)2(ox)2] (1) (Im = imidazole anion) and [Cu(HIm)6](ox) (2). Formation of the two compounds depends on the molar ratio of the reactants and a secondary reagent, indicating a rich chemistry of the combination of CuII/ox/HIm components. Structural analyses of the two compounds show that 1 is a linear trinuclear cluster, in which the two Im anions act as bridging ligands and 2 shows a rare 6:1 HIm/Cu stoichiometry with a CsCl-type packing structure. There are numerous N–H···O and O–H···O hydrogen bonds among the ox, HIm, and/or H2O components, leading to complicate supramolecular hydrogen-bond networks. Temperature-dependent dielectric constants of the compounds show no dielectric anomalies between 100–345 K, suggesting no structural phase transitions in the measured temperature range.

Run-Qiang Zhu

Papers

Tunable and switchable dielectric constant in an amphidynamic crystal.

Crystal Structure and Dielectric Property of (p‐CH3OC6H4NH3)+(18‐crown‐6)·H2PO4–·2H3PO4

Distinct dielectric transitions in 1-ethylimidazole-based cadmium(II) complexes

Synthesis, Structural and Dielectric Characterization of two Imidazole‐ and Oxalate‐containing Copper(II) Complexes