Control of Polar/Antipolar Layered Organic Semiconductors by the Odd-Even Effect of Alkyl Chain

Some rodlike organic molecules exhibit exceptionally high layered crystallinity when composed of a link between π-conjugated backbone (head) and alkyl chain (tail). These molecules are aligned side-by-side unidirectionally to form self-organized polar monomolecular layers, providing promising 2D materials and devices. However, their interlayer stacking arrangements have never been tunable, preventing the unidirectional arrangements of molecules in whole crystals. Here, it is demonstrated that polar/antipolar interlayer stacking can be systematically controlled by the alkyl carbon number n, when the molecules are designed to involve effectively weakened head-to-head affinity. They exhibit remarkable odd-even effect in the interlayer stacking: alternating head-to-head and tail-to-tail (antipolar) arrangement in odd-n crystals, and uniform head-to-tail (polar) arrangement in even-n crystals. The films show excellent field-effect transistor characteristics presenting unique polar/antipolar dependence and considerably improved subthreshold swing in the polar films. Additionally, the polar films present enhanced second-order nonlinear optical response along normal to the film plane. These findings are key for creating polarity-controlled optoelectronic materials and devices.

Quantum Liquid States of Spin Solitons in a Ferroelectric Spin-Peierls State

In this study, we performed high-magnetic-field magnetization, dielectric, and ultrasound measurements on an organic salt showing a ferroelectric spin-Peierls (FSP) state, which is in close proximity to a quantum critical point. In contrast to the sparsely distributed gaslike spin solitons typically observed in conventional spin-Peierls (SP) states, the FSP state exhibits dense liquidlike spin solitons resulting from strong quantum fluctuations, even at low fields. Nevertheless, akin to conventional SP systems, a magnetic-field-induced transition is observed in the FSP state. In conventional high-field SP states, an emergent wave vector results in the formation of a spin-soliton lattice. However, in the present high-field FSP state, the strong quantum fluctuations preclude the formation of such a soliton lattice, causing the dense solitons to remain in a quantum-mechanically melted state. This observation implies the realization of a quantum liquid-liquid transition of topological particles carrying spin and charge in a ferroelectric insulator.

Competition of polar and antipolar states hidden behind a variety of polarization switching modes in hydrogen-bonded molecular chains

Switchable π-electron systems are very powerful fragments to emphasize ferroelectric or antiferroelectric polarizations up to record-high levels among organic molecular crystals. According to the Cambridge Structural Database, many azole crystals such as imidazoles and tetrazoles contain polar and bistable hydrogen-bonded molecular sequences suitable for ferroelectricity or antiferroelectricity. Indeed, polarization hysteresis experiments on the 5-phenyl-1H-tetrazole (PHTZ) family combined with single crystal structural analysis have revealed one ferroelectric, two antiferroelectrics, and two hybrid-like dielectrics. Here, the rich variations for the interrelation between the hydrogen-bonding states and the polarization switching modes are interpreted by density functional theory (DFT) calculations with an excellent consistency. Large switchable polarizations are theoretically confirmed, and, as expected, the largest contribution is the switchable π-electron systems. By mapping the energy levels of polar/antipolar states, the disordered hydrogen bonds always appear when the ground state is accompanied by a nearly degenerate state. The straightforward case is the hybrid-like dielectric caused by the competition between the polar and antipolar states. However, contrastive behaviors are observed when the switchable dipoles are involved in competition between the different antipolar arrangement. For example, the PHTZ crystal exhibits typical antiferroelectric switching regardless of the hydrogen disorder, whereas polarization switching is silent in the imidazole derivatives. The latter is explained by the switching field increase with depth of the ground state relative to the energy level of the polar state.

A straightforward method using the sign of the piezoelectric coefficient to identify the ferroelectric switching mechanism

Some organic ferroelectrics have two possible switching modes: molecular reorientation and proton transfer. Typical examples include 2,5-dihydroxybenzoic acid (DHBA) and Hdabco-ReO[Formula: see text] (dabco = diazabicyclo[2.2.2]octane). The direction and amplitude of the expected polarization depends on the switching mode. Herein a straightforward method to identify the ferroelectric switching mechanism is demonstrated. First, the relationship between the polarization vectors corresponding to the two modes is illustrated using the Berry phase. Second, the theoretical background for the sign of the piezoelectric coefficient is used to decide which mode occurs. Finally, comparing the theoretically calculated piezoelectric coefficients to the experimental results confirms the switching mode of each compound.

Large polarization and record-high performance of energy storage induced by a phase change in organic molecular crystals

Dielectrics that undergo electric-field-induced phase changes are promising for use as high-power electrical energy storage materials and transducers. We demonstrate the stepwise on/off switching of large polarization in a series of dielectrics by flipping their antipolar or canted electric dipoles via proton transfer and inducing simultaneous geometric changes in their π-conjugation system. Among antiferroelectric organic molecular crystals, the largest-magnitude polarization jump was obtained as 18 μC cm⁻² through revisited measurements of squaric acid (SQA) crystals with improved dielectric strength. The second-best polarization jump of 15.1 μC cm⁻² was achieved with a newly discovered antiferroelectric, furan-3,4-dicarboxylic acid. The field-induced dielectric phase changes show rich variations in their mechanisms. The quadruple polarization hysteresis loop observed for a 3-(4-chlorophenyl)propiolic acid crystal was caused by a two-step phase transition with moderate polarization jumps. The ferroelectric 2-phenylmalondialdehyde single crystal having canted dipoles behaved as an amphoteric dielectric, exhibiting a single or double polarization hysteresis loop depending on the direction of the external field. The magnitude of a series of observed polarizations was consistently reproduced within the simplest sublattice model by the density functional theory calculations of dipole moments flipping over a hydrogen-bonded chain or sheet (sublattice) irrespective of compounds. This finding guarantees a tool that will deepen our understanding of the microscopic phase-change mechanisms and accelerate the materials design and exploration for improving energy-storage performance. The excellent energy-storage performance of SQA was demonstrated by both a high recoverable energy-storage density W_r of 3.3 J cm⁻³ and a nearly ideal efficiency (90%). Because of the low crystal density, the corresponding energy density per mass (1.75 J g⁻¹) exceeded those derived from the highest W_r values (∼8–11 J cm⁻³) reported for several bulk antiferroelectric ceramics , without modification to relaxor forms.

Electric-field induced phase changes, which are promising for use in high-power electrical energy storage, can be realized in a series of organic dielectrics by flipping the antipolar or canted electric dipoles via proton transfer.

Metaelectric multiphase transitions in a highly polarizable molecular crystal

Metaelectric transition, i.e. an abrupt increase in polarization with an electric field is just a phase change phenomenon in dielectrics and attracts increasing interest for practical applications such as electrical energy storage and highly deformable transducers. Here we demonstrate that both field-induced metaelectric transitions and temperature-induced phase transitions occur successively on a crystal of highly polarizable bis-(1H-benzimidazol-2-yl)-methane (BI2C) molecules. In each molecule, two switchable polar subunits are covalently linked with each other. By changing the NH hydrogen location, the low- and high-dipole states of each molecule can be interconverted, turning on and off the polarization of hydrogen-bonded molecular ribbons. In the low-temperature phase III, the tetragonal crystal lattice comprises orthogonally crossed arrays of polar ribbons made up of a ladder-like hydrogen-bond network of fully polarized molecules. The single-step metaelectric transition from this phase III corresponds to the forced alignment of antiparallel dipoles typical of antiferroelectrics. By the transition to the intermediate-temperature phase II, the polarity is turned off for half of the ribbons so that the nonpolar and polar ribbons are orthogonal to each other. Considering also the ferroelastic-like crystal twinning, the doubled steps of metaelectric transitions observed in the phase II can be explained by the additional switching at different critical fields, by which the nonpolar ribbons undergo "metadielectric" molecular transformation restoring the strong polarization. This mechanism inevitably brings about exotic phase change phenomena transforming the multi-domain state of a homogeneous phase into an inhomogeneous (phase mixture) state.

Regioisomeric control of layered crystallinity in solution-processable organic semiconductors

The construction and control of 2D layered molecular packing motifs with functionally substituted π-electron cores are crucial for developing organic electronic materials and devices. We investigated a regioisomeric structure–property relationship in high-performance and solution-processable layered organic semiconductors based on mono-octyl-substituted benzothieno[3,2-b]naphtho[2,3-b]thiophene (mono-C8-BTNT). We demonstrated that an isomorphous bilayer-type layered herringbone packing motif is obtainable in a series of four positional isomers of mono-C8-BTNTs whose π-electron core is substituted by an octyl chain at one of the four most peripheral positions with roughly keeping the rod-like molecular shape. These regioisomeric compounds exhibited systematic variations in the solvent solubility and liquid-crystalline phase transitions at elevated temperatures. The analysis of intermolecular interaction energies in the crystals based on dispersion-corrected DFT calculations revealed that the crystals of 2- and 8-mono-C8-BTNTs are more stable than those of 3- and 9-mono-C8-BTNTs owing to the higher ordering of alkyl chain layers in the crystals. Such differences of the stability in their crystal formation are closely correlated with TFT performances, where the single-crystal devices of the 2- and 8-mono-C8-BTNTs substituted at the most peripheral positions exhibit high-performance TFT characteristics with a mobility of approximately 10 cm² V⁻¹ s⁻¹.

An isomorphous bilayer-type layered herringbone crystal packing is reported for a series of four positional isomers of mono-C8-BTNTs, where the single-crystal devices with the isomers exhibit high-performance TFT characteristics.

Architecting layered molecular packing in substituted benzobisbenzothiophene (BBBT) semiconductor crystals

The construction of layered molecular packing structures in a non-layered crystalline material, benzobisbenzothiophene (BBBT), was achieved by employing long-alkyl and phenyl substituents, leading to high-performance organic thin-film transistors.

Topological charge transport by mobile dielectric-ferroelectric domain walls

The concept of topology has been widely applied in condensed matter physics, leading to the identification of peculiar electronic states on three-dimensional (3D) surfaces or 2D lines separating topologically distinctive regions. In the systems explored so far, the topological boundaries are built-in walls; thus, their motional degrees of freedom, which potentially bring about new paradigms, have been experimentally inaccessible. Here, working with a quasi-1D organic material with a charge-transfer instability, we show that mobile neutral-ionic (dielectric-ferroelectric) domain boundaries with topological charges carry strongly 1D-confined and anomalously large electrical conduction with an energy gap much smaller than the one-particle excitation gap. This consequence is further supported by nuclear magnetic resonance detection of spin solitons, which are required for steady current of topological charges. The present observation of topological charge transport may open a new channel for broad charge transport-related phenomena such as thermoelectric effects.

Coexistence of normal and inverse deuterium isotope effects in a phase-transition sequence of organic ferroelectrics

Supramolecular cocrystals of anilic acids with 2,2′-bipyridines exhibit successive phase transitions as well as unusual isotope effects. Ferroelectricity driven by a cooperative proton transfer along the supramolecular chains is accompanied by huge permittivity (a maximum of 13 000) at the Curie point, as well as a large spontaneous polarization (maximum 5 μC cm⁻²) and a low coercive field ranging from 0.5 to 10 kV cm⁻¹. Deuterium substitutions over the hydrogen bonds smoothly raise the Curie point and simultaneously reduce other phase-transition temperatures by a few tens of degrees. The coexistence of opposite isotope effects reduces the temperature interval of the intermediate paraelectric phase from 84 to 10 K for the 5,5′-dimethyl-2,2′-bipyridinium bromanilate salt. The bipyridine molecules exhibit interplanar twisting, which represents the order parameter relevant to the high-temperature phase transitions. The normal and inverse temperature shifts are ascribed to the direct and indirect effects, respectively, of the lengthened hydrogen bonds, which adjusts the molecular conformation of the flexible bipyridine unit so as to minimally modify their adjacent intermolecular interactions.

Deuterium substitutions of the hydrogen-bonded ferroelectrics smoothly raise the Curie point and simultaneously reduce other phase-transition temperatures by a few tens of degrees.

Evidence for solitonic spin excitations from a charge-lattice-coupled ferroelectric order

Topological defects have been explored in different fields ranging from condensed matter physics and particle physics to cosmology. In condensed matter, strong coupling between charge, spin, and lattice degrees of freedom brings about emergent excitations with topological characteristics at low energies. One-dimensional (1D) systems with degenerate dimerization patterns are typical stages for the generation of topological defects, dubbed "solitons"; for instance, charged solitons are responsible for high electrical conductivity in doped trans-polyacetylene. Here, we provide evidence based on a nuclear magnetic resonance (NMR) study for mobile spin solitons deconfined from a strongly charge-lattice-coupled spin-singlet ferroelectric order in a quasi-1D organic charge-transfer complex. The NMR spectral shift and relaxation rate associated with static and dynamic spin susceptibilities indicate that the ferroelectric order is violated by dilute solitonic spin excitations, which were further demonstrated to move diffusively by the frequency dependence of the relaxation rate. The traveling solitons revealed here may promise the emergence of anomalous electrical and thermal transport.

Field-Induced Antipolar-Polar Structural Transformation and Giant Electrostriction in Organic Crystal

The field-induced antipolar-polar structural transition in an organic antiferroelectric 2-trifluoromethylnaphthimidazole crystal is investigated by performing synchrotron X-ray diffraction. The polarities of all of the hydrogen-bonded chains become parallel with each other in the presence of an external electric field. The switching is accompanied by a giant electrostriction, which provides 1 order of magnitude larger strain than the piezoelectric strain of the organic ferroelectrics: croconic acid and poly(vinylidene fluoride); however, it is comparable to those of typical commercial piezoelectric ceramics. The crystal structure analysis with electric field shows that the origin of the observed giant electrostriction can be attributed to the shear strain that emerges from the polarity switching of the hydrogen-bonded chains. The antipolar-polar structural transition in antiferroelectrics could be employed for the development of high-performance electrostrictive organic materials.

Strong polarization switching with low-energy loss in hydrogen-bonded organic antiferroelectrics

The electric-field-induced phase transition from antipolar to polar structures is at the heart of antiferroelectricity. We demonstrate direct evidence of antiferroelectricity by applying a strong electric field to two antipolar crystals of squaric acid (SQA) and 5,5'-dimethyl-2,2'-bipyridinium chloranilate. The field-induced polarization of SQA is quite large and reasonably explained by the theoretically calculated polarization on the hydrogen-bonded sheet sublattice. The pseudo-tetragonal lattice of SQA permits unique switching topologies that produce two different ferroelectric phases of low and high polarizations. By tilting the applied field direction, the electrical switching mechanism can be attributed to a 90° rotation of the sheet polarization. From the viewpoint of applications, the strong polarization, high switching field, and quite slim hysteresis observed in the polarization versus electric field curve for SQA are advantageous for excellent-efficiency energy storage devices.

Shift current photovoltaic effect in a ferroelectric charge-transfer complex

Shift current is a steady-state photocurrent generated in non-centrosymmetric single crystals and has been considered to be one of the major origins of the bulk photovoltaic effect. The mechanism of this effect is the transfer of photogenerated charges by the shift of the wave functions, and its amplitude is closely related to the polarization of the electronic origin. Here, we report the photovoltaic effect in an organic molecular crystal tetrathiafulvalene-p-chloranil with a large ferroelectric polarization mostly induced by the intermolecular charge transfer. We observe a fairly large zero-bias photocurrent with visible-light irradiation and switching of the current direction by the reversal of the polarization. Furthermore, we reveal that the travel distance of photocarriers exceeds 200 μm. These results unveil distinct features of the shift current and the potential application of ferroelectric organic molecular compounds for novel optoelectric devices.The bulk photovoltaics refers to an effect whereby electrons move directionally in non-centrosymmetric crystals upon light radiation. Here, Nakamura et al. observe this effect in a ferroelectric organic charge-transfer complex, which shows large diffusion distance of photogenerated electrons over 200 µm.