Organic Conductors Research Articles

Emergent atomistic Kondo resonances in strained carbon-benzene single chains

Using real-space generalized gradient approximation plus dynamical mean-field theory ($\mathrm{GGA}+\mathrm{DMFT}$) we explore the effect of uniaxial strain on the correlated electronic structure reconstruction of atomistic carbon patterned out in carbon-benzene single chains. Firstly, based on GGA we show how strain can trigger a valley electronic structure with similarities akin to narrow-band semiconductors. The correlated electronic structure we derive is promising in the sense that it enables the observation of emergent Kondo quasiparticle resonances for future valleytronics based on low-dimensional organic conductors. Our proposal is a key step to iteratively understand the intricate and interdependent changes in orbital and electronic degrees of freedom at the atomistic level.

Unraveling Doping Capability of Conjugated Polymers for Strategic Manipulation of Electric Dipole Layer toward Efficient Charge Collection in Perovskite Solar Cells

AbstractDeveloping electrical organic conductors is challenging because of the difficulties involved in generating free charge carriers through chemical doping. To devise a novel doping platform, the doping capabilities of four designed conjugated polymers (CPs) are quantitatively characterized using an AC Hall‐effect device. The resulting carrier density is related to the degree of electronic coupling between the CP repeating unit and 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4‐TCNQ), and doped PIDF‐BT provides an outstanding electrical conductivity, exceeding 210 S cm−1, mainly due to the doping‐assisted facile carrier generation and relatively fast carrier mobility. In addition, it is noted that a slight increment in the electron‐withdrawing ability of the repeating unit in each CP diminishes electronic coupling with F4‐TCNQ, and severely deteriorates the doping efficiency including the alteration of operating doping mechanism for the CPs. Furthermore, when PIDF‐BT with high doping capability is applied to the hole transporting layer, with F4‐TCNQ as the interfacial doping layer at the interface with perovskite, the power conversion efficiency of the perovskite solar cell improves significantly, from 17.4% to over 20%, owing to the ameliorated charge‐collection efficiency. X‐ray photoelectron spectroscopy and Kelvin probe analyses verify that the improved solar cell performance originates from the increase in the built‐in potential because of the generation of electric dipole layer.

Optical Study of Anisotropy in Organic Conductors D4[MBr4](Solvent)

The polarized reflectance and optical conductivity spectra of layered organic conductors D4[MBr4](Solvent): θ-(BEDT-TTF)4HgBr4(DCB), θ-(BEDT-TTF)4CdBr4(DBB), θ-(d8-BEDT-TTF)4CdBr4(DBB), and θ-(BETS)4CdBr4(PhBr) (DCB and DBB = o-dichloro and o-dibromo-benzenes, BEDT-TTF and BETS are bis(ethylenedithio)tetrathia- and bis(ethylenedithio)tetraselena-fulvalene) are studied. The adjacent conducting layers of these compounds differ in the direction of their stacks of radical cations (the directions of the stacks are mutually perpendicular). It is shown that these conductors are characterized by high isotropy of the electronic system in the conducting plane and the quasi-two-dimensional nature of the electronic conductivity, which is in good agreement with the crystal structure.

Spin-density wave in the vicinity of superconducting state in λ−(BETS)2GaBrxCl4−x probed by C13 NMR spectroscopy

This paper identifies the electronic state just close to the superconducting phase in a system of organic conductors lambda-(BETS)2GaBrxCl4-x as a spin-density-wave state, using nuclear magnetic resonance and resistivity measurements. This finding explains the unexplainable enhancement of spin fluctuation at low temperatures in lambda-(BETS)2GaCl4 and suggests that the superconductivity of lambda-type salts is mediated by spin-density-wave fluctuations.

Improvement of the Optoelectrical Properties of a Transparent Conductive Polymer via a Simple Mechanical Pressure Treatment.

In this study, a simple and novel mechanical pressure treatment (MPT) was used to effectively improve the electrical and optical properties of ethylene glycol (EG)-doped PEDOT:PSS (EG-PEDOT:PSS) thin films, one of the most successful organic conductor materials ever which is are widely used in organic electronics because of their admirable film-forming property, high light transmittance, and excellent thermal stability. It is found that the conductivity of the EG-PEDOT:PSS films increased by 32% due to dramatically enhanced carrier mobility because an MPT improves the phase separation between PEDOT and PSS and then yields an interpenetrating conductive network. Meanwhile, the transmittance of the EG-PEDOT:PSS films in the near-infrared band was enhanced, and the surface roughness was reduced. These thin films retain their incredible flexibility as well; after 5000 times of 180° bending, the sheet resistance is basically unchanged. Considering that this MPT approach is already well developed in industrial applications, it is very hopeful to extend this technique in the field of organic electronics.

Finite Temperature Properties of Geometrically Charge Frustrated Systems

We theoretically study finite temperature properties of interacting fermion systems under geometrical frustration in the charge degree of freedom. Physical quantities such as charge structure factors, the specific heat, and the entropy, of the two-dimensional model of interacting spinless fermions on an anisotropic triangular lattice are numerically calculated using the thermal pure quantum state. By considering the Coulomb interactions up to the next-nearest-neighbor bonds, we elucidate that in the highly frustrated region where a long-period stripe-type charge order (CO) is the ground state, fluctuations of different stripe-type CO patterns become large at finite temperatures. When we further introduce $1/r$-type long-range Coulomb interactions, the ground state unexpectedly recovers the non-stripe-type 3-fold CO pattern characteristic of triangular lattice models with short-range interactions. Our results imply that the BEDT-TTF-based organic conductors exhibiting glass-like behavior locates in the region of the intermediate strength of long-range interactions, where both the stripe- and non-stripe-type CO fluctuations are prominent.

Quantum Confined Tomonaga-Luttinger Liquid in Mo6Se6 Nanowires Converted from an Epitaxial MoSe2 Monolayer.

Confining interacting particles in one-dimension (1D) changes the electronic behavior of the system fundamentally, which has been studied extensively in the past. Examples of 1D metallic systems include carbon nanotubes, quasi-1D organic conductors, metal chains, and domain boundary defects in monolayer thick transition-metal dichalcogenides such as MoSe2. Here single and bundles of Mo6Se6 nanowires were fabricated through annealing a MoSe2 monolayer grown by molecular-beam epitaxy on graphene. Conversion from two-dimensional (2D) MoSe2 film to 1D Mo6Se6 nanowire is reversible. Mo6Se6 nanowires form preferentially at the Se-terminated zigzag edges of MoSe2 and stitch to it via two distinct atomic configurations. The Mo6Se6 wire is metallic and its length is tunable, which represents one of few 1D systems that exhibit properties pertinent to quantum confined Tomonaga-Luttinger liquid, as evidenced by scanning tunneling microscopic and spectroscopic studies.

The Key Role of Side Chain Linkage in Structure Formation and Mixed Conduction of Ethylene Glycol Substituted Polythiophenes.

Functionalizing conjugated polymers with polar ethylene glycol side chains enables enhanced swelling and facilitates ion transport in addition to electronic transport in such systems. Here, we investigate three polythiophene homopolymers (P3MEET, P3MEEMT, and P3MEEET) having differently linked (without spacer and with methyl and ethyl spacer, respectively) diethylene glycol side chains. All the polymers were tested in organic electrochemical transistors (OECTs). They show drastic differences in the device performance. The highest μOECT C* product of 11.5 F/cm·V·s was obtained for ethyl-spaced P3MEEET. How the injection and transport of ions is influenced by the side-chain linkage was studied with electrochemical impedance spectroscopy, which shows a dramatic increase in volumetric capacitance from 80 ± 9 up to 242 ± 17 F/cm3 on going from P3MEET to P3MEEET. Thus, ethyl-spaced P3MEEET exhibits one of the highest reported volumetric capacitance values among p-type polymers. Moreover, P3MEEET exhibits in dry thin films an organic field-effect transistor (OFET) hole mobility of 0.005 cm2/V·s, highest among the three, which is one order of magnitude higher than that for P3MEEMT. The extracted hole mobility from OECT (oxidized swollen state) and the hole mobility in solid-state thin films (OFET) show contradictory trends for P3MEEMT and P3MEEET. In order to understand exactly the properties in the hydrated and dry states, the crystal structure of the polymers was investigated with wide-angle X-ray scattering (WAXS) and grazing incidence WAXS, and the water uptake under applied potential was monitored using electrochemical quartz crystal microbalance with dissipation monitoring (E-QCMD). These measurements reveal an amorphous state for P3MEET and a semicrystalline state for P3MEEMT and P3MEEEET. On the other hand, E-QCMD confirms that P3MEEET swells 10 times more than P3MEEMT in the oxidized state. Thus, the importance of the ethyl spacer toward crystallinity and mixed-conduction properties was clearly demonstrated, emphasizing the impact of side chain linkage of diethylene glycol. This detailed study offers a better understanding of how to design high-performance organic mixed conductors.

Thermopower in an anisotropic two-dimensional Weyl semimetal

We investigate the generation of an electric current from a temperature gradient in a two-dimensional Weyl semimetal with anisotropy, in both the presence and absence of a quantizing magnetic field. We show that the anisotropy leads to doping dependences of thermopower and thermal conductivities which are different from those in isotropic Dirac materials. Additionally, we find that a quantizing magnetic field in such systems leads to an interesting magnetic field dependence of the longitudinal thermopower, resulting in unsaturated thermoelectric coefficients. Thus, the results presented here will serve as a guide to achieving high thermopower and a thermoelectric figure-of-merit in graphene-based materials, as well as organic conductors such as $\alpha$-BEDT-TTF$_2$I$_3$.

One-dimensional electronic systems: metal-chain complexes and organic conductors.

One-dimensional (1D) metal-chain complexes and organic conductors show many similarities as well as striking differences in structural and electronic properties, although constituent elements and orbitals that contribute to charge transfer in these systems are quite different. In this review, we highlighted the structural and electronic properties of neutral MMX-chain complexes (M = Pt2+/3+, X = I-) and tetramethyltetrathiafulvalene-based cation radical salts as typical examples of each group while comparing them with each other. This review primarily aims to construct a coherent body of knowledge of 1D electronic materials that might have been separately investigated. We have proposed future directions for the exploration of new and more advanced electronic materials not only having 1D character, but also residing in the dimensional crossover regime.

Phase transformations of iodine-containing dual-layered conductor θ-(ET)4CdI4(PhCl) upon cooling

The effect of transformation of conducting layers with decreasing temperature on transport properties was studied for new organic conductor θ-(ET)4CdI4(PhCl).

Mixed Ionic and Electronic Conduction in Appropriately Substituted Phthalocyanine Thin Films Allows Fast and Reversible Electrochromic Switching

Thin films of phthalocyanines can be considered established materials as organic semiconductors and as electrocatalytically active electrodes. As a consequence of their redox chemistry, however, they also have been proposed as electrochromic layers. For this purpose, they have to perform as organic mixed ionic and electronic conductors (OMIEC). Phthalocyanines with electron-withdrawing substituents have shown reversible reduction and re-oxidation. Fluorination has proven particularly useful since chemically reversible reactions can be achieved, even in contact to aqueous electrolytes [1]. The degree of fluorination determines the intermolecular coupling and, hence, the movement of electrons and ions in thin films. For films of perfluorinated copper phthalocyanine (F16PcCu) facile transport of electrons was established, but the rate of the electrochromic reaction was limited by ion transport in the films. For a derivative in which all 8 peripheral F atoms were replaced by perfluoroisopropyl groups (F64PcCu), more facile ion transport was observed, but the electron mobility was strongly suppressed and then limited the rate of electrochromic switching [2].In this work, a new type of phthalocyanine with only 4 peripheral F atoms replaced by perfluoroisopropyl groups (F40PcCu) is studied. Notably, the distribution of the bulky, perfluoroalkyl group is on adjacent 2 phthalocyanine quadrants, leaving two quadrants able to pi-stack, a feature not present in F64PcCu. Physical vapor deposition yielded homogeneous thin films of 1 nm - 50 nm average film thickness. The dependence of the intermolecular coupling on the film thickness was analyzed by in situ UV/Vis spectroscopy. The films showed constantly small spectral splitting from the monolayer regime throughout the studied range of thicknesses, well into the range of bulk structure. A packing of molecules that leads to weak electronic coupling of adjacent phthalocyanine cores was thereby indicated. The electrochromic characteristics were investigated by electrochemical and spectroelectrochemical measurements with an aqueous solution of KCl as electrolyte. The optical absorption spectra revealed reversible changes of the films upon reduction with intercalation of the K+ counter ions and re-oxidation with extraction of the counter ions. As typical for phthalocyanines, absorption in the Soret- and Q-band was strongly attenuated in the reduced state and an additional absorption band was established between the two, around 500 nm. Therefore, the films showed intense changes of color upon reduction and re-oxidation. The films provided a well-balanced, equally fast transport of electrons and ions with effective diffusion coefficients in the range of 10-10 cm2 s-1. Fast and stable electrochromic switching to 90 % of the final state was obtained within about 0.3 s after applying a potential difference of about 1.5 V upon either reduction or re-oxidation. Reversible and stable switching of the films was achieved over at least 200 cycles. We consider F40PcCu to have an optimized substitution pattern to provide equally fast transport of electrons and ions in the films and, therefore, to provide very attractive switching characteristics which might be of interest as electrochromic layers in smart windows and smart mirrors. 1. S. Nagel, M. Lener, C. Keil, R. Gerdes, Ł. Łapok, S. M. Gorun and D. Schlettwein, "Electrochromic Switching of Evaporated Thin Films of Bulky, Electronic Deficient Metallo-Phthalocyanines", J. Phys. Chem. C, 2011, 115, 8759–8767.2. J. Weissbecker, A. Loas, S. M. Gorun and D. Schlettwein, "Switching of the Rate-limiting Step in the Electrochromic Reduction of Fluorinated Phthalocyanine Thin Films by Decreased Intermolecular Coupling", Electrochimica Acta, 2015, 157, 232–244.

Organic-based inverters: basic concepts, materials, novel architectures and applications.

While organic materials have demonstrated industry-leading performances in a wide array of electronic applications (including OLEDs and OPVs), their use for integration into electronic circuits has been so far limited, in spite of their potential for portable, flexible, light-weight, low-cost applications. However, recent advances in organic (semi)conductors and novel designs in organic field-effect transistors and hybrid systems have reaffirmed the potential of organic logic circuits. This review article provides an overview of organic-based inverter operation and considers all aspects of such circuits including their active layer, processing methods, hybrid organic/inorganic inverters, novel architectures and potential applications.

Flexible energy generation and storage devices: focus on key role of heterocyclic solid-state organic ionic conductors.

An evolving trend toward the ever-growing market of portable and wearable electronics has accelerated development in the construction of multifunctional energy generation and storage systems that can be twisted and folded to multiple deformations while retaining their electrochemical performance. The latest advances and well developed approaches for the design of heterocyclic solid-state organic ionic conductors (SOICs) in flexible energy generation and storage devices are discussed here. The development of SOICs with improved physical, optical, and electrochemical properties provides new prospects for flexible photoelectrochemical cells and supercapacitors. Equipped with a better knowledge of SOICs' multifunctional properties, researchers have made considerable progress in their development that allows modification according to the requirements of different types of flexible energy devices. Within this review, we highlight the design of efficient SOICs and their incorporation into flexible energy generation and storage devices, and address exciting instances that profile the multifunctionality of SOICs such as three-dimensional (3D) ionic channels, excellent thermal stability, dual functionality (hole/ions transportation), one-dimensional (1D) lamellar network, light-harvesting, and non-toxicity to mention a few. It is expected that innovative and customizable properties utilized in the development of multifunctional SOICs will provide a forum for future advancement in flexible and wearable energy generation and storage devices.

Nanoscale Mapping of Extrinsic Interfaces in Hybrid Solid Electrolytes

Summary Inorganic-organic hybrid solid electrolytes are promising material systems for all solid-state batteries (ASSBs). These electrolytes contain numerous solid | solid interfaces that govern transport pathways, electrode | electrolyte compatibility, and durability. This paper evaluates the role that electrode | electrolyte interfaces and electrolyte structure have on electrochemical performance. Atomic force microscopy techniques reveal how mechanical, adhesion, and morphological properties transform in a series of model hybrid solid electrolytes. These measurements are mapped to sub-surface microstructural features using synchrotron nano-tomography. Hybrid solid electrolytes with shorter polymer chains demonstrate a higher adhesion ( > 100 nN), Young's Modulus (6.4 GPa), capacity (114.6 mAh/g), and capacity retention (92.9%) than hybrid electrolytes with longer polymer chains (i.e., higher molecular weight). Extrinsic interfacial properties largely dictate electrochemical performance in ASSBs. Microstructural control over inorganic constituents may provide a means for tailoring interfacial properties in hybrid solid electrolytes.

Mechanism of Superconductivity in Two-Dimensional Organic Conductors

Mechanism of Superconductivity in Two-Dimensional Organic Conductors

Rising advancements in the application of PEDOT:PSS as a prosperous transparent and flexible electrode material for solution-processed organic electronics

ABSTRACTAn organic conductive polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), is an attractive candidate for a low-cost, low-temperature, and solution-processed electrode material for achieving high-performance flexible and stretchable thin-film devices. Unlike most organic materials, this water-soluble conjugated polymer is highly stable against chemical and physical exposure. It exhibits the most superior mechanical flexibility and the highest optical transparency and electrical conductivity among all organic conductors. Therefore, this conductive polymer is among the most promising alternatives to the expensive, rigid, and brittle metal oxide- and even metal-based electrode materials, such as indium tin oxide (ITO) and gold, in the future solution-processed electronic devices. Nevertheless, the intrinsic conductivity of PEDOT:PSS is typically below 1 S cm−1, which is too low for such devices. Fortunately, the material properties of PEDOT:PSS, including its conductivity, are easily tuned by employing a large number of simple approaches. In this paper, the reports on the successful application of PEDOT:PSS to a wide range of solution-processed organic devices, such as organic light-emitting diodes (OLEDs), organic thin-film transistors (OTFTs), and organic photovoltaics (OPVs), are reviewed. The recent progress in the development of highly conductive PEDOT:PSS-based films for electrode applications in the field of organic electronics is the main focus of the discussion herein.

Effects of electron correlations and chemical pressures on superconductivity of β′′ -type organic compounds

We investigate low-temperature electronic states of the series of organic conductors $\beta^{\prime\prime}$-(BEDT-TTF)$_4$[(H$_3$O)M(C$_2$O$_4$)$_3$]G, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene, and M and G represent trivalent metal ions and guest organic molecules, respectively. Our structural analyses reveal that the replacement of M and G give rise to systematic change in the cell parameters, especially in the $b$-axis length, which has positive correlation with the superconducting transition temperature $T_{\rm c}$. Analyses of temperature and magnetic field dependences of the electrical resistance including the Shubnikov-de Haas oscillations elucidates that the variation of charge disproportionation, effective mass and the number of itinerant carriers, can be systematically explained by the change of the $b$-axis length. The changes of the transfer integrals induced by stretching/compressing the $b$-axis are confirmed by the band calculation. We discuss that electron correlations in quarter-filled electronic bands lead to charge disproportionation and the possibility of a novel pairing mechanism of superconductivity mediated by charge degrees of freedom.

Universal Fermi-surface anisotropy renormalization for interacting Dirac fermions with long-range interactions

Recent experimental [I. Jo et al., Phys. Rev. Lett. 119, 016402 (2017)] and numerical [M. Ippoliti, S. D. Geraedts, R. N. Bhatt, Phys. Rev. B 95, 201104 (2017)] evidence suggests an intriguing universal relationship between the Fermi surface anisotropy of the noninteracting parent 2-dimensional (2D) electron gas and the strongly correlated composite Fermi liquid formed in a strong magnetic field close to half-filling. Inspired by these observations, we explore more generally the question of anisotropy renormalization in interacting 2D Fermi systems. Using a recently developed [H. -K. Tang et al., Science 361, 570 (2018)] nonperturbative and numerically exact projective quantum Monte Carlo simulation as well as other numerical and analytic techniques, only for Dirac fermions with long-range Coulomb interactions do we find a universal square-root decrease of the Fermi-surface anisotropy. For the [Formula: see text] composite Fermi liquid, this result is surprising since a Dirac fermion ground state was only recently proposed as an alternative to the usual Halperin-Lee-Read state. Our proposed universality can be tested in several anisotropic Dirac materials including graphene, topological insulators, organic conductors, and magic-angle twisted bilayer graphene.

Effects of finite-range interactions on the one-electron spectral properties of TTF-TCNQ

The electronic dispersions of the quasi-one-dimensional organic conductor TTF-TCNQ are studied by angle-resolved photoelectron spectroscopy (ARPES) with higher angular resolution and accordingly smaller step width than in previous studies. Our experimental results suggest that a refinement of the single-band 1D Hubbard model that includes finite-range interactions is needed to explain these photoemission data. To account for the effects of these finite-range interactions we employ a mobile quantum impurity scheme that describes the scattering of fractionalized particles at energies above the standard Tomonaga-Luttinger liquid limit. Our theoretical predictions agree quantitatively with the location in the $(k,\omega)$ plane of the experimentally observed ARPES structures at these higher energies. The nonperturbative microscopic mechanisms that control the spectral properties are found to simplify in terms of the exotic scattering of the charge fractionalized particles. We find that the scattering occurs in the unitary limit of (minus) infinite scattering length, which limit occurs within neutron-neutron interactions in shells of neutron stars and in the scattering of ultracold atoms but not in perturbative electronic condensed-matter systems. Our results provide important physical information on the exotic processes involved in the finite-range electron interactions that control the high-energy spectral properties of TTF-TCNQ. Our results also apply to a wider class of 1D and quasi-1D materials and systems that are of theoretical and potential technological interest.