Polaronic Charge Carriers Research Articles

DFT and EPR study of spin-assisted processes in poly(3-alkylthiophene) oligomers and their composites

Correlations of spin, electronic and magnetic resonance parameters of charge carriers stabilized in poly(3-alkylthiophene) oligomers with their band structure, polymerization degree, side substitutes, and conformation established by using the density functional theory and electron paramagnetic resonance methods. The comparison of calculated and experimental data identified spin distribution on oligomer chains with different conformations. The type and size of alkyl substituent of polymer matrix governs the band gap, mobility, and spin-orbit coupling of polaron charge carriers. The introduce of small aromatic additives between oligomer chains initiates their self-assembly crystallization and formation of organic:organic spinterfase which accelerate spin-assisted electronic and magnetic processes in the composite. The research revealed that embedding aromatic hydrocarbons into oligomer bulk allows continuous and discrete handling of electronic spin-assisted processes carrying out in respective organic/organic composites. The key role of spinterfacial effects in the development of a new generation of molecular devices with actively tuned spin-assisted functionality is described.

Low-dimensional lead chromate-based hybrid system for capacitance and polarization performances: a flexible device for pressure-induced voltage generator

AbstractOrganic molecule–functionalized lead chromate nanoparticle was synthesized using a complexation strategy route. The structural characterization revealed the single-phase lead chromate system belongs to monazite-type monoclinic structure with the space group of P21/n. The unit cell consists of an alternating chain of polyhedra (PbO9) and tetrahedra (CrO4) units, connected via edge and corner-sharing arrangement. A flexible device of lead chromate was fabricated and investigated dielectric capacitance, electrical impedance, conductivity, and field dependent polarization performances under different frequency and temperature environments. The device displayed a maximum dielectric constant (εʹ) value ~ 2400 at 120°C under 100 Hz frequency condition. High dielectric constant value is originated from the orientation polarization of CrO4 unit and space charge contribution in the system. AC conductivity of the device suggested the polaronic charge carriers control the overall conduction process via localized hopping mechanism. A fatigue-free polarization behavior was observed in the device under the applied field of 4.0 kV/mm, for 103 cycles and retained the polarization value ~ 0.26 μC/cm2 over the switching cycles. Lead chromate–based device also showed pressure-induced voltage generation under different pressure conditions and could have the potential application as a voltage generator.

Insights into the polaron physics of high temperature superconductors from optical and time-domain spectroscopy

Polarons and percolation were mentioned as guiding concepts in the paper by Bednorz and Müller in their report on the discovery of superconductivity in Ba–La–Cu–O. Both of these concepts seem to have survived, but their role in high-temperature superconductivity is somewhat different than originally envisaged. Optical and time-resolved experiments have been extremely useful for understanding the roles and dynamical interplay of polaronic and itinerant charge carriers in the different states of matter that appear in the cuprate phase diagram. The present article outlines some of the connections between the original concepts, systematics and apparent dichotomies, and discusses them in the light of more recent experimental observations, leading to some important theoretical considerations on the mechanism for superconductivity.

Electrochemical and Solvent-Driven Swelling in a Conducting Polymer Film

Poly(ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is likely the most widely used conducting polymer and a champion material for emerging electrochemical technologies. However, the fundamental polymer science describing this material in an electrochemical context remains an active area of investigation. We employ in situ UV–vis absorption and in situ spectroscopic ellipsometry to measure changes to a cross-linked PEDOT:PSS film under an applied electrochemical potential. Using multiple component regression data analyses, we find that two polaronic charge carriers and two neutral forms of PEDOT are necessary to adequately describe the data. Systematic studies of swelling of PEDOT:PSS films in a range of non-aqueous solvents demonstrate that a traditional Flory–Huggins model is suitable to describe film thickness changes. Electrochemical oxidation and reduction also cause dramatic thickness changes to PEDOT:PSS films. In-operando spectroscopic ellipsometry shows that film thickness increases upon reduction and decreases during oxidation, indicating that cation transport is of primary importance in the electrochemistry of PEDOT:PSS. The changes in film thickness in response to electrochemically driven swelling can also be approached using a Flory–Huggins approach when a voltage dependence is included in the enthalpy of mixing.

Temperature-dependent electrical transport, Hall effect, and Seebeck properties of bulk chemically reduced graphene oxide with bipolar charge carrier materials

The temperature-dependent electrical transport, Hall effect, and Seebeck properties of bulk-reduced graphene oxide (rGO) prepared by a chemical reduction process were investigated in a temperature range of 310–475 K. The bulk rGO contained bipolar charge carriers with p-type to n-type switching at a temperature of 420 K. The materials illustrated a p-type characteristic in the temperature range of 310–420 K and n-type characteristic in the temperature range of 420–475 K. The charge transport mechanism was that of the graphene-derived 2D material in the p-type regime and governed by polaronic charge carriers.

Design of Donor–Acceptor Polymer Semiconductors for Optimizing Combinations with Dopants to Maximize Thermoelectric Performance

Several conjugated polymers have been designed for applications in thermoelectrics (TEs) to improve doping efficiency and charge transport. However, optimizing the compatibility of the polymer with molecular dopants through judicious design to achieve high TE performance remains a challenge. By designing a novel donor–acceptor (D-A)-based conjugated polymer, poly(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)-co-(4,7-bis(4,4-dihexyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophen-2-yl)-5,6-difluorobenzo[c][1,2,5] thiadiazole) (PBDTC2FBT), compatibility with molecular dopants was maximized, resulting in enhanced TE properties. PBDTC2FBT was designed to include two types of electron donor building blocks with one acceptor in a repeat unit; this structure formed local short-range ordered crystals. Two molecular doping processes employing FeCl3 and tris(pentafluorophenyl)borane (BCF)–water complexes generated polaronic charge carriers in PBDTC2FBT while minimizing crystal deterioration. The transport behavior of the charge carriers in PBDTC2FBT differed depending on the dopant. Studies using the temperature-dependent electrical conductivity and Kang–Snyder charge transport models indicated that the weak Coulomb binding energy with the dopant counterion in BCF-doped PBDTC2FBT facilitated charge transport compared with that in the FeCl3-doped sample at the low doping region. Finally, BCF-doped PBDTC2FBT exhibited an optimized power factor of 35.1 μW m–1 K–2, which is extremely close to the theoretical maximum (36.3 μW m–1 K–2) that can be achieved with PBDTC2FBT. This study provides useful guidelines for maximizing the TE performance of D-A conjugated polymers through judicious design.

A polypyrrole hybrid thin film with [sn(dmit)3]2−, [tris(1,3-dithiole-2-thione-4,5-dithiolate)stannate]2−: Surface analysis

It is essential to understand the formation of the interface and the morphological design of the surface of electrodes and devices to control the structure and adapt the material to different applications. To study the interface, thin films of a PPy/[Sn(dmit)3]2 were electrochemically grown with the aid of a microcell typically used in scanning tunneling microscopy assays. Raman and FTIR spectroscopies showed the effect of dimensionality on those materials, especially on the vibrational modes of dmit ring and on the 75 nm thick PPy/[Sn (dmit)3]2 film, which presents an optimized polymer chain conjunction length and a 20 % greater contribution to the polaron charge carriers. X-ray Photoelectron Spectroscopy (XPS) analyses confirmed the polaron/bipolaron ratio data, allowing us to determine other contributions to the surface charge density. Atomic Force Microscopy (AFM) images show the hybrid thin film's different growth modes and the roughness evolution as a function of the electrochemical signal used in the synthesis.

Charge transfer excitons in π-stacked thiophene oligomers and P3[Alkyl]T crystals: CIS calculations and electroabsorption spectroscopy.

Poly(3-alkylthiophenes) (P3[Alkyl]T) exhibit high mobility and efficiency of formation of polaronic charge carriers generated by light absorption, thus finding applications in field effect devices. Excited states of π-stacked dimers of tetra-thiophene oligomers (T4), infinite isolated polythiophene (PT) chains, and P3[Alkyl]T crystals are modeled using configuration interaction singles (CIS) calculations. Excited states in cofacial T4 dimers are mostly localized Frenkel states except for two low energy charge transfer (CT) exciton states, which become the ionization potential and electron affinity levels of T4 molecules at large dimer separation. The lowest excited states in infinite, isolated PT chains and P3[Alkyl]T crystals are intra-chain excitons where the electron and hole are localized on the same chain. The next lowest excited states are interchain, CT excitons in which the electron and hole reside on neighboring chains. The former capture almost all optical oscillator strength and the latter may be a route to efficient formation of polaronic charge carriers in P3[Alkyl]T systems. Changes in optical absorption energies of T4 dimers as a function of molecular separation are explained using CIS calculations with four frontier orbitals in the active space. Shifts in optical absorption energy observed on going from isolated chains to P3[Alkyl]T lamellar structures are already present in single-particle transition energies induced by direct π-π interactions at short range. The electroabsorption spectrum of T4 dimers is calculated as a function of dimer separation and states that are responsible for parallel and perpendicular components of the spectrum are identified.

Effects of vanadium doping on the charge ordering and low-temperature spin-glass phase in Pr0.45Ca0.55MnO3

We report structural, magnetic, and dielectric properties of the Pr0.45Ca0.55Mn1-xVxO3 (x = 0.05 and 0.1) polycrystalline compounds, prepared by solid state reaction method. Pnma space group and orthorhombic crystal structure of the compounds are confirmed by the Rietveld refinement of X-ray diffraction data. Magnetic study reveals a multiple-phase-separated metastable magnetic behavior with charge ordered phase followed by a spin-glass state at the low-temperature region. For a single change of vanadium content from x = 0.05–0.1, the charge ordering temperature decreases from 245 K to 239 K, the antiferromagnetic ordering temperature decreases from 131 K to 127 K, and the ferromagnetic cluster-glass ordering temperature increases from 41 K to 44 K. This indicates the weakening of charge ordering strength and development of ferromagnetic components in the antiferromagnetic background, which is also confirmed by isothermal hysteresis. Colossal dielectric constant is observed in both samples. In the measured temperature range, an implication of a ferroelectric phase transition around 253 K is observed in Pr0.45Ca0.55Mn0.90V0.10O3 compound. The calculated results indicate that the low-temperature relaxation follows variable range hopping behavior, whereas the high-temperature one takes place according to the Arrhenius model. The fitting parameters ascribe the low-temperature relaxation process to the hopping of polaron charge carriers at localized sites, whereas the high-temperature relaxation is related to Maxwell-Wagner relaxation, caused by blocking of charge carriers at grain boundaries.

Electrical Tuning of Plasmonic Conducting Polymer Nanoantennas.

Nanostructures of conventional metals offer manipulation of light at the nanoscale but are largely limited to static behavior due to fixed material properties. To develop the next frontier of dynamic nano-optics and metasurfaces, this study utilizes the redox-tunable optical properties of conducting polymers, as recently shown to be capable of sustaining plasmons in their most conducting oxidized state. Electrically tunable conducting polymer nano-optical antennas are presented, using nanodisks of poly(3,4-ethylenedioxythiophene:sulfate) (PEDOT:Sulf) as a model system. In addition to repeated on/off switching of the polymeric nanoantennas, the concept enables gradual electrical tuning of the nano-optical response, which wasfound to be related to the modulation of both density and mobility of the mobile polaronic charge carriers in the polymer. The resonance position of the PEDOT:Sulf nanoantennas can be conveniently controlled by disk size, here reported down to a wavelength of around 1270 nm. The presented concept may be used for electrically tunable metasurfaces, with tunable farfield as well as nearfield. The work thereby opens for applications ranging from tunable flat meta-optics to adaptable smart windows.

Enhanced Dielectric Relaxation in Self-Organized Layers of Polypeptides Coupled to Platinum Nanoparticles: Temperature Dependence and Effect of Bias Voltage

Using alternative current impedance spectroscopy, we investigate the dynamical conductivity of hybrid nanomaterials composed of helical polypeptide layers containing platinum nanoparticles (PtNP). The electrical characteristics of the self-organized poly(γ-benzyl-l-glutamate) (PBGL) in bidimensional lamellar assembly in the presence of Pt nanoparticles are well modeled and described by a single equivalent circuit of parallel resistance and capacitance. The latter are determined using a comparison between the measured and calculated Nyquist plots, which allows extracting the characteristic relaxation time and frequency of the dipolar relaxation process. We found that the relaxation frequency in the PBLG–PtNP hybrid materials is enhanced by 4 orders of magnitudes compared to pure PBLG, which indicates a much faster dielectric relaxation in PBLG–PtNP due to dipole orientation and dipole–dipole interactions. The temperature dependence of the relaxation time is analyzed using Arrhenius plots, from which the activation energy of the relaxation process is found to be around 0.1 eV. Such a value close to the peptide vibration energy of the PBLG indicates a vibration-assisted relaxation process and a polaronic charge transport mechanism. An advantage of the PBLG–PtNP nanocomposite material is that the activation energy can be finely tuned by the PBLG degree of polymerization. Finally, an important outcome of this work is the investigation of the dielectric relaxation process in PBLG–PtNP under applied DC bias. We found that the activation energy decreases with increasing bias voltage for all degrees of polymerization of the PBLG molecule. This effect is interpreted in terms of electric field-induced alignment of the dipoles and of increased mobility of the polaronic charge carriers. The presence of piezoelectricity in the hybrid material gives the possibility to use the DC bias as a simple mean of monitoring the dynamical conductivity involving polaronic states.

In Situ Monitoring of Polaron Localization and Cell Activity with Charge Accumulation Spectroscopy

AbstractLocalization of charge carriers directly affects charge transport and local potential fluctuations and is a critical factor dictating the performance of organic bioelectronic devices. Understanding how the polaron localization at the solid−water interface is influenced by the microscopic structural ordering of the organic semiconducting films can provide complementary information to achieve better control on the electrochemical stability of bioelectronic devices. Here, charge accumulation spectroscopy is utilized to explore the structure‐dependent polaron localization in an electrolyte‐gated organic field‐effect transistor structure. The results reveal that the ordered and closely packed systems exhibit much higher absorption cross‐sections (σp) than disordered and isolated molecules. Therefore, the charge‐induced absorption feature can be utilized to monitor in situ the microscopic morphology variations of organic semiconducting films under electrolyte and trace the entire cell detachment process noninvasively. This study suggests that the spectroscopic signature of polaronic charge carriers, which is highly sensitive to the local environment surrounding the probed chromophores, serves as an effective probe to detect the structure‐dependent charge transport features in organic bioelectronic devices.

Influence of Backbone Curvature on the Organic Electrochemical Transistor Performance of Glycolated Donor–Acceptor Conjugated Polymers

AbstractTwo new glycolated semiconducting polymers PgBT(F)2gT and PgBT(F)2gTT of differing backbone curvatures were designed and synthesised for application as p‐type accumulation mode organic electrochemical transistor (OECT) materials. Both polymers demonstrated stable and reversible oxidation, accessible within the aqueous electrochemical window, to generate polaronic charge carriers. OECTs fabricated from PgBT(F)2gT featuring a curved backbone geometry attained a higher volumetric capacitance of 170 F cm−3. However, PgBT(F)2gTT with a linear backbone displayed overall superior OECT performance with a normalised peak transconductance of 3.00×104 mS cm−1, owing to its enhanced order, expediting the charge mobility to 0.931 cm2 V−1 s−1.

Influence of Backbone Curvature on the Organic Electrochemical Transistor Performance of Glycolated Donor-Acceptor Conjugated Polymers.

Two new glycolated semiconducting polymers PgBT(F)2gT and PgBT(F)2gTT of differing backbone curvatures were designed and synthesised for application as p‐type accumulation mode organic electrochemical transistor (OECT) materials. Both polymers demonstrated stable and reversible oxidation, accessible within the aqueous electrochemical window, to generate polaronic charge carriers. OECTs fabricated from PgBT(F)2gT featuring a curved backbone geometry attained a higher volumetric capacitance of 170 F cm−3. However, PgBT(F)2gTT with a linear backbone displayed overall superior OECT performance with a normalised peak transconductance of 3.00×104 mS cm−1, owing to its enhanced order, expediting the charge mobility to 0.931 cm2 V−1 s−1.

Thermodynamic and kinetic aspects of charge transfer inside conducting polymer films

A new variant of the theory of charging processes for electroactive polymer films on electrode surfaces was recently developed for conducting polymers containing cation-radicals (polarons) of identical sizes. Compared with the previously proposed models, the new approach explicitly takes into account the polaron charge delocalization over several monomer units of a polymer chain. Although the obtained theoretical predictions appeared to be in a qualitative agreement with experimental results, it has not been explored earlier whether the developed approach is consistent with thermodynamic principles. With that in mind, we complete the previously derived expressions for fluxes of charged quasi-particles and then treat them within the framework of the non-equilibrium thermodynamics. As a result, it has been established in the limit of the quasi-reversible character of charging/discharging processes that such a treatment allows one to achieve complete accordance of these expressions with thermodynamic requirements. This confirms validity of the previous predictions and, at the same time, creates premises for treating the electrochemical properties of more complicated systems than those considered in this paper. In parallel to these general conclusions, we have calculated quasi-equilibrium voltammetry curves of electrodes modified with polymer films containing polaron and bipolaron charge carriers of equal sizes. A primary comparison of such calculated curves with experimental ones confirms their qualitative agreement.

EFFECT OF PRIMARY DOPANTS ON THE CONDUCTIVITY OF POLYANILINE SYNTHESIZED BY ELECTROCHEMICAL POLYMERIZATION

Polyaniline salts were synthesized at room temperature (300 C) by electrochemical oxidation using primary dopants such as HCL, HNO3, H2SO4 and CH3COOH. All the synthesized PANI have resulted in emeraldine salt form as indicated by dark green colour of the salts. Various characterization techniques employed were UV-Visible spectroscopy, FT-IR and Cyclic voltammetry studies. All these techniques confirm the various properties of PANI. The characteristic peaks in the UV-Visible spectra of the samples indicate that effective doping has occurred in the synthesized polymer polymers. Quinoid and benzenoid bands at 1553-1596 cm-1 and 1437 – 1496 cm-1 respectively were observed in all the samples. The band gaps for all the samples were obtained using the absorption spectra and Stern relation. PANI-H2SO4 had the least band gap whereas PANI-CH3COOH had the highest band gap. The voltammograph showed the various oxidation states during the electrochemical deposition at a potential of 0.8 V. Four probe Method was used to study the conductivity of both thin films deposited on ITO slides. The conductivity values for PANI-H2SO4 were found to be 3.424 S.cm−1 for thin film. By comparing the four samples’ π - π* transition, from the strongest to the weakest were in the order: , PANI-H2SO4 followed by PANI-HCl then PANI-HNO3 and the weakest been PANI-CH3COOH. This suggests that polaron charge carriers are responsible for the high conductivity in H2SO4 doped PANI.

Conductive polymer nanoantennas for dynamic organic plasmonics.

Being able to dynamically shape light at the nanoscale is one of the ultimate goals in nano-optics1. Resonant light-matter interaction can be achieved using conventional plasmonics based on metal nanostructures, but their tunability is highly limited due to a fixed permittivity2. Materials with switchable states and methods for dynamic control of light-matter interaction at the nanoscale are therefore desired. Here we show that nanodisks of a conductive polymer can support localized surface plasmon resonances in the near-infrared and function as dynamic nano-optical antennas, with their resonance behaviour tunable by chemical redox reactions. These plasmons originate from the mobile polaronic charge carriers of a poly(3,4-ethylenedioxythiophene:sulfate) (PEDOT:Sulf) polymer network. We demonstrate complete and reversible switching of the optical response of the nanoantennas by chemical tuning of their redox state, which modulates the material permittivity between plasmonic and dielectric regimes via non-volatile changes in the mobile charge carrier density. Further research may study different conductive polymers and nanostructures and explore their use in various applications, such as dynamic meta-optics and reflective displays.

Redox-State-Dependent Interplay between Pendant Group and Conducting Polymer Backbone in Quinone-Based Conducting Redox Polymers for Lithium Ion Batteries

Conducting redox polymers (CRPs) have attracted increased interest in recent years because of the possibility of combining the favorable electron-transport properties of conducting polymers with the additional functionality provided by the redox active pendant groups (PGs). Herein we present a series of quinone-substituted PEDOT-CRPs where the quinone PGs have been substituted by electron-withdrawing substituents. Introducing electron-withdrawing substituents leads to an increase of the quinone formal potential, making, for example, CF3-substituted CRPs, a promising high-voltage cathode material for lithium ion batteries with a well-defined charge/discharge plateau around 3 V vs Li+/Li0. Interestingly, we find a shift in conductance onset potential concomitant with the quinone formal potential shift, indicating that the polymer backbone conductance is intimately associated with the PG redox chemistry. Through in situ UV–vis, electron paramagnetic resonance (EPR), and electrochemical quartz crystal microbalance experiments as well as by experiments in lithium- and tert-butyl-ammonium-based electrolytes, we show that the conductance delay is caused by the reduced lithiated quinone state, most likely by localizing the polaron charge carrier as indicated by EPR and UV–vis experiments.

Controlling the Optical and Electronic Properties of Polyaniline (PAni) Using Vapor Phase Infiltration of Titanium Tetrachloride

This talk with discuss our use of vapor phase infiltration (VPI) to alter the optoelectronic properties of poly(aniline) (PAni). VPI is similar to atomic layer deposition (ALD), but instead of cycling viscous flows of co-reactants, VPI doses a single reactant vapor in a static atmosphere followed by a pump-purge cycle. ALD typically provides conformal surface coatings while VPI enables extensive sub-surface diffusion and reactions within organic substrates. In particular to our study, PAni was VPI treated with a single cycle of TiCl4­­-H2O, and the resulting hybrid films showed unique optoelectronic properties that have not been previously reported. As VPI hold time increased, PAni films turned from blue to nearly transparent to green. This color transition is because of the formation of polaronic charge carriers concomitant with the formation of TiOx; the polaronic carriers absorb the red-NIR-MIR spectra (700- 5000 nm) while TiO­x absorb the blue-UV region (450 - 350 nm). This combination of optical absorbances in PAni has not been observed before. Furthermore, these polaronic carriers enabled electrical conductivities as high at 0.2 S/cm- ca. 3 orders of magnitude higher than prior PAni VPI doping studies. By varying the TiCl4 exposure time and measuring ex-situ­ optical signatures, an effective diffusion coefficient of ca. 1·10-15 cm2/s was measured; this value is self-consistent with the measured film thicknesses and optical peak intensity evolution. Additionally, we demonstrated that the relative ratio of UV and IR absorbing species can be controlled by varying reaction temperature. Lastly, XPS measurements elucidated a reaction mechanism for the formation of TiO­x and PAni polarons. By better understanding this reaction mechanism and kinetics, ALD and VPI can be optimized for future organic semiconductor doping processes.

Effect of Heteroatom and Doping on the Thermoelectric Properties of Poly(3‐alkylchalcogenophenes)

AbstractThis study reports on the thermoelectric properties of poly(3‐alkylchalcogenophene) thin films (500 nm) as a function of heteroatom (sulfur, selenium, tellurium), and how these properties change with dopant (ferric chloride) concentration. UV–vis–NIR spectroscopy shows that polaronic charge carriers are formed upon doping. Poly(3‐alkyltellurophene) (P3RTe) is most easily doped followed by poly(3‐alkylselenophene) (P3RSe) and poly(3‐alkylthiophene) (P3RT), where R = 3,7‐dimethyloctyl chain is the pendant alkyl group. Thermoelectric properties vary as functions of the heteroatom and doping level. At low dopant concentrations (≈1 × 10−3 m), P3RTe shows the highest power factor of 10 µW m−1 K−2, while, at higher dopant concentrations (≈5 × 10−3 m), P3RSe shows the highest power factor of 13 µW m−1 K−2. Most notably, it is found that the measured properties are consistent with Mott's polaron hopping model and not consistent with other transport models. Additionally, temperature‐dependent conductivity measurements show that for a given dopant concentration, the activation energies for electronic transport decrease as the heteroatom is changed from sulfur to selenium to tellurium. Overall, this work presents a systematic study of poly(chalcogenophenes) and indicates the potential of polymers beyond P3HT by tuning the heteroatom and doping level for optimized thermoelectric performance.