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In this work, we use density functional theory to investigate the electronic structure of poly(3,4-ethylenedioxythiophene) (PEDOT) oligomers with co-located AlCl4- anions, a promising combination for energy storage. The 1980s bipolaron model remains the dominant interpretation of the electronic structure of PEDOT despite recent theoretical progress that has provided new definitions of bipolarons and polarons. By considering the influence of oligomer length, oxidation or anion concentration and spin state, we find no evidence for many of the assertions of the 1980s bipolaron model and so further contribute to a new understanding. No self-localisation of positive charges in PEDOT is found, as predicted by the bipolaron model at the hybrid functional level. Instead, our results show distortions that exhibit a single or a double peak in bond length alternations and charge density. Either can occur at different oxidation or anion concentrations. Rather than representing bipolarons or polaron pairs in the original model, these are electron distributions driven by a range of factors. Distortions can span an arbitrary number of nearby anions. We also contribute a novel conductivity hypothesis. Conductivity in conducting polymers has been observed to reduce at anion concentrations above 0.5. We show that at high anion concentrations, the energy of the localised, non-bonding anionic orbitals approaches that of the system HOMO due to Coulombic repulsion between anions. We hypothesize that with nucleic motion in the macropolymer, these orbitals will interfere with the hopping of charge carriers between sites of similar energy, lowering conductivity.
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Hole-doped cuprates exhibit partially coexisting pseudogap (PG), charge ordering (CO) and superconductivity; we show that there exists a class of systems in which they have a single nature as it has recently been supposed. Since the charge-ordered phase exhibits large frozen deformation of the lattice, we develop a method for calculating the phase diagram of a system with strong long-range (Fröhlich) electron-phonon interaction. Using a variational approach, we calculate the free energy of a two-liquid system of carriers with cuprate-like dispersion comprising a liquid of autolocalized carriers (large polarons and bipolarons) and Fermi liquid of delocalized carriers. Comparing it with the free energy of pure Fermi liquid and calculating (with standard methods of Bose liquid theory) a temperature of the superfluid transition in the large-bipolaron liquid we identify regions in the phase diagram with the presence of PG (caused by the impact of the (bi)polarons potential on delocalized quasiparticles), CO and superconductivity. They are located in the same places in the diagram as in hole-doped cuprates, and, as in the latter, the shape of the calculated phase diagram is resistant to wide-range changes in the characteristics of the system. As in cuprates, the calculated temperature of the superconducting transition increases with the number of conducting planes in the unit cell, the superfluid density decreases with doping at overdoping, the bipolaron density (and bipolaronic plasmon energy) saturates at optimal doping. Thus, the similarity of the considered system with hole-doped cuprates is not limited to the phase diagram. The results obtained allow us to discuss ways of increasing the temperature of the superfluid transition in the large-bipolaron liquid and open up the possibility of studying the current-carrying state and properties of the bipolaron condensate.
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This research aims to deepen the understanding of the relationship between conductivity and morphology in polypyrrole (PPy) via a comparison of the bipolaron to polaron ratios with a focus on the C-H deformation area. PPy samples were synthesized with different surfactants: sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), and tween 80 (TW). This study revealed that SDS significantly altered the bipolaron and polaron in the C-H deformation region and showed higher conductivity than other surfactants. Notably, the morphological shifts to a sheet-like structure when using ammonium sulfate (APS) contrasted with the particle-like form observed with ferric chloride (FeCl3). These results showed that if the oxidant changed, the bipolaron and polaron ratios in C-H deformation were unrelated to PPy morphology. However, this work showed a consistent relationship between SDS use, the bipolaron and polaron ratios in the C-H deformation, and the conductivity properties. Moreover, the natural positive charge of PPy and negatively charged SDS molecules may lead to an electrostatic interaction between PPy and SDS. This work assumes that this interaction might cause the transformation of polaron to bipolaron in the C-H deformation region, resulting in improved conductivity of PPy. This work offers more support for the future investigation of PPy characteristics.
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The origin of the pseudogap in many strongly correlated materials has been a longstanding puzzle. Here, we present experimental evidence that many-body interactions among small Holstein polarons, i.e., the formation of bipolarons, are primarily responsible for the pseudogap in (TaSe4)2I. After weak photoexcitation of the material, we observe the appearance of both dispersive (single-particle bare band) and flat bands (single-polaron sub-bands) in the gap by using time- and angle-resolved photoemission spectroscopy. Based on Monte Carlo simulations of the Holstein model, we propose that the melting of pseudogap and emergence of new bands originate from a bipolaron to single-polaron crossover. We also observe dramatically different relaxation times for the excited in-gap states in (TaSe4)2I (â¼600 fs) compared with another 1D material Rb0.3MoO3 (â¼60 fs), which provides a new method for distinguishing between pseudogaps induced by polaronic or Luttinger-liquid many-body interactions.
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Conjugated polymers are increasingly used as organic mixed ionic-electronic conductors in electrochemical applications for neuromorphic computing, bioelectronics, and energy harvesting. The design of efficient electrochemical devices relies on large modulations of the polymer conductivity, fast doping/dedoping kinetics, and high ionic uptake. In this work, structure-property relations are established and control of these parameters by the co-existence of order and disorder in the phase morphology is demonstrated. Using in situ time-resolved spectroelectrochemistry, resonant Raman, and terahertz (THz) conductivity measurements, the electrochemical doping in the different morphological domains of poly(3-hexylthiophene) (P3HT) is investigated. The main finding is that bipolarons are found preferentially in disordered polymer regions, where they are formed faster and are thermodynamically more favored. On the other hand, polarons show a preference for ordered domains, leading to drastically different bipolaron/polaron ratios and doping/dedoping dynamics in the distinct regions. A significant enhancement of the electronic conductivity is evident when bipolarons start forming in the disordered regions, while the presence of bipolarons in the ordered regions is detrimental for transport. This study provides significant advances in the understanding of the impact of morphology on the electrochemical doping of conjugated polymers and the induced increase in conductivity.
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The structure and packing of organic mixed ionic-electronic conductors have an especially significant effect on transport properties. In operating devices, this structure is not fixed but is responsive to changes in electrochemical potential, ion intercalation, and solvent swelling. Toward this end, the steady-state and transient structure of the model organic mixed conductor, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), is characterized using multimodal time-resolved operando techniques. Steady-state operando X-ray scattering reveals a doping-induced lamellar expansion of 1.6 Å followed by 0.4 Å relaxation at high doping levels. Time-resolved operando X-ray scattering reveals asymmetric rates of lamellar structural change during doping and dedoping that do not directly depend on potential or charging transients. Time-resolved spectroscopy establishes a link between structural transients and the complex kinetics of electronic charge carrier subpopulations, in particular the polaron-bipolaron equilibrium. These findings provide insight into the factors limiting the response time of organic mixed-conductor-based devices, and present the first real-time observation of the structural changes during doping and dedoping of a conjugated polymer system via X-ray scattering.
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Organic spintronics is a new emerging field that deals with the spin-related phenomena of organic materials under the influence of a magnetic field. However, there remain some challenges in organic spintronics including (i) low conductivity and massive disorders of organic thin films blocking the way to controllable spin transport, (ii) relatively low magnetic-field sensitivity of organic magnetoresistance (OMAR) devices with tangled working mechanisms and short of methods for sensitivity improvement. Here, we report the realization of OMAR devices based on organic single crystals. The lesser amount of impurities and defects in crystals guarantees a reduction in spin and charge scatterings, so that the OMAR devices exhibit both a small Lorentz function fitting parameter B0 of 2.3 mT and a non-Lorentz function fitting parameter B1 of 0.86 mT in the strictly limited bipolaron model. Moreover, we demonstrate the effect of aggregation and intrinsic trap states, pointing out a way for the improvement of the sensitivity.
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The conductive properties of polypyrrole chains doped with ClO4- or MoO42- anions and the existence of polarons and bipolarons in these doped polypyrrole chains were investigated by performing computational calculations based on density functional theory (DFT). Doping with these anions was found to decrease the band gap of the polypyrrole. Theoretical calculations revealed that changing the type of oxidative agent applied does not affect the conversion of polypyrrole into a conducting polymer, but the conductivity of the doped polypyrrole does depend on the ratio of oxidant to polypyrrole.
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To simultaneously increase the electrical conductivity and Seebeck coefficient of poly(3,4-ethylenedioxythiophene):polystyrenesulfonate ( PEDOT: PSS) was a challenge for realizing efficient organic thermoelectrics. In this study, for the first time, we report both increased electrical conductivities and Seebeck coefficients, hence, enhanced thermoelectric properties of PEDOT: PSS thin films by doped with binary secondary dopants, dimethyl sulfoxide (DMSO) and poly(ethylene oxide) (PEO). Without modifying film morphology, the molar ratios of PEDOT to PSS are tuned by PEO, resulting in increased proportions of PEDOT in the bipolaron states. Our study provides a facile route to optimizing thermoelectric properties of PEDOT: PSS thin films.
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We show that each surface atom of heavily boron-doped, (111)-oriented silicon with a â3 × â3 reconstruction has electrically switchable two charge states due to the strong electron-lattice coupling at this surface. The structural and electronic properties of the two charge states as well as their energetics are uncovered by employing scanning tunneling microscopy measurements and density functional theory calculations, which reveals that one of the two is a two-electron bound state or surface bipolaron. We also execute the single-atom bit operations on individual surface atoms by controlling their charge states while demonstrating implementation of the atomic scale memory at a silicon surface with an unprecedented recording density.