Polaron Band Research Articles

Direct observation of the electronic structure and many-body interactions of low-mobility carriers in perylene diimide derivative

Despite the rapid progresses in the field of organic semiconductors, aided by the development of high-mobility organic materials, their high carrier mobilities are often unipolar, being sufficiently high only for either electrons or holes. Yet, the basic mechanisms underlying such significant mobility asymmetry largely remains elusive. We perform angle-resolved photoelectron spectroscopy to reveal the occupied band structures and the many-body interactions for low-mobility hole carriers in a typical n-type semiconductor perylene diimide derivative. The band dispersion exhibits strong renormalization to the calculated non-interacting electronic structure. The analysis including many-body interactions elucidate that the significant mass enhancement can be understood in terms of strong charge–phonon coupling, leading to an important mechanism of polaron band transport of low intrinsic carrier mobility in organic semiconductors.

Effects of Planarization of the Triphenylamine Unit on the Electronic and Transport Properties of Triarylamine-Fluorene Copolymers in Both Doped and Undoped Forms.

Triarylamine-alt-fluorene (TAF) copolymers are widely used for hole injection and transport in organic electronics. Despite suggestions to planarize the triphenylamine moiety, little research has been conducted. Here, we report a comprehensive investigation of the effects of planarization on the electronic and transport properties of a model TAF polymer semiconductor core. We compared the conventional twisted-propeller N-4-methoxyphenyl-N,N-diphenylamine-4',4″-diyl (TA) unit and its planarized bridged analogue (bTA) where adjacent o,o'-positions are linked by 1,1-dimethylmethylene. We studied both polyelectrolyte and non-polyelectrolyte forms of this core in both doped and undoped states. We found that planarization leads to an unprecedented trap-free transport of holes, and a pronounced enhancement of their mobility in the undoped state though less so in the doped state. Planarization also induces a slight reduction in the ionization energy of the undoped polymer, consequently lowering the work function of the doped polymer. This is accompanied by small spectral shifts: a red shift in the first absorption band of the undoped polymer and a blue shift in the first absorption band of the polaron. Furthermore, this study unveils new fundamental features of TAF polymers: (i) Doping induces the formation of three polaron bands within the subgap. (ii) Absorption of both neutral and polaron segments exhibit a linear intensity relationship with doping level. (iii) Electrical conductivity reaches a maximum at the half-doped state, varying as σ ∼ (x (1 - x))3 for 0.1 ≲ x ≲ 0.9, where x is the doping level. Finally, we demonstrate the successful integration of these self-compensated hole-doped TAF polymers as efficient hole injection layers in organic semiconductor diodes.

Spin-Dependent ππ^{*} Gap in Graphene on a Magnetic Substrate.

We present a detailed analysis of the electronic properties of graphene/Eu/Ni(111). By using angle- and spin-resolved photoemission spectroscopy and abinitio calculations, we show that the intercalation of Eu in the graphene/Ni(111) interface gives rise to a gapped freestanding dispersion of the ππ^{*} Dirac cones at the K[over ¯] point with an additional lifting of the spin degeneracy due to the mixing of graphene and Eu states. The interaction with the magnetic substrate results in a large spin-dependent gap in the Dirac cones with a topological nature characterized by a large Berry curvature and a spin-polarized Van Hove singularity, whose closeness to the Fermi level gives rise to a polaronic band.

Polyaniline-(dinonylnapthyl disulphonic acid) hydrogel showing enhanced supercapacitor and photo-current performance by in situ growth of AgNPs

Emulsion polymerization of aniline in presence of dinonylnapthyl disulphonic acid (DNNDSA) produces 3D-fibrillar network and invariant storage and loss modulus with frequency suggest (PANI-DNNDSA, PD) gel formation. The x-ray diffraction and x-ray photoelectron spectroscopy confirm in-situ Ag NP formation. The PD gel exhibits excellent specific capacitance of 1100 F/g, however, in-situ growth of AgNPs (PDAg1.2) exhibits specific capacitance 1350 F/g at 1 A/g current density in three-electrode technique. Two electrode measurement with flexible carbon cloth using poly(vinyl alcohol)-H2SO4 gel electrolyte shows a specific capacitance value of 646 F/g and Ragon plot exhibits maximum power density 7000 W/kg at energy density of 34.7 Wh/kg. The PDAg1.2 gel exhibit highly improved photocurrent with simultaneous presence of photocurrent memory and photocurrent rectification which are absent in pure PD gel. Due to synergistic interaction between silver plasmon bands and polaronic bands of PANI very good photocurrent and stable photocurrent switching is noticed.

Ab initio study of the topological itinerant transport properties observed between excited edge states in a 2D compound with the Mn15B16Ni composition.

We theoretically demonstrate how the competition between band inversion and spin-orbit coupling (SOC) results in the nontrivial topology of band evolution, using two-dimensional (2D) Mn16B16 as a matrix. This study utilizes the ab initio method with the generalized gradient approximation (GGA+U scheme) and Wannier functions to investigate the topological and transport properties of the Ni-doped structure. The Ni atom induces dynamical antilocalization, which appears due to the phase accumulation between time-reversed fermion loops. A key observation is that when band inversion dominates over SOC, "twin" Weyl cones appear in the band structure, in which the Weyl cones caused by the large Berry curvature coupling with the net magnetization lead to the significantly enhanced anomalous Hall conductivity (AHC). Interestingly, the nested small polaron and energy band inversion coexist with SOC. An analysis of the projected energy band shows that the doped Ni atom induces a strong spin wave for both spin up and spin down.

Chemical Doping of Organic and Coordination Polymers for Thermoelectric and Spintronic Applications: A Theoretical Understanding.

ConspectusThe controlled doping of organic semiconductors (OSCs) is crucial not only for improving the performance of electronic and optoelectronic devices but also for enabling efficient thermoelectric conversion and spintronic applications. The mechanism of doping for OSCs is fundamentally different from that of their inorganic counterparts. In particular, the interplay between dopants and host materials is complicated considering the low dielectric constant, strong lattice-charge interaction, and flexible nature of materials. Recent experimental breakthroughs in the molecular design of dopants and the precise doping with high spatial resolution call for more profound understandings as to how the dopant interacts with the charge introduced to OSCs and how the admixture of dopants alters the electronic properties of host materials before one can exploit controllable doping to realize desired functionalities.By employing state-of-the-art computational tools, we revealed the effects of doping in representative and emerging organic and coordination polymers aiming toward thermoelectric and spintronic applications. We showed that dopants and hosts should be taken as an integrated system, and the type of charge-transfer interaction between them is the key for spin polarization. First, we found doping-induced modifications to the electronic band in a potassium-doped coordination polymer, an n-type thermoelectric material. The charge localization due to the Coulomb interaction between the completely ionized dopant and the injected charge on the polymer backbone and also the polaron band formation at low doping levels are responsible for the nonmonotonic temperature dependence of the conductivity and Seebeck coefficient observed in recent experiments. The mechanistic insights gained from these results have provided important guidelines on how to control the doping level and working temperature to achieve a high thermoelectric conversion efficiency. Next, we demonstrated that the ionized dopants scatter charge carriers via screened Coulomb interactions, and it may become a dominant scattering mechanism in doped polymers. After incorporating the ionized dopant scattering mechanism in PEDOT:Tos, a p-type thermoelectric polymer, we were able to reproduce the measured Seebeck coefficient-electrical conductivity relationship spanning a wide range of doping levels, highlighting the importance of ionized dopant scattering in charge transport.In the two cases described above, charge injection is enabled by integral charge transfer between the dopant and host polymers. In a third example, we showed that a novel type of stacked two-dimensional polymer, conjugated covalent organic frameworks (COFs) with closed-shell electronic structures, can be spin polarized by iodine doping via fractional charge transfer even at high doping levels. We then manifested that magnetization can be attained in nonmagnetic materials lacking metal d electrons and further designed two new COFs with tunable spintronic structure and magnetic interactions after the iodine doping. These findings have suggested a practical route to enable spin polarization in nonradical materials by chemical doping via orbital hybridization, which holds great promise for flexible spintronic applications.

Probing Polaron Environment in a Doped Polymer via the Photoinduced Stark Effect

Conjugated polymers (CPs) constitute a unique class of materials with applications ranging from energy storage to bioelectronics, thanks in part to their ability to be easily doped to produce mobile charge carriers. While methods for doping CPs are well-developed, a deeper understanding of the interplay between the local environment and polaronic properties is currently lacking. Using transient absorption (TA) spectroscopy, we demonstrate that photoexciting polarons in a lightly doped 3,4-propylenedioxythiophene-co-3,4-ethylenedioxythiophene (ProDOT-co-EDOT) polymer film produces a Stark effect that is sensitive to their local environments. The resulting electroabsorption (EA) signal, which originates from a shift in the band gap of nearby polymer chains due to the photoinduced electric field, indicates that polaron excitation spatially separates the hole from its counterion as the hole thermalizes within 200 fs. Changes in the spectral position of the EA signal when tuning the excitation wavelength across the P1 and P2 polaron bands, combined with a dynamic blue shift, suggest a variation in field strength that senses different extents of the local environment. We propose the photoinduced Stark effect in TA experiments as a sensitive probe of the local environment of polarons in doped CPs that can also be used to refine electronic structure models describing their polaronic states and transitions.

Is it Photodegradation or other Electronic Mechanisms Responsible for Photoluminescence Quenching under Nonenvironmental Conditions in Poly(3-octyltiophene) Electrochemically Synthesised?

Aims: This work investigates the photoluminescence properties of poly(3- octylthiophene), namely P3OT films, under excitation power and time light expositions. When exposed to monochromatic illumination, the degradation mechanism of P3OT films is precise in environmental conditions. However, in some environmental controlled or non-environmental conditions, the quenching luminescence of P3OT subject to monochromatic light excitation is not fully understood. In this context, it is necessary to understand what causes quenching luminescence of P3OT films under environmental conditions. Methods: The P3OT was synthesized by chronocoulometry, using tetraethylammonium tetrafluoroborate and lithium perchlorate as supporting electrolytes, and after it is deposited on a conductive substrate of fluoride-doped tin oxide (FTO). The films were synthesized and maintained under an argon atmosphere, and their thickness is dependent on the charge. The characterisation was achieved by applying techniques like UV-Vis absorption spectroscopy, photoluminescence, and emission ellipsometry. Results: The UV-Vis absorption measurements demonstrated a different interaction of polymer/ dopant due to the polaronic band. The results showed that photoluminescence quenching with time is a behaviour caused by energy transfer between quinone chains and pristine chains of the P3OT. The polarised emission measurements corroborate the hypothesis of energy transfer between chains. The emission ellipsometry technique was used to understand energy transfer processes and other properties. The self-absorption phenomenon and integrated PL investigation of the P3OT emission elucidated the mechanism involved in the observed properties. Conclusion: Besides that, the results show that photoluminescence quenching happens in nonenvironmental and environmental conditions, corroborating the hypothesis that PL quenching occurs because self-absorption phenomena influence the energy transfer mechanism.

Beta-borophene under the circularly polarized radiation: polaritonic and polaronic dynamic band structure

We study the effect of circularly polarized electromagnetic radiation on the optical polarons in a monolayer \({\beta}\)-borophene. We focus our attention on the off-resonant regime of large driving frequencies, which allows us to set up an effectively time-independent Hamiltonian describing the radiation field up to the second perturbational order. The contributions of optical surface phonons are incorporated according to the Lee–Low–Pines theory, via a sequence of appropriate unitary transformations. A central object in our investigation is the electronic band gap, which is shown to be widely controllable via the variation of light irradiation and substrate properties. We also show that the polaron self-energy contribution gives more contribution than the combination of the polaronic and polaritonic effects.

Electron removal spectral function of a polaron coupled to dispersive optical phonons

We explore the ground state and thermodynamic properties of the polaron coupled to quantum dispersive optical phonons in one spatial dimension. Calculations are performed using the finite--temperature Lanczos method augmented by a highly efficient construction of the variational Hilbert space. We focus on the electron removal spectral function as relevant for the angle-resolved photoemission experiments. We show that photoemission spectroscopy can be used to measure the phonon dispersion relation in a dilute system of polarons. The spectral weight of observed phonon bands is proportional to the phonon contribution to the wavefunction at finite phonon momentum. In addition, we demonstrate that when removing an electron from a polaron ground state, the polaron band does not appear in the spectral function. The latter becomes observable only at elevated temperatures.

Charge carrier mobilities of organic semiconductors: ab initio simulations with mode-specific treatment of molecular vibrations

The modeling of charge transport in organic semiconductors usually relies on the treatment of molecular vibrations by assuming a certain limiting case for all vibration modes, such as the dynamic limit in polaron theory or the quasi-static limit in transient localization theory. These opposite limits are each suitable for only a subset of modes. Here, we present a model that combines these different approaches. It is based on a separation of the vibrational spectrum and a quantum-mechanical treatment in which the slow modes generate a disorder landscape, while the fast modes generate polaron band narrowing. We apply the combined method to 20 organic crystals, including prototypical acenes, thiophenes, benzothiophenes, and their derivatives. Their mobilities span several orders of magnitude and we find a close agreement to the experimental mobilities. Further analysis reveals clear correlations to simple mobility predictors and a combination of them can be used to identify high-mobility materials.

Investigation of Electrochemical Performance of PmAP/WO3 Composite

PmAP and PmAP/WO3 composite are synthesized by chemical oxidation polymerization process. Synthesized materials are investigated by UV Visible, SEM and cyclic-voltagram (CV) techniques. SEM study of as prepared materials is indicated formation of different sized crystals (small and aggregated). In UV Visible spectra analyses, both transitioned (i.e., π-π* transition and charge transfer of polaron band) are observed. Tauc expression is used to estimate optical band gap. Estimated optical band gap is to be 3.09 eV. Oxidation-reduction potential profile is detected in cyclic voltammetry (CV) analyses of PmAP and PmAP/WO3 composite.

Comparison of the canonical transformation and energy functional formalisms for ab initio calculations of self-localized polarons

In materials with strong electron-phonon ($e$-ph) interactions, charge carriers can distort the surrounding lattice and become trapped, forming self-localized (small) polarons. We recently developed an ab initio approach based on canonical transformations to efficiently compute the formation and energetics of small polarons [N.-E. Lee et al., Phys. Rev. Mater. 5, 063805 (2021)]. A different approach based on a Landau-Pekar energy functional has been proposed in the recent literature [W. H. Sio et al., Phys. Rev. Lett. 122, 246403 (2019); Phys. Rev. B 99, 235139 (2019)]. In this work, we analyze and compare these two methods in detail. We show that the small polaron energy is identical in the two formalisms when using the same polaron wave function. We also show that our canonical transformation formalism can predict polaron band structures and can properly treat zero- and finite-temperature lattice vibration effects, although at present using a fixed polaron wave function. Conversely, the energy functional approach can compute the polaron wave function, but as we show here, it neglects lattice vibrations and cannot address polaron self-localization and thermal band narrowing. Taken together, this work relates two different methods developed recently to study polarons from first-principles, highlighting their merits and shortcomings and discussing them both in a unified formalism.

METAL-INSULATOR TRANSITIONS IN DOPED La-BASED SUPER CONDUCTORS WITH SMALL-RADIUS DOPANTS

n this work, we study the possibility of realizing two distinct mechanisms of metal-insulator transitions in hole-doped cuprates induced by the localization of charge carriers near the small-radius impurities and in a deformable lattice (i.e. in the absence of impurities). The purpose of this research is to determine the criteria (i.e. conditions) for the existence of the localized states of hole carriers and solve the problem of metal-insulator transitions in La-based cuprates. The advantage of La-based cuprate versus other types of cuprates is that two distinct metal-insulator transitions in La-based cuprates driven by the strong carrier-impurity-phonon and carrier-phonon interactions occur simultaneously in a wider doping range from the lightly doped to heavily doping regime. We show that at very low doping, the separate levels of hole carriers localized near impurities and in a deformable lattice are formed in the charge-transfer gap of the cuprates. As the doping level increases towards underdoped region, the energy levels of such charge carriers start to form energy bands which gradually broaden with increasing doping. We propose a new two-carrier cuprate superconductor model for studying two distinct metal-insulator transitions occurring simultaneouslyin hole-doped La-based cuprate compounds. We demonstrate that when hole carriers reside in impurity and polaron bands, these metal-insulator transitions in La-based superconductors with small-radius dopants occur accordingly in a wide doping range and relatively lower doping levels.

Chemically dedoped PEDOT:PSS films with an amidine/polymer overcoating and their applications as a semiconducting channel in organic field-effect transistors and inverters

We investigate the properties of a conducting polymer composite of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) after chemical dedoping with an amidine base and demonstrate the applications of the dedoped films as a semiconducting channel in a transistor and an inverter. We observe that PEDOT:PSS can be chemically transformed from a conductor to a semiconductor through a post-treatment with 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), which can donate electrons to the oxidized PEDOT chains and screen negative charges of PSS by salt formation. Both appearance of visible absorption bands and disappearance of (bi)polaronic absorption bands are observed in the DBN-treated PEDOT:PSS film, suggesting the effective dedoping. Compositional depth profiling and morphological analyses show that two distinct layers evolve after chemical dedoping and that the majority of internal PEDOT chains are dedoped over the whole film. We also introduce an overcoating layer using polyvinylpyrrolidone (PVP) with DBN to get more stable PEDOT:PSS films in a dedoped state. Organic field-effect transistors (OFETs) with the dedoped PEDOT:PSS film as a semiconducting channel show the typical p-type enhancement-mode characteristics without any doping-induced shift of threshold voltage. The pinch-off, hard saturation, and distinguishable switching characteristics with on-to-off current ratio of 104 are observed without redoping. We also demonstrate a logic circuit from the OFET as a proof-of-concept of semiconducting nature of PEDOT:PSS.

Nonadiabatic Dynamics of Polaron Hopping and Coupling with Water on Reduced TiO2.

By interplay between first-principles molecular dynamics and nonadiabatic molecular dynamics simulations based on the decoherence-induced surface-hopping approach, we investigate and quantify the mechanisms through which different electron polaron hopping regimes in the reduced anatase TiO2(101) surface influence recombination of photogenerated charge carriers, also in the presence of adsorbed water (H2O) molecules. The simulations reveal that fast hopping regimes promote ultrafast recombination of photogenerated charge-carriers. Conversely, charge recombination is delayed in the presence of slower polaron hopping and even more so if the polaron is pinned at one Ti-site, as typical following adsorption of H2O on the anatase(101) surface. These trends are related to the observed enhancement of the space and energy overlap between conduction band minimum and polaron band gap states, and the ensuing nonadiabatic couplings (NAC) strengths, during a polaronic hop. We expect these insights on the beneficial role of polaron diffusion pinning for the extended lifetime of photoexcitations in TiO2 to sustain ongoing developments of photocatalytic strategies based on this substrate.

On the photoconductivity behavior of emeraldine-salt polyaniline films

Deep and shallow traps play a fundamental role in the photoresponse profile of organic devices. Here we present an investigation on emeraldine-salt polyaniline films concerning structural, optical, and electrical properties by X-rays diffraction, optical and electron microscopies, and optical spectroscopies as Fourier Transform Infrared Spectroscopy (FTIR), absorption, and photoluminescence (PL). Photoconductivity measurements show signatures related to localized states near-edge absorption and the efficient transition between polaron bands. The photoconductive profile is dependent on the excitation intensity and presents negative photoconductivity (NPC) for higher intensities. A rate equations model attributes the NPC to shallow traps while the increase of response and recovery times are associated with deep traps.

Oxidative Polymerization of Polyaniline (PANI) Colloids with Different Oxidizing Agents

We synthesized PANI thin films by chemical oxidative polymerization technique using aniline monomer, oxidizing agent, and dopant acid. The effect of different oxidizing agents such as ammonium dichromate (ADC), chloroauric acid (CAA), and potassium permanganate (PPM) on the properties of PANI was investigated. The as-prepared PANI colloids were thoroughly characterized using a range of characterization techniques. Raman spectra demonstrate that all the intense bands associated to the emeraldine salt form of PANI. Optical absorption spectra revealed that the absorption peaks corresponds to π–π*, polaron and bipolaron transitions of emeraldine salt form of PANI. PL spectra demonstrate that the relative intensity of the peak arises due to the polaron band de-excitation increases for the PANI film synthesized using PPM due to the highest redox oxidation potential. A clear change in the surface morphology was observed for the samples grown with different oxidizing agents. A long fiber-like structure detected with CAA is proposed to be responsible for the development of the highly conducing emeraldine salt of PANI. The PANI sample prepared with CAA not only measured good electrical conductivity but the optical and morphological properties also showed promising characteristics for various device applications.

Understanding the Temperature Dependence of the Seebeck Coefficient from First-Principles Band Structure Calculations for Organic Thermoelectric Materials

Understanding the Temperature Dependence of the Seebeck Coefficient from First-Principles Band Structure Calculations for Organic Thermoelectric Materials

Abnormal Seebeck effect in doped conducting polymers

The Seebeck effect or thermopower relates the temperature gradient to the electric voltage drop. Seebeck coefficient α measures the transport entropy, which could either linearly increase with temperature T like metallic conducting or decrease as 1/T like semiconducting behavior. It could become more complicated in the temperature dependence for a number of disordered systems but still in a monotonic way. However, several recent experiments reported the “abnormal” non-monotonic temperature dependence of the Seebeck coefficient in doped conducting polymers, for instance, first increasing and then decreasing. Through a one-dimensional tight-binding model coupled with the Boltzmann transport equation, we investigate theoretically the doping effect for the Seebeck coefficient. We find that the abnormal behavior comes from multi bands' contribution and a two-band model (conduction or valence band plus a narrow polaronic band) can address such an abnormal Seebeck effect, namely, if there exists (i) a small bandgap accessible for thermal activation between the two bands; and (ii) a large difference in the bandwidth between the polaronic band and the conduction band (or valence band), then the Seebeck coefficient increases with temperature first, then levels off, and finally drops down.