Polaron Transport Research Articles

Polaronic transport and thermoelectricity in Fe1−xCoxSb2S4 ( x=0 , 0.1, and 0.2)

We report a study of Co-doped berthierite Fe$_{1-x}$Co$_x$Sb$_2$S$_4$ ($x$ = 0, 0.1, and 0.2). The alloy series of Fe$_{1-x}$Co$_x$Sb$_2$S$_4$ crystallize in an orthorhombic structure with the Pnma space group, similar to FeSb$_2$, and show semiconducting behavior. The large discrepancy between activation energy for conductivity, $E_\rho$ (146 $\sim$ 270 meV), and thermopower, $E_S$ (47 $\sim$ 108 meV), indicates the polaronic transport mechanism. Bulk magnetization and heat capacity measurements of pure FeSb$_2$S$_4$ ($x$ = 0) exhibit a broad antiferromagnetic (AFM) transition ($T_N$ = 46 K) followed by an additional weak transition ($T^*$ = 50 K). Transition temperatures ($T_N$ and $T^*$) slightly decrease with increasing Co content $x$. This is also reflected in the thermal conductivity measurement, indicating strong spin-lattice coupling. Fe$_{1-x}$Co$_x$Sb$_2$S$_4$ shows relatively high value of thermopower (up to $\sim$ 624 $\mu$V K$^{-1}$ at 300 K) and thermal conductivity much lower when compared to FeSb$_{2}$, a feature desired for potential applications based on FeSb$_{2}$ materials.

Nature of mixed electrical transport in Ag2O–ZnO–P2O5 glasses containing WO3 and MoO3

This study reports on the nature of electrical transport and role of structural changes induced by different type and content of TMO in Ag–containing glasses of xTMO–(30–0.5x)Ag2O–(30–0.5x)ZnO–40P2O5 (TMO = MoO3/WO3, 0 ≤ x ≤ 60 mol%) composition. Raman spectra show clustering of WO6 units in glasses with high WO3 content while the addition of MoO3 induces a gradual change of MoO6 octahedra to MoO4 tetrahedra both being cross-linked with phosphate units without clustering. For WO3 glasses, minimum in DC conductivity is observed at 30–40 mol% of WO3 for temperatures from 303 to 513 K, followed by an increase in conductivity with further WO3 addition due to an increase in polaronic contribution. Observed turnover suggests a distinct transition from predominantly ionic to predominantly polaronic transport. On the contrary, for MoO3 glasses, conductivity decreases in the whole mixed compositional range indicating that the nature of transport is dominated by ionic component throughout the measured temperature range. A comparative study of Ag+, Li+, Na+ transport in MoO3/WO3 glasses reveals a strong correlation between pre-exponential factor and activation energy, which allows detection of the prevalence of conduction mechanism. Finally, the results demonstrate that Ag2O–WO3–ZnO–P2O5 glass system is a promising electrically tunable material with significant contributions of ionic or polaronic conductivity depending on composition.

Photoinduced Slowdown of Charge Carrier Transport and Phonon Emission in Conjugated Polymers

In a real bulk heterojunction polymer solar cell, once the photoactive polymer layer undergoes photoexcitation, the formed electron–hole pair (or exciton) diffuses to the fabricated heterojunction to be dissociated, and then the resultant charge carriers move toward the cathode and anode. During the negative polaron transport, the external light field excites the polaron and makes it transport slowly where the excitation selects three localized phonon modes with even parity and higher frequency, not only greatly deepening the local distortion along alternating bonds but also leading to blue shift in the infrared phonon spectrum. It is furthermore discovered that the slowdown of the charged polaron’s velocity is contributed by the occurrence of the phonon modes and the blue shift of the phonon spectrum. Controlling the phonon modes and the frequency blue shift of the phonon spectrum in a composite conjugated polymer solar cell presents an opportunity for modifying the properties of polymer solar cells.

From Linear to Angular Isomers: Achieving Tunable Charge Transport in Single-Crystal Indolocarbazoles Through Delicate Synergetic CH/NH⋅⋅⋅π Interactions.

Weak intermolecular interaction in organic semiconducting molecular crystals plays an important role in molecular packing and electronic properties. Here, four five-ring-fused isomers were rationally designed and synthesized to investigate the isomeric influence of linear and angular shapes in affecting their molecular packing and resultant electronic properties. Single-crystal field-effect transistors showed mobility order of 5,7-ICZ (3.61 cm2 V-1 s-1 ) >5,11-ICZ (0.55 cm2 V-1 s-1 ) >11,12-ICZ (ca. 10-5 cm2 V-1 s-1 ) and 5,12-ICZ (ca. 10-6 cm2 V-1 s-1 ). Theoretical calculations based on density functional theory (DFT) and polaron transport model revealed that 5,7-ICZ can reach higher mobilities than the others thanks to relatively higher hole transfer integral that links to stronger intermolecular interaction due to the presence of multiple NH⋅⋅⋅π and CH⋅⋅⋅π(py) interactions with energy close to common NH⋅⋅⋅N hydrogen bonds, as well as overall lower hole-vibrational coupling owing to the absence of coupling of holes to low frequency modes due to better π conjugation.

Electron–Phonon Coupling and Polaron Mobility in Hybrid Perovskites from First Principles

Density functional theory electronic structures, maximally localized Wannier funcitons and linear response theory are used to compute the electron and hole mobility, μ, of both inorganic, Cs-containing, and hybrid, CH3NH3-containing, lead bromide perovskites. When only phonon scattering is considered, we find hole mobilities at room temperature in the 40–180 cm2 V–1 s–1 range, in good agreement with experimental data for highly ordered crystals. The electron mobility is about an order of magnitude larger, because low-energy phonons are ineffective over the Pb 6p shell. Most importantly, our parameter-free approach, finds a T–3/2 power-law temperature dependence of μ, which is a strong indication of polaronic transport in these compounds. Our work then offers an independent theoretical validation of the many hypotheses about the polaronic nature of the charge carriers in lead halide perovskites.

Defect Chemistry and Transport within Dense BaCe0.7Zr0.1Y0.1Yb0.1O3 − δ (BCZYYb) Proton-Conducting Membranes

This paper uses previously published conductivity measurements as the basis for establishing thermodynamic and transport properties for BaCe0.7Zr0.1Y0.1Yb0.1O3 − δ (BCZYYb) proton-conducting membranes. BCZYYb is mixed ionic-electronic conducting (MIEC) doped perovskite that is dominantly a proton-conducting ceramic. In addition to protons, BCZYYb permits transport of oxygen vacancies and O-site polarons. The present analysis considers the effects of immobile polaron traps. The paper reports enthalpies ΔH° and entropies ΔS° for the defect reactions, and defect diffusion coefficients in Arrhenius form. The paper incorporates the thermodynamic and transport properties into a Nernst–Planck model to characterize BCZYYb performance as a protonic-ceramic membrane for electrolyzer and fuel-cell applications. Especially at temperatures below 600°C, BCZYYb can achieve high faradaic efficiencies (i.e., low electronic leakage) as an electrolyzer membrane.

Dielectric features, relaxation dynamics and a.c. conductivity studies on Ag+ mixed lead arsenate glass ceramics

The objective of this study is to investigate the influence of silver ions on a.c. conductivity and dielectric properties of lead arsenate glass ceramics. PbO–As2O3 glasses mixed with different concentrations of Ag2O were prepared by conventional melt quenching technique and later were ceramicized by heat treatment at crystallization temperature for about 72 h. The prepared samples were characterized by XRD, SEM and DSC techniques. The SEM studies have indicated the samples are embedded with different crystal phases of varying sizes from 0.2 to 1.0 µm and the fraction of crystal phases as well as the size of the crystallites increased with increase of Ag2O concentration. The X–ray diffraction studies have revealed that the samples composed of different crystal phases viz., Ag3AsO4, AgPb4(AsO4)3, Pb2As2O5 and PbAs2O4 entrenched in the residual amorphous phase. The spectroscopic studies viz., optical absorption and IR spectroscopic studies have revealed a gradual increase in the degree of depolymerization of the glass network with increase in the concentration of Ag2O. Photoluminescence studies have indicated that the investigated glass ceramics contain silver ion clusters; the concentration of such clusters is predicted to be higher in the samples containing higher content of Ag2O. Finally, we have carried out extensive studies on dielectric properties viz., dielectric constant, loss, electric moduli, impedance spectra and a.c. conductivity over broad ranges of continuous frequencies (3–100 kHz) and temperatures (298–600 K). The obtained results were quantitatively analyzed in terms of different polarization mechanisms. The relaxation effects exhibited by the electric moduli were analyzed in a detailed way by different methods. Spreading of relaxation times for dipoles was established and the possible dipoles responsible for such relaxation effects were suggested. The observed increase of a.c. conductivity with the concentration of Ag2O is attributed to the contribution of both polaronic and ionic transport. Finally, it is concluded that the samples crystallized with higher content of Ag2O exhibit predominantly ionic conductivity and hence such glass ceramics may be useful as solid electrolytes for solid state batteries.

Novel insights into electrical transport mechanism in ionic‐polaronic glasses

AbstractTransformation of electrical transport from ionic to polaronic in glasses, which are a potential class of new cathode materials, has been investigated in four series containing WO3/MoO3 and Li+/Na+ ions, namely: xWO3–(30−0.5x)Li2O–(30−0.5x)ZnO–40P2O5, xWO3–(30−0.5x)Na2O–(30.5x)ZnO–40P2O5, xMoO3–(30−0.5x)Li2O–(30−0.5x)ZnO–40P2O5, and xMoO3–(30−0.5x)Na2O–(30−0.5x)ZnO–40P2O5, 0 ≤ x ≤ 60, (mol%). This study reports a detailed analysis of the role of structural modifications and its implications on the origin of electrical transport in these mixed ionic‐polaron glasses. Raman spectra show the clustering of WO6 units by the formation of W–O–W bonds in glasses with high WO3 content while the coexistence of MoO4 and MoO6 units is evidenced in glasses containing MoO3 with no clustering of MoO6 octahedra. Consequently, DC conductivity of tungstate glasses with either Li+ or Na+ exhibits a transition from ionic to polaronic showing a minimum at about 20‐30 mol% of WO3 as a result of ion‐polaron interactions followed by a sharp increase for six orders of magnitude as WO3 content increases. The formation of WO6 clusters involved in W‐O‐W linkages for tungsten glasses plays a key role in significant increase in DC conductivity. On the other hand, DC conductivity is almost constant for glasses containing MoO3 suggesting an independent ionic and polaronic transport pathways for glasses containing 10‐50 mol% of MoO3.

Polaronic transport in Ag-based quaternary chalcogenides

Low temperature resistivity measurements on dense polycrystalline quaternary chalcogenides Ag2+xZn1-xSnSe4, with x = 0, 0.1, and 0.3, indicate polaronic type transport which we analyze employing a two-component Holstein model based on itinerant and localized polaron contributions. Electronic structure property calculations via density functional theory simulations on Ag2ZnSnSe4 for both energetically similar kesterite and stannite structure types were also performed in order to compare our results to those of the compositionally similar but well known Cu2ZnSnSe4. This theoretical comparison is crucial in understanding the bonding that results in polaronic type transport for Ag2ZnSnSe4, as well as the structural and electronic properties of both crystal structure types. In addition to possessing this unique electronic transport, the thermal conductivity of Ag2ZnSnSe4 is low and decreases with increasing silver content. This work reveals unique structure-property relationships in materials that continue to be of interest for thermoelectric and photovoltaic applications.

Effect of defects on the small polaron formation and transport properties of hematite from first-principles calculations

Hematite (α-Fe2O3) is a promising candidate as a photoanode material for solar-to-fuel conversion due to its favorable band gap for visible light absorption, its stability in an aqueous environment and its relatively low cost in comparison to other prospective materials. However, the small polaron transport nature in α-Fe2O3 results in low carrier mobility and conductivity, significantly lowering its efficiency from the theoretical limit. Experimentally, it has been found that the incorporation of oxygen vacancies and other dopants, such as Sn, into the material appreciably enhances its photo-to-current efficiency. Yet no quantitative explanation has been provided to understand the role of oxygen vacancy or Sn-doping in hematite. We employed density functional theory to probe the small polaron formation in oxygen deficient hematite, N-doped as well as Sn-doped hematite. We computed the charged defect formation energies, the small polaron formation energy and hopping activation energies to understand the effect of defects on carrier concentration and mobility. This work provides us with a fundamental understanding regarding the role of defects on small polaron formation and transport properties in hematite, offering key insights into the design of new dopants to further improve the efficiency of transition metal oxides for solar-to-fuel conversion.

On the AC-conductivity mechanism in nano-crystalline Se79−xTe15In6Pbx (x = 0, 1, 2, 4, 6, 8 and 10) alloys

In-depth analysis of complex AC-conductivity for nano-crystalline Se79-xTe15In6Pbx (x = 0, 1, 2, 4, 6, 8 and 10at wt%) alloys is made in the temperature range 308–423K and over the frequency range 10−1–107Hz, to understand the conduction mechanism. The investigated nano-crystalline alloys were prepared by melt-quench technique. Sharp structural peaks in X-ray diffraction pattern indicate the nano-crystalline nature, which is also confirmed by FESEM. The AC conductivity shows universal characteristics and at higher frequency a transition from dc to dispersive behavior occurs. Moreover, it is confirmed that ac conductivity (σac) obeys the Jonscher power law as ~ ωs (s< 1). The obtained results are analyzed in the light of various theoretical models. The correlated barrier hopping (CBH) model associated with non-intimate valence alternation pairs (NVAP's) is found most appropriate to describe the conduction mechanisms in these alloys. In addition, the CBH model description reveals that the bipolaron (single polaron) transport dominates at lower (higher) temperature. The density of localized states has also been deduced.

Phase-breaking effect on polaron transport in organic conjugated polymers

Despite intense investigations and many accepted viewpoints on theory and experiment, the coherent and incoherent carrier transport in organic semiconductors remains an unsettled topic due to the strong electron-phonon coupling. Based on the tight-binding Su-Schrieffer-Heeger (SSH) model combined with a non-adiabatic dynamics method, we study the effect of phase-breaking on polaron transport by introducing a group of phase-breaking factors into π-electron wave-functions in organic conjugated polymers. Two approaches are applied: the modification of the transfer integral and the phase-breaking addition to the wave-function. Within the former, it is found that a single site phase-breaking can trap a polaron. However, with a larger regular phase-breaking a polaron becomes more delocalized and lighter. Additionally, a group of disordered phase-breaking factors can make the polaron disperse in transport process. Within the latter approach, we show that the phase-breaking can render the delocalized state in valence band discrete and the state in the gap more localized. Consequently, the phase-breaking frequency and intensity can reduce the stability of a polaron. Overall, the phase-breaking in organic systems is the main factor that degrades the coherent transport and destroys the carrier stability.

Polaron spin echo envelope modulations in an organic semiconducting polymer

Theoretical treatment of the electron spin echo envelope modulation (ESEEM) spectra from polarons in a semiconducting $\pi$- conjugated polymer is presented. The contact hyperfine coupling and the dipolar interaction between the polaron and proton spins are found to have distinct contributions in the ESEEM spectra. However, since the two contributions are spaced very closely, and the dipolar contribution is dominant, the detection of the contact hyperfine interaction is difficult. To resolve this problem, a recipe of probing the contact hyperfine and dipolar interactions selectively is proposed, and a method for detecting the polaron contact hyperfine interaction is formulated. The ESEEM decay due to the polaron random hopping is analyzed, and the robustness of the method against this decay is verified. Moreover, this decay is linked to the transport properties of polarons, providing an auxiliary probe for the polaron transport.

Bloch oscillations in organic and inorganic polymers.

The transport of polarons above the mobility threshold in organic and inorganic polymers is theoretically investigated in the framework of a one-dimensional tight-binding model that includes lattice relaxation. The computational approach is based on parameters for which the model Hamiltonian suitably describes different polymer lattices in the presence of external electric fields. Our findings show that, above critical field strengths, a dissociated polaron moves through the polymer lattice as a free electron performing Bloch oscillations. These critical electric fields are considerably smaller for inorganic lattices in comparison to organic polymers. Interestingly, for inorganic lattices, the free electron propagates preserving charge and spin densities' localization which is a characteristic of a static polaron. Moreover, in the turning points of the spatial Bloch oscillations, transient polaron levels are formed inside the band gap, thus generating a fully characterized polaron structure. For the organic case, on the other hand, no polaron signature is observed: neither in the shape of the distortion-those polaron profile signatures are absent-nor in the energy levels-as no such polaron levels are formed during the simulation. These results solve controversial aspects concerning Bloch oscillations recently reported in the literature and may enlighten the understanding about the charge transport mechanism in polymers above their mobility edge.

Intermolecular Charge Transfer Parameters, Electron–Phonon Couplings, and the Validity of Polaron Hopping Models in Organic Semiconducting Crystals: Rubrene, Pentacene, and C60

We evaluate the validity of the commonly assumed polaron hopping model for some of the most popular organic semiconductors, rubrene, pentacene, and C60. This model is based on the assumption that the charge carrier is localized, i.e., forms a polaron that hops from one molecule to the next. We have calculated the relevant intermolecular charge transfer parameters that determine whether a polaron forms or not: electronic coupling matrix element and reorganization energy for the above materials using quantum chemical calculations and molecular dynamics simulations. We find that neither for rubrene nor pentancene the hopping model is justified due to the relatively large electronic couplings between molecules in the respective herringbone layers. For C60 the coupling matrix elements are smaller, and a small but finite barrier for charge transport exists in any transport direction. Despite the theoretical problems surrounding the polaron transport model, we find that mobilities based on this model (as obtaine...

Large magnetocaloric and magnetoresistance effects in metamagnetic Sm0.55(Sr0.5Ca0.5)0.45MnO3 manganite

The metamagnetic transition, magnetocaloric and magnetoresistance effects are investigated in polycrystalline Sm0.55(Sr0.5Ca0.5)0.45MnO3 (SSCMO) manganite. A sharp magnetization jump at Curie temperature (TC) 73.5K with large thermal hysteresis is observed. Magnetic measurements and Arrott plots analysis indicate that the transition is first order in nature. Under a low magnetic field change of 1T, the magnetic entropy change exhibits a peak value of −ΔSMmax(T)=4.01J/kg K with relative cooling power (RCP) value of 44.1J/kg in the vicinity of TC. The mechanism of charge conduction in insulator phase is polaron transport. Large negative magnetoresistance ratio with value of ~99% is obtained within a broad temperature range below metal insulator transition temperature under 1T magnetic field. These results indicate the potential applications of SSCMO in magnetic refrigeration and spintronics devices.

Polaronic transport in iron phosphate glasses containing HfO2 and CeO2

The electrical and dielectric properties of three series of glasses, xHfO2-(40 - x)Fe2O3-60P2O5, 0 ≤ x ≤ 8 mol%, xCeO2-(40 - x)Fe2O3-60P2O5, 0 ≤ x ≤ 8 mol%, and xHfO2-(38 - x)Fe2O3-2B2O3-60P2O5 2 ≤ x ≤ 6 mol%, have been investigated by impedance spectroscopy over a wide frequency and temperature range. As expected, these glasses exhibit polaronic conductivity which strongly depends on the fraction of ferrous ions, Fe2+/Fetot. Following a detailed discussion on the DC conductivity, we use the MIGRATION concept to model their conductivity spectra. It is found that in each series of glasses, the shape of the conductivity isotherms remains the same indicating that the time-temperature superposition principle is satisfied and that the mechanism of conductivity is the same. Returning to a model-free scaling procedure, namely Summerfield scaling, it is found that while conductivity isotherms for each composition yield a master curve, we need to suitably shift individual master curves on the frequency axis to generate a super-master curve. We examine the dependence of the DC conductivity and the shift factors on the number density of charge carriers. Next, using the fact that the dielectric strength of relaxation for each isotherm is well-defined in these systems, we scale the conductivity isotherms using the Sidebottom scaling procedure. This procedure yields a super-master curve, implying that length scales for polaronic transport also change with composition. Further, using the scaling features of permittivity spectra, we extract in a straightforward way the characteristic spatial extent of localized hopping of polarons and find that it decreases with increasing number density of charge carriers. The magnitude of these values obtained from permittivity spectra lies in the same range as those for the polaron radius calculated using the equation proposed by Bogomolov and Mirilin.

Substitution of Re7+ into CaMnO3: an efficient free electron generation dopant for tuning of thermoelectric properties.

Highly dense CaMn1-xRexO3 (0 ≤ x ≤ 0.04) samples were prepared by solid-state synthesis. The effect of Re doping was assessed by the characterisation of crystal structure, oxygen content, and electrical and thermal transport properties. The oxidation state of the substituted Re was determined by X-ray absorption near edge spectra to be Re7+, and led to expansion of the lattice and an increase in electron carrier concentration due to the formation of Mn3+. The thermal behaviour of the electrical conductivity and the thermopower over a wide temperature range allowed identification of different conduction mechanisms: (1) below 110 K, 3D variable range hopping, (2) between 110 and 650 K, small polaron transport, and (3) above 650 K, activation of carriers over a mobility edge. Evaluation of the power factor expected for different dopant oxidation states as a function of dopant concentration shows that the doping strategy using a heavy heptavalent ion allows accessibility of the peak power factor at lower dopant concentrations, lowering the amount of non-ionised impurities, and therefore improves the electronic substitution efficiency, the ratio of activated carriers over the nominal doping concentration, compared to previously studied dopants. An increased power factor and a reduced lattice thermal conductivity are obtained with a peak figure of merit ZT = 0.16(3) at 947 K for CaMn0.98Re0.02O3. This is an approximately two-fold increase compared to undoped CaMnO3, and is comparable to the highest values reported for highly dense B-site doped CaMnO3.

Mixed Ion-Polaron Glasses as New Cathode Materials

Electrical transport in mixed ion-polaron glasses has been investigated in four series of glasses containing transition metal oxides (TMO) namely WO3/MoO3, and Na+/Ag+ ions with the composition xWO3- (30-0.5x)Na2O-(30-0.5x)ZnO-40P2O5, xWO3-(30- 0.5x)Ag2O-(30-0.5x)ZnO-40P2O5, xMoO3-(30-0.5x)Na2O- (30-0.5x)ZnO-40P2O5, and xMoO3-(30-0.5x)Ag2O-(30- 0.5x)ZnO-40P2O5, 0 ≤ x ≤ 60 (mol%). The DC conductivity of Na-glasses up to 30 mol% of WO3 and MoO3 is almost identical due to the dominance of ionic conductivity. In this compositional region, the introduction of tungstate and molybdate units increases the mobility of sodium ions and compensates the decrease in sodium number density. On the other hand, with increasing WO3 and MoO3 content and decreasing Ag+ ion concentration in Ag-glasses the DC conductivity decreases for few orders of magnitude. A significant difference in conductivity is observed for glasses with higher WO3 and MoO3 content. While for glasses containing WO3 the conductivity rapidly increases due to a huge polaronic contribution indicating a turnover from predominantly ionic to polaronic transport, the conductivity for MoO3- glasses decreases in the entire mixed compositional range suggesting dominant ionic transport. The changes in the conduction mechanism with the systematic alternation of the glass composition have been analyzed in correlation with the structural modifications and variations of molybdenum and tungsten in different oxidation states.

Polaron dynamics in anisotropic Holstein-Peierls systems.

Polaron dynamics in anisotropic organic molecular semiconductors is theoretically investigated and simulated in the framework of a semi-classical Holstein-Peierls model. Our computational protocol is presented and applied to studies of a two-dimensional molecular crystal. The intermolecular (Peierls) parameters for a particular crystal direction are systematically changed in order to study the effect of anisotropy in the system. The usefulness of this methodology is highlighted by studying the polaron dynamics on a picosecond timescale, which provides a microscopic insight into the influence of the interplay between different intramolecular parameters on the charge transport mechanism. Our results show that the polaron mobility is substantially reduced in going from an anisotropic to an isotropic relationship between the Peierls parameters for different directions in the crystal. Interestingly, the molecular charge distribution presents three different signatures corresponding to a one-dimensional polaron, a two-dimensional polaron, and an intermediate state for which the polaron localization depends on the degree of anisotropy. Importantly, the two-dimensional polaron, which is present in the essentially isotropic system, is immobile whereas the other two types of polarons are mobile. This, in order for polaron transport to occur in a two-dimensional molecular based system, this system has to be anisotropic.