Polaron Density Research Articles

Image-enhanced bipolaron formation at organic semiconductor/electrode interfaces

We explore the image charge interaction for organic semiconductor bipolarons near a conducting interface and find that the cross term between one of the constituent charges and the image of its neighbor stabilizes the bipolaron by up to \ensuremath{\sim}0.3 eV, dramatically increasing the concentration of this species near the interface. Using density functional theory calculations for the common hole transport molecule $N,N$\ensuremath{'}-bis(3-methylphenyl)-$N,N$\ensuremath{'}-diphenylbenzidine, we validate a simple point charge description of this effect and incorporate it within an interface energy level alignment model to predict the density of polarons and bipolarons near the interface. We find that the image effect greatly enhances bipolaron formation in the first few monolayers, leading to the expectation that bipolarons account for more than 1% of the total interface charge in many cases of practical interest. This result reinforces the notion that bipolarons are robust near the contacts of many organic semiconductor devices and thus helps to rationalize their involvement in the phenomenon of unipolar organic magnetoresistance.

Comprehensive Model of the Degradation of Organic Light-Emitting Diodes and Application for Efficient, Stable Blue Phosphorescent Devices with Reduced Influence of Polarons

We present a comprehensive model to analyze, quantitatively, and predict the process of degradation of organic light-emitting diodes (OLEDs) considering all possible degradation mechanisms, i.e., polaron, exciton, exciton-polaron interactions, exciton-exciton interactions, and a newly proposed impurity effect. The loss of efficiency during degradation is presented as a function of quencher density, the density and generation mechanisms of which were extracted using a voltage rise model. The comprehensive model was applied to stable blue phosphorescent OLEDs (PhOLEDs), and the results showed that the model described the voltage rise and external quantum efficiency (EQE) loss very well, and that the quenchers in emitting layer (EML) were mainly generated by dopant polarons. Quencher formation was confirmed from a mass spectrometry. The polaron density per dopant molecule in EML was reduced by controlling the emitter doping ratio, resulting in the highest reported LT50 of 431 hours at an initial brightness of 500 cd/m2 with CIEy<0.25 and high external quantum efficiency (EQE) >18%.

Low potassium mobility in iron pyrophosphate glasses

Structural and electrical properties of glasses of the molar composition xK2O−(40−x/2)Fe2O3−(60−x/2)P2O5, x = 0–36 mol%, are investigated by Raman, Mössbauer and Impedance spectroscopy. All glasses show pyrophosphate structure with a growing tendency for depolymerization with increasing K2O content. The DC conductivity changes in a non-monotonic fashion, exhibiting a broad flat maximum as K2O gradually substitutes Fe2O3 and P2O5. The observed conductivity trend could be related to the changes in the number density of polarons which depends on Fe2O3 and fraction of ferrous ions, indicating dominant polaronic conduction mechanism in all glasses. Interestingly, a relatively small addition of K2O, up to 12 mol%, has a positive effect on the polaronic transport due to slight modifications of glass network. Potassium ions are weakly mobile in iron pyrophosphate glass network and have a negligible contribution to the electrical transport even when their molar fraction is as high as 36 mol%.

Study of exciton-polaron interaction in pentacene field effect transistors using high sensitive photocurrent measurements

Luminescence quenching in the presence of polarons is one of the major challenges in organic light emitting devices. In this work, exciton quenching in the presence of polarons is studied using phase sensitive photocurrent measurements on pentacene field effect transistors. The enhancement of conduction in the organic field effect transistors on light illumination is studied using photocurrent spectral response measurements and corresponding optical simulations. The photocurrent is shown to be governed by the polaron mobility and the exciton quenching efficiency, both of which depend on the polaron density in the channel. Two models are proposed on the exciton dynamics in the presence of gate induced polarons in the transistor channel. The first model simulates the steady-state exciton concentration profile in the presence of exciton-polaron interaction. The second one is a three-dimensional steady state exciton-polaron interaction model, which supports the findings from the first model. It is shown that the excitons quench by transferring its energy to polarons, thereby promoting the latter to high energy states in the density of states manifold. The polarons move in the higher energy states with greater microscopic mobility before thermalizing, thereby leading to an enhancement of conduction. It is observed that for the present system, where charge carrier transport is by hopping, all polarons interact with excitons. This implies that for low mobility systems, the interaction is not limited to deep trapped polarons.

Role of spin, phonon and plasmon dynamics on ferromagnetism in Cr doped 3C-SiC

The defect induced magnetism has initiated a lot of interest in field of spintronics. In this regard, SiC is a promising material because of its unique properties under extreme conditions. Hence it will be very much interesting to investigate the interaction between defects and itinerant carrier in doped SiC for spintronics application. We report the structural stability and magnetic interaction in Cr doped 3C-SiC synthesized by Thermal Plasma Technique. The EPR spectrum of undoped 3C-SiC shows a sharp resonance line corresponding to $g=2.00$ associated with the defects present in the system. Anomalous temperature evolution tendency of the relative intensity of EPR spectra can be attributed to magnetically correlated defects in the host matrix. For the first time we report the detailed quantitative analysis of X-band and Q-band EPR study in Cr doped 3C-SiC which reveals that Cr can be in multivalent state. The non monotonous variation of Longitudinal Mode (LO) of the Raman spectra has been explained based on the interaction between carriers and surface plasmon using Longitudinal optical plasmon coupling model (LOPC). The carrier density calculated by using LOPC fit with experimental data varies from $1.8\times10^{15}$ to $4.2\times10^{17}$ cm$^{-3}$. Room temperature magnetic measurements exhibit ferromagnetic behavior with non-zero coercivity for all the samples up to 7 T field. We, for the first time, show the Curie temperature to be above 760 K. Quantitative analysis of magnetic interaction validates the applicability of Bound Magnetic Polaron Model (BMP) which probably arises from the exchange interaction of Cr$^{3+}$ ions with related (Si, C) defects. The polaron density estimated from the BMP fit agrees well with the carrier density obtained from the line shape fitting of Raman spectra.

Charge transport layers manage mobility and carrier density balance in light-emitting layers influencing the operational stability of organic light emitting diodes

Abstract Organic light emitting diodes (OLEDs) consist of several organic layers, including the charge injection layer, charge transport layer, and light emitting layer (EML). Of these layers, the charge transport layer is crucial for ensuring device longevity, but its overall effects on charge transport and corresponding device stability are poorly understood. Herein we report the factors influencing differences in lifetime between two OLEDs with different hole transporting layers (HTLs). Comprehensive electrical analysis of the materials and the devices reveals that the mobility, accumulation, trapping, and the transport path of holes in the EML are totally changed by the HTLs. The charge transport layers affect mobility and carrier density balance in the EML through the modification of the charge transport path and the energetic barrier. This results in a reduction of overbalanced polaron density, which is critical for bond dissociation in excitonic interactions. Consequently, device lifetime is increased sevenfold through modification of the HTL structure without any alteration of the EML. These results imply that the analysis of polaronic transport through impedance spectroscopy is a crucial step in determining the requisite electrical properties for charge transport layers, with a view to maximizing the operational stability of OLEDs.

Impact of Molecular Order on Polaron Formation in Conjugated Polymers

The nature of polaron formation has profound implications on the transport of charge carriers in conjugated polymers, but still remains poorly understood. Here, we develop in situ electrochemical resonant Raman spectroscopy, a powerful structural probe that allows direct observation of polaron formation. We report that polaron formation in ordered poly(3-hexyl)thiophene (P3HT) polymer domains (crystalline phase) results in less pronounced changes in molecular conformation, indicating smaller lattice relaxation, compared to polarons generated in disordered polymer domains (amorphous phase) for which we observe large molecular conformational changes. These conformational changes are directly related to the effective conjugation length of the polymer. Furthermore, we elucidate how blending the P3HT polymer with phenyl C-61 butyric acid methyl ester affects polaron formation in the polymer. We find that blending disturbs polymer crystallinity, reducing the density of polarons that can form upon charge injecti...

Enhanced thermoelectric properties of two-dimensional conjugated polymers

Organic thermoelectric materials have drawn great attention in the past years. In this study, we report the thermoelectric properties of ferric salt bis(trifluoromethane)sulfonimide (TFSI−) doped two-dimensional (2D) conjugated polymer, poly[[4,8-bis[(5-ethylhexyl)thienyl]benzo[1,2-b;3,3-b]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7-DT). It is found that the electrical conductivities of TFSI− doped 2D PTB7-DT are nearly 100 times larger than those of TFSI− doped one-dimensional (1D) corresponding conjugate polymer, poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) at the same doping levels. Moreover, TFSI− doped 2D PTB7-DT possess over 20 times larger power factor (79.8 μW/mK2) than that of TFSI− doped 1D PTB7. The enhancement of electrical conductivities of TFSI− doped 2D PTB7-DT is attributed to enhanced charge carrier densities and polaron densities. The studies of grazing incidence wide angle X-ray scattering further indicate that the π-π stacking distances of TFSI− doped 2D PTB7-DT along the in-plane direction are largely reduced compared to that of TFSI− doped 1D PTB7, which would facilitate the inter-chain and the intra-chain charge carrier transport, resulting in enhanced electrical conductivities. All these results demonstrate that 2D conjugated polymers are promising candidate materials for approaching high performance organic thermoelectric electronics.

Thermal, Structural, AC Conductivity, and Dielectric Properties of Ethyl-2-amino-6-ethyl-5-oxo-4-(3-phenoxyphenyl)-5,6-dihydro-4H-pyrano[3,2-c]quinoline-3-carboxylate Thin Films

Thermal, structural, alternating-current (AC) conductivity (σAC), and dielectric properties of ethyl-2-amino-6-ethyl-5-oxo-4-(3-phenoxyphenyl)-5,6-dihydro-4H-pyrano[3,2-c]quinoline-3-carboxylate (HPQC) thin films have been studied. Thermogravimetry analysis and differential scanning calorimetry confirmed the thermal stability of HPQC over a wide temperature range. Fourier-transform infrared spectroscopy and x-ray diffraction analysis were carried out on HPQC in powder form and as-deposited thin film. The crystal system and space group type were determined for HPQC in powder form. The AC conductivity and dielectric properties were determined in the frequency range from 0.5 kHz to 5 MHz and temperature range from 296 K to 443 K. The AC electrical conduction of HPQC thin film was found to be governed by the small-polaron tunneling mechanism. The polaron hopping energy (WH), tunneling distance (R), and density of states (N) near the Fermi level were determined as functions of temperature and frequency. The dielectric properties of HPQC thin film were studied by analysis of Nyquist diagrams, the dissipation factor (tan δ), and real (e′) and imaginary (e″) parts of the dielectric constant.

Linear and non-linear infrared response of one-dimensional vibrational Holstein polarons in the anti-adiabatic limit: Optical and acoustical phonon models.

The theory of linear and non-linear infrared response of vibrational Holstein polarons in one-dimensional lattices is presented in order to identify the spectral signatures of self-trapping phenomena. Using a canonical transformation, the optical response is computed from the small polaron point of view which is valid in the anti-adiabatic limit. Two types of phonon baths are considered: optical phonons and acoustical phonons, and simple expressions are derived for the infrared response. It is shown that for the case of optical phonons, the linear response can directly probe the polaron density of states. The model is used to interpret the experimental spectrum of crystalline acetanilide in the C=O range. For the case of acoustical phonons, it is shown that two bound states can be observed in the two-dimensional infrared spectrum at low temperature. At high temperature, analysis of the time-dependence of the two-dimensional infrared spectrum indicates that bath mediated correlations slow down spectral diffusion. The model is used to interpret the experimental linear-spectroscopy of model α-helix and β-sheet polypeptides. This work shows that the Davydov Hamiltonian cannot explain the observations in the NH stretching range.

Influence of quasi-particle density over polaron mobility in armchair graphene nanoribbons.

An important aspect concerning the performance of armchair graphene nanoribbons (AGNRs) as materials for conceiving electronic devices is related to the mobility of charge carriers in these systems. When several polarons are considered in the system, a quasi-particle wave function can be affected by that of its neighbor provided the two are close enough. As the overlap may affect the transport of the carrier, the question concerning how the density of polarons affect its mobility arises. In this work, we investigate such dependence for semiconducting AGNRs in the scope of nonadiabatic molecular dynamics. Our results unambiguously show an impact of the density on both the stability and average velocity of the quasi-particles. We have found a phase transition between regimes where increasing density stops inhibiting and starts promoting mobility; densities higher than 7 polarons per 45 Å present increasing mean velocity with increasing density. We have also established three different regions relating electric field and average velocity. For the lowest electric field regime, surpassing the aforementioned threshold results in overcoming the 0.3 Å fs-1 limit, thus representing a transition between subsonic and supersonic regimes. For the highest of the electric fields, density effects alone are responsible for a stunning difference of 1.5 Å fs-1 in the mean carrier velocity.

Förster-type triplet-polaron quenching in disordered organic semiconductors

Triplet-polaron quenching (TPQ) is a major cause of the efficiency loss at large current densities in phosphorescent organic light-emitting diodes. The nature of the interaction process is presently not well understood. In this paper, we study TPQ due to F\orster-type triplet-polaron interactions in energetically disordered organic semiconductors with a Gaussian polaron density of states. A continuum theory, which neglects the spatial inhomogeneity and energetic disorder, is from a kinetic Monte Carlo approach shown to correctly predict that the effective steady-state TPQ rate coefficient ${k}_{\text{TPQ,eff}}$ depends only sensitively on the polaron diffusion in a rather narrow range of diffusion coefficients. However, in this regime, significant discrepancies between the two approaches are found, in particular for realistic values of the TPQ-F\orster radius, around 3 nm, and for systems with strong energetic disorder. Both approaches show that ${k}_{\text{TPQ,eff}}$ is not constant but can depend on the polaron density and the electric field. Various methods for deducing the TPQ mechanism from experiment are discussed, including an approach which utilizes the shape of the time-dependent photoluminescence after pulsed illumination.

Effect of polaron diffusion on exciton-polaron quenching in disordered organic semiconductors

Exciton-polaron quenching (EPQ) is a major efficiency loss process in organic optoelectronic devices, in particular at high excitation densities. Within commonly used models, the rate is assumed to be given by the product of the exciton density, the polaron density, and a constant EPQ rate coefficient, which is proportional to the polaron diffusion coefficient and an EPQ capture radius. In this work, we study the effects of polaron diffusion on the EPQ rate in energetically disordered materials with a Gaussian density of states using kinetic Monte Carlo simulations, and show that the effective rate coefficient can depend strongly on the polaron concentration and on the electric field. We furthermore find that under realistic conditions, the effective value of the capture radius can exceed the expected value of $\ensuremath{\sim}1$ nm by up to two orders of magnitude. To a first approximation, the simulation results can be understood from macroscopic diffusion theory, adapted at finite electric fields to include the observed ``polaron wind'' effect. However, for strongly disordered systems we find distinct deviations from that theory, related to the very small time and spatial scales involved in the capture process.

Characterization of recombination in P3HT:fullerene blends: Clarifying the influence of interfacial states

The influence of an interfacial dipole at the donor/acceptor interface on the generation and bulk recombination in P3HT:fullerene blends is clarified. We use P3HT:PC60BM and P3HT:ICBA blends which have a large and small interfacial dipole, respectively. We show that polarons are generated using above and below gap excitations in both blends. The polaron density generated using below gap excitation is lower in P3HT:ICBA blends which is suggested to be due to a higher lying CT state compared to P3HT:PC60BM blends. Using intensity and frequency dependent measurements, we show that the bulk recombination is surprisingly similar in both blends. The bulk recombination is dominated by bimolecular recombination at all light-intensities for the delocalized polarons, while for the localized polaron trap-assisted recombination dominates at low intensities which becomes bimolecular at high intensities. For the localized polaron we find that the characteristic energy for the trap depth to be Ech=24.4±0.7meV and Ech=21.3±0.3 for the P3HT:ICBA and P3HT:PC60BM blends, respectively. Comparing the ratio between the bimolecular recombination constant and the mobility, we find the recombination to be practically the same in the two blends. However, while the localized polaron does show a difference of a factor of two, suggesting faster bimolecular recombination and stronger trap-assisted recombination in the P3HT:ICBA blend having a small interfacial dipole, we conclude that interfacial dipoles have little or no effect on the bulk recombination of long-lived photoexcitations in these blends.

Free charge localization and effective dielectric permittivity in oxides

This review will deal with several types of free charge localization in oxides and their consequences on the effective dielectric spectra of such materials. The first one is the polaronic localization at the unit cell scale on residual impurities in ferroelectric networks. The second one is the collective localization of free charge at macroscopic interfaces like surfaces, electrodes and grain boundaries in ceramics. Polarons have been observed in many oxide perovskites mostly when cations having several stable electronic configurations are present. In manganites, the density of such polarons is so high as to drive a net lattice of interacting polarons. On the other hand, in ferroelectric materials like BaTiO3 and LiNbO3, the density of polarons is usually very small but they can influence strongly the macroscopic conductivity. The contribution of such polarons to the dielectric spectra of ferroelectric materials is described. Even residual impurities as for example Iron can induce well-defined anomalies at very low temperatures. This is mostly resulting from the interaction between localized polarons and the highly polarizable ferroelectric network in which they are embedded. The case of such residual polarons in SrTiO3 will be described in more detail, emphasizing the quantum polaron state at liquid helium temperatures. Recently, several nonferroelectric oxides have been shown to display giant effective dielectric permittivity. It is first shown that the frequency/temperature behavior of such parameters is very similar in very different compounds (donor-doped BaTiO3, CaCu3Ti4O12, LuFe2O4, Li-doped NiO, etc.). This similarity calls for a common origin of the giant dielectric permittivity in these compounds. A space charge localization at macroscopic interfaces can be the key for such extremely high dielectric permittivity.

First-Principles Study of the Nuclear Dynamics of Doped Conjugated Polymers

Infrared-active vibrational (IRAV) modes are specific optical fingerprints to probe the density, dynamics, and spatial distribution of polarons in π-electron conjugated polymers. So far, the description of IRAV mode activation and selection rules, resulting from the local breaking of spatial symmetry induced by charge carriers, has been restricted to phenomenological lattice-dynamics models. Overcoming the classical picture, here we combine first-principles calculations with vibrational spectroscopy to study the nuclear dynamics of a model polymer system, poly(3-hexylthiophene) (P3HT). We assign and reproduce quantitatively the transition energies and intensities of vibrational normal modes in the ground and excited electronic states. By comparing the ground, polaronic, and excitonic states of regioregular (RR-) and regiorandom (RRa-) chains, we identify, for the first time, the vibrational fingerprints of neutral singlet excitations in the IRAV spectra of P3HT and highlight structure–property correlation...

Interfacial and bulk polaron masses in Zn1−xMgxO/ZnO heterostructures examined by terahertz time-domain cyclotron spectroscopy

The cyclotron resonance of polarons in Zn1−xMgxO/ZnO heterostructures (with 0.15&amp;lt;x&amp;lt;0.22) was studied by terahertz time-domain spectroscopy. Low-temperature magnetoconductivity spectra of the 2D electron gas at the Zn1−xMgxO/ZnO interface determined the polaron density, mass, and scattering rate. The cyclotron mass of 2D polarons was found to increase significantly with magnetic field B from 0.24 me at B = 2 T to 0.37 me at B = 7.5 T. A nonlinear cyclotron frequency with B was also observed for 3D polarons in ZnO. The findings are discussed in the context of polaron mass renormalization driven by the electron-LO-phonon and electron-acoustic phonon interactions.

Optical measurement of doping efficiency in poly(3-hexylthiophene) solutions and thin films

The doping mechanism of 2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane (F4TCNQ) doped poly(3-hexylthiophene) (P3HT) thin films and solutions is studied by UV-visible and IR absorption spectral analysis. We show that integer charge transfer between the two molecules is observed in this system with 63% of the dopants producing a fully transferred charge in thin film. The primary P3HT doped species in solution and thin film are shown to be bipolarons and polarons, respectively. The absorption signatures of polarons and bipolarons are identified and their cross sections are measured, which can be used to evaluate the influence of processing conditions on the density of impurity dopant-induced polarons in nominally ``neat'' P3HT films.

Decoherence of Polaron in Asymmetric Quantum Dot Qubit Under an Electromagnetic Field

In this paper, we investigate the time evolution of the quantum mechanical state of a polaron using the Pekar type variational method on the electric-LO-phonon and the magnetic-LO-phonon strong coupling in a quantum dot. We obtain the Eigen energies and the Eigen functions of the ground state and the first excited state, respectively. In a quantum dot, this system can be viewed as a two level quantum system qubit. The superposition state polaron density oscillates in the quantum dot with a period τ_0when the polaron is in the superposition of the ground and the first-excited states. The spontaneous emission of phonons causes the decoherence of the qubit. We show that the density matrix of the qubit decays with the time while the coherence term of the density matrix element 〖 p〗_01 (〖 or p〗_10) decays with the time as well for different coupling strengths, confinement lengths, and dispersion coefficients. The Shannon entropy is evaluated in order to investigate the decoherence of the system.

Elucidating the localized plasmonic enhancement effects from a single Ag nanowire in organic solar cells.

The origins of performance enhancement in hybrid plasmonic organic photovoltaic devices are often embroiled in a complex interaction of light scattering, localized surface plasmon resonances, exciton-plasmon energy transfer and even nonplasmonic effects. To clearly deconvolve the plasmonic contributions from a single nanostructure, we herein investigate the influence of a single silver nanowire (NW) on the charge carriers in bulk heterojunction polymer solar cells using spatially resolved optical spectroscopy, and correlate to electrical device characterization. Polarization-dependent photocurrent enhancements with a maximum of ∼ 36% over the reference are observed when the transverse mode of the plasmonic excitations in the Ag NW is activated. The ensuing higher absorbance and light scattering induced by the electronic motion perpendicular to the NW long axis lead to increased exciton and polaron densities instead of direct surface plasmon-exciton energy transfer. Finite-difference time-domain simulations also validate these findings. Importantly, our study at the single nanostructure level explores the fundamental limits of plasmonic enhancement achievable in organic solar cells with a single plasmonic nanostructure.