Bipolar Charge Transport Research Articles

Cyclohexane-Coupled Bipolar Host Materials with High Triplet Energies for Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence.

Thermally activated delayed fluorescence-based organic light-emitting diodes (TADF-OLEDs) have recently attracted tremendous research interest as next-generation optoelectronic devices. However, there are a limited number of host materials with an appropriately high lowest-excited triplet energy (ET) and bipolar charge transport properties for high-efficiency TADF-OLEDs. Moreover, these host materials should have high thermal and morphological stabilities. In this study, we develop novel bipolar host materials consisting of an electron-donating 9-phenylcarbazole unit and an electron-accepting triphenylphosphine oxide, triphenylphosphine sulfide, or 2,4,6-triphenyl-1,3,5-triazine unit linked by a nonconjugated cyclohexane core. These bipolar host materials possess high glass-transition temperatures of over 100 °C and high ET values of approximately 3.0 eV. TADF-OLEDs employing these bipolar host materials could achieve high external electroluminescence quantum efficiencies of up to 21.7% together with reduced efficiency roll-off characteristics, because of expansion of the charge-recombination zone within the emission layer arising from the bipolar charge transport ability of these host materials.

Effect of DC Prestressing on Periodic Grounded DC Tree in Cross-Linked Polyethylene at Different Temperatures

Periodic grounded dc trees in cross-linked polyethylene under various dc prestressing times are investigated in the temperature range of 20 °C–80 °C. Space charge behaviors in the samples during dc prestressing are simulated based on the bipolar charge transport model. The results reveal that the dc prestressing time has different effects on the tree growth at different temperatures, which is because that the space charge behavior in the sample during dc prestressing is closely related to the dc prestressing time and the temperature, and it has both promotion effect and impedance effect on the tree growth. The moderate promotion effect and impedance effect under 160 s dc prestressing at 20 °C, and the strongest promotion effect and the weakest impedance effect under 240 s dc prestressing at 40 °C result in the largest tree lengths, widths, and accumulated damages under each corresponding condition. At 60 °C and 80 °C, the lengths, widths, and accumulated damages are the largest under 240 s dc prestressing, smaller under 80 s dc prestressing, and the smallest under 160 s dc prestressing, and the reason is believed to be the growth rates reversal during the tree growth. Also it is found that the dc prestressing time has significant effects on the tree shape at 60 °C and 80 °C, which is believed to be related to the wider charge diffusion range under longer dc prestressing time. The electrical tree characteristics are discussed in detail combined with the space charge behaviors.

Modelling of dielectric breakdown through charge dynamics for polymer nanocomposites

A dielectric breakdown model consisted of bipolar charge transport and free volume breakdown criterion is used to investigate the dc breakdown properties of polymer nanocomposites. In the model, we consider charge injection from electrodes, carrier migration with a constant mobility, charge trapping and detrapping associated with deep traps, charge recombination, and energy gained for charge carriers from the local distorted electric field. Since incorporating nanoparticles into a polymer can change its density and/or energy of deep traps, we calculate the dielectric breakdown properties of low-density polyethylene nanocomposites characterized by various densities and energies of deep traps. The simulated results show that the breakdown strength increases with an increase in the density and energy of deep traps, which is consistent with the experimental results. It is shown that the accumulation of homocharges, the distortion of electric field, and the energy gain of free carriers are regulated to improve the performance of breakdown.

Blue Light Emitting Polyphenylene Dendrimers with Bipolar Charge Transport Moieties

Two light-emitting polyphenylene dendrimers with both hole and electron transporting moieties were synthesized and characterized. Both molecules exhibited pure blue emission solely from the pyrene core and efficient surface-to-core energy transfers when characterized in a nonpolar environment. In particular, the carbazole- and oxadiazole-functionalized dendrimer (D1) manifested a pure blue emission from the pyrene core without showing intramolecular charge transfer (ICT) in environments with increasing polarity. On the other hand, the triphenylamine- and oxadiazole-functionalized one (D2) displayed notable ICT with dual emission from both the core and an ICT state in highly polar solvents. D1, in a three-layer organic light emitting diode (OLED) by solution processing gave a pure blue emission with Commission Internationale de l’Éclairage 1931 CIE xy = (0.16, 0.12), a peak current efficiency of 0.21 cd/A and a peak luminance of 2700 cd/m2. This represents the first reported pure blue dendrimer emitter with bipolar charge transport and surface-to-core energy transfer in OLEDs.

Simulation of low-energy electron beam irradiated PTFE based on bipolar charge transport model

Studies on charge transport properties of space grade insulating materials become popular recently because it is closely related to the safe operation of spacecraft. Taking charge carriers retrapping and hole injection into consideration, we simulated the surface charging and the space charge accumulation properties of PTFE under lowenergy electron-beam irradiation using bipolar charge transport model in this paper. Parameters for simulation were based on experiments and previous studies. The secondary electron yields of PTFE irradiated by electron-beam of different primary electron energies were measured, and experimental data were fitted with an empirical equation. Thermally stimulated current was measured, and the trap parameters of PTFE were analyzed from the current-temperature curve by initial-rise method. In order to investigate the space charge distribution and the surface potential of steady state, factors like primary electron energy, sample thickness, temperature and trap density were selected as variables separately. It is found that the steady surface potential is nearly linear with the primary electron energy, which also fits the experimental results. The simulation results show the sample thickness affects the charging rate of surface potential mostly, and the charging rate is slower in thinner samples. The steady surface potential fluctuates slightly in the case that temperature is not high, but it increases significantly when temperature is higher than 400 K. With the increase in trap density, the space charge accumulation tends to focus on the surface and the dielectric/electrode interface, and the steady surface potential decreases.

Space Charge Modulated Electrical Breakdown.

Electrical breakdown is one of the most important physical phenomena in electrical and electronic engineering. Since the early 20th century, many theories and models of electrical breakdown have been proposed, but the origin of one key issue, that the explanation for dc breakdown strength being twice or higher than ac breakdown strength in insulating materials, remains unclear. Here, by employing a bipolar charge transport model, we investigate the space charge dynamics in both dc and ac breakdown processes. We demonstrate the differences in charge accumulations under both dc and ac stresses and estimate the breakdown strength, which is modulated by the electric field distortion induced by space charge. It is concluded that dc breakdown initializes in the bulk whereas ac breakdown initializes in the vicinity of the sample-electrode interface. Compared with dc breakdown, the lower breakdown strength under ac stress and the decreasing breakdown strength with an increase in applied frequency, are both attributed to the electric field distortion induced by space charges located in the vicinity of the electrodes.

Modelling space charge in a cable geometry

Fluid models including charge generation and transport function of time and space need to be further developed, especially when the object under study is a HVDC cable. In the present paper, we propose an evolution of a 1D fluid bipolar charge transport model, taking into account the cylindrical geometry, and its associated effects, i.e. a non-homogeneity of the field and the temperature within the dielectric. Simulations are performed using this one-dimensional cylindrical model, featuring charge injection, transport, trapping, detrapping and recombination, in order to highlight the effects of the cylindrical geometry, i.e. a temperature and a field gradient, on the space charge profiles. A long term simulation is performed until stationary state, to study the capabilities of the model in describing the behaviour of space charge in a MV cable, and the future improvements that need to be performed.

Simulation of Space Charge Dynamic in Polyethylene Under DC Continuous Electrical Stress

The space charge dynamic plays a very important role in the aging and breakdown of polymeric insulation materials under high voltage. This is due to the intensification of the local electric field and the attendant chemical–mechanical effects in the vicinity around the trapped charge. In this paper, we have investigated the space charge dynamic in low-density polyethylene under high direct-current voltage, which is evaluated by experimental conditions. The evaluation is on the basis of simulation using a bipolar charge transport model consisting of charge injection, transports, trapping, detrapping, and recombination phenomena. The theoretical formulation of the physical problem is based on the Poisson, the continuity, and the transport equations. Numerical results provide temporal and local distributions of the electric field, the space charge density for the different kinds of charges (net charge density, mobile and trapped of electron density, mobile hole density), conduction and displacement current densities, and the external current. The result shows the appearance of the negative packet-like space charge with a large amount of the bulk under the dc electric field of 100 kV/mm, and the induced distortion of the electric field is largely near to the anode, about 39% higher than the initial electric field applied.

Modeling Bipolar Charge Transport in Layered Polymer Films with Graded Permittivity Layers

Modeling Bipolar Charge Transport in Layered Polymer Films with Graded Permittivity Layers

Ambipolar Phosphine Derivatives to Attain True Blue OLEDs with 6.5% EQE.

A family of new branched phosphine derivatives {Ph2N-(C6H4)n-}3P → E (E = O 1-3, n = 1-3; E = S 4-6, n = 1-3; E = Se 7-9, n = 1-3; E = AuC6F5 4-6, n = 1-3), which are the donor-acceptor type molecules, exhibit efficient deep blue room temperature fluorescence (λem = 403-483 nm in CH2Cl2 solution, λem = 400-469 nm in the solid state). Fine tuning the emission characteristics can be achieved varying the length of aromatic oligophenylene bridge -(C6H4)n-. The pyramidal geometry of central R3P → E fragment on the one hand disrupts π-conjugation between the branches to preserve blue luminescence and high triplet energy, while on the other hand provides amorphous materials to prevent excimer formation and fluorescence self-quenching. Hence, compounds 2, 3, 5, and 12 were used as emitters to fabricate nondoped and doped electroluminescent devices. The luminophore 2 (E = O, n = 2) demonstrates excellently balanced bipolar charge transport and good nondoped device performance with a maximum external quantum efficiency (EQEmax) of 3.3% at 250 cd/m(2) and Commission International de L'Eclairage (CIE) coordinates of (0.15, 0.08). The doped device of 3 (E = O, n = 3) shows higher efficiency (EQEmax of 6.5, 6.0 at 100 cd/m(2)) and high color purity with CIE (0.15, 0.06) that matches the HDTV standard blue. The time-resolved electroluminescence measurement indicates that high efficiency of the device can be attributed to the triplet-triplet annihilation to enhance generation of singlet excitons.

Charge Transport in LDPE Nanocomposites Part II-Computational Approach.

A bipolar charge transport model is employed to investigate the remarkable reduction in dc conductivity of low-density polyethylene (LDPE) based material filled with uncoated nanofillers (reported in the first part of this work). The effect of temperature on charge transport is considered and the model outcomes are compared with measured conduction currents. The simulations reveal that the contribution of charge carrier recombination to the total transport process becomes more significant at elevated temperatures. Among the effects caused by the presence of nanoparticles, a reduced charge injection at electrodes has been found as the most essential one. Possible mechanisms for charge injection at different temperatures are therefore discussed.

Selectively Modulating Triplet Exciton Formation in Host Materials for Highly Efficient Blue Electrophosphorescence.

The concept of limiting the triplet exciton formation to fundamentally alleviate triplet-involved quenching effects is introduced to construct host materials for highly efficient and stable blue phosphorescent organic light-emitting diodes (PhOLEDs). The low triplet exciton formation is realized by small triplet exciton formation fraction and rate with high binding energy and high reorganization energy of triplet exciton. Demonstrated in two analogue molecules in conventional donor-acceptor molecule structure for bipolar charge injection and transport with nearly the same frontier orbital energy levels and triplet excited energies, the new concept host material shows significantly suppressed triplet exciton formation in the host to avoid quenching effects, leading to much improved device efficiencies and stabilities. The low-voltage-driving blue PhOLED devices exhibit maximum efficiencies of 43.7 cd A(-1) for current efficiency, 32.7 lm W(-1) for power efficiency, and 20.7% for external quantum efficiency with low roll-off and remarkable relative quenching effect reduction ratio up to 41%. Our fundamental solution for preventing quenching effects of long-lived triplet excitons provides exciting opportunities for fabricating high-performance devices using the advanced host materials with intrinsically small triplet exciton formation cross section.

Synthesis of a dibenzothiophene/carboline/carbazole hybrid bipolar host material for green phosphorescent OLEDs

A bipolar host material for green phosphorescent organic light-emitting diodes (OLEDs) was designed and synthesized through a combination of dibenzothiophene, α-carboline, and carbazole (DTCC) moieties. The synthesized host material showed a sufficient HOMO/LUMO bandgap (3.49eV) and triplet energy (2.67eV) for green emitting bis[2-(2-pyridinyl-N)phenyl-C] (acetylacetonato) iridium(III) [Ir(ppy)2(acac)]. From the results of a single charge device for DTCC, the hole current density of DTCC was similar to the electron current density, indicating that DTCC possesses bipolar charge transport properties, confirming its bipolar nature and thus its applicability as the host of PHOLED. Thus, the DTCC host showed efficient energy transfer to the Ir(ppy)2(acac) dopant in the device. A maximum external quantum efficiency of 18.9% was obtained using DTCC as the host material and the color coordinate of the green PHOLED was (CIEx,y=0.34, 0.62) at 10% doping concentration.

Synthesis and Optoelectronic Properties of a Solution-Processable Anthraquinone/Fluorene Hybrid Bipolar Fluorescent Material

The solution-processable, anthraquinone-based, fluorene bipolar fluorescent material 2-(9,9'-bis (2-ethylhexyl)-9H-fluoren-2-yl)anthracene-9,10-dione (FAA) was synthesized via a Suzuki reaction. The photophysical properties of FAA were subsequently investigated by acquiring absorption and photoluminescence spectra, and its optical properties were studied using computational density functional theory methods. Data obtained from single-carrier devices incorporating FAA demonstrated its well-matched bipolar charge-transport characteristics. The electroluminescence performance of this material was also examined by doping FAA into a 1,3-di(9H-carbazol-9-yl)benzene (mCP) matrix as the light-emitting layer via spin coating to produce an organic light-emitting diode (OLED) with an indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene:poly(styrenesulfonate) (PEDOT:PSS)/mCP:FAA/3,3'-(5'-(3-(pyridin-3-yl)phenyl)-[1,1':3',1 ''-terphen-yl]-3,3 ''-diyl)dipyridine (TmPyPb)/LiF/Al structure. This device exhibited a maximum luminance of 1719 cd.m(-2) with a turn-on voltage of 7.4 V, along with maximum current and power efficiencies of 1.66 cd.A(-1) and 0.56 Im.W-1, respectively. The electroluminescence mechanism of the OLED is discussed based on the energy level diagrams of the functional layers.

Effective permittivity of nanocomposites from 3D charge transport simulations

ABSTRACTThe effective permittivity and stored energy in nanocomposites incorporating dielectric and conducting nanofillers are computed by simulating bipolar charge injection, transport, attachment, and recombination through amorphous polymer using a self‐consistent 3D particle‐in‐cell model with nanofillers treated as extensions to the classical electrical double layer. Effective permittivities computed using an energy conserving scheme is shown to have excellent agreement with the Lichtenecker, Bruggeman, and Maxwell‐Garnett mixing rules especially at low volume fraction, low permittivity contrast, and small Clausius‐Mossotti factor, and lie well within the Wiener bounds. The energy conserving scheme with Maxwell‐Garnett E field interpolation combines the best of the Maxwell‐Garnett and fundamental Lichtenecker rules and results in broad validity over the entire volume fraction range. Computed stored energies show monotonic increase with dielectric fillers and a peak at 25 vol % for conducting fillers, attributed to the competing effects of higher energy with increasing E field modification and lower energy with decreasing binder volume. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43300.

Mechanism of space charge formation in cross linked polyethylene (XLPE) under temperature gradient

A bipolar charge transport model is used to simulate the formation of space charge in XLPE plaques placed under a temperature gradient. The model is used to assess the relative importance of charge migration and blocked extraction for three possible effects of the temperature gradient. The simulations were carried out for samples with different thicknesses and the results verified by comparison with experiment. In this way it has been shown that not only does the temperature gradient influence space charge accumulation through its introduction of a conductivity gradient, but also through differences in the injecting and extracting processes at electrodes of different temperature.

A new theoretical formulation of coupling thermo-electric breakdown in LDPE film under dc high applied fields

In this paper, we present a new theoretical and numerical formulation for the electrical and thermal breakdown phenomena, induced by charge packet dynamics, in low-density polyethylene (LDPE) insulating film under dc high applied field. The theoretical physical formulation is composed by the equations of bipolar charge transport as well as by the thermo-electric coupled equation associated for the first time in modeling to the bipolar transport problem. This coupled equation is resolved by the finite-element numerical model. For the first time, all bipolar transport results are obtained under non-uniform temperature distributions in the sample bulk. The principal original results show the occurring of very sudden abrupt increase in local temperature associated to a very sharp increase in external and conduction current densities appearing during the steady state. The coupling between these electrical and thermal instabilities reflects physically the local coupling between electrical conduction and thermal joule effect. The results of non-uniform temperature distributions induced by non-uniform electrical conduction current are also presented for several times. According to our formulation, the strong injection current is the principal factor of the electrical and thermal breakdown of polymer insulating material. This result is shown in this work. Our formulation is also validated experimentally.

Modeling effect of nanofillers on charge transport in composite polymer films for energy storage

The effects of dielectric and conducting nanofiller size, loading, and electric field on bipolar charge injection, transport, and recombination (or electroluminescence) through amorphous polymer are studied. Versions of 3D particle-in-cell model based on the classical electrical double layer representation are used to treat the conducting and dielectric nanoparticles. Metal–polymer charge injection assumes Schottky emission and Fowler–Nordheim tunneling, migration through field-dependent Poole–Frenkel mobility, and recombination with Monte Carlo selection based on collision probability. A boundary integral equation method is used for solution of the Poisson equation coupled with a second-order predictor–corrector scheme for robust time integration of the equations of motion. The stability criterion of the explicit algorithm conforms to the Courant–Friedrichs–Levy limit. Trajectories for charge that traverse the film are curvilinear paths that meander through the interspaces of the interaction zone. Compared to dielectric nanofillers, composite films with conducting nanofillers have: larger peaks and higher steady-state amplitudes for both injected currents and electrode E fields; <1/6 of the attached charge fraction; >2x of the conduction charge fraction; >2x of the recombined charge fraction; all charge fractions change very rapidly at sizes below 10nm; much higher loading of 21vol.% versus 2vol.%; and higher leakage conductivities of 0.5×10−14S/cm and 0.75×10−14S/cm with bipolar and unipolar charge, respectively, where the value for the dielectric nanofiller is ∼0.2×10−14S/cm. Effective permittivities computed using a new energy conservation method is shown to have excellent agreement compared with established Lichtenecker, Bruggeman, and Maxwell-Garnett mixing rules. Computed stored energies show monotonic increase with dielectric fillers and a peak at 25vol.% for conducting fillers which may be attributed to the competing effects of higher energy with increasing field modification and lower energy with decreasing binder volume.

Phenylcarbazole/diphenylphosphine oxide-based alcohol soluble host materials for efficient solution-processed multilayer blue electrophosphorescent OLEDs

Two novel phenylcarbazole/diphenylphosphine oxide derived, alcohol soluble host materials for solution-processed, blue phosphorescent organic light-emitting devices were designed and synthesized. The thermal stability, photophysical and electrochemical properties of these host materials have been investigated in detail. These hosts both exhibited high triplet energy levels, which ensure efficient energy transfer from hosts to the triplet emitter iridium(III) bis(4,6-difluorophenylpyridinato)picolinate. The new host compounds also showed good film morphology and bipolar charge transport properties. Multilayer solution-processed blue electrophosphorescent organic light-emitting diodes using the new host compounds with the triplet emitter iridium(III) bis(4,6-difluorophenylpyridinato)picolinate exhibited high luminance efficiency of 27.8 cd A−1, which was outstanding with respect to other work reported for solution-processed blue PhOLEDs.

Modeling effect of ferroelectric nanofiller size on bipolar charge transport in nanocomposite film

Bipolar charge injection and field-dependent mobility transport through nanocomposite film comprised of ferroelectric ceramic nanofillers in an amorphous polymer matrix is simulated using a 3D particle-in-cell model which extends the classical electrical double layer by substitution of a dipolar core for the nanofiller. The stability criterion of the explicit algorithm conforms to the Courant–Friedrichs–Levy limit. Simulation results for BaTiO3 nanofiller in amorphous polymer matrix indicate that antiparallel polarization results in the highest leakage conduction and lowest level of charge trapping in the interaction zone. Theoretical considerations validated simulation prediction in identifying a size range of 80 to 100 nm to minimize attachment and maximize conduction. The largest difference is in attached charge in the antiparallel case where fractions go from 2.2 to 97% as nanofiller size is decreased from 150 to 60 nm. Computed conductivity of 0.4 × 10−14 S/cm is in agreement with published data for PVDF. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1380–1390