Transient Electroluminescence Research Articles

Markedly Different Rise and Fall Times in Transient Electroluminescence of 4,4'-N,N'-Dicarbazolylbiphenyl:fac-Tris(2-phenylpyridinate) Iridium(III)-Based Organic Light Emitting Diodes

The transient electroluminescence of organic light-emitting diodes (OLEDs) with 4,4'-N,N'-dicarbazolylbiphenyl (CBP): fac-tris(2-phenylpyridinate) iridium(III) [Ir(ppy)3] as an emission layer (EML) was investigated. The rise time of these OLEDs was nearly two orders of magnitude longer than the fall time. The highest occupied molecular orbital levels of Ir(ppy)3, lying above that of CBP, act as deep traps for holes, which creates a situation where the hole mobility is much lower than the electron mobility in EML. Semilog plots of the rise time versus voltage applied to OLEDs were almost linear, which could be explained using the dynamic equation of holes and the observed relationship between current and voltage. When the forward bias was applied before the main voltage, the rise time decreased until the bias voltage increased to a certain level. The accumulated charge at the bias onset was consistent with the calculation.

Inversion of magnetic field effects on electrical current and electroluminescence in tri-(8-hydroxyquinoline)-aluminum based light-emitting diodes

The magnetic field effects on electrical current and electroluminescence have been investigated for tri-(8-hydroxyquinoline)-aluminum based organic light emitting diodes through the transient electroluminescence method. By applying rectangular pulse to drive the devices, the influence of the degradation is dramatically decreased; and a full-range evolvement for the magnetoconductance (MC) and magnetoelectroluminescence (MEL) with the driving voltage has been obtained. The transition voltages for MC (1.6 V) from the negative to positive value and MEL (7 V) from the positive to negative value are different, which suggests that the magnetic field effect on the MC and MEL may have different mechanisms.

Study on the Transient Electroluminescence of Phosphorescent OLEDs

We have studied the turn-off dynamics of phosphorescent organic light-emitting diodes (OLEDs) by investigating the temperature, pulse width, and voltage dependences of the radiative triplet lifetime with the dispersive parameter γ, calculated from the second luminescence of the transient electro-luminescence. This behavior is caused by the redistribution and recombination of electrons and holes accumulated near the interfaces. The lifetime increases with increasing carrier density and with decreasing temperature. This is due to the decrease in the hopping and the triplet-triplet annihilation rates. The experimental data satisfy the relationship between the dispersive parameter γ and temperature T, γ − 1 ∝ exp(−A/T ), with a constant A.

Transient electroluminescence and anomalous dispersion of charge carriers in thin polymer films

The electroluminescence and charge carrier transport in an organic light-emitting diode based on a thin polymer film are investigated. The initial kinetics of transient electroluminescence is analyzed theoretically. Both the theoretical and experimental curves are characterized by an anomalous dispersion and universality, which can be considered a consequence of the nonequilibrium field-assisted dispersion of charge carriers. It is demonstrated that the drift mobility of charge carriers should be determined from the time corresponding to the half-height of the steady-state electroluminescence intensity rather than from the delay time of the electroluminescence.

Transient Electroluminescence of White Organic Light-Emitting Diodes with Blue Phosphorescent and Red Fluorescent Emissive Layers

The transient electroluminescence (EL) of white organic light-emitting diodes (OLEDs) with blue phosphorescent and red fluorescent emissive layers was investigated by applying voltage pulses on the devices. The transient properties of a blue phosphorescence EL signal are strongly affected by the phosphorescence recombination lifetime. As a result, the transient properties of a white OLED were almost identical to those of a phosphorescent emissive layer. Utilizing the differences in transient characteristics between the fluorescent materials and Ir complexes, the emission color has been successfully tuned by varying applied pulse width and frequency.

Degradation Analysis of Blue Phosphorescent Organic Light Emitting Diode by Impedance Spectroscopy and Transient Electroluminescence Spectroscopy

SUMMARY We carried out degradation analysis of a blue phosphorescent organic light emitting diode by both impedance spectroscopy and transient electroluminescence (EL) spectroscopy. The number of semicircles observed in the Cole-Cole plot of the modulus became three to two after the device was operated for 567 hours. Considering the effective layer thickness of the initial and degraded devices did not change by degradation and combining the analysis of the Bode-plot of the imaginary part of the modulus, the relaxation times of emission layer and hole-blocking with electron transport layers changed to nearly the same value by the increase of the resistance of emission layer. Decay time of transient EL of the initial device was coincident with that of the degraded one. These phenomena suggest that no phosphorescence quenching sites are generated in the degraded device, but the number of the emission sites decrease by degradation.

Initial Rise of Transient Electroluminescence in Organic Films

Theoretical analysis of transient electroluminescence in thin organic films controlled by non-equilibrium charge transport is carried out including space-charge effects. Results are compared with experimental data for a 100-nm-thick film of a phenylenevinylene co-polymer. Normalized transients are highly dispersive and universal at the initial time interval, which is considered as a manifestation of the non-equilibrium field-assisted dispersion of holes. Transit time is the half-rise time rather than the delay time.

The role of poly(3,4-ethylenedioxythiophene):poly(styrenesulphonate) as a hole injection layer in a blue-emitting polymer light-emitting diode

The authors study the role of the conducting polymer poly(3,4-ethylenedioxythiophene): poly(styrenesulphonate) (PEDOT:PSS) in determining the transient and steady-state operation of a blue-emitting polymer light-emitting diode. Combining the results from photoemission spectroscopy, time-of-flight photocurrent measurements, and studies on hole-only devices reveals a significant barrier for the injection of holes into the polymer. Simulations with a numerical drift-diffusion model, however, show that the injection currents determined from single-carrier devices cannot account for the rapid transient luminance onset and the efficient steady-state luminance output of the corresponding bipolar light-emitting devices. It is shown that the transient electroluminescence traces measured at different external bias can be well reproduced when assuming the presence of a weak barrier for electron extraction at this interface, which is attributed to electron accumulation at a thin phase-segregated PSS-rich layer at the surface of PEDOT:PSS. In addition, interface conditioning, presumably due to electron trapping near PEDOT:PSS, renders the anode-polymer interface nearly Ohmic. This conditioning, however, occurs on intermediate time scales normally not addressed by either transient or steady-state measurements.

Spin-Assembled Nanolayer of a Hyperbranched Polymer on the Anode in Organic Light-Emitting Diodes: The Mechanism of Hole Injection and Electron Blocking

We introduced a spin-assembled nanolayer of hyperbranched poly(ether sulfone) with sulfonic acid terminal on top of an indium-tin oxide anode in organic light-emitting diodes. This results in great improvement in luminous efficiency, better than that of devices using a commercially available conducting polymer composition as a hole-injection layer. The effect of the nanolayer was investigated by impedance spectroscopy, photovoltaic measurement for built-in-potential, and transient electroluminescence. We concluded that the high luminous efficiency resulted from the efficient electron-blocking by the nanolayer and hole-injection assisted by the accumulation of electrons at the interface. This result implies that, for an efficient hole-injection layer, the electron-blocking capability should be incorporated in addition to the hole-injection and -transport capability.

Transient Electroluminescence in Organic Light-Emitting Diode with Optical Microcavity Structure

A comparison of the transient electroluminescence (EL) between a microcavity organic light-emitting diode (MC-OLED) and a conventional OLED was performed and their carrier dynamics were discussed. We found that the behavior of transient EL in a MC-OLED can be accurately described using rate equations without exciton-carrier interaction for a conventional OLED, suggesting that carrier dynamics is not modified using a MC structure.

Kinetics of transient electroluminescence in organic light emitting diodes

Mathematical simulation on the rise and decay kinetics of transient electroluminescence (EL) in organic light emitting diodes (OLEDs) is presented. The transient EL is studied with respect to a step voltage pulse. While rising, for lower values of time, the EL intensity shows a quadratic dependence on (t − tdel), where tdel is the time delay observed in the onset of EL, and finally attains saturation at a sufficiently large time. When the applied voltage is switched off, the initial EL decay shows an exponential dependence on (t − tdec), where tdec is the time when the voltage is switched off. The simulated results are compared with the transient EL performance of a bilayer OLED based on small molecular bis(2-methyl 8-hydroxyquinoline)(triphenyl siloxy) aluminium (SAlq). Transient EL studies have been carried out at different voltage pulse amplitudes. The simulated results show good agreement with experimental data. Using these simulated results the lifetime of the excitons in SAlq has also been calculated.

Observation of Carrier Behavior in Organic Field-Effect Transistors with Electroluminescence under AC Electric Field

The carrier behavior in a tetracene thin film under AC electric field was examined by electroluminescence (EL) using the structure of organic field-effect transistors (OFETs). A square wave voltage was applied to the source electrode, whereas the drain and gate electrodes were connected to the ground. Electrons and holes were injected from the Au source electrode alternately, and they recombined at the interface between the tetracene thin film and the source electrode. A series of EL pulses was observed in accordance with the applied AC voltage, i.e., a transient EL was observed in a half cycle of AC. This result indicates that alternating carrier injection causes EL. Furthermore, EL intensity increased with the frequency and amplitude of AC voltage. It was suggested that the amount of electrons and holes injected from the source electrode in a unit time is controlled by the frequency and amplitude of AC voltage.

Mixed ligands 8-hydroxyquinoline aluminum complex with high electron mobility for organic light-emitting diodes

A mixed ligands 8-hydroxyquinoline complex (Alq2A) with improved electron mobility was designed for organic light-emitting diodes. The electron mobility in Alq2A was determined via transient electroluminescence (EL) from bilayer devices with structure of indium tin oxide/4,4′-bis[N-(1-naphthyl)-N-Phenyl amino]biphenyl/Alq2A∕LiF∕Al. It was found that the electron mobility in Alq2A is between (2.7and4.4)×10−6cm2∕Vs at electric fields ranging between 1.42×106 and 2.40×106V∕cm, which is higher than that in Alq3. The Alq2A also shows a higher EL efficiency in steady-state EL studies, which is considered to be derived from (1) improved electron mobility, (2) high fluorescene efficiency, and (3) good film-forming.

Electron mobility and electroluminescence efficiency of blue conjugated polymers

We study systematically the underlying property which determines the electroluminescence efficiency of blue polymers based on polyfluorene and its copolymers. The photoluminescence efficiency is high (40–50%) for all the selected polymers, but the electroluminescence efficiency varies in a wide range. The hole mobility is measured by transient electroluminescence. The electron mobility is obtained from the current in electron-only device using space-charge-limited current equation. All polymers share roughly the same results, whereas electron mobility is the critical factor for electroluminescence. In general, low efficiency results from the low electron mobility.

Effects of interfacial stability between electron transporting layer and cathode on the degradation process of organic light-emitting diodes

The authors have demonstrated that the increase of electron injection barrier height between tris(8-hydroxyquinoline)aluminum (Alq3) and LiF∕Al cathode is one of the most critical parameters to determine the reliability of organic light-emitting diode with the typical structure of indium tin oxide/N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl) benzidine/Alq3∕LiF∕Al. The electrical properties of several devices (hole only, electron only, and integrated double-layered devices) have been measured in the function of operating time to analyze the bulk and interface property changes. Bulk properties of trap energy and mobility in an organic layer have been estimated by using trap-charge-limited currents and transient electroluminescence measurements.

Study of electron mobility in small molecular SAlq by transient electroluminescence method

The study of electron mobility of bis(2-methyl 8-hydroxyquinoline) (triphenyl siloxy) aluminium (SAlq) by transient electroluminescence (EL) is presented. An EL device is fabricated in bilayer, ITO/N,N′-diphenyl-N, N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD)/SAlq/LiF/Al configuration. The temporal evaluation of the EL with respect to the step voltage pulse is characterized by a delay time followed by a fast initial rise, which is followed by a slower rise. The delay time between the applied electrical pulse and the onset of EL is correlated with the carrier mobility (electron in our case). Transient EL studies for SAlq have been carried out at different temperatures and different applied electric fields. The electron mobility in SAlq is found to be field and temperature dependent and calculated to be 6.9 × 10−7 cm2 V−1 s−1 at 2.5 × 106 V cm−1 and 308 K. The EL decays immediately as the voltage is turned off and does not depend on the amplitude of the applied voltage pulse or dc offset.

Nonequilibrium transport of charge carriers and transient electroluminescence in organic light-emitting diodes

An analytic theory of nonequilibrium hopping charge transport in disordered organic materials includes quasiequilibrium (normal) and extremely nonequilibrium (dispersive) regimes as limiting cases at long and short times, respectively. In the intermediate interval of time quasiequilibrium value of mobility is nearly established while the coefficient of field-assisted diffusion continues to increase (quasidispersive regime). Therefore, normalized time dependencies of transient current in time-of-flight (TOF) conditions are practically independent of field strength and sample thickness, in good agreement both with data of TOF experiments for molecularly doped polymers and results of numerical simulations of Gaussian disorder model. An analytic model of transient electroluminescence (TEL) is developed on the base of the mentioned theory. Strong asymmetry of mobilities is presumed. In analogy with TOF transients, dispersion parameter of normalized TEL intensity is anomalously large and almost field independent in the quasidispersive regime of transport. The method for determination of mobility from TEL data is proposed.

Sensing electron transport in a blue-emitting copolymer by transient electroluminescence

A variation of the transient electroluminescence technique is introduced which allows us to selectively study the electron transport in a thin polymer layer. It relies on the formation of an insoluble interlayer from a formerly solvable polymer and enables probing of unipolar electron transport despite of injection barriers. It opens up possibilities to gain insight into the operation of light-emitting diodes. Applicability to a blue-emitting spirobifluorene-based copolymer is shown by comparison to time-of-flight results for electron and hole transport and evidence supplied for an intermixing of electron and hole dynamics through blocking of electrons at the polymer/anode interface.

Nanostructure‐Dependent Vertical Charge Transport in MEH‐PPV Films

AbstractThe correlation between morphology and charge‐carrier mobility in the vertical direction in thin films of poly(2‐methoxy‐5‐(2′‐ethylhexyloxy)‐1,4‐phenylenevinylene) (MEH‐PPV) is investigated by a combination of X‐ray reflectivity (XRR), field‐emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), fluorescence optical microscopy (FOM), photoluminescence spectroscopy (PL), photoluminescence excitation spectroscopy (PLE), as well as time‐of‐flight (TOF) and transient electroluminescence (TrEL) techniques. The mobility is about two orders of magnitude greater for drop‐cast films than for their spin‐cast counterparts. Drop‐casting in the presence of a vertical static electric field (E‐casting) results in films with an additional increase in mobility of about one order of magnitude. While PL and PLE spectra vary with the method of film preparation, there is no correlation between emission spectra and charge‐carrier mobility. Our XRR measurements on spin‐cast films indicate layering along the film depth while no such structure is found in drop‐cast or E‐cast films, whereas FESEM examination indicates that nanodomains within drop‐cast films are eliminated in the E‐cast case. These observations indicate that carrier transport is influenced by structure on two different length scales. The low mobility observed in spin‐cast films is a direct result of a global layered structure with characteristic thickness of ca. 4 nm: in the absence of this layered structure, drop‐cast films with inherent nanoscale heterogeneities (ca. 20 nm in size) exhibit much better hole mobility. Elimination of nanodomains via electric‐field alignment results in further improved charge mobility.

The Electroluminescent Decay Mechanism of Rare-Earth Ions in OLEDs Based on a Terbium Complex

The organic light-emitting diodes (OLEDs) based on terbium (Tb) complexes show sharp green emission spectrum with excellent color purity. However, the brightness of Tb-OLEDs is generally weak. Here, the electroluminescent (EL) decay mechanism of TB-OLEDs is studied by arbitrarily using tris-(1-phenyl-3-methyl-4-isobutyryl-5-pyrozolone)-bis(triphenyl phosphine oxide) terbium as the emitting layer. The device shows high EL efficiency at low current density but rapid reduction of device efficiency at higher current density. The transient EL is investigated for understanding the decay process of excited Tb3+ ions. Together with theoretical studies, exciton quenching is proposed to explain the decay of the Tb-OLEDs which is important for optimizing and engineering the material and device structures. The EL from the mixed layer of the Tb and europium (Eu) complexes is also studied. We find that the EL performance and transient decay of the excited Tb ions are modified by energy transfer from Tb to Eu in the OLEDs.