W. Stampor

Impact of high electric fields on the charge recombination process in organic light-emitting diodes

A study of electric field dependent spectra and the quantum electroluminescence yield of single-layer (SL) and double-layer (DL) light-emitting diodes (LEDs) based on a combination of an aromatic diamine (TPD) and an oxadiazole derivative (PBD) was carried out. It was shown that a high electric field weakens the recombination of Coulombically correlated electron-hole pairs in the bulk (SL LEDs) and enhances the process at the TPD-doped polymer/PBD interface (DL LEDs). The effects are discussed in terms of electric field mediated interplay between populations of localized (monomolecular) excitons, exciplexes and electroplexes in conjuction with their local environments.

Electromodulation of Luminescence in Organic Photoconductors

The modulating electric field effect on the luminescence (EFEL) of a series of organic photoconductors is observed and results discussed in terms of various theoretical models. A physical interpretation of EFEL is proposed on the basis of the Onsager and macrotrap models. It is found that EFEL is, in major part, due to the field-induced change in the hopping rate of optically excited charge carriers between localized sites (microtraps) forming an extended defect (macrotrap) for molecular solids with excited states characteristic of neutral Frenkel excitons, whereas the field modification of the carrier diffusion in the separation process (Onsager model) seems to be of crucial importance for the photoconductors revealing low-energy charge-transfer excitons formed upon optical excitation.

Electric field effect on luminescence efficiency in 8-hydroxyquinoline aluminum (Alq3) thin films

Electric field-induced luminescence quenching in thin films made from common organic electroluminescent material of aluminum (III) 8-hydroxyquinoline (Alq3) is reported. The dependence of luminescence quenching on excitation wavelength and electric field is attributed to field-assisted hopping separation of charge in localized excited states. The effect extrapolated to high electric fields can reduce the luminescence yield by as much as 60% limiting electroluminescence quantum efficiency in high-field-driven light emitting diodes based on the Alq3 emitter.

Electroabsorption study of excited states in tris 8-hydroxyquinoline aluminum complex

Electric field-modulated absorption spectra, from 280–490 nm, of vacuum-evaporated films of tris (8-hydroxyquinoline) aluminum (Alq3) have been studied to determine the polarity of the electronic excited states. Semiempirical molecular modelling has been employed to relate the energetics and molecular electrical dipole moments of the isolated molecule to those resulting from the electroabsorption experiment, interpreted in terms of the second-order Stark effect. The results indicate the lowest excited singlets with relatively small dipole moments to be well localized on individual quinolate ligands, and those with higher energy and large dipole moments to show delocalization by charge transfer to the nearest-neighbour ligands of different molecules. The implications for the electroluminescent behaviour of Alq3 are discussed.

Electromodulation of fluorescence in a crystalline organic photoconductor (thionaphthenindole)

The influence of electric fields on the fluorescence of polycrystalline layers of thionaphthenindole has been studied by an electromodulation technique that allows internal electric fields to be accounted for the effect. The in‐phase first harmonic (1ω) and out‐of‐phase second harmonic (2ω) of the fluorescence response to a modulating (sinusoidal) electric field of frequency ω reveal different electric‐field behavior, the 2ω response providing nearly internal field‐free signal suitable for verification of theoretical models. Fluorescence quenching with this signal has been observed and attributed to electric‐field modulation of the probability of charge separation within an excited state as a precursor. It was found that fluorescence quenching data could not be well explained using theoretical models formulated by Onsager, Poole–Frenkel, and Noolandi–Hong–Popovic for the charge separation via delocalized charge‐transfer excitons. The macrotrap model which reconciles both the experimental data and their ph...

Electric field-assisted dissociation of singlet excitons in tris-(8-hydroxyquinolinato) aluminum (III)

Results based on the electric-field dependence of electro-modulated fluorescence and steady-state photoconduction in films of tris-(8-hydroxyquinolinato) aluminum (III) (Alq3) are consistent with a model where singlet excitons dissociate into electron–hole pairs through charge transfer state. Dissociation into free carriers or predominantly geminate recombination results from this intermediate state. The quantum efficiency of charge separation is in agreement with predictions based on a model using the three-dimensional Onsager theory of geminate recombination combined with volume (bimolecular) recombination of the photogenerated space charge. This model is used to account for the decrease in the electroluminescence quantum efficiency of Alq3-based organic light-emitting diodes at high electric fields.

Electroabsorption study of excited states in hydrogen-bonding solids: epindolidione and linear trans-quinacridone

Abstract Electroabsorption (EA) spectra within the first absorption band of vacuum-evaporated films of epindolidione (EPI) and linear trans-quinacridone (QAC) have been studied in search of the hydrogen-bonding effect on electronic excited states. Whereas the EPI EA spectra are determined by well-localized molecular excitons, the EA spectra of QAC can be rationalized only by the involvement of the low-energy charge-transfer (CT) excitons. The lowest CT state is found to appear within the first dominant absorption transition at 2.22 ± 0.02 eV. Near resonance between an intramolecular (Frenkel type) excitation and nearest-neighbour CT states is suggested to underlie the strong absorption and electroabsorption features in this spectral range. The EA effect is ascribed to the electric-field-induced mixing of the hybridized Frenkel/symmetric CT excitons with anti-symmetric CT states. Intermolecular hydrogen bonding of the NH⋯O type is suggested to play an important role in creation of the CT states in QAC.

Electromodulation of fluorescence in hole-transporting materials (TPD, TAPC) for organic light-emitting diodes

Abstract Electric-field-modulated fluorescence (EMF) was measured in films of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) and 1,1′-bis(di-4-tolylaminophenyl) cyclohexane (TAPC), the diamine compounds commonly used as hole-transporting materials in organic light-emitting diodes. External electric fields of 2×106 V/cm reduce the integral emission intensity from about 0.3% down to 10% dependent on the excitation photon energy and spectral emission range. The analysis of the experimental data provides evidence that the EMF in TPD is due to the field imposed changes of the dissociation rate of localized molecular excited states. The observed difference in the EMF for the short-wavelength (monomolecular) and long-wavelength (excimer) emission bands of TAPC can be ascribed to the different field response of these two types of emitting species. All these results are consistent with a kinetic scheme that includes formation of molecular excited states, their localization and relaxation into charge pairs within extended trapping domains, and formation of excimer states at a field-dependent rate mediated by the field-assisted dissociation of the charge pairs. The 3D-Onsager theory of dissociation satisfactorily explains the electric field and excitation wavelength dependence of EMF in TPD and that for the excimer emission of TAPC. The quadratic Stark effect on fluorescence quantum yield must have been invoked to understand the EMF characteristics of monomolecular emission of TAPC.

Injection-controlled and volume-controlled electroluminescence in organic light-emitting diodes

Abstract Two types of electroluminescence (EL), injection-controlled (IC) with τ rec >τ, and volume-controlled (VC) with τ rec T , in organic light-emitting diodes (LEDs) are discussed using the notions of recombination (τ rec ) and transit (τ T ) times of charge carriers. The results of experimental and theoretical studies presented show that the EL intensity (Φ EL ) is, in general, a nonlinear function of the current density ( j ) in an EL diode. Often it can be approximated by a power-type function Φ EL α j n , with the power, n , dependent on LED structure, the nature of light-emitting material, injection and transport mechanisms of the charge carriers. In the LEDs with carrier mobilities independent of electric field 1 ≤ n ≤ 2 for both ICEL and VCEL operation modes. In the ICEL, n = 1 + ω 1 /ω 2 , where ω 1 and ω 2 are quantities characterizing various charge injection mechanisms into the EL sample. Field-dependent mobility and charge carrier trapping lead Φ EL to be a sublinear or supralinear function of j . Selected examples of experimental Φ EL ( j ) relationships on single-layer ITO/Alq 3 /Mg/Ag (8-hydroxyquinoline aluminium complex, Alq 3 ), double-layer ITO/TPD/Alq 3 /Mg/Ag (aromatic diamine, TPD), ITO/QAC/Alq 3 /Mg/Ag (quinacridone, QAC), and triple-layer ITO/QAC/Alq 3 /PBP/Mg/Ag (perylene bisimide pigment, PBP) are presented briefly to illustrate various types of operation modes of organic LEDs.

High-electric-field quantum yield roll-off in efficient europium chelates-based light-emitting diodes

The europium chelates-based light-emitting-diodes (LEDs) have been fabricated showing the maximum electroluminescence (EL) quantum efficiency (QE) up to 5%photons∕carrier at electric fields slightly below F=1MV∕cm and current density j≅0.01mA∕cm2. Their line-like emission QE drops, however, rapidly within the high drive voltage range. This roll-off effect, exceeding one order of magnitude at F≅1.6MV∕cm(j≅15mA∕cm2), is shown to be underlain by the electric field-assisted dissociation of electron-hole pair precursors of europium ion-localized emissive states. The high field dependence of EL QE fits the Onsager model of geminate recombination well. This is at variance with triplet-triplet annihilation mechanism assumed previously to reduce QE in such LEDs.