V. Fattori

Unusual disparity in electroluminescence and photoluminescence spectra of vacuum-evaporated films of 1,1-bis ((di-4-tolylamino) phenyl) cyclohexane

The electroluminescence (EL) and photoluminescence (PL) spectra of thin films of 1,1-bis((di-4-tolyloamino)phenyl) cyclohexane (TAPC) appear to be completely different. Whereas the broad PL spectra reveal major maxima at ≅370 and 450 nm, a strong regular band at 580 nm is characteristic of the EL spectra. It is shown that in contrast to PL, which is composed of molecular exciton (monomer) and excimer emission, EL can be explained by the direct cross recombination transition between electrons and holes trapped on tritolylamine (TTA) subunits of different TAPC molecules. Such a pair of trapped carriers, formed selectively under recombination of statistically independent (here injected at opposite electrodes) electrons and holes, is considered as a particular excited state (“electromer”) responsible also for the EL emission of some other compounds containing TTA or triphenylamine moieties, as for example an amino-substituted triphenylbenzene.

Single-dopant organic white electrophosphorescent diodes with very high efficiency and its reduced current density roll-off

Very high-efficiency organic white light electrophosphorescent diodes (WLEDs) have been fabricated using an efficient N∧C∧N-coordinated platinum (II) complex phosphor dopant. Their white light emanation is underlain by the simultaneous emission of monomer in blue and excimer in red. By optimizing the phosphor concentration and confining the electron-hole recombination zone to the emitter layer in the devices, the authors achieve their unusually high forward viewing external quantum efficiency (QE) up to 15.5±0.2% and 13.0±0.2% photons/electron at low and high drive current densities, corresponding to 40 and 1300cd∕m2, respectively. The current density where QE drops by half of its peak value is greater than three times that of the highest efficiency single-dopant WLEDs reported hitherto. The performance parameters of the presented devices can be further improved by using efficient dopants with the emission spectrum shifted towards blue.

Efficient exciplex emitting organic electroluminescent devices

Organic electroluminescent (EL) single-layer (SL) and double-layer (DL) light-emitting diodes were fabricated based on 4,4′,4″-tris[3-methylphenyl(phenyl)amino] triphenylamine (m-MTDATA) and 2-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) molecules incorporated in bisphenol-A-polycarbonate (PC) matrix. The external EL quantum efficiency of the dominating exciplex emission of the indium tin oxide (ITO)/m-MTDATA:PBD:PC/Ca SL structure is ≅0.4% photon/electron and that from the ITO/m-MTDATA:PC/PBD/Ca DL device exceeds 1% photon/electron. The operating voltage of the DL device at maximum efficiency is 4.8 V and the onset voltage of the EL is as low as 2.0 V. The photoluminesce quantum efficiency of the exciplex is greater than 17%. The high exciplex emission-based EL yield from the DL device is attributed to the narrow recombination zone imposed by the high energy barriers for the electron and hole transfer at the (m-MTDATA:PC)/PBD interface.

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.

Multicomponent emission from organic light emitting diodes based on polymer dispersion of an aromatic diamine and an oxadiazole derivative

Abstract The electroluminescence (EL) spectra of single layer (SL) and double layer (DL) light emitting diodes (LEDs) based on N , N ′-diphenyl- N , N ′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′ diamine (TPD) and 2-(4-biphenyl)-5-(4- tert. -butylphenyl)1,3,4-oxadiazole (PBD) molecules incorporated in bisphenol-A–polycarbonate (PC) matrix were investigated. The EL spectra of both SL and DL ((TPD+PC)/PBD) LEDs reveal several components up to four emission bands for DL structure devices. Only the blue band can be ascribed to emission of one of the dopants (TPD), the origin of other red-shifted bands show characteristics suggesting formation of excited hetero-molecular aggregates such as exciplexes and electroplexes including their modification by local energetic conditions.

Highly efficient near-infrared organic excimer electrophosphorescent diodes

The authors report the fabrication of very high efficiency near-infrared (NIR) organic light-emitting diodes (LEDs) based on a series of terdentate cyclometallated phosphorescent Pt(II) complexes PtLCl as the emitting layer. The LEDs exhibit exclusive NIR excimeric phosphorescence peaking between 705 and 720nm for three different organic ligands (L). Due to the high excimer emission quantum yields of these Pt complexes and to confinement of the recombination zone within the emission layer, unusually high external quantum efficiencies from 9.8% to 10.7% photons/electron and a high forward light output exceeding 15mW∕cm2 were achieved.

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.

Voltage‐tunable‐color multilayer organic light emitting diode

Organic electroluminescent diodes with a double‐heterostructure of indium‐tin‐oxide (ITO) substrate/aromatic diamine (TPD)/8‐hydroxyquinoline aluminum (Alq3)/perylene bisimide pigment (PBP)/Mg/Ag have been fabricated by successive vapor deposition. Their electroluminescence (EL) for thin layers of PBP varies from the orange dominated by the red emission of PBP to greenish‐yellow with a strong contribution of the green emission of Alq3 as voltage increases. The relative contribution of the red emission increases with increasing voltage for the structures with thick PBP layers. The tuning of the EL color is explained in terms of electric field‐induced quenching of excited states and voltage evolution of the recombination zone in PBP.

Color-variable highly efficient organic electrophosphorescent diodes manipulating molecular exciton and excimer emissions

A simple way of tuning the emission color and electroluminescence efficiency from vacuum-deposited emitters of phosphorescent organic light emitting diodes (LEDs) is demonstrated. For each color a single-emissive layer consisting of a blend of two materials, one of Pt(N∧C∧N) complex series [where (N∧C∧N)=di(2-pyridinyl)benzene-based tridentate ligands] as either the low-concentration bluish green (molecular) phosphorescence emitter or high-concentration red (excimer) phosphorescence emitter, and (4,4′,4″-tris(N-carbazolyl-triphenylamine) as the host was employed. By adjusting the relative amount of blue and red emissive species, the color of the light emission was tuned from bluish green through green and white up to red. Very high external quantum efficiency (up to 18.3±0.5%) and current efficiency (up to 44.8±0.5 cd/A) at ∼500 cd/m2 four-layer devices were achieved with white and greenish light emitting layers, respectively. It is found that the introduction of electron-withdrawing fluorine atoms at the...

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.