G. Giro

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.

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.

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.

The nature of emitting states in electroluminescence of polymeric films doped with anthracene and anthracene-based supramolecules

Abstract Electroluminescent studies of anthracene (A) dispersed in a polycarbonate (PC) matrix show a long-wavelength emission band in addition to the broad band underlain by excimer states. Through studies of absorption, photoluminescence and electroluminescence of PC dispersions of A-based supramolecules we demonstrate that this band results from so-called “electromer” (EC) states constituting charge pair states with the inter-ion separation ranged between 0.7 and 2.4 nm, and formed from electron–hole pairs with more distant charges. Holes and electrons can be either injected from electrodes of the electroluminescent cells or produced by dissociation of optically excited states.

EXCITON DYNAMICS IN AN AROMATIC DIAMINE AT THE INTERFACE WITH 8-HYDROXYQUINOLINE ALUMINUM

Abstract Exciton dynamics parameters such as diffusion length, l d , and surface quenching velocity, ν, in an amorphous film of an aromatic diamine (TPD) have been measured utilizing both the quenching effect exerted on its fluorescence by a thin layer of 8-hydroxyquinoline aluminum (Alq 3 ) deposited on the film as well as the exciton detector layer radiative emission itself. It is found that l d = (30 ± 10) nm and ν = (2 ± 1) × 10 3 cm/s. Using independent data on the intrinsic lifetime in TPD yields the diffusion coefficient of excitons D = (5±1) × 10 −3 cm 2 /s. The results are discussed in connection with the operation mechanism of the double-layer electroluminescent structure based on the TPD/Alq 3 junction.

Photoconduction in solid films of C60

Abstract Photoconduction in solid films of C 60 has been investigated as a function of applied field, excitation wavelength and light intensity. Exposure of the films to ultraviolet and visible light (200–800 nm), and externally applied voltage produced photocurrent in the range 10 −11 –10 −6 A/ cm 2 ; the magnitude depended on the polarity of the illuminated electrode. This asymmetry is a function of electric field and light penetration depth. It is attributed to a combined effect of varying excitonic electron injection from the Al cathode and a difference in mobility of holes and electrons. Spectral features of the photocurrent vary from sample to sample, but a notable number of recurrent bands can be distinguished, in addition to principal features such as their antibatic behaviour with the absorption spectrum and higher photogeneration yield in the long-wavelength part of the spectra above 400 nm. This behaviour can be explained by a vast range of orbitally and spin forbidden electron transitions identified in high-resolution absorption spectra of C 60 . Correspondingly, a complex relationship between photocurrent and light intensity is observed.

Electrochemical polymerization of thionaphthene−indole

The polymer deriving from thionaphthene-indole has been prepared by electrochemical oxidation of the monomer in methylene chloride on platinum electrode. The oxidized polymer so obtained has been reduced on the same electrode to a neutral polymer, which is processable and soluble in several solvents. Both oxidized and neutral polymers have been characterized by UV, IR and mass spectrometry. The electrical conductivity of the oxidized material has been found to be nine orders of magnitude greater than that of the reduced product.

Absorption tail photoconductivity in solid films of C60

Abstract Steady-state photoconductivity measurements in solid C 60 films are reported. The observed action spectra are antibatic with the absorption spectrum and show well-resolved structure in the long-wavelength tail above 500 nm. They are analyzed in terms of the recombination limit model for the bulk-generated charge carriers. It is concluded that photocarrier production occurs by autoionization of localized intramolecular excited states.