I. Sokolik

Bimolecular reactions of singlet excitons in tris(8‐hydroxyquinoline) aluminum

We have studied singlet excited state dynamics in thin films of tris(8‐hydroxyquinoline) aluminum (Alq3), a well known emitter for organic electroluminescent devices. Our experimental results show that bimolecular recombination dominates the singlet exciton decay in pristine films at high intensities, thus decreasing the photoluminescence quantum efficiency and the singlet lifetime. Because of the relatively low carrier concentration at which light emitting diodes operate the electroluminescence efficiency is not affected. The measured rate constant of singlet–singlet annihilation in Alq3 films is γss=(3.5±2.5)×10−11 cm3 s−1. The diffusion coefficient and the diffusion length of singlet excitons estimated from γss are Ds=(1.2±0.8)×10−5 cm2 s−1 and L=98±40 A, respectively.

Photoluminescence and electroluminescence of new lanthanide-(methyoxybenzoyl)benzoate complexes

New luminescent thermally stable lanthanide-organic complexes, namely europium tris(4-methoxy benzoylbenzoate), Eu(MeOBB)3, and terbium tris(4-methoxy benzoylbenzoate), Tb(MeOBB)3, were synthesized and characterized. The UV absorption bands in these compounds are attributed to the ligand while the emission originates from the lanthanide ions’ electronic transitions. The quantum efficiency of photoluminescence, Φ, measured in 5×10−4 M methylene chloride solutions is relatively large in both compounds: Φ=0.27 for Tb(MeOBB)3 and Φ=0.16 for Eu(MeOBB)3. The narrow-band electroluminescence from these complexes is studied by fabricating and characterizing single- and multi-layer organic light emitting diodes. Energy transfer between the organic components of these devices is also discussed.

Synthesis and characterization of electroluminescent organo-lanthanide(iii) complexes

New luminescent thermally stable lanthanide-(benzoyl)benzoate complexes are synthesized and characterized. The photoluminescence in these complexes originates from the radiative transitions of the lanthanide ions which are excited via intersystem crossing from the initially excited ligands. The electroluminescent properties of the terbium-(benzoyl)benzoates were studied by fabricating and characterizing single layer light emitting diodes containing poly(N-vinylcarbazole) doped with the lanthanide complexes and butyl-PBD. The energy transfer between the organic components of the single layer devices is also discussed.

BLUE PHOTO- AND ELECTROLUMINESCENCE FROM POLY(BENZOYL-1,4-PHENYLENE)

Optical and electroluminescent properties of a new poly(p-phenylene)-type polymer, poly(benzoyl-1, 4-phenylene) (PBP) were studied. PBP is soluble in common organic solvents, has high thermal stability and shows bright blue photoluminescence. Light-emitting diodes fabricated with PBP as the active layer emit blue electroluminescence with a peak wavelength at 446 nm and a brightness of about 100 cd/m2.

Photo-, and electroluminescence studies of 2,5-bis[2'-(4-(6-hexoxy benzyl))-1'-ethenyl]-3,4-dibutyl thiophenes

Abstract We have synthesized a thiophene-based compound 2,5-bis[2′-(4″-(6-hexoxy benzyl))-1′-ethenyl]-3, 4-dibutyl thiophene (HBDT) and a copolymer poly(2,5-bis(2′-(4″-(6-hexoxy benzyl))-1′-ethenyl)-3,4-dibutyl thiophene-1,6-diisocyanatohexane) (HBDT-PU) consisting of alternating HBDT and urethane spacer units. Absorption and photoluminescence (PL) spectra of the compounds coincide indicating that emission in HBDT-PU occurs from the thiophene containing unit. PL is emitted in the blue–green region with a maximum at 460 nm. Concentration quenching occurs in pure materials: PL efficiency is strongly enhanced when the compounds are dispersed in a polymer matrix like poly( N -vinyl carbozole) (PVK) or poly(methyl metacrylate) (PMMA). Light emitting devices were fabricated using a PVK:2-(4-biphenylyl)-5-(4- tert -butylphenyl)-1,3,4-oxadiazole (PBD) blend doped with HBDT as the emitter material. Efficient energy transfer from PVK:PBD to HBDT molecules takes place in blended films. The electroluminescence (EL) spectra coincided with the PL spectra of HBDT indicating that EL emission comes solely from HBDT molecules. The influence of the doping concentration on the EL efficiency was found to coincide with the concentration dependence of the PL quantum yield.

Photooxidation effects on picosecond photoluminescence and photoconductivity in tris-(8-hydroxyquinoline) aluminum (Alq3).

Abstract The photogeneration of radiative excited states and charge carriers in pristine and photooxidized tris-(8-hydroxyquinoline) aluminum (Alq 3 ) was studied by measuring both its photoluminescence and photoconductive response simultaneously, with subnanosecond resolution. 25 psec laser pulses at 355nm were used for excitation of vacuum evaporated 2–4 μm thick films of Alq 3 . We observed that photooxidation of Alq 3 with oxygen and UV light substantially decreases the fluorescence lifetime and quantum yield as well as the photoconductive response. This decrease is shown to be a result of quenching of singlet molecular excitations by products of photooxidation of Alq 3 .

Dynamics of photoexcited states and charge carriers in organic thin films: Alq3

We demonstrate a method for observing and analyzing the dynamics of photoexcited molecular states and charge carriers in organic thin films, by measuring simultaneously the transient photoluminescence and photoconductivity. We show experimentally in tris‐(8‐hydroxyquinoline) aluminum (Alq3) that exciton–exciton annihilation dictates the behavior of the photoluminescence and photoconductivity at high excitation intensities. The experimental results are corroborated by calculations based on an exciton‐exciton annihilation model.

Photoluminescence and electroluminescence of a soluble PPP-type polymer

Abstract The optical properties of a new soluble PPP-type polymer, poly(benzoyl-1,4-phenylene) (PBP), were studied. PBP films show bright blue photoluminescence (PL) with a maximum emission at 433 nm. The PL quantum yield of PBP in dilute chloroform solution is approximately 15%. Electroluminescence (EL) was investigated using PBP as the active layer in single and multilayer LEDs. The EL of all fabricated devices lies in the blue region with a maximum brightness of about 100 Cd/m 2 . These results show that PBP might be a promising material for optoelectronic device applications.

Simultaneous measurement of the transient photoconductivity and photoluminescence in Alq3

Abstract We present a unique and simple technique which allows one to measure the transient photoluminescence (PL) and transient photoconductivity (PC) of thin film materials simultaneously. This technique will be useful in characterizing thin film materials that have a relatively high PL quantum yield, such as those used in EL devices (i.e. metal chelates, polymers, dyepolymer composite, etc.). The photogeneration of charge carriers and radiative excited states in pristine tris-(8-hyaroxyquinoline) aluminum (Alq 3 ) were studied by measuring both its photoconductive and photoluminescence response simultaneously, with subnanosecond resolution. 25 psec laser pulses at 355nm were used for excitation of an Auston switch with a 2–4 μm thick film of Alq 3 vacuum-evaporated across the active region. Quantitative measurements of the dependence on laser intensity, and applied electric field were performed. A scheme for the photoprocess in Alq3 is presented.

Reactions of singlet excitons in tris-(8-hydroxyquinoline) aluminum*

Abstract Singlet excited state reactions in thin films of tris (8-hydroxyquinoline) aluminum (Alq 3 ), a well known emitter for organic electroluminescent devices, are described. Bimolecular recombination dominates the singlet exciton decay in pristine films at high intensities, thus decreasing the photoluminescence quantum efficiency and singlet lifetime. The measured rate constant of the singlet-singlet annihilation in Alq 3 films is γ ss = (3.5±2.5) × 10 −11 cm 3 s −1 . The value of the diffusion coefficient of singlet excitons estimated from γ ss is D s = (1.2±0.8) × 10 −5 cm 2 s −1 .