Luminescence quantum yield of molecular aggregates and excitons in α-sexithienyl thin films at variable temperature

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.

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.

Polyelectrolyte multilayers containing electrochemically active polythiophenes have been constructed on ITO/glass substrates using the layer-by-layer adsorption deposition technique. Electrochemically active layers of poly(cyclopentadithienyl-alkylsulfonate), poly(cyclopentadithienyl-alkylammonium), and R,ω-bis(carboxyhexyl)sexithiophene were deposited with nonelectroactive layers of polyallylamine and polystyrenesulfonate. The first sequential adsorption of multilayers in which both the polycation and the polyanion are based on the same polythiophene is reported. UV-vis spectroscopy and cyclic voltammetry indicate a linear dependence of the amount of deposited polymer on the number of deposition cycles. The rate of deposition depends on the oxidation state of the polythiophene being 3 times lower for the oxidized polycationic polymer. Atomic force microscopy characterization of the layers has shown that flat monolayers are deposited with a progressive increase of roughness. Interposition of multiple nonelectroactive layers between the electrode and a single external electroactive polythiophene layer has allowed determination by cyclic voltammetry that each polythiophene layer interpenetrates the confining layers up to a three-layer distance. The conductivities of the multilayers along the surface and perpendicular to it are anisotropic depending on the layer alternation. The photoluminescence properties of the R,ω-bis-(carboxyhexyl)sexithiophene multilayers are the same for vacuum-deposited polycrystalline layers.

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