Two-layer Light-emitting Diodes Research Articles

Polarized Electroluminescence in Double-Layer Light-Emitting Diodes with Perpendicularly Oriented Polymers

Polarized light over a large spectral region is provided by the novel procedure described in this work. The active material of these light-emitting diodes (LEDs) is formed by two polymer layers that are oriented perpendicularly to each other. The orientation is obtained using the rubbing technique combined with thermal annealing. The Figure represents the electroluminescence (EL) emission from the two-layer LED and its structure (see also inside front cover).

Nitrogen- and sulfur-containing conjugated polymers as the hole-transporting materials

A series of novel nitrogen- and sulfur-containing conjugated polymers have been synthesized via an acid-induced self polycondensation of functional monomers with methyl sulfinyl group. They exhibit good solubility in common solvents, such as CHCl3, THF, DMF, DMSO, et al; and thus show excellent film-forming properties. They are used as hole-transport layer (HTL) in two-layer light-emitting diodes (ITO/polymer/Alq3/Mg:Ag). The typical turn-on voltage of these diodes is about 4 similar to 5V. The maximum brightness is about 3440cd/m(2) at 20 V. The maximum efficiency is estimated to be 0.15 Im/W at 10V.

Organic Two-Layer Light-Emitting Diodes Based on High-Tg Hole-Transporting Polymers with Different Redox Potentials

A series of soluble arylamine-based hole-transporting polymers with glass transition temperatures in the range of 130−150 °C have been synthesized. The synthetic methodology allows facile substitution of the aryl groups on the amine with electron-withdrawing and electron-donating moieties, which permits tuning of the redox potential of the polymer. These polymers have been used as hole-transport layers (HTLs) in two-layer light-emitting diodes ITO/HTL/Alq/Mg [ITO = indium tin oxide, Alq = tris(8-quinolinato)aluminum]. The maximum external quantum efficiency of the device increases if the redox potential of the HTL is increased to facilitate reduction of the positive charge carriers at the HTL/Alq interface. A fluorinated hole-transport polymer with a relatively large redox potential (390 mV vs ferrocenium/ferrocene) yielded the device with the highest external quantum efficiency of 1.25% photons/e-. The device stability, however, follows the opposite trend. The device with the most electron-rich HTL exhibited the best performance after prolonged usage.

Phenylquinoxaline Polymers and Low Molar Mass Glasses as Electron-Transport Materials in Organic Light-Emitting Diodes

We present a new synthetic approach to both phenylquinoxaline polymers and low molar mass glasses. A palladium-catalyzed coupling of arylalkynes and bromobenzenes and subsequent oxidation of the triple bonds lead to the corresponding benziles. Reaction with diaminobenzidine yields poly(phenylquinoxalines) (PPQs), whereas the reaction with 1,2-diaminobenzenes leads to low molar mass bis(phenylquinoxalines) (BPQs) and tris(phenylquinoxalines) (TPQs). Both PPQs and TPQs carry tert-butyl or CF3− substituents and are fully soluble in chlorinated hydrocarbons. The starburst TPQs are able to form stable, low molar mass glasses. Cyclic voltammetry reveals that the TPQs have low-lying lowest unoccupied molecular orbitals levels at about −3.6 eV and are attractive as electron-transport materials in organic light-emitting diodes (LEDs). Two-layer LEDs with poly(phenylenevinylene) were fabricated that show a maximum brightness of 450 cd/m2.

Electron Injection Process in Light-Emitting Diodes Made from Polysilanes as Manifested by Incorporation of Electron Injection Layers

The electron injection process was investigated for poly(methylphenylsilane) (PMPS) in two-layer light-emitting diodes (TL-LEDs) composed of a PMPS film and a vacuum-deposited film of oxadiazole molecules (OXDs). The latter acts as an electron injection layer. The basic characteristics of the TL-LEDs were markedly better than those of single-layer LEDs. The magnitude of the accompanying red-shift in the EL spectrum coincides with the energy difference between the conduction band and/or the one-dimensional exciton level of PMPS, and the conduction band of an OXD layer. This study has revealed that electron injection in PMPS is an energy selective process occurring via surface defect levels which exist below the conduction band of the OXD layer in terms of energy, and that the electron-hole recombination takes place in the PMPS layer near the interface with the OXD layer.

New Triarylamine-Containing Polymers as Hole Transport Materials in Organic Light-Emitting Diodes: Effect of Polymer Structure and Cross-Linking on Device Characteristics

A series of poly(norbornenes) with pendant triarylamine (TPA) groups has been synthesized by ring-opening metathesis polymerization and investigated as hole transport materials in organic two-layer light-emitting diodes (LEDs). Efficient device fabrication through spin casting of the hole transport layer (HTL) was possible, since the polymers exhibited excellent film formation properties. LEDs of the form ITO/poly(norbornene)-TPA/Alq3/Mg (ITO = indium tin oxide, Alq3 = tris(8-quinolinato)aluminum) showed bright green emission with external quantum efficiencies of up to 0.77% (1.30 lm/W) for 20 nm thick HTL films. The length and polarity of the linker between the triarylamine functionality and the polymer backbone were varied systematically. The device performance was found to depend strongly on these structural differences. Substitution of ester groups by less polar ether functionalities greatly enhances external quantum efficiencies, lowers the operating voltage, and improves the stability of the device. Further improvement of the device characteristics is achieved by reducing the length of the alkyl linker. The HTL can be conveniently cross-linked by UV irradiation. Cross-linking was found to decrease device performance. A maximum external quantum efficiency of 0.37% was achieved for an Alq3-LED with cross-linked HTL.

Two‐layer light‐emitting diodes based on sexithiophene and derivatives

The performance of oligothiophene‐based organic LEDs can be dramatically improved (by up to four orders of magnitude) by using a two‐layer device made of unsubstituted sexithiophene (6T) and a substituted 6T (see Fig.), although the substituted 6T materials do not produce electroluminescence in single‐layer devices. Yellow emission with a quantum yield of 0.15% has been achieved. magnified image