Peter Pösch

Polymeric Light-Emitting Diodes Based on Poly(p-phenylene ethynylene), Poly(triphenyldiamine), and Spiroquinoxaline

In this paper polymeric light-emitting diodes (LEDs) based on alkoxy-substituted poly(p-phenylene ethynylene) EHO-OPPE as emitter material in combination with poly(triphenyldiamine) as hole transport material are demonstrated. Different device configurations such as single-layer devices, two-layer devices, and blend devices were investigated. Device improvement and optimization were obtained through careful design of the device structure and composition. Furthermore, the influence of an additional electron transporting and hole blocking layer (ETHBL), spiroquinoxaline (spiro-qux), on top of the optimized blend device was investigated using a combinatorial method, which allows the preparation of a number of devices characterized by different layer thicknesses in one deposition step. The maximum brightness of the investigated devices increased from 4 cd/m2 for a device of pure EHO-OPPE to 260 cd/m2 in a device with 25 % EHO-OPPE + 75 % poly(N,N′-diphenylbenzidine diphenylether) (poly-TPD) as the emitting/hole-transporting layer and an additional electron-transport/hole-blocking spiro-qux layer of 48 nm thickness.

Heterolayer light-emitting diodes based on new oxadiazole polymers

Abstract Monolayer light-emitting diodes (LEDs) from poly(1,4-phenylene vinylene) (PPV) usually exhibit relatively low quantum efficiencies. Thus, the external efficiency of an indium-tin oxide (ITO)/PPV/A1 LED is typically 0.001%. In order to increase the quantum yield heterolayer devices with new oxadiazole polymers have been fabricated. These polymers with the electron-withdrawing oxadiazole units facilitate electron injection and transport in bilayer LEDs with PPV as hole transport and emitting layer. Thus, LEDs with external quantum efficiencies up to 0.2% have been achieved. Compared to conventional PPV LEDs no temperature dependence of the quantum efficiency is detectable, indicating improved balanced charge carrier injection at room temperature.

Perylenediimides with electron transport moieties for electroluminescent devices

Abstract Low molecular weight and polymeric imides are synthesized by attaching perylene units as substituents or by incorporating in aromatic main chain copolymers to achieve high photoluminescent efficiency. In order to improve electron injection and transport properties 1,3, 4-oxadiazole comonomers with high electron affinities are used. The best solid state photoluminescence is obtained in copolymers with 0.5 mol% perylene content. The low molecular weight compounds exhibit high electron affinity with LUMO values in the range of −3.9 eV as determined by cyclic voltammetry. Electroluminescent devices with copolymers in PVK blends show characteristic red emission from perylene which is nearly identical with its photoluminescence and the efficiency of this device, ITO/copolymer+PVK blend(100nm)/Al, depends upon the PVK content.

Efficient screening of electron transport material in multi-layer organic light emitting diodes by combinatorial methods

A combinatorial approach combining vapor deposition of organic molecules and a mask technique was used to prepare on one substrate a matrix of 49 organic light emitting diodes (OLEDs) with different configurations and layer thicknesses. A landscape library with two orthogonal, linear gradients of an emitter and a hole blocking electron transport material on top of a hole transport layer of constant thickness was prepared. The aim of this experiment was to investigate the influence of an additional electron transport material on the efficiency. Using a semi-automated measurement set-up, the device parameters for each of the 49 OLEDs were evaluated. The existence of an optimum Alq3 layer thickness for ITO/TPD/Alq3/Al two-layer devices was confirmed and such an optimized two-layer structure could not be improved by adding an additional hole blocking layer to the optimum Alq3 layer. However, an improvement in photometric efficiency can be achieved by replacing the optimum Alq3 layer thickness by certain combinations of Alq3/spiro-quinoxaline layers.

Intermolecular energy transfer after vibrational excitation of a perylene dye in solution, in polymer binder, and in a side-chain copolymer

Skeletal modes between 1550 cm−1 and 1750 cm−1 of the perylene chromophor are vibrationally excited in different surroundings via resonant absorption of ultrashort IR pulses. The vibrational energy transfer is monitored via electronic transitions to the S1 state with the help of a second, delayed visible pulse. In liquid chloroform solution of the dye an intermolecular energy transfer time of τe=(15±1) ps was determined. In the solid dye/PMMA blend the intermolecular energy transfer time was with τe=(10,5±1) ps substantially smaller. In the copolymer a similar time constant of τe=(9±1) ps is found. Obviously the spacer decouples in the copolymer not only the electronic systems but also the vibrational manifolds.

A comparison of hole blocking/electron transport polymers in organic LEDs

Three main-chain aromatic polyethers with different electroactive heterocyclic moieties, 1,4-quinoxaline, 1,3,4-oxadiazole and 1,3,5-triazine, have been synthesized. The polymers are amorphous with glass transition temperatures above 200 C. The polymers with these high electron affinity units were used as hole blocking/electron transport layers (HBETL) in light-emitting diodes (LEDs) having the HBETL casted on top of a hole transport/emitting PPV layer. In order to compare the influence of the different polyethers on the LED characteristics, three multilayer devices (ITO/PPV/HBETL/AI) with different HBETLs were investigated. Relative to the single layer PPV device, quantum efficiencies were improved by two orders of magnitude in all multilayer devices and power efficiency was increased using poly(quinoxaline ether) as HBETL. To investigate the electrochemical behavior of the three HBETLs, cyclic voltammetry measurements were carried out and the HOMO/LUMO energy values determined from redox potentials were used to understand the hole blocking property. Lowering the onset voltage using the poly(quinoxaline ether) as HBETL in two-layer devices is compatible with the high electron affinity of this polymer.