Characteristics Of Polymer Light-emitting Diodes Research Articles

Improved characteristics of polymer light-emitting diode based on poly(alkylfluorene) with functional layer fabricated by contact printing method

In the organic light-emitting diode, the multilayer structure is useful to achieve the effective carrier injection and transport, and improve the emission efficiency. The characteristics of the polymer light-emitting device with a functional layer by contact printing method are investigated. Hole transport material, the guest and electron transport material are mixed at each layer with the same host. Poly (9,9-dioctylfluorene) (F8) and Tris[1-phenylisoquinolinato-C2,N]iridium(III) (Ir(piq)3) are used as the host and guest materials. We focused on the effects of a functional layer with 1,3,5-Tris (1-phenyl-1H-benzimidazol-2-yl)benzene(TPBi) as the electron transport material doped in F8. For a non-heterostructure device with F8:TPBi functional layer, electrons are injected from the cathode interface to TPBi and transported through F8. TPBi acts as the effective hole blocking and electron transport layer as TPBi is mixed in F8 at the cathode side. Non-heterostructure device exhibits improved emission efficiency and lower driving voltage.

Photocurrent hysteresis by ion motion within conjugated polyelectrolyte electron transporting layers

The change in photocurrent characteristics of polymer light-emitting diodes containing cationic conjugated polyelectrolyte electron transport/injection layers depends on the nature of the counterions and the rate of bias sweep. We attribute this feature to the motion of counteranions and differences in the rates of ion migration.

Modeling and numerical study of electrical characteristics of polymer light-emitting diodes containing an insulating buffer layer

In order to improve the electrical characteristic of polymer light-emitting diodes, a simple model for the device characteristic with an insulating buffer layer at cathode is proposed. This model is based on Fowleer–Nordhein tunneling mechanism and Poission's equation. An additional tunneling factor which characterises the tunneling effect of buffer layer is introduced. The simulated current–voltage characteristic indicates how an insulating buffer layer with suitable thickness decreases the barrier height at the cathode and therefore increases the electron injection. The model is validated by experimental results of devices with BaO as the buffer material and poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] as the emission material. An optimum thickness of the buffer layer is also obtained from the model, which provides a guide to device design.

Mechanisms of Performance Improvement for Polymer Light-Emitting Diodes Fabricated on (NH[sub 4])[sub 2]S[sub x]-Treated Indium-Tin Oxide Substrates

Photoelectron spectroscopy measurements show that (NH 4 ) 2 S x surface treatment may lead to an increase in the work function of indium-tin oxide (ITO), due to the occupation of oxygen vacancies (V O ) by sulfur and a reduction in the amount of V O near the ITO surface region. Comparison between the current-luminance-voltage characteristics of polymer light-emitting diodes fabricated on ITO substrates with and without (NH 4 ) 2 S x treatment, shows that (NH 4 ) 2 S x treatment led to the enhancement of external quantum efficiency, due to the reduction of forward-biased leakage current (resulting from the reduction of V O -related defects near the ITO surface region).

Improved Performance by Modification of Cathode in PolymerLight-Emitting Diode Based on a Novel Se-Containing ElectroluminescentCopolymer

The electroluminescent characteristics of polymer light-emitting diodes(PLEDs) with the structure ITO/PEDOT/ PFO-Selenophenecopolymer/cathode were investigated. Various cathodes, such as Al,Li/Al, Ba/Al, LiF/Al and LiF/Ba/Al, were used for testing the deviceperformances. The strongest light output (1112 cd/m2) and thehighest quantum efficiency (0.74%) were achieved by using theLiF/Ba/Al cathode. A possible working mechanism for improving the PLEDperformance was discussed.

Anomalous current-voltage characteristics of polymer light-emitting diodes

Light-emitting diodes based on an alternating copolymer containing triphenylamine and phenylene units (TPA-PPA) were prepared, and electrically and optically characterized. The diode with a structure of indium tin oxide/TPA-PPV/Al exhibited a ``current anomaly'' phenomenon. We demonstrated that the ``current anomaly'' is caused by a reverse internal electric field owing to the reabsorption of electroluminescent light rather than the changes in the aluminum-doping concentration during operation.

Efficiency and stability of polymer light-emitting diodes

The operation characteristics of polymer light-emitting diodes (PLEDs) are strongly dependent on materials, processing and the structure of the device. The device structure developed at Philips Research is presented together with some typical results for brightness, efficiency, response times and stability. The PLEDs typically operate at a voltage of 3–4 V for a brightness of 100 cd m-2 and have an efficiency ranging from 2 cd A-1 for orange emitting polymers (610 nm) up to 16 cd A-1 for green emitting polymers (550 nm). The response time under conditions for display operation is determined by the charge carrier transport properties and amounts to 43 ns. Lifetimes of several thousand hours have been obtained for large orange emitting devices of 8 cm2 for daylight visibility at room temperature.

Device characteristics of polymer light-emitting diodes

A model describing the device characteristics of polymer light-emitting diodes (PLEDs) is presented. The recent observation that in poly(p-phenylene vinylene) (PPV) the hole current is space-charge-limited whereas the electron current is strongly reduced by traps is taken into account. It is calculated that such an unbalanced electron and hole transport gives rise to a bias dependent efficiency. By comparison with experiment it is found that the recombination process in PPV is mainly non-radiative. Only 5% of the total number of recombinations contributes to the light-output of the PLED.