High brightness multilayer polymeric light-emitting diodes (GPC.6909)

Project nummer: gpc6909

Omschrijving van het onderzoek

In order to strongly increase the efficiency of polymeric light-emitting diodes (PLEDs) multilayer stuctures have to be developed, comparable to the heterojunction based LEDs and laser diodes in inorganic III-V semiconductors. The active part of present state-of-the-art PLEDs consists of only a single layer. In such a device holes are injected from the anode and electrons from the cathode into the polymer layer. The use of only a single electro-optic layer has a large fundamental disadvantage: due to the reduced electron transport in conjugated polymers most of the light in a PLED is generated close to the metallic cathode. This metallic cathode acts as a quenching site for the generated excitons, thereby strongly reducing the efficiency of the PLEDs. One way to further enhance the efficiencies of the state-of-the-art PLEDs would be to improve the charge transport properties of the conducting polymer. Such an improvement would lower the operation voltage of the PLED and as a result is is beneficial for the power efficiency (light output/consumed electrical power). However, recent studies revealed that an improvement of the charge transport properties also leads to an increase of the exciton quenching at the metallic cathode. Therefore, the materials presently used in single layer PLEDs are the best compromise between these two processes. In stead of using high mobility polymers low operating voltages are achieved in state-of-the-art PLEDs by keeping the active layer thickness limited to only 80 nm, which makes the devices very vulnerable to electrical shorts.
For the next generation of highly efficient PLEDs these fundamental limitations can be circumvented by using devices consisting of a number of active layers, each optimized for its own functionality. In an optimized device electrons and holes are efficiently transported via high mobility layers towards a highly luminescent layer. The energy levels of the polymers used will be designed in such a way that the hole transport layer also serves as a blocking layer for electron transport and vice versa, similar to the inorganic heterostructure LEDs. Using a heterostructure electrons and holes can not leave the device without recombining, leading to an enhanced efficiency. A major problem why polymer based multilayer devices have not been realized so far is the solubility of the materials used; a multilayer can not be fabricated when a spin casted layer dissolves in the solvent of the subsequent layer. In the project charge transport and luminescent layers will be developed with compatible solubility, which enable the construction of organic heterojunctions. The electrical and optical characterization of the various layers will be accompanied by extensive device modeling. Specifically, charge transport across polymeric heterojunctions has not been addressed so far and knowledge about this process needs to be developed. From the modeling studies insight will be acquired in the electric field and charge carrier distributions inside the multilayer devices, providing routes to optimize the device performance. The aim of the project is to construct, characterize and understand the next generation of highly efficient polymeric LEDs based on a multilayer structure. By the use of multi-layer structures relatively thick PLEDs of several hundreds of nanometers can be fabricated, which enable high brightnesses with corresponding high stability as required for display and lighting applications.

Gebruikers

Two companies are involved in this project.

Projectleider

Prof.dr.ir. P.W.M. Blom

Rijksuniversiteit Groningen
Wiskunde en Natuurwetenschappen
Materials Science Centre

Nijenborgh 4
9747 AG Groningen

Status van het project

Gestart	: 01-10-2005
Einddatum	: 01-10-2008

Trefwoorden

Display, Light-emitting diode (LED), Polymeer, Solid state lighting.

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