A review was presented to demonstrate a historical description of the synthesis of light-emitting conjugated polymers for applications in electroluminescent devices. Electroluminescence (EL) was first reported in poly(para-phenylene vinylene) (PPV) in 1990 and researchers continued to make significant efforts to develop conjugated materials as the active units in light-emitting devices (LED) to be used in display applications. Conjugated oligomers were used as luminescent materials and as models for conjugated polymers in the review. Oligomers were used to demonstrate a structure and property relationship to determine a key polymer property or to demonstrate a technique that was to be applied to polymers. The review focused on demonstrating the way polymer structures were made and the way their properties were controlled by intelligent and rational and synthetic design.

Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices

This article provides a review about electroluminescence from organic materials and deals in detail with organic light-emitting diodes (OLEDs), light-emitting electro-chemical cells (LECs) and electrogenerated chemilumi-nescence (ECL) reflecting different electrooptical appli-cations of conjugated materials. It is written from an organic chemist's point of view and pays particular attention to the development of organic materials involved in corresponding devices. In recent years a substantial amount of both academic and industrial research has been directed to organic electroluminescence in an effort to improve the processability and tunability of organic materials and the longevity of OLEDs and LECs. On the eve of the commercialization of organic electrolumi-nescence this review provides an overview of lifetimes and efficiencies attained and reflects materials and device concepts developed over the last decade. In this context electrogenerated chemiluminescence is discussed with respect to its importance as a versatile tool to simulate the fundamental electrochemical processes in OLEDs.

The electroluminescence of organic materials

Intermolecular energy transfer processes typically involve an exothermic transfer of energy from a donor site to a molecule with a substantially lower-energy excited state (trap). Here, we demonstrate that an endothermic energy transfer from a molecular organic host (donor) to an organometallic phosphor (trap) can lead to highly efficient blue electroluminescence. This demonstration of endothermic transfer employs iridium(III)bis(4,6-di-fluorophenyl)-pyridinato-N,C2′)picolinate as the phosphor. Due to the comparable energy of the phosphor triplet state relative to that of the 4,4′-N,N′-dicarbazole-biphenyl conductive host molecule into which it is doped, the rapid exothermic transfer of energy from phosphor to host, and subsequent slow endothermic transfer from host back to phosphor, is clearly observed. Using this unique triplet energy transfer process, we force emission from the higher-energy, blue triplet state of the phosphor (peak wavelength of 470 nm), obtaining a very high maximum organic light-emi...

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Endothermic energy transfer: A mechanism for generating very efficient high-energy phosphorescent emission in organic materials

Emissive phosphorescent organometallic compounds are described that produce improved electroluminescence, particularly in the blue region of the visible spectrum. Organic light emitting devices employing such emissive phosphorescent organometallic compounds are also described. Also described is an organic light emitting layer including a host material having a lowest triplet excited state having a decay rate of less than about 1 per second; a guest material dispersed in the host material, the guest material having a lowest triplet excited state having a radiative decay rate of greater than about 1×10 5 or about 1×10 6 per second and wherein the energy level of the lowest triplet excited state of the host material is lower than the energy level of the lowest triplet excited state of the guest material.

Organometallic compounds and emission-shifting organic electrophosphorescence

The modification and control of exciton–photon interactions in semiconductors is of both fundamental1,2,3,4 and practical interest, being of direct relevance to the design of improved light-emitting diodes, photodetectors and lasers5,6,7. In a semiconductor microcavity, the confined electromagnetic field modifies the optical transitions of the material. Two distinct types of interaction are possible: weak and strong coupling1,2,3,4. In the former perturbative regime, the spectral and spatial distribution of the emission is modified but exciton dynamics are little altered. In the latter case, however, mixing of exciton and photon states occurs leading to strongly modified dynamics. Both types of effect have been observed in planar microcavity structures in inorganic semiconductor quantum wells and bulk layers1,2,3,4,5,6,7,8. But organic semiconductor microcavities have been studied only in the weak-coupling regime9,10,11,12,13,14,15,16,17,18. Here we report an organic semiconductor microcavity that operates in the strong-coupling regime. We see characteristic mixing of the exciton and photon modes (anti-crossing), and a room-temperature vacuum Rabi splitting (an indicator of interaction strength) that is an order of magnitude larger than the previously reported highest values for inorganic semiconductors. Our results may lead to new structures and device concepts incorporating hybrid states of organic and inorganic excitons19, and suggest that polariton lasing20,21,22 may be possible.

Strong exciton–photon coupling in an organic semiconductor microcavity

Efficient blue electroluminescence, peaked at 436 nm, is demonstrated from polymer light-emitting diodes operating at high brightness A dioctyl-substituted polyfluorene was used as the emissive layer in combination with a polymeric triphenyldiamine hole transport layer The luminance reaches 600 cd/m2 at a current density of 150 mA/cm2 for a bias voltage of 20 V, corresponding to an efficiency of 025 cd/A and a luminosity of 004 lm/W These values are optimized at a critical emissive layer thickness

High brightness and efficiency blue light-emitting polymer diodes

We report on the photophysical properties of a dioctyl substituted polyfluorene, with a focus on the effects of intermolecular interactions on absorption and emission spectra. We discuss these results in context of the strong current interest in the nature of aggregation phenomena and their role in controlling the emissive properties of conjugated polymers.

Influence of aggregation on the optical properties of a polyfluorene

Improved performance of light-emitting devices is reported for devices containing the newly synthesized, triazole-containing polymer TRIDSB (see Figure) This fully conjugated material has a high solid-state photoluminescence quantum efficiency, unlike other triazole polymers investigated previously, and emits blue light TRIDSB holds promise as an electron-transporting material

A blue‐emitting triazole‐based conjugated polymer

Polymer light emission: control of properties through chemical structure and morphology

Both soluble and insoluble conjugated polymers have advantages and disadvantages for the construction of light-emitting diodes (LEDs). A route is reported for the formation of optical quality films of insoluble (I) MEHPPV, a poly(phenylenevinylene) derivative whose soluble form is widely used in single-layer devices. The incorporation of I-MEHPPV into a multilayer device is described, in which the electron-transport/hole-blocking layer was deposited on top of the I-MEHPPV layer from a solvent in which the standard soluble MEHPPV would dissolve.

Alan W. Grice

Papers

High brightness and efficiency blue light-emitting polymer diodes

Influence of aggregation on the optical properties of a polyfluorene

A blue‐emitting triazole‐based conjugated polymer

Polymer light emission: control of properties through chemical structure and morphology

Insoluble Poly [2-(2′-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] for Use in Multilayer Light-Emitting Diodes†