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

FOLLOWING the discovery1 of electroluminescence in poly(p-phenylenevinylene) (PPV), considerable effort has been directed towards the realization of optoelectronic devices based on semiconducting conjugated polymers of this type2–6. But the viability of these materials for such applications depends critically on the nature of the photoexcited states—in particular, whether they are predominantly non-emitting interchain species7–9 or emitting intrachain species10. One way to study this fundamental issue is in a device structure known as a microcavity11, which offers the possibility of using quantum electrodynamic effects to alter (and hence probe the nature of) spontaneous and stimulated emission in these materials12–17. Here we make use of such a structure to demonstrate optically driven laser activity in devices based on solid films of PPV. This demonstration of lasing provides direct support for a model10 in which the main photoexcitation in PPV is an emissive intrachain species, and opens the possibility of electrically driven polymer-based lasers.

Lasing from Conjugated-Polymer Microcavities

Organic semiconductor lasers.

Electroluminescence from organic materials has the potential to enable low-cost, full-color flat-panel displays, as well as other emissive products. Some materials have now demonstrated adequate efficiencies (1 to 15 lumens/watt) and lifetimes (>5000 hours) for practical use; however, the factors that govern lifetime remain poorly understood. This article provides a brief review of device principles and applications requirements and focuses on the understanding of reliability issues.

https://science.sciencemag.org/content/sci/273/5277/884.full.pdf

Organic Electroluminescent Devices

Transient photoluminescence measurements in poly($p$-phenylene vinylene) illustrate that exciton diffusion to quenching centers explains the nonexponential decay dynamics. The structure of these defects is studied by vibrational spectroscopy and the reason they act as quenchers is explained. In their absence, we observe nearly exponential photoluminescence decay with lifetime g 1 ns. The new picture of photoexcitations which emerges and its implications for polymer electroluminescent devices are discussed.

Defect quenching of conjugated polymer luminescence.

We observe competition between picosecond photoinduced absorption and stimulated emission in poly(paraphenylenevinylene) which demonstrates that they derive from two different pathways. We conclude that the photoinduced absorption originates from nonemissive ``spatially indirect'' singlet excitons which are formed with quantum yield as high as 0.9. Previous arguments about maximum possible electroluminescence device efficiencies and about lasers in conjugated polymer films are reexamined in light of the prevalence of this species.

Spatially indirect excitons as primary photoexcitations in conjugated polymers.

Operating lifetime is the main problem that complicates the use of polymeric light-emitting diodes (LEDs). A class of electron transport (ET) polymers [poly(aryl acrylate) and poly(aryl ether)s] is reported in which moieties with high electron affinities are covalently attached to stable polymer backbones. Devices based on poly(p-phenylenevinylene) (PPV) prepared with these materials exhibited a 30-fold improvement in stability and, in one case, dramatically lower (10 volts versus about 30 volts) operating voltage relative to those having conventional ET layers. The current-carrying capacity of indium tin oxide-PPV-polymeric ET layer-aluminum LEDs was also increased by a factor of 30. These improvements lead to an enhancement in power efficiency of nearly an order of magnitude. Choosing polymers with high glass transition temperatures increases device lifetime.

https://science.sciencemag.org/content/sci/267/5206/1969.full.pdf

Design and Application of Electron-Transporting Organic Materials

Upon irradiating poly(p-phenylene vinylene) thin films with white light in air (total exposure \ensuremath{\sim}5 J/${\mathrm{cm}}^{2}$), an enhancement of about forty times in photocarrier generation and a similar reduction in the photoluminescence intensity are observed. The wavelength and electric-field dependence of the photoconductivity are, however, essentially unchanged. These results demonstrate the sensitivity of the conjugated polymers to photo-oxidation and the importance of extrinsic photogeneration processes in these materials. The microscopic physics of carrier generation and luminescence quenching are established, and we discuss the implications for various conjugated polymer devices.

F. Papadimitrakopoulos

Papers

Defect quenching of conjugated polymer luminescence.

Spatially indirect excitons as primary photoexcitations in conjugated polymers.

Design and Application of Electron-Transporting Organic Materials

Enhanced carrier photogeneration by defects in conjugated polymers and its mechanism.

Effects of Polymeric Electron Transporters and the Structure of Poly(p-phenylenevinylene) on the Performance of Light-Emitting Diodes