Research in the use of organic polymers as the active semiconductors in light-emitting diodes has advanced rapidly, and prototype devices now meet realistic specifications for applications. These achievements have provided insight into many aspects of the background science, from design and synthesis of materials, through materials fabrication issues, to the semiconductor physics of these polymers.

Electroluminescence in conjugated polymers

Organic electronics are beginning to make significant inroads into the commercial world, and if the field continues to progress at its current, rapid pace, electronics based on organic thin-film materials will soon become a mainstay of our technological existence. Already products based on active thin-film organic devices are in the market place, most notably the displays of several mobile electronic appliances. Yet the future holds even greater promise for this technology, with an entirely new generation of ultralow-cost, lightweight and even flexible electronic devices in the offing, which will perform functions traditionally accomplished using much more expensive components based on conventional semiconductor materials such as silicon.

The path to ubiquitous and low-cost organic electronic appliances on plastic

We demonstrate very high efficiency electrophosphorescence in organic light-emitting devices employing a phosphorescent molecule doped into a wide energy gap host. Using bis(2-phenylpyridine)iridium(III) acetylacetonate [(ppy)2Ir(acac)] doped into 3-phenyl-4(1′-naphthyl)-5-phenyl-1,2,4-triazole, a maximum external quantum efficiency of (19.0±1.0)% and luminous power efficiency of (60±5) lm/W are achieved. The calculated internal quantum efficiency of (87±7)% is supported by the observed absence of thermally activated nonradiative loss in the photoluminescent efficiency of (ppy)2Ir(acac). Thus, very high external quantum efficiencies are due to the nearly 100% internal phosphorescence efficiency of (ppy)2Ir(acac) coupled with balanced hole and electron injection, and triplet exciton confinement within the light-emitting layer.

Nearly 100% internal phosphorescence efficiency in an organic light emitting device

Direct printing of functional electronic materials may provide a new route to low-cost fabrication of integrated circuits. However, to be useful it must allow continuous manufacturing of all circuit components by successive solution deposition and printing steps in the same environment. We demonstrate direct inkjet printing of complete transistor circuits, including via-hole interconnections based on solution-processed polymer conductors, insulators, and self-organizing semiconductors. We show that the use of substrate surface energy patterning to direct the flow of water-based conducting polymer inkjet droplets enables high-resolution definition of practical channel lengths of 5 micrometers. High mobilities of 0.02 square centimeters per volt second and on-off current switching ratios of 10 5 were achieved.

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High-Resolution Inkjet Printing of All-Polymer Transistor Circuits

The development of white organic light-emitting diodes (OLEDs) holds great promise for the production of highly efficient large-area light sources. High internal quantum efficiencies for the conversion of electrical energy to light have been realized. Nevertheless, the overall device power efficiencies are still considerably below the 60-70 lumens per watt of fluorescent tubes, which is the current benchmark for novel light sources. Although some reports about highly power-efficient white OLEDs exist, details about structure and the measurement conditions of these structures have not been fully disclosed: the highest power efficiency reported in the scientific literature is 44 lm W(-1) (ref. 7). Here we report an improved OLED structure which reaches fluorescent tube efficiency. By combining a carefully chosen emitter layer with high-refractive-index substrates, and using a periodic outcoupling structure, we achieve a device power efficiency of 90 lm W(-1) at 1,000 candelas per square metre. This efficiency has the potential to be raised to 124 lm W(-1) if the light outcoupling can be further improved. Besides approaching internal quantum efficiency values of one, we have also focused on reducing energetic and ohmic losses that occur during electron-photon conversion. We anticipate that our results will be a starting point for further research, leading to white OLEDs having efficiencies beyond 100 lm W(-1). This could make white-light OLEDs, with their soft area light and high colour-rendering qualities, the light sources of choice for the future.

White organic light-emitting diodes with fluorescent tube efficiency

Ink-jet printing was used to directly deposit patterned luminescent doped-polymer films. The luminescence of polyvinylcarbazol (PVK) films, with dyes of coumarin 6 (C6), coumarin 47 (C47), and nile red was similar to that of films of the same composition deposited by spin coating. Light emitting diodes with low turn-on voltages were also fabricated in PVK doped with C6 deposited by ink-jet printing.

Ink-jet printing of doped polymers for organic light emitting devices

The emission intensity of an organic light-emitting diode at normal viewing angle and the total external emission efficiency have been increased by factors of 9.6 and 3.0, respectively, by applying spherically shaped patterns to the back of the device substrate. The technique captures light previously lost to waveguiding in the substrate and, with proper choice of substrate, light previously lost to waveguiding in the organic/anode layers. A method of applying the technique using laminated films and an optical model for evaluating coupling efficiency are also presented.

Improvement of output coupling efficiency of organic light-emitting diodes by backside substrate modification

The external coupling efficiency in planar organic light-emitting devices is modeled based on a quantum mechanical microvavity theory and measured by examining both the far-field emission pattern and the edge emission of light trapped in the glass substrate. The external coupling efficiency is dependent upon the thickness of the indium–tin–oxide layer and the refractive index of the substrate. The coupling efficiency ranges from ∼24% to ∼52%, but in general it is much larger than the 18.9% expected from classical ray optics.

External coupling efficiency in planar organic light-emitting devices

High-index-of-refraction substrates are shown theoretically and experimentally to increase the external coupling efficiency of organic light-emitting devices (OLEDs) by using a quantum mechanical microcavity model This increase is due to the elimination of those modes waveguided in the ITO/organic layer Bi-layer OLEDs were fabricated on standard soda lime glass and high-index glass substrates, and their far-field intensity pattern was measured Among the devices optimized for external efficiency, those on shaped high-index substrates exhibited a 53% improvement in external quantum efficiency over the devices on shaped standard glass substrates, and an increase by a factor of 2-3 times over those on planar glass substrates This principle is applicable to any backside patterning technique in conjunction with other OLED structural improvements

Improved external coupling efficiency in organic light-emitting devices on high-index substrates

In this paper the fundamental properties underlying the transfer of organic fluorescent dyes to local areas in polymer thin films by both liquid phase (ink-jet printing) and evaporation/diffusion transport methods are examined, with the goal of achieving full color displays based on organic light emitting diodes made from such polymers. Ink-jet printing offers a simple non-contact method for forming patterns, but a critical issue is the redistribution of dyes and other molecules in the liquid droplet before it dries. Masked large area evaporations allows one to rapidly pattern large areas, but its rate depends on the ability of dyes to diffuse through polymer films.

M. H. Lu

Papers

Ink-jet printing of doped polymers for organic light emitting devices

Improvement of output coupling efficiency of organic light-emitting diodes by backside substrate modification

External coupling efficiency in planar organic light-emitting devices

Improved external coupling efficiency in organic light-emitting devices on high-index substrates

Printing approaches for large-area color organic LED displays