Brighter outlook for PolyLEDs

Philips Components Pty Ltd
Thursday, 05 August, 2004

Scientists at Philips have developed new methods to significantly increase the efficiency of PolyLED polymer OLED (organic light-emitting diode) displays. This opens the way to lower power consumption and will further strengthen the advantages of polymer OLED for mobile and other applications, namely high brightness and contrast, wide viewing angle and excellent video capabilities but also enables the use of polymer OLEDs in solid-state lighting applications.

The past 10 years have seen OLEDs evolve into a truly powerful display technology capable of rivalling LCDs. Philips and other manufacturers are already using OLED displays in several new products such as its latest 639 mobile phone with 'Magic Mirror'. OLED displays are light, durable and efficient, which makes them suitable for portable applications.

Being intrinsically emissive displays, they require no backlight and can be manufactured in exceptionally small form-factors, including displays no thicker than a pane of glass and flexible displays, with no compromise on display characteristics.

Compared with small-molecule based OLED displays, Philips' PolyLED polymer OLED technology has significant advantages in manufacturing and scalability.

In 2002, it became the first company to launch polymer-based OLED displays for consumer applications.

Efficiency is a major focus in current polymer OLED research since higher efficiency opens the way to low cost, large-scale manufacturing. There are two major developments aimed at increasing quantum efficiency in polymer OLEDs are both likely to have important consequences for the future of polymer OLED research.

The first relates to the development of a novel anode layer that reduces losses due to imbalances in the hole and electron partial currents. With present anode layers, hole current can far exceed electron current resulting in significant energy wastage since the excess holes cannot combine with electrons within the polymer to generate light.

The new anode layer introduces a barrier to hole injection to reduce the number of excess holes. Results reported by Philips Research show good balance between holes and electrons at high voltage, and this is supported by huge increases in quantum efficiency to around 12%. (from between 2 to 4% for standard devices).

This translates into a luminous efficacy of 35 cd/A for a yellow light-emitting polymer and 20 cd/A for a blue-emitting polymer - the latter is currently claimed to be a world record in luminous efficacy for blue polymer emitters.

The second development relates to improving polymer OLED efficiency by using both fluorescence and phosphorescence. Phosphorescence is caused by the excited electrons returning to the ground state from a long-lived triplet state (in contrast to fluorescence in which the transition to ground is from a short-lived single state). By dispersing phosphorescent 'guest' material into a light-emitting polymer 'host', it is possible to use all excited states for the emission of light - provided the triplet energy gap of the host is higher than that of the guest.

This condition is particularly hard to meet for higher-energy green and blue emitters. Until now the only polymers capable of hosting blue and green phosphorescent emitters have proved unpractical in polymer OLED applications.

In collaboration with TNO Industrial Technology, however, Philips Research has produced a new copolymer material suitable for hosting a green triplet emitter and providing a high luminous efficacy of 24 cd/A.

Still higher efficacies and efficient blue emission are expected in the future with further optimisation of the copolymer composition.

"These developments point the way for the future in which we fully expect to see increases approaching 10-fold in polymer OLED efficiency without any compromise of the technology's inherent simplicity and easy manufacturability," says Eric Meulenkamp, principal scientist at Philips Research.

"Along with other developments in the pipeline, this could lead to polymer OLED technology overtaking LCD to become one of the major display technologies of the future."

So what exactly is PolyLED technology? It is a display based on polymer light-emitting diode (polyLED) technology, a low-weight display that is very flat and thin. It uses a low driving voltage and it offers high resolution and a wide greyscale range.

The technology can be used to make segmented as well as matrix displays. A polyLED display is an emissive display and consequently has a perfect viewing angle.

It offers high brightness, high contrast, and a fast response time suitable for displaying video. A strong point of a polyLED is its simple layout, which makes processing of these devices relatively easy and cheap. A polyLED device consists of the following parts:

  1. glass substrate;
  2. transparent anode (indium-tin oxide);
  3. hole transporting layer (PEDOT:PSS);
  4. light-emitting polymer;
  5. low-workfunction cathode (eg. Ba).

The entire device is encapsulated with a metal (or glass) seal.

A polyLED is made up of a thin (100 nm) layer of polymer material sandwiched between two electrodes: a cathode of low workfunction metal, and an anode of high work-function polymer material.

When the diode is forward biased (anode positive, cathode negative), the cathode injects electrons into the high energy conduction band of the polymer, from where they travel towards the positive anode.

The anode on the other hand attracts electrons from the valence band of the polymer, leaving behind positively charged holes that travel towards the cathode under the action of the applied electric field.

This gives rise to the phenomenon known as electroluminescence, where the holes and electrons recombine in the polymer material, releasing energy in the form of photons of light of a characteristic colour.

When a polyLED is irradiated with light of a characteristic energy, electrons will be excited from the valence band to the conduction band to form electron-hole pairs that break up under the action of an electric field to produce a small photocurrent.

With the device in forward bias, the photocurrent would be swamped by the diode current, but in reverse bias, it is easily detectable and can be used to measure the intensity of the light falling on the polyLED.

So by rapidly switching a matrix of polyLEDs between forward and reverse bias, it is possible to produce an interactive display that responds to a light pen or reflective pointer.

Clean room opens

Philips has extended its Microsystems Plaza facility for innovation in materials, devices, and microsystems in Eindhoven Holland, with world-class clean room, laboratory and materials-analysis services. The MiPlaza clean room ranks as one of the largest multi-purpose research clean rooms in the world.

This clean room is a multi-purpose, multi-technology facility. It occupies of 2650 m2. It also offers a broad range of process equipment capable of handling substrates of any shape, in sizes up to 200 mm.

The clean room will be used by Phillips for its internal strategic innovation programs, concentrating on topics such as materials and devices for molecular medicine, solid-state lighting, system-in-package solutions for healthcare, lifestyle and technology applications, sensors and actuators and new types of displays.

As a first step towards fostering open innovation, ie, the notion that progress in the industrial sphere can best be achieved through sharing knowledge and competencies with academic and industrial partners, Phillips recently opened up its High Tech Campus in Eindhoven to other high tech companies and 'technology accelerators'. Companies are encouraged to make use of both the infrastructure and the expertise and experience available on the campus and to work, with Phillips research personnel.

As a further step in fostering open innovation, the clean room facilities are offered to third party researchers and engineers active in the field of materials, devices and microsystems. First users include Phillips Research, Phillips CFT, startup companies such as micro-filtration specialists FluXXion and the Dutch Foundation for Fundamental Research on Matter (FOM).

The polyLED's sensing function is also suitable for power management in, for example, mobile phones by adjusting the display brightness in response to changes in ambient light levels.

The centrepiece of a polyLED is the light-emitting polymer layer. This as well as the hole transporting layer can be applied by spin coating or an ink jet process.

The former can be used for the preparation of monochrome displays, while the latter should be used for full colour devices.

Applying a small voltage across the device results in the injection of charge carriers that drift through the light-emitting polymer layer under the influence of the applied electric field.

At some point, the charge carriers can recombine and release energy in the form of photons which are emitted through the transparent anode. This process occurs millions of times per second and gives the device its high brightness.

The emission colour of the polymers strongly depends on their chemical composition. By chemical modification of the polymer structure, a range of soluble light-emitting polymers emitting in the range from 400 to 800 nm is available.

This means that any colour in the visible spectrum can be obtained.

Important for the emission colour of the light-emitting polymers are the type of polymer and the nature of the side-groups (which are also important for the solubility of the polymer). Well-known examples of light-emitting polymers are poly(p-phenylenevinylene), and poly(fluorene).

A monochrome polyLED was introduced into the market in mid 2002 by Philips as information display in the Sensotec men's shaver.

Next-generation displays will be full colour, manufactured by ink jet print technology. Demonstrators of such displays have already been shown with images such as the sunflower.

Regarding full colour polyLEDs, the use of emissive additives (also known as dyes) is an interesting option for influencing the emission colour of light-emitting polymers. By adding a small amount of a suitable dye to a polymer, energy can be transferred from the polymer to the dye and light will only be emitted from the dye.

By using different dyes, the colour from the device can be tuned. For example, a green dye in a blue polymer will give green light while a red dye in a blue polymer will give red light.

Energy transfer can be studied with time-resolved optical spectroscopy. When energy transfer is incomplete, white light can be obtained, which is of interest for lighting.

The external efficiency (the number of emitted photons per injected electron) of a polyLED device is determined by several factors: the photoluminescence efficiency, light outcoupling, the singlet-triplet ratio, the recombination efficiency and charge balance.

For a standard yellow-emitting poly-(phenylenevinylene) polymer, the product of these factors results in an external efficiency of 4%.

For all colours, much effort is invested into optimising these factors to obtain higher efficiencies.

Ofcourse, for a full colour device the colours red, green, and blue are required. Much progress in the efficiencies of these polymers has been made over the last couple of years. Major improvements in device efficiency can be obtained by using so-called triplet emitters as dyes.

Besides efficiency, another important issue for polyLEDs is their stability.

This can be enhanced by improving the stability of the light-emitting material, by improving the stability of the device (eg, the lifetime strongly depends on the type of cathode), and/or by improving the processing conditions.

By improving the materials and the device properties, the stability of polyLEDs has improved over the years up to 40,000 hours for the standard yellow material at room temperature.

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