Increasing OLED efficiency

Scientists at Philips have developed two new methods to increase the efficiency of PolyLED polymer OLED (organic light-emitting diode) displays. Efficiency is a major focus in current polymer OLED research, since higher efficiency opens the way to low-cost, large-scale manufacturing.

The first development relates to the development of a novel anode layer that significantly 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 currently the 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 singlet 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 proven 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 breakthrough 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.”

Reports of this work will be featured in the Journal of the American Chemical Society and are being presented this week by Eric Meulenkamp at the SPIE Photonics Europe Conference.

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