Polymer organic light-emitting diode (P-OLED) device technology was invented in the Cavendish Laboratory at Cambridge University in 1989, and subsequently acquired by Cambridge Display Technology (CDT) to further develop and commercialize the fundamental technology. The early discoveries centered around light-emitting polymer (LEP) materials which combined the unique optical and electrical properties of semiconductors with the ease-of-processing and printability of inks. While the initial efficiencies of the first P-OLED devices were low, the discovery was significant because it enabled a new self-emissive, thin-film display technology with potentially improved display characteristics and cost performance over LCD and plasma display  technologies .
LEP materials have since come to demonstrate tremendous advancement in operating lifetime and efficiency rivaling many inorganic emitter systems. Their success is largely attributable to the pioneering efforts of CDT, Merck (www.merck-oled.de), Sumation (www.sumation.co.uk) and others in the FPD industry.
The basic principle of operation for polymer Organic Light-Emitting Diodes (P-OLEDs) is the following:
- A thin film of LEP material is stacked between two electrodes, one of which is transparent, typically the anode.
- Electronic charges are injected into the LEP layer, with the cathode providing electrons and the anode providing holes.
- Light is produced through radiative recombination of bound electrons and hole pairs in the LEP layer at a wavelength characteristic of the LEP structure.
- The radiated light is emitted from the device through the transparent anode and substrate producing near Lambertian, wide-viewing angle, area emission.
Conventional OLED Stacks
A traditional RGB OLED device is shown below and is generalized representative of the type of OLED structures used in high-resolution display applications. The RGB P-OLED device structure is unmatched for achieving superior operating lifetime, brightness and efficiency in glass-based configurations. Applications are primary displays used in TV’s, laptops and mobile appliances. The RGB OLED device requires very careful control of layer thickness, uniformity, purity and composition in order to achieve efficient, long-life operation. Most notably, the high-efficiency approach uses an unstable, low-work function metal for its cathode. This low-work function metal can oxidize rapidly in the presence of oxygen and moisture and must therefore be deposited in a dry, vacuum-based environment using relatively expensive tooling. Degradation of these less-stable materials can also lead to black spot formation and device failure from the ingress of moisture into encapsulated devices, limiting shelf-life for similar displays on flexible substrates. This is because the barrier properties of substrates and encapsulants used in flexible displays are typically lower those glass-based materials and approaches.
So, traditional RGB OLED displays are a superior choice if the aim is superior power efficiency and operating lifetime. However, some of the device layers must still be deposited using high-finesse, multi-million dollar tooling. In addition, RGB OLEDs are particularly sensitive to ingress of moisture and oxygen into the display package. For these reasons, most RGB OLEDs use glass substrates and employ complex and costly manufacturing tooling and processing that limits its application to low-resolution display applications.
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