is thin enough, one can make devices that are 75 percent or more transparent. By replacing the metal electrode with an electron injection layer, devices have been made that are 95-percent transparent. Because the OLEDs themselves are transparent, different-colored OLEDs can be stacked to combine colors; placing green on top of blue on top of red can create an OLED “sandwich” that gives white light when all three of the devices are turned on. In addition, transparency allows OLEDs to be overlaid on windshields or other transparent substrates where the light can be turned on as needed; when it is off, the view is clear.

Transparency is also valuable in terms of function. Dr. Thompson’s laboratory has demonstrated approximately 80-percent efficiency for monochromatic lights. In certain applications, such as games, they have achieved external luminosities as high as 60 lumens per watt for monochromatic green. Lamp lifetimes of 10,000-hour lifetimes are becoming common, and ultimate lifetimes of approximately 100,000 hours (~12 years) are anticipated. Some devices are very bright, with demonstrations as high as a million candelas per square meter, compared with 100 candelas per square meter for a cathode ray tube and 800 candelas for a fluorescent panel. Turn-on voltages as low as 3 volts have been demonstrated.

OLEDs are Specialized

No single device has all of those desirable characteristics; it is unlikely that any single device has even two of them. More typically, devices are designed around particular parameters, reflecting decisions about whether long-lifetime, high-efficiency brightness or some other feature is primary. There are different choices of structure and materials and within the device itself one can use various electrodes, transporting layers, and emissive layers. For the application of lighting, efficiency is paramount, and other qualities may have to be sacrificed.

OLED Function

Dr. Thompson offered a simple description of OLED function, displaying a device package with a two-layer structure. When a fixed voltage is applied, electrons pile up on one side of the organic interface, electron holes3 on the other. The recombination of electrons and electron holes at the interface leads to a formation of excitons, or excited states, consisting of bound electrons and holes. When the exciton energy is transferred from the host material to the dopant, the dopant emits radiation in the decay process to the ground state.4

3  

In physics a hole (or electron hole) is a vacant position in a substance left by the absence of an electron, especially a position in a semiconductor that acts as a carrier of positive electric charge.

4  

A dopant is a substance, such as phosphorus, added in very small amounts to a semiconductor or an OLED to improve the quantum efficiency of the material.



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