CLEO Postdeadline: Aluminum-Alloy Contacts Make High-Brightness Organic LEDs

Novel cathode material lowers the barrier height for electron injection in organic LEDs, producing devices with high brightness and high efficiency

By: Yvonne Carts-Powell

Organic light-emitting diodes (OLEDs) are an attractive alternative to inorganic materials for display applications. Compared with inorganic materials, which often require expensive deposition techniques, with attention paid to crystal lattice-matching during the growth process, organic materials can be manufactured with relative ease in large areas, and on a wide variety of substrates. In the past, however, the inefficient transportation of charge carriers through the diodes, especially electron injection from the cathode, has offset these advantages.

In the CLEO '98 postdeadline sessions, Bernard Kippelen, along with colleagues G. E. Jabbour and N. Peyghambarian at the University of Arizona (Tucson, AZ), reported making an OLED that is extremely bright and efficient by using a aluminum and lithium fluoride metal alloy (Al:LiF) for the cathode (paper #CPD21).

Light output versus bias voltage for OLEDs with cathodes made of Al:LiF alloy (dot); Al and LiF layers (upward triangle); Mg (square); and Al (downward triangle).

The device consists of a transparent indium tin oxide anode, a 200-nm-thick organic hole-transport layer, a 100-nm-thick light-emitting layer of 8-tris-hydroxylquinoline aluminum (Alq₃), and a 100-nm-thick cathode made of aluminum mixed with 1.3% to 3% (by weight) LiF. The resulting OLED emitted 20,000 cd/m² (for comparison, a typical display emits 200 cd/m²).

Cathode development
Efficient cathodes have been made in the past from magnesium, calcium, and lithium, but these materials are unstable in air. Silver, while not as efficient, is somewhat more stable. Aluminum is both cheap and stable, but even less effective as a charge injector, producing OLEDs with reduced brightness and efficiency. Previous work showed that a thin film of an insulator, such as lithium fluoride, deposited between the organic layer and the aluminum cathode, improved the efficiency of the devices. Researchers explored making cathodes with an alloy formed of just aluminum and lithium, but these devices were difficult to reproduce because of the fine control required during the evaporation process.

The Al:LiF-cathode OLEDs demonstrate lower operating voltages than versions with aluminum, aluminum and LiF thin-film, or even magnesium cathodes (see figure at top). The University of Arizona device emitted green light at voltages as low as 3 V. At 7 V, the device emitted 830 cd/m² and showed an external quantum efficiency around 1.4%. The researchers believe that the increased efficiency of the device is a result of the lithium fluoride bending the energy bands of the organic layer, thus lowering the barrier height for electron injection at the Al/Alq₃ interface.

The group is currently developing three-layer organic structures and devices with luminescent dopants. They are also working on an aluminum alloy doped with cesium fluoride, which promises to be an even more efficient cathode material than the current alloy.