ORGANIC LIGHT-EMITTing diodes

Researchers at Princeton University (Princeton, NJ) and the University of Southern California (Los Angeles, CA) have developed a stacked organic light-emitting diode (SOLED) structure that they hope will eventually improve the resolution and fill factor available with flat-panel displays made from OLEDs. Using the same materials, they have also built a solid-state, organic thin-film laser.

Jan 1st, 1998

ORGANIC LIGHT-EMITTing diodes

Stacked LEDs may improve color displays

Researchers at Princeton University (Princeton, NJ) and the University of Southern California (Los Angeles, CA) have developed a stacked organic light-emitting diode (SOLED) structure that they hope will eventually improve the resolution and fill factor available with flat-panel displays made from OLEDs. Using the same materials, they have also built a solid-state, organic thin-film laser.

In standard full-color OLED designs, red, green, and blue pixels are placed next to each other, requiring three different pixel spaces to represent all of the primary colors. By using transparent OLED (TOLED) materials, however, the Princeton researchers are able to stack all three colors vertically, in a single pixel space. In addition, the structural complexity of the new device is not much different from that of the standard design, according to Stephen Forrest, chair of the electrical engineering department at Princeton, who presented the results last October at the 44th National Symposium of the American Vacuum Society in San Jose, CA.

Each of the TOLED layers in the Princeton device consists of a transparent anode made of a combination of a thin, semitransparent magnesium:silver (Mg:Ag) and fully transparent indium tin oxide cathode, deposited on an electron transporting layer of tris(8-hydroxy quinoline) or Alq3. This combination is then layered onto a hole transport layer made of 4,4`-bis[N-(1-napthyl)-N-phenyl-amino]biphenyl (a-NPD) and an electron transport layer made of doped aluminum quinoline (Alq3). The electron transport layer is also often used for the dual purpose of emitting light, Forrest said.

5,10, 15, 20-tetraphenyl-21H,23H- porphine (TPP) doped Alq3 is used for red emission. Undoped Alq3 is used for green emission. And bis-(8-hydroxy)quinaldine aluminum phenoxide (Alq2`Oph) provides blue light.

The transparency in the TOLED results from the fact that the luminescence wavelengths in many organic compounds are red-shifted from the absorption region. TOLED layers originally fabricated by the Princeton team were between 60% and 70% transparent, Forrest said. More-recent devices appear totally transparent unless they are viewed from the edge.

When the layers are stacked into a half-micron-thick, full-color SOLED, the brightness and intensity of the primary color in each TOLED can be controlled separately, because each is contacted separately. In addition, the stacking of TOLED layers in a SOLED does not significantly increase the structural complexity of the device over standard full-color OLED designs. In standard designs a separate pixel is required for each of the primary colors, separate layers are required for each different color emitter and separate contacts are required for each color layer.

The next steps in developing the SOLED devices will be to improve the color rendition and to resolve manufacturing issues such as how to avoid short circuits between the thin films and how to increase lifetimes to acceptable levels for high-information-content displays on lapto¥computers.

Thin-film laser

The Princeton researchers have also extended their work with Alq3 to fabricate a solid-state, organic thin-film laser with about 50 W of peak (pulsed) output power in the 600- to 650-nm range, when pumped with an ultraviolet source. An early version of the device was demonstrated at the 1997 Conference on Lasers and Electro-Optics in Baltimore, MD, Forrest said.

The device was made using an indium phosphide (InP) substrate and a silicon dioxide cladding layer. The In¥substrate was used to allow fabrication of cleaved facets to define the laser resonator. The facets were made by depositing Alq3 doped with about 1% of the standard laser dye DCM.

The Alq3 doped with DCM creates a classical, four-level laser system, in which the ultraviolet pumping raises the Alq3 from the ground state to the excited state, which then excites the DCM. The 500- to 600-nm photoluminescence range of the Alq3 falls right on to¥of the absorption spectrum of DCM. The DCM then emits in the 600-700 nm range (see figure). Because the DCM forms discrete molecular states, the device basically acts as a quantum dot laser, Forrest said.

By building the devices with a double heterostructure, the Princeton team has increased the output quantum efficiency from 30% to 70%. They have also achieved stable operation u¥to 140°C and have managed to achieve vertical-cavity emission by using distributed-feedback Bragg reflector mirrors. The major ste¥in developing a workable device is electrical pumping, and because of low threshold energies, on the order of 1 µJ/cm2, Forrest said the prospects of doing that with this device are good. But many issues must be resolved on the way to electrical pumping. So, Forrest added, he`s saving his excitement until something actually happens.

"We have to be careful in this field not to get our promises in front of our capabilities," he said. "A lot of claims have been made, and I think now people have got to sit down and be rather sober in assessing how to go about actually making an electrically pumped organic laser."

Hassaun Jones-Bey

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