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Compound White OLED Proves Efficient

Photonics Spectra
Dec 2006
Lauren I. Rugani

Researchers at Princeton University in New Jersey and at the University of Michigan in Ann Arbor have fabricated a compound fluorescent/phosphor-sensitized fluorescent white OLED capable of achieving 100 percent internal quantum efficiency. The device employs red and blue fluorescent dopants and a green phosphor that result in high luminance and efficiency comparable to previous fluorescent-phosphorescent structures.

The white OLEDs were generated by organic layer deposition on glass substrates coated with a 150-nm anode layer of indium tin oxide. The researchers then grew a 50-nm-thick hole-transport layer consisting of 4-4’-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl. They followed with a symmetric emission layer comprising a 10-nm fluorescent blue region doped with 4,4'-bis(9-ethyl-3-carbazovinylene)-1,1'-biphenyl, a 2-nm undoped conductive host spacer, a 15-nm green-red emission layer doped with a green emitting phosphor sensitizer, an orange fluorescent compound, another 2-nm 4,4'-N,N'-dicarbazole-biphenyl spacer and a second 10-nm blue region.

A 30-nm 4,7-diphenyl-1,10-phenanthroline layer topped the emission layer and was itself capped by a cathode layer comprising 0.8-nm-thick lithium fluoride and 50-nm-thick aluminum.

Excitons formed between the blue fluorescent regions of the emission layer and the surrounding electron- and hole-transporting layers. The conductive host spacer between the fluorescent regions and the phosphor-doped center region prevented the transfer of singlets from the blue to the green-red zones. However, with an increased concentration of Ir(ppy)3, triplets are effectively transferred to the red fluorescent zone.

By carefully limiting the amount of the phosphor sensitizer, the green-to-red emission ratio is balanced to achieve the desired white color temperature.

The total external efficiency for the experimental device was 13.1 ±0.5 percent, and it had a power conversion efficiency of 20.2 ±0.7 lm/W at a luminance of 800 cd/m2. The measured spectral dependence on current density was nearly constant, suggesting the presence of balanced carrier injection within the device.

The results reveal a means for producing high-brightness and very high efficiency light sources and enable many fluorescent and phosphorescent dopant combinations.

Applied Physics Letters, Oct. 2, 2006, 143516.


GLOSSARY
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
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