Compound White OLED Proves Efficient

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.