Organic LED Features Quantum Dots

Daniel S. Burgess

A quantum-dot sandwich on the menu at Massachusetts Institute of Technology in Cambridge promises to boost the performance of organic LEDs. The device, which features a layer of CdSe nanocrystals squeezed between two organic thin films, displays a luminescence efficiency 25 times higher than other quantum-dot LEDs and a much narrower emission spectrum than standard organic devices.

Organic LEDs are quickly making inroads into consumer applications where liquid crystal had dominated, particularly as flat panel displays for handheld devices such as cellular phones and personal data assistants. Market analysts at DisplaySearch of Austin, Texas, estimate that organic LEDs produced revenues of $85 million in 2002, and they forecast revenues of $3 billion by 2007.

Today's organic display materials, however, produce relatively broad emission peaks, with a full width half maximum of 50 to 100 nm, yielding less than optimal color saturation. Quantum dots, in contrast, which effectively confine charge carriers in three dimensions on the order of the de Broglie wavelength, offer much sharper peaks and extremely high quantum efficiencies. And because the spectral properties of these "artificial atoms" are dependent on their physical size, they promise to ease the production of color displays by enabling manufacturers to tune the emission wavelength by controlling the diameters of the dots.

Benefits of both

The new devices combine the benefits of both materials by inserting quantum dots into the organic LED structure. The researchers spin-cast onto glass substrates coated with indium tin oxide a chloroform solution of N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine (TPD) and 3-nm-diameter CdSe nanocrystals that are coated with ZnS and capped with trioctylphosphine oxide.

Spontaneous phase segregation accompanies this process, resulting in a monolayer of the quantum dots atop a 35-nm-thick layer of TPD. The researchers thermally evaporate on the quantum dot layer a thin film of tris-(8-hydroxyquinoline)aluminum (Alq₃) with or without a mediating layer of 3-(4-biphenylyl)-4-phenyl-5-t-butylphenyl-1,2,4-triazole, which suppresses electroluminescence by Alq₃, and deposit an Mg:Ag cathode and Ag cap on the stack.

They thus far have produced 1 x 1-cm LEDs with a 32-nm-wide spectral peak at 562 nm but suggest that optimizing the production of the quantum dots could enable linewidths of 25 nm. At a current density of 125 mA/cm², the devices display a brightness of 2000 cd/m², or a luminescence efficiency of 1.6 cd/A.

Seth A. Coe, a graduate student at MIT and member of the research team, suggested that quantum-dot organic LEDs might find applications as flat panel displays and in solid-state lighting. He noted that the new technique employs processes that already are used in the manufacture of polymer organic LEDs. "It has zero negative impact," he said.