OLEDs Today, OLETs Tomorrow?

dd oled 0510

dd-oled-0510OLEDs have their drawbacks, but they are now widely recognized as cutting edge displays in their commercial growth phase. OLED stands for Organic Light Emitting Diode. But what if you could make an Organic Light Emitting Transistor – an OLET? Would that offer advantages for displays? Let’s find out.

In an interview with the publication Nanowerk, Michele Muccini, Head of Research and team leader at the Institute of Nanostructured Materials (Bologna, Italy), stated that the architecture of the OLET device combines the switching mechanism of a thin-film transistor with that of an electroluminescent device. Like an OLED, the OLET is also a planar light source.

Muccini’s team, along with colleagues at flexible electronics maker Polyera Corp. (Skokie, IL), reported their findings in a paper entitled “Organic Light-Emitting Transistors with an Efficiency that Outperforms the Equivalent Light-Emitting Diodes” published in the May 2, 2010 online edition of Nature Materials.

Referring to this work, Muccini states that, “We show that the same organic emitting layer leads to more efficient device emission when it is incorporated in the OLET structure than in the OLED one.”

The OLET structure is properly called a trilayer heterostructure. To build the device, the team started with a glass substrate, on top of which was deposited a layer of indium tin oxide as the transistor’s gate. The next layer was a dielectric material called poly(methyl methacrylate) or PMMA. Vapor deposition was then used to deposit the active region that consisted of three stacked organic layers. The first, in contact with the PMMA was a 7-nm thick, field effect electron transporting or n-type layer. The middle layer was a 40-nm thick light emitting host-guest matrix, a fluorescent polymer semiconductor. The third layer was a 15-nm thick hole transporting or p-type semiconductor. Finally, 50-nm thick gold contacts were deposited as the source and the drain.

In this structure, holes flow horizontally through the top layer and electrons flow horizontally through bottom layer. Any carriers that enter into the central layer recombine and emit photons. Since each type of carrier is restricted to its own layer of material, recombined carriers, known as singlets, do not encounter other carriers. This quenching effect is one of the major factors limiting the efficiency of conventional OLEDs.

The new OLET configuration was found capable of producing a device that is more than 100 times more efficient than an equivalent OLED, over twice the efficiency of an OLED that has been optimized through the use of the same emitting layer, and over ten times the efficiency of any other OLET device yet reported.

More specifically, the new OLET device demonstrated an external quantum efficiency of 5 percent. An OLED based on the same material has a quantum efficiency of only 2 percent. Previous OLET designs had an efficiency of only 0.6 percent.

Light emitted by the new OLET device peaked at a wavelength of about 600 nm (red-orange). In order for the device to emit other wavelengths, researchers need to develop charge transport layers that have a different band gap.

In the OLET device configuration, light is emitted as a stripe along the emissive layer. This is in contrast to the OLED in which light is emitted up and through the contacts. This point is significant in that OLEDs suffer a light loss in the range of 20 to 30 percent due to the light traveling through the contacts. Another favorable feature of the OLET as compared to the OLED is that the shape of the light output by the device is reportedly easier to couple into a waveguide or other optical structure.

Organic transistors (not light emitting) can be used to control display devices like electrophoretic displays or more conventional OLED layers. As noted, others have worked on OLETs before, and despite the breakthrough reported above, the current generation of OLETs still have a way to go before reaching commercial viability.

The first problem is insufficient brightness. To address this issue, Muccini explained that ongoing research is focused on controlling the photonic processes within the device to improve light confinement, guiding and extraction.

Another problem is that the operating voltage of OLETs is undesirably high. One approach under investigation to lower the operating voltage is the use of high capacitance gate insulators.

DD_artNot the least of the challenges to commercialization will be the requirement to demonstrate device reliability and lifetime under real world operating conditions.

The incentive for research into OLETs is that the pay off for success could be very big. With their planar form factor, OLETs could be integrated into chips. Such light based circuits could switch much faster than equivalent electron based circuits and do so with less power to dissipate. The benefits of these features can be left to the imagination of the reader.

Art Berman is an Insight Media consultant. Reach him at aberman@insightmedia.info