Organic light-emitting transistors outperforming OLEDs

OLEDs – organic light-emitting diodes – are full of promise for a range of practical applications. OLED technology is based on the phenomenon that certain organic materials emit light when fed by an electric current and it is already used in small electronic device displays in mobile phones, MP3 players, digital cameras, and also some TV screens. With more efficient and cheaper OLED technologies it will possible to make ultra flat, very bright and power-saving OLED televisions, windows that could be used as light source at night, and large-scale organic solar cells. In contrast to regular LEDs, the emissive electroluminescent layer of an OLED consists of a thin-film of organic compounds. What makes OLEDs so attractive is that they do not require a backlight to function and therefore require less power to operate; also, since they are thinner than comparable LEDs, they can be printed onto almost any substrate.

Nonetheless, exciton quenching and photon loss processes still limit OLED efficiency and brightness. Organic light-emitting transistors (OLETs) are alternative, planar light sources combining, in the same architecture, the switching mechanism of a thin-film transistor and an electroluminescent device. Thus, OLETs could open a new era in organic optoelectronics and serve as test beds to address general fundamental optoelectronic and photonic issues.

"OLET is a new light-emission concept, providing planar light sources that can be easily integrated in substrates of different nature – silicon, glass, plastic, paper, etc. – using standard microelectronic techniques," Michele Muccini, a researcher at the Institute of Nanostructured Materials (ISMN) in Bologna, Italy, explains to Nanowerk. "The focus of OLET development is the possibility to enable new display/light source technologies, and exploit a transport geometry to suppress the deleterious photon losses and exciton quenching mechanisms inherent in the OLED architecture."

Trilayer OLET device structure and active materials forming the heterostructure. Schematic representation of the trilayer OLET device with the chemical structure of each material making up the device active region. The field-effect charge transport and the light-generation processes are also sketched.

Motivated by the need to unravel the full potential of field-effect transistors as a photonic technology platform, Muccini and his team have now demonstrated that OLETs enable the control of quenching and electrode-induced photon loss processes in an organic light-emitting device. These fundamental processes are those that still limit efficiency and brightness of OLED technology.

Reporting their findings in a paper in the May 2, 2010 online edition of Nature Materials ("Organic light-emitting transistors with an efficiency that outperforms the equivalent light-emitting diodes"), the scientists demonstrated the advantages of using an OLET versus an OLED configuration, and enabled OLETs with the highest efficiency reported so far.

"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" says Muccini. "Our devices provide planar micrometer-size light sources that might enable organic photonic applications like integrated on-chip bio-sensing and high resolution display technology with embedded electronics."

The Italian team introduced the concept of using a p-channel/emitter/n-channel tri-layer semiconducting heterostructure in OLETs providing a novel approach to dramatically improve OLET performance. According to Muccini, these devices are more than 100 times more efficient than the equivalent OLED, over 2 times more efficient than the optimized OLED with the same emitting layer, and over ten times more efficient than any other reported OLET.

The trilayer heterostructure OLETs used by the researchers were fabricated on glass/indium tin oxide/PMMA substrates. The active region consists of the superposition of three organic layers. The first, in contact with the PMMA dielectric, and the third layers are field-effect electron-transporting (n-type) and hole-transporting (p-type) semiconductors, respectively, whereas the middle layer is a light-emitting hostguest matrix. These three layers are 62nm thick. The device structure is completed by the deposition of 50nm gold contacts as source and drain.

"To enable the vertical charge diffusion process, the basis of the OLET electroluminescence mechanism, energetic compatibility between the materials forming the heterostructure is required," explains Muccini. "Furthermore, the morphology of these films must allow the formation of a continuous multistack. Meeting these requirements is not trivial and it took us several attempts to identify the appropriate film material."

"The new trilayer heterostructure field-effect concept unravels the full potential of the light-emitting field-effect technology and restricts the limitation of OLEDs to only materials-related issues," he continues. "Improvements in the top-layer field-effect mobility at high current density coupled to the use of triplet emitters will enable OLETs with even higher EQE and brightness."

He notes that ongoing research directions include the control of photonic processes within the device to improve light confinement, guiding and extraction. In addition device reliability and lifetime under operational conditions need to be thoroughly addressed.

"A critical parameter to be addressed for the future development of the OLET technology is the device operating voltage" says Muccini. "The power efficiency at a given voltage is an essential figure-of-merit of any light-emitting device. Lower operating voltages are to be targeted using high-capacitance gate insulators. However, despite the necessary technical improvements, we believe that our tri-layer OLETs represent a viable route towards practical organic light-emitting devices with unprecedented performances."

Jean Lucas Mendez

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