There is huge interest in flexible electronics that can be produced in large volumes and at low cost, and a number of companies are now manufacturing organic thin film devices based on printed semiconductor polymer inks. These are entirely suitable for applications that do not require fast switching – such as the eletrophoretic displays used to display static images in electronic books – but the performance of current flexible organic devices cannot match that of silicon electronics.
Video displays, for example, have refresh rates of 60 Hz, and employ pixel drivers that provide sufficient current to the LEDs to deliver the desired light output. This requires semiconductors with electron mobilities two orders of magnitude higher than those of current polymer inks.
Using a room-temperature process, Bernard Kippelen and his colleagues at the Georgia Institute of Technology in the US have created high-performance field effect transistors from thin films of fullerene molecules. The new transistors have electron mobility values higher than those of amorphous silicon, opening the way to high-performance and low-cost flexible electronics.
The Georgia Tech work builds on that of Imperial College London physicist Thomas Anthopoulos and others, who in 2006 reported on the development of fullerene transistors based on C60 films grown by hot wall epitaxy. Anthopoulos et al. showed electron mobility to be strongly dependent on substrate temperature, and their process used temperatures of up to 250°C. The resulting mobilities are impressive, but the temperature is too high for flexible plastic substrates. “If you want to deposit transistors on a plastic substrate, you really can’t have any process at a temperature of more than 150°C,” says Kippelen.
Kippelen’s transistors, while not as fast as those of the Imperial College team, are produced with physical vapour deposition at room temperature, with obvious benefits for industrial scale manufacture. The devices also display on-off ratios greater than 106, and are stable under multiple cycling and continuous bias.
The Georgia Tech researchers produced the transistors by depositing C60 molecules in vapour form into a thin film on top of a silicon substrate pre-fabricated with a gate electrode and gate dielectric. The source and drain electrodes of the transistors were then deposited on the fullerene films through a shadow mask.
“The higher mobility values obtained by our group and that of Thomas Anthopoulos illustrate the potential of organic semiconductors for video display applications,” says Kippelen. “In other applications such as RFID, there is a need to operate wireless devices based on printed organic semiconductors at higher frequencies, which requires circuits with higher clock speeds.”
So far, the room-temperature fullerene transistors have been fabricated on silicon for convenience. “The challenge in moving to an organic substrate is to develop a process for the fabrication of the gate dielectric that is at lower temperature while maintaining similar performance,” says Kippelen.
Semiconductors with higher electron mobilities will enable higher frequency, and therefore higher performance, applications. “So far, good semiconductors were known for p-channel transistors,” says Kippelen. “For low-power electronics, complementary designs are desirable that use both p-channel and n-channel transistors. The work on C60 transistors provides a path for high performance n-channel transistors.”
Kippelen, together with collaborators Xiao-Hong Zhang and Benoit Domercq, plans to follow-up the development of fullerene transistors with other electronic components such as inverters, ring oscillators, logic gates, and drivers for active matrix displays and imaging devices. “The goal is to increase the complexity of the circuits to see how high mobility can be used to make more complex structures with unprecedented performance.”
“High performance n-channel organic field-effect transistors and ring oscillators based on C60 fullerene films”, Anthopoulos et al., Appl. Phys. Lett. 89, 213504 (2006).
“High-performance and electrically stable C60 organic field-effect transistors”, Zhang et al., Appl. Phys. Lett. 91, 092114 (2007).
Article first published in Nanomaterials News.