The inverters use a small-molecule organic semiconductor in a carrier polymer, both dissolved in tetralin to form an ink branded tru-Flex by SmartKem.
Spin-coating results in the small molecule material forming thin polycrystalline ‘platelets’ separate from the polymer. These platelets are generally larger than the 4.6µm transistor channel length, minimising the effect of grain boundaries on transistor operation, said the team in its Advanced Electronic Materials paper.
The transistors are five-mask top-gate fets with gold source and drain electrode at the bottom of the stack.
Although described as ‘solution processed’, only the semiconducting ink and organic gate insulator are spin-coated. Photolithography was used to define the electrodes and the spin-coated layers were oxygen plasma etched following photolithography.
Complememtary transistors are not available, so inverters each consist of two transistors, one wired as an active load (Vth=0, wired source-to-drain) for the other.
For testing, dozens of five inverters ring-oscillator were made, each fitting in 1.4mm2. Powered with a Vdd of 20-30V, many routinely oscillated at 250-350kHz, and the best at over 500kHz.
“It is expected that with trivial optimisation of the circuit design, oscillator frequencies well above 1MHz can be easily attained,” said SmartKem.
Modelling is in reasonable agreement with actual performance, and mechanisms have been proposed to explain any differences.
The firm intends to develop organic thin-film transistors that canbe used in RFID tags and in backplanes for conformal LCD and flexible OLED displays.
“Narrow bezels can be achieved by the use of gate drivers fabricated on the display backplane,” said the firm, claiming: “Whilst it was once thought that this could only be achieved using inorganic materials such as low-temperature polysilicon, this new result shows that tru-FLEX would allow the manufacture of integrated gate drivers that would offer true physical flexibility.”
Bangor University was also involved in the research.
The materials involved are:
- small molecule: 1,4,8,11-tetramethyl-6,13-bis-triethylsilyl ethynyl pentacene
- polymer: amorphous polytriarylamine
- solvent: 1,2,3,4-tetrahydronaphthalene (tetralin)