Researchers in Japan have designed a smectic liquid crystal that overcomes many of the challenges posed by organic field effect transistor materials. Crystalline organic semiconductors have attracted a lot of interest as they offer convenient low-cost fabrication by printed electronics.
Development to date has been slow because of the low thermal durability and reproducibility of these materials, however, researchers at Tokyo Institute of Technology and the Japan Science and Technology Agency have been able to design a liquid crystal molecule that produces high-performance organic field effect transistors (FETs) that provide good temperature resilience and relatively low device variability in addition to high mobility.
The molecule developed by researchers can incorporate a number of qualities, in particular the smectic E phase. Low ordered liquid crystal phases form droplets at their melting temperature, but the smectic E phase has the advantage of retaining the thin-film shape.
Fabricated organic FETs can be created by spin coating a solution of their material at 110 °C before allowing it to cool. Comparison of the FET characteristics before and after mild annealing revealed a phase transition. Using atomic force microscopy the researchers identified that at around 120°C in the crystal formed a bilayer crystal phase.
The mobility of a bottom gated FET made from the material was around 12cm2Vs comparable to single-crystal devices.
"Considering that it could potentially be necessary to fabricate millions of FETs for display applications, polycrystalline OFETs may have an advantage over single-crystal OFETs," said the research team. The devices also exhibited a minimal variability of just 1.2cm2Vs, which is likely an advantage from the smoothness of the obtained film.
According to the research, "The discovery of a dramatic enhancement of FET mobility up to 13.9cm2Vs, resulting from the phase transition from a monolayer to a bilayer crystal structure in mono-alkylated liquid crystalline molecules may lead to the possibility of designing new materials for the burgeoning field of printed electronics."
Author
Neil Tyler
Source: www.newelectronics.co.uk