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Performance and durability combine in liquid crystal transistors
Crystalline organic semiconductors have attracted a lot of interest for convenient low-cost fabrication by printed electronics. However progress has been stymied by the low thermal durability and reproducibility of these materials. We have designed a liquid crystal molecule that produces high-performance organic field effect transistors (FETs) with good temperature resilience and relatively low device variability in addition to high mobility.
We designed a molecule that would incorporate a number of desirable liquid crystal 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.
We then fabricated organic FETs 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 12cm2V-1s-1 comparable to single-crystal devices. We point out "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," in a report of the work. The devices also exhibited a minimal variability of just 1.2cm2V-1s-1, which is likely an advantage from the smoothness of the obtained film.
We conclude, "The discovery of a dramatic enhancement of FET mobility up to 13.9cm2V-1s-1, 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."
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