Dry growth of n-octylphosphonic acid monolayer for low-voltage organic thin-film transistors

Dry method for monolayer deposition of n-octylphosphonic acid (C8PA) on the surface of aluminium oxide (AlOx) is presented. Vacuum thermal evaporation is employed to deposit initial thickness corresponding to several C8PA monolayers, followed by a thermal desorption of the physisorbed C8PA molecules. AlOx functionalized with such C8PA monolayer exhibits leakage current density of ∼10−7 A/cm2 at 3 V, electric breakdown field of ∼6 MV/cm, and a root-mean-square surface roughness of 0.36 nm. The performance of low-voltage pentacene thin-film transistors that implement this dry AlOx/C8PA gate dielectric depends on C8PA desorption time. When the desorption time rises from 25 to 210 min, the field-effect mobility increases from ∼0.02 to ∼0.04 cm2/V s, threshold voltage rises from ∼−1.2 to ∼−1.4 V, sub-threshold slope decreases from ∼120 to ∼80 mV/decade, off-current decreases from ∼5 × 10−12 to ∼1 × 10−12 A, on/off current ratio rises from ∼3.8 × 104 to ∼2.5 × 105, and the transistor hysteresis decreases from 61 to 26 mV. These results collectively support a two stage model of the desorption process where the removal of the physisorbed C8PA molecules is followed by the annealing of the defect sites in the remaining C8PA monolayer.

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► Monolayer of n-octylphosphonic acid (C8PA) was formed by vacuum evaporation.
► C8PA surface exhibits RMS surface roughness of 0.36 nm and water contact angle of 107.4°.
► Aluminium oxide functionalized with C8PA monolayer forms a gate dielectric in low-voltage organic thin-film transistors.
► Completely dry process for bottom-gate, top-contact pentacene thin-film transistors is presented.
► Transistor performance is comparable to transistors implementing solution processed short-chain alkyl-PA monolayers.