Low-voltage p-channel organic thin-film transistors based on [n]phenacene (n = 5, 6 or 7) were fabricated on glass and on flexible poly(ethylene 2,6-naphthalate) (PEN) substrates. For the first time, these phenacenes were combined with two ultrathin gate dielectrics based on aluminium oxide and a monolayer of octadecyl-phosphonic acid in three different transistor structures. Regardless of the substrate and the transistor structure, the field-effect mobility is found to increase with increasing length of the conjugated [n]phenacene core, leading to the best performance for [7]phenacene. The largest average field-effect mobility we have obtained is 0.27 cm 2 /Vs for transistors on glass and 0.092 cm 2 /Vs for transistors on flexible PEN.

Monolayers of six alkylphosphonic acids ranging from C8 to C18 were prepared by vacuum evaporation and incorporated into low-voltage organic field-effect transistors based on dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT). Similar to solution-assembled monolayers, the molecular order for vacuum-deposited monolayers improved with increasing length of the aliphatic tail. At the same time, Fourier transform infrared (FTIR) measurements suggested lower molecular coverage for longer phosphonic acids. The comparison of FTIR and vibration frequencies calculated by density functional theory indicated that monodentate bonding does not occur for any phosphonic acid. All monolayers exhibited low surface energy of ∼17.5 mJ/m(2) with a dominating Lifshitz-van der Waals component. Their surface roughness was comparable, while the nanomechanical properties were varied but not correlated to the length of the molecule. However, large improvement in transistor performance was observed with increasing length of the aliphatic tail. Upon going from C8 to C18, the mean threshold voltage decreased from -1.37 to -1.24 V, the field-effect mobility increased from 0.03 to 0.33 cm(2)/(V·s), the off-current decreased from ∼8 × 10(-13) to ∼3 × 10(-13) A, and for transistors with L = 30 μm the on-current increased from ∼3 × 10(-8) to ∼2 × 10(-6) A, and the on/off-current ratio increased from ∼3 × 10(4) to ∼4 × 10(6). Similarly, transistors with longer phosphonic acids exhibited much better air and bias-stress stability. The achieved transistor performance opens up a completely "dry" fabrication route for ultrathin dielectrics and low-voltage organic transistors.

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Organic thin-film transistors (OTFTs) with multi-finger contacts based on dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) exhibit near-zero turn-on voltage, hysteresisfree behavior, and high transconductance of 30-80 A at V DS = V GS = −2 V. [1] In addition, common-source amplifiers based on such transistors deliver voltage gain even when the supply voltage is limited to 5 V, making them attractive for flexible/ wearable analog sensors. This paper presents the results of compact modeling, implemented in Matlab Simulink, applied to such transistors. The measured transistor transfer characteristics are used to extract the parameters for the semiempirical model. The model was validated in 3 ways on 8 OTFTs with varied geometries and substrates (glass or PEN). The validation included calculations of (a) transistor output characteristics, (b) a.c. drain currents for 1 Hz sinusoidal gate voltages, and (c) output voltages of the common-source amplifier, and their comparison to the measured data.

While the OTFTs exhibit large on-state drain current and a.c. transconductance, smaller L leads to a slightly reduced mobility. In addition, the OTFTs with the largest W of 18.23 mm possess the lowest off-state drain current and subthreshold slope.