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.
Keywords: DNTT; alkylphosphonic acids; bias stress; monolayers; organic field-effect transistors.