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The planarization of bottom‐contact organic field‐effect transistors (OFETs) resulting in dramatic improvement in the nanomorphology and an associated enhancement in charge injection and transport is reported. Planar OFETs based on regioregular poly(3‐hexylthiophene) (rr‐P3HT) are fabricated wherein the Au bottom‐contacts are recessed completely in the gate‐dielectric. Normal OFETs having a conventional bottom‐contact configuration with 50‐nm‐high contacts are used for comparison purpose. A modified solvent‐assisted drop‐casting process is utilized to form extremely thin rr‐P3HT films. This process is critical for direct visualization of the effect of planarization on the polymer morphology. Atomic force micrographs (AFM) show that in a normal OFET the step between the surface of the contacts and the gate dielectric disrupts the self‐assembly of the rr‐P3HT film, resulting in poor morphology at the contact edges. The planarization of contacts results in notable improvement of the nanomorphology of rr‐P3HT, resulting in lower resistance to charge injection. However, an improvement in field‐effect mobility is observed only at short channel lengths. AFM shows the presence of well‐ordered nanofibrils extending over short channel lengths. At longer channel lengths the presence of grain boundaries significantly minimizes the effect of improvement in contact geometry as the charge transport becomes channel‐limited.

Enthalpies of solution (AHS) of a variety of straight-chain, branched, and cyclic alkanes and alkenes have been measured in methanol, TV.iV-dimethylformamide (DMF), benzene, and cyclohexane. AHS values and enthalpies of evaporation {AHV) have been combined to give enthalpies of transfer from the vapor to each solvent [ //( -S)]. These solvation enthalpies are more exothermic for alkenes than for the corresponding alkanes in transfers to methanol, DMF, and benzene, by about 0.3, 0.6, and 0.3 kcal/mol, respectively, but are similar in transfers to cyclohexane. This suggests that solute-solvent interactions in cyclohexane are mainly dispersion forces. The more exothermic interaction of DMF with alkenes relative to alkanes results mainly from dipole-induced dipole interactions of the solvent dipoles with the highly polarizable bonds. Only very large steric factors inhibit the approach of solvent dipoles to the tr bond, as in trans-1,2-di-terr-butylethylene, where the alkene-alkane difference in AH(v -* S) is diminished. AH(v -* S) of alkanes and alkenes are reduced by nearly l kcal/mol for each quaternary carbon atom present, possibly arising from steric hindrance to solvation by dispersion forces.

Charge injection and transport in bottom-contact regioregular-poly(3-hexylthiophene) (rr-P3HT) based field-effect transistors (FETs), wherein the Au source and drain contacts are modified by self-assembled monolayers (SAMs), is reported at different channel length scales. Ultraviolet photoelectron spectroscopy is used to measure the change in metal work function upon treatment with four SAMs consisting of thiol-adsorbates of different chemical composition. Treatment of FETs with electron-poor (electron-rich) SAMs resulted in an increase (decrease) in contact metal work function because of the electron-withdrawing (-donating) tendency of the polar molecules. The change in metal work function affects charge injection and is reflected in the form of the modulation of the contact resistance, R(C). For example, R(C) decreased to 0.18 MΩ in the case of the (electron-poor) 3,5-bis-trifluoromethylbenzenethiol treated contacts from the value of 0.61 MΩ measured in the case of clean Au-contacts, whereas it increased to 0.97 MΩ in the case of the (electron-rich) 3-thiomethylthiophene treated contacts. Field-effect mobility values are observed to be affected in short-channel devices (<20 μm) but not in long-channel devices. This channel-length-dependent behavior of mobility is attributed to grain-boundary limited charge transport at longer channel lengths in these devices.

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