Orientation-dependent electronic structures and charge transport mechanisms in ultrathin polymeric n-channel field-effect transistors.

We investigated the role of metal/organic semiconductor interface morphology on the charge transport mechanisms and energy level alignment of the n-channel semiconductor poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (P(NDI2ODT2)). Variable-temperature study of well-ordered edge-on-oriented P(NDI2OD-T2) monolayer and multilayer field-effect transistors fabricated via Langmuir-Schäfer (LS) method reveals a higher activation energy for the edge-on morphology when compared to that extracted for the face-on oriented P(NDI2OD-T2) spin-coated films, which showed a weaker temperature dependence. Near-ultraviolet inverse photoemission and low-energy electron transmission spectroscopies are utilized to study these microstructurally defined polymeric films. The cross correlations of these techniques with the device characterization reveals the role of the molecular orientation at the semiconductor/contact interface in shifting the charge injection barrier. Finally, we demonstrate that the injection barrier for electrons is higher for the LS/edge-on than in the spin-coated/face-on films.