High Hole Mobility and Thickness‐Dependent Crystal Structure in α,ω‐Dihexylsexithiophene Single‐Monolayer Field‐Effect Transistors

Monolayer‐thickness two‐dimensional layers of α,ω‐dihexylsexithiophene (α,ω‐DH6T) exhibit field‐effect hole mobility of up to 0.032 cm 2 V −1 s −1 , higher than previously reported for monolayers of other small‐molecule organic semiconductors. In situ measurements during deposition show that the source‐drain current saturates rapidly after the percolation of monolayer‐high islands, indicating that the electrical properties of α,ω‐DH6T transistors are largely determined by the first molecular monolayer. The α,ω‐DH6T monolayer consists of crystalline islands in which the long axes of molecules are oriented approximately perpendicular to the plane of the substrate surface. In‐plane lattice constants measured using synchrotron grazing‐incidence diffraction are larger in monolayer‐thickness films than the in‐plane lattice constants of several‐monolayer films and of previously reported thick‐film structures. Near‐edge X‐ray absorption fine structure spectroscopy (NEXAFS) reveals that the larger in‐plane lattice constant of single‐monolayer films arises from a larger tilt of the molecular axis away from the surface normal. NEXAFS spectra at the C 1s and S 2p edges are consistent with a high degree of molecular alignment and with the local symmetry imposed by the thiophene ring. The high mobility of holes in α,ω‐DH6T monolayers can be attributed to the reduction of hole scattering associated with the isolation of the thiophene core from the interface by terminal hexyl chains.