Cyano-substituted oligothiophenes: A new approach to n-type organic semiconductors

A series of α,ω-cyano oligothiophenes with three to six rings, as well as seven β,β′-substituted cyano terthiophenes have been synthesized using a palladium-catalyzed coupling reaction via organotin or organozinc intermediates. The structure of an oliothiophene trimer has been determined by X-ray crystallography; its space group is monoclinic (C2/c) with four molecules per unit cell (Z=4). The molecules adopt the π-π stacking structure. UV-vis spectra of these materials as thin films show a bathochromic shift compared to unsubstituted oligothiophenes. These bathochromic shifts are interpreted in the light of charge transfer exciton. Cyano end-capped sexithiophene (CN-6T-CN) sandwiched between various metals (metal/CN-6T-CN/metal), to form Schottky diode structures, were fabricated by vapor deposition. The electron injection and rectification ratio strongly depend on the metal contact, namely the work function of the metal is compatible with the electron affinity of the organic material. The current-voltage results are compatible with n-type conduction in CN-6T-CN.