Theoretical investigation of charge injection and transport properties of novel organic semiconductor materials - Cyclic oligothiophenes

By employing the Marcus electron transfer theory coupled with two-state model, the Brownian diffusion assumption and density functional theory (DFT), we have investigated the charge injection and transport properties for three types of the cyclic oligothiophenes (A, B and C in Fig. 1), and their corresponding linear molecules (linear-A, linear-B and linear-C in Fig. 1). By indentifying 13 distinct nearest-neighbor hopping pathways based on the crystal structures of the molecules reported (A and B), we predicted the electronic coupling matrix elements for a wide variety of charge transfer pathways using the "energy splitting in dimer" (ESD) method and their carrier mobility. The theoretical results indicate that they possess large hole carrier mobility, importantly, very outstanding properties of electron transport. The major reason should be that (1) the closed ring structure restricts the rotation of the thiophene rings in charge transfer process, (2) the introduction of the alkynyl and double bonds weakens the coulomb repulsion between lone electron pairs of sulfur atoms, the ring stain and the steric effect resulting from the butyls, more importantly, (3) their introduction stabilizes LUMO level, in result to decrease electronic organization energy (λ_e) and thereby improve their ability of electron transport. The temperature dependences of the carrier mobility were theoretically investigated for molecules A and B and analyzed within the hopping mechanism.