First-Principles Study of Rectification in Bis-2-(5-ethynylthienyl)ethyne Molecular Junctions

Using density functional theory (DFT) combined with the first-principles nonequilibrium Green’s function (NEGF), we investigated the electron-transport properties and rectifying behaviors of several molecular junctions based on the bis-2-(5-ethynylthienyl)ethyne (BETE) molecule. To examine the roles of different rectification factors, asymmetric electrode–molecule contacts and donor–acceptor substituent groups were introduced into the BETE-based molecular junction. The asymmetric current–voltage characteristics were obtained for the molecular junctions containing asymmetric contacts and donor–acceptor groups. In our models, the computed rectification ratios show that the mode of electrode–molecule contacts plays a crucial role in rectification and that the rectifying effect is not enhanced significantly by introducing the additional donor–acceptor components for the molecular rectifier with asymmetric electrode–molecule contacts. The current–voltage characteristics and rectifying behaviors are discussed in terms of transmission spectra, molecular projected self-consistent Hamiltonian (MPSH) states, and energy levels of MPSH states.