Switch of conducting orbital by bias-induced electronic contact asymmetry in a bipyrimidinyl-biphenyl diblock molecule: Mechanism to achieve a pn directional molecular diode

We have both experimentally and theoretically studied low voltage rectification phenomena in an asymmetric single diblock molecular junction and have analyzed its mechanism. First-principles nonequilibrium Greens function (NEGF) theory was applied to electric current calculations. Then we identified a conducting molecular orbital based on the Breit-Wigner formula combined with the effective molecular projected state Hamiltonian (MPSH) analysis, where real and complex energy terms are renormalized explicitly into the standard MPSH by a Feshbach projection operator technique. The analysis discusses voltage dependences of conducting molecular orbitals as well as of site energy alignment defined in terms of molecular fragments. We found quantitative results of (i) electronic switching of the conducting molecular orbital caused by bias-dependent intrinsic electronic couplings with electrodes and (ii) change of site energy alignment driven by bias voltage sweep without significant additional charge injection. On the basis of these findings, we conclude that a mechanism characterized by (i) and (ii) is responsible for rectification and its pn-like forward direction in this specific diblock molecular system.