Polaron vs coherent mechanism of the carrier transport through molecular layers sandwiched by electrodes
Tsukada, Masaru; Mitsutake, Kunihiro
Japan

Carrier transport through molecular layers or molecular bridges connected to electrodes attract much interest as a prototype system of molecular devices both from basic science as well as technological application. The carrier transport behaviour is determined by the combination of coherent and dissipative processes, and controlled by various parameters as the electron-molecular vibration coupling, electron transfer integrals between the molecular orbitals, applied electric field, HOMO-LUMO levels of the molecule relative to the Fermi level of the electrodes and the temperature. The novel theoretical approach proposed by us recently combines ab initio molecular orbital method with analytical many-boson model and developed for the analysis and prediction of the carrier injection and transport properties of organic molecular layers (bridges) sandwiched between the metallic electrodes.
As the application of this method, we analysed the conductance of a long chain model of the oligo-acene series. First the eigen-states are solved by a variation approach. Mixed states of moderately extended molecular orbital states mediated and localised by dressing of molecular vibration quanta are solved as eigen-states. All the excited states accompanied by multiple quanta of vibration can be obtained, and the overall carrier transport properties including the conductance, mobility, dissipation spectra are analyzed by solving the master equation with the transition rates estimated by the Fermi's golden rule. We also found how the carrier injection rates is governed by the relative molecular levels to the electrode Fermi level, and how the overall transport mechanism changes from a dominantly coherent transport to the dissipative hopping transport. Based on these analyses, the dependence of the conductance of the oligo-acene series on the molecular size and bias voltage is clarified.
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