We considered these mechanisms previously for bis-thienyl benzene devices in the thickness range of 8−12 nm, but rejected them due to inconsistency with the temperature dependence of the JV curves.25
2017
Key concepts
Scholarcy highlights
- Poole− Frenkel transport between Coulombic traps and Schottky emission at interfaces can both predict linearity of ln J with V1/2, due to changes in the respective barriers with increases in electric field. We considered these mechanisms previously for bis-thienyl benzene devices in the thickness range of 8−12 nm, but rejected them due to inconsistency with the temperature dependence of the JV curves. Figure 3 shows ln J vs V1/2 plots for comparison to ln J vs V in Figure 2b, and statistics for linear fits of ln J to V, ln J to V1/2, ln J/V to V1/2 and other possible functional forms are listed in Supporting Information section 3 and Table S2
- Plots of ln J vs d for conjugated and aromatic MJs are often reported to extend from d < 1 nm to d = 5 nm, depending on molecular structure and experimental paradigm
- Figure 1a shows a junction schematic and cross section of a completed device as well as the structure of FL and its oligomer. After describing their JV behavior, temperature dependence, and UV−vis absorption, additional MJs containing NAB, AQ, and bis-thienyl benzene were examined for comparison, before proposing a transport mechanism
- Figure 1b shows the four aromatic molecules used in the current work, with the density functional theory
- Efficient transport does not correlate with the energies of molecular orbitals relative to the electrode Fermi levels, but is directly controlled by the HOMO−LUMO energy gap in the molecular layer interior
- The unusual linearity of ln J with V1/2 observed for all four molecules is consistent with lowering of the H-L tunneling barrier in the applied electric field, and the weak temperature dependence is expected for a multistep, sequential tunneling mechanism
- At least for the case of strong electronic coupling between carbon electrodes and aromatic molecular layers, the findings provide useful guidance for rational design of molecular electronic devices with desired electronic behaviors
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