Structure Controlled Long-Range Sequential Tunneling in Carbon-Based Molecular Junctions

Abstract

Carbon-based molecular junctions consisting of aromatic oligomers between conducting sp hybridized carbon electrodes exhibit structure-dependent current densities (J) when the molecular layer thickness (d) exceeds ∼5 nm. All four of the molecular structures examined exhibit an unusual, nonlinear ln J vs bias voltage (V) dependence which is not expected for conventional coherent tunneling or activated hopping mechanisms. All molecules exhibit a weak temperature dependence, with J increasing typically by a factor of 2 over the range of 200-440 K. Fluorene and anthraquinone show linear plots of ln J vs d with nearly identical J values for the range d = 3-10 nm, despite significant differences in their free-molecule orbital energy levels. The observed current densities for anthraquinone, fluorene, nitroazobenzene, and bis-thienyl benzene for d = 7-10 nm show no correlation with occupied (HOMO) or unoccupied (LUMO) molecular orbital energies, contrary to expectations for transport mechanisms based on the offset between orbital energies and the electrode Fermi level. UV-vis absorption spectroscopy of molecular layers bonded to carbon electrodes revealed internal energy levels of the chemisorbed films and also indicated limited delocalization in the film interior. The observed current densities correlate well with the observed UV-vis absorption maxima for the molecular layers, implying a transport mechanism determined by the HOMO-LUMO energy gap. We conclude that transport in carbon-based aromatic molecular junctions is consistent with multistep tunneling through a barrier defined by the HOMO-LUMO gap, and not by charge transport at the electrode interfaces. In effect, interfacial "injection" at the molecule/electrode interfaces is not rate limiting due to relatively strong electronic coupling, and transport is controlled by the "bulk" properties of the molecular layer interior.

Photocurrent, Photovoltage, and Rectification in Large‐Area Bilayer Molecular Electronic Junctions

“…Figure D displays the average JV behavior collected under vacuum of the four MJ structures in the present study with the observed RR values ( RR  | J −3.5 V / J +3.5 V |) of the bilayer MJs. All JV curves were observed to be similar to those previously reported, independent of scan rate in the bias region studied, and stable for at least thousands of JV scans. The bilayer JV curves require larger applied bias to achieve current densities similar to the single‐layer MJs due mainly to increased thickness.…”

“…Proposed mechanism of charge transport through C) BTB or D) AQ excitation. Energy levels were determined from DFT and do not include level shifts by electronic coupling with the contacts[11b] or between the AQ and BTB layers.…”

“…Transport through the relatively thick molecular layers is likely mediated by sequential tunneling between molecular subunits with a low activation energy, as discussed recently. [11a] Dissociation and charge transport throughout the bilayer are likely driven by the photo‐induced electric field indicated by the significant V OC present in illuminated bilayers. A second contribution to the PC initiated by AQ absorption at ≈300 nm is shown in Figure D and may be responsible for the PC shoulder at 300 nm in Figure B and Figure S4 in the Supporting Information.…”

Orbital Control of Photocurrents in Large Area All-Carbon Molecular Junctions

“…36 However, the minor changes in the UV−vis spectrum with increasing layer thickness implies relative weak coupling between molecular subunits, and a limited conjugation length within the interior layer. 6 The main question in the current report is a correlation of the absorption and PC spectra, to determine the origin of PC generation. Figure 3 shows overlays of the absorption and PC spectra for FL, AQ, NAB, and NDI, and additional examples are provided in Figures S5−S12.…”

“…We concluded that transport was by multistep tunneling through the H−L barrier in the interior of the MJ, and the smaller barriers at the electrode interfaces were not controlling the current. 6 We proposed that the tunneling barriers are likely located between the molecular subunits, where the electronic interactions between subunits of the molecular layer are weak and delocalization length is limited to one or at most two attached molecules. The direct correlation between optical absorption and PC implies that photon absorption permits carriers to traverse the H−L tunneling barrier and result in measurable PC under illumination.…”