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Light‐Stimulated Charge Transport In Bilayer Molecular Junctions For Photodetection

S. K. Saxena, S. R. Smith, Mustafa Supur, R. McCreery
Published 2019 · Materials Science

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DOI: 10.1002/adom.201901053 been studied extensively.[1,3] The field is driven in part by the possibility of new electronic functions in molecular devices and the wide variety of molecular structures available, presumably with a broad range of electronic behaviors.[4] “Large area” or “ensemble” MJs with partially transparent contacts enable the use of optical spectroscopy for characterization and monitoring of molecular electronic devices using infrared absorption,[5] Raman,[6] and UV-vis spectroscopy.[4d,7] While optical spectroscopy is normally a probe of device structure during fabrication and operation, the field of “molecular optoelectronics” investigates stimulation of transport with light or generation of light in response to an external bias.[8] Photocurrent (PC) generation and changes in MJ conductance by incident light have been reported for single molecules and molecular ensembles and examined theoretically.[4a,9] We have reported PCs and photoconductance changes in carbon-based MJs, mediated by internal photoemission (IPE)[10] and optical absorption in the molecular layer.[11] The observed PC tracks the in situ UV-vis absorption spectrum of the molecular layer when the transport distance exceeds 5 nm, and the PC polarity (i.e., the charge transport direction) and induced photovoltage correlate strongly with which orbitals mediate transport (highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO)).[11b] Carbon-based MJs also exhibit photoemission from hot electrons,[8c,12] and transport in thiol-based large-area MJs can couple to plasmons in the contacts to emit light.[8d,13] For PCs in unbiased, Au/carbon/molecule/carbon/Au MJs with identical top and bottom electrodes, the direction of illumination did not cause a change in PC sign, indicating that there must be some inherent asymmetry in the device. We attributed the asymmetry in single-component carbon-based MJs to a difference in electronic coupling at the two electrode contacts, one of which is covalent and the other physisorbed.[11b] This effect was predicted theoretically by Galperin et al.[8b,14] Since the internal electric field generated by the effect is small (10–30 mV), the resulting PCs are also small for single-component MJs, and would likely be absent if the electronic coupling to both electrodes were identical. We recently reported a different approach in which a molecular bilayer was used to create asymmetry, leading to significantly larger PCs.[15] Successive reduction of two diazonium reagents[16] resulted in a covalent Eleven bilayer molecular junctions (MJs) consisting of two different 5–7 nm thick molecular layers between conducting contacts are investigated to determine how orbital energies and optical absorbance spectra of the oligomers affect the photocurrent (PC) response, the direction of photoinduced charge transport, and maximum response wavelength. Photometric sensitivity of 2 mA W−1 and a detection limit of 11 pW are demonstrated for MJs, yielding an internal quantum efficiency of 0.14 electrons per absorbed photon. For unbiased MJs, the PC tracks the absorption spectrum of the molecular layer, and is stable for >5 h of illumination. The organic/organic (O/O) interface between the molecular layers within bilayer MJs is the primary determinant of PC polarity, and the bilayer MJ mechanism is conceptually similar to that of a single O/O heterojunction studied in bilayers of much greater thickness. The charge transport direction of the 11 MJs is completely consistent with hole-dominated transport of photogenerated carriers. For MJs illuminated while an external bias is applied, the PC greatly exceeds the dark current by factors of 102 to 105, depending on bias, bilayer structure, and wavelength. The bilayer MJs are amenable to flexible substrates, and may have applications as sensitive, wavelength-specific photodetectors.
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