The alignment of the electrode Fermi level with the valence or conduction bands of organic semiconductors is a key parameter controlling the efficiency of organic light-emitting diodes, solar cells, and printed circuits. Here, we introduce a class of organic molecules that form highly robust dipole layers, capable of shifting the work function of noble metals (Au and Ag) down to 3.1 eV, that is, ∼1 eV lower than previously reported self-assembled monolayers. The physics behind the considerable interface dipole is elucidated by means of photoemission spectroscopy and density functional theory calculations, and a polymer diode exclusively based on the surface modification of a single electrode in a symmetric, two-terminal Au/poly(3-hexylthiophene)/Au junction is presented. The diode exhibits the remarkable rectification ratio of ∼2·10(3), showing high reproducibility, durability (>3 years), and excellent electrical stability. With this evidence, noble metal electrodes with work function values comparable to that of standard cathode materials used in optoelectronic applications are demonstrated.

or inorganic [ 13,18 ] interlayers have been introduced to protect ultrathin molecular layers from invasive, vapor deposited contacts. These approaches represent a promising pathway toward robust molecular/polymer circuits; however they require an additional organic/inorganic layer that inevitably masks the intrinsic electrical response of the molecules under investigation. A possible solution has been proposed by McCreery and co-workers, involving molecular layers that are sandwiched between carbon and copper electrodes forming stable and highly reproducible molecular junctions. [ 19 ] Remarkably, these large area junctions show high yields, endurance, and temperature stability, even though the requirement of using pyrolytic carbon as a bottom electrode might limit their applicability.Here, we demonstrate that by integration of Fe II -terpyridine redox complex oligomers [20][21][22] into large area solid-state junctions, molecular thin-fi lm devices of outstanding mechanical and electrical robustness are realized. Notwithstanding the metallic crossbar junctions are deposited in a conventional thermal evaporation process, Fe II -terpyridine oligomers are operational over a period of more than two and a half years and resist to temperatures ranging from 150-360 K. The oligomer layers show a high electron mobility ( µ e = 0.1 cm 2 V −1 s −1 ) and, most remarkably, electrical transport follows an ideal RichardsonSchottky (RS) injection behavior, as demonstrated by means of complementary experimental and theoretical investigations.Bottom electrodes are prepared by thermal evaporation of an array of eight parallel Au electrodes (each 100 µm wide) on native silicon using a shadow mask. Subsequently, metal center oligomers (MCOs) are deposited by a stepwise sequential coordination reaction of a Fe II redox center by a conjugated 1,4-di(2;2′;6′;2″-terpyridine-4′-yl)benzene (TPT) ligand ( Figure 1 a), [ 21 ] as schematically depicted in Figure 1 b. In our work, oligomers of three different lengths have been assembled by incorporation of 15, 20, and 30 Fe II metal centers (MC), yielding MCO layers with a thickness of 15, 20, and 30 nm. This allows a detailed study of their electrical characteristics as a function of molecular length. A symmetric contact of the oligomers to both Au electrodes is established by using 4′-(4-mercaptophenyl)terpyridine (MPTP) as the fi rst and last ligands of the stepwise coordination.From density functional theory (DFT) calculations, a length of 1.55 nm is derived for the repeat unit of the MCO chain (Figure 1 a). A constant increment in fi lm thickness as a function of the coordination number is determined from AFM data (Figure 1 c), following a linear regression with a slope of ≈1.08 nm per coordination step. These data and the coordination effi ciency known for the stepwise coordination process [ 23 ]

A new process is presented that combines nanoimprint lithography and soft lithography to assemble metal–bridge–metal crossbar junctions at ambient conditions. High density top and bottom metal electrodes with half‐pitches down to 50 nm are fabricated in a parallel process by means of ultraviolet nanoimprint lithography. The top electrodes are realized on top of a sacrificial layer and are embedded in a polymer matrix. The lifting of the top electrodes by dissolving the sacrificial layer in an aqueous solution results in printable electrode stamps. Crossbar arrays are noninvasively assembled with high yield by printing the top electrode stamps onto bare or modified bottom electrodes. A semiconducting and a quasi metal like conducting type of polymer are incorporated in the cross points to form metal‐polymer‐metal junctions. The electrical characterization of the printed junctions revealed that the functional integrity of the electrically addressed conductive polymers is conserved during the assembling process. These findings suggest that printing of electrodes represents an easy and cost effective route to highly integrated nanoscale metal‐bridge‐metal junctions if imprint lithography is used for electrode fabrication.

We have investigated bipolar resistive switching of Cu/CuTCNQ/Al cross-junctions in both vacuum and different gas environments. While the generally observed S-shaped I-V hysteresis was reproduced in ambient air, it was reversibly suppressed in well-degassed samples in vacuum and in dry N2. The OFF-switching currents in ambient air peaked when approximately +2.6 V was applied to the Al electrode at low voltage sweep rates. OFF-switching at constant bias was accelerated in humid and oxygen-rich atmospheres. For unbiased samples stored in air, ON-state (RON) and OFF-state (ROFF) resistances increased with time, and RON surpassed the initial ROFF after approximately one week. Retention times were enhanced for samples stored in vacuum and those with a larger cross-junction area. We suggest that resistive switching occurs in a hydrated native alumina layer at the CuTCNQ/Al interface that grows in thickness during exposure to ambient humidity: ON-switching by electrochemical metallization of free Al and/or Cu ions and OFF-switching by anodic oxidation of the Al electrode and previously grown metal filaments.

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