The role of van der Waals forces in the performance of molecular diodes
TL;DR: It is shown experimentally that subtle changes in the intermolecular van der Waals interactions in the active component of a molecular diode dramatically impact the performance of the device.
Abstract: One of the main goals of organic and molecular electronics is to relate the performance and electronic function of devices to the chemical structure and intermolecular interactions of the organic component inside them, which can take the form of an organic thin film, a self-assembled monolayer or a single molecule. This goal is difficult to achieve because organic and molecular electronic devices are complex physical–organic systems that consist of at least two electrodes, an organic component and two (different) organic/inorganic interfaces. Singling out the contribution of each of these components remains challenging. So far, strong π–π interactions have mainly been considered for the rational design and optimization of the performances of organic electronic devices, and weaker intermolecular interactions have largely been ignored. Here, we show experimentally that subtle changes in the intermolecular van der Waals interactions in the active component of a molecular diode dramatically impact the performance of the device. In particular, we observe an odd–even effect as the number of alkyl units is varied in a ferrocene–alkanethiolate self-assembled monolayer. As a result of a more favourable van der Waals interaction, junctions made from an odd number of alkyl units have a lower packing energy (by ∼0.4–0.6 kcal mol–1), rectify currents 10 times more efficiently, give a 10% higher yield in working devices, and can be made two to three times more reproducibly than junctions made from an even number of alkyl units.
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TL;DR: This Review covers the major advances with the most general applicability and emphasizes new insights into the development of efficient platform methodologies for building reliable molecular electronic devices with desired functionalities through the combination of programmed bottom-up self-assembly and sophisticated top-down device fabrication.
Abstract: Creating functional electrical circuits using individual or ensemble molecules, often termed as “molecular-scale electronics”, not only meets the increasing technical demands of the miniaturization of traditional Si-based electronic devices, but also provides an ideal window of exploring the intrinsic properties of materials at the molecular level. This Review covers the major advances with the most general applicability and emphasizes new insights into the development of efficient platform methodologies for building reliable molecular electronic devices with desired functionalities through the combination of programmed bottom-up self-assembly and sophisticated top-down device fabrication. First, we summarize a number of different approaches of forming molecular-scale junctions and discuss various experimental techniques for examining these nanoscale circuits in details. We then give a full introduction of characterization techniques and theoretical simulations for molecular electronics. Third, we highlig...
949 citations
TL;DR: A monolithic perovskite/silicon tandem with a certified power conversion efficiency of 29.15% is reported, made possible by a self-assembled, methyl-substituted carbazole monolayer as the hole-selective layer in the perovSKite cell.
Abstract: Tandem solar cells that pair silicon with a metal halide perovskite are a promising option for surpassing the single-cell efficiency limit. We report a monolithic perovskite/silicon tandem with a certified power conversion efficiency of 29.15%. The perovskite absorber, with a bandgap of 1.68 electron volts, remained phase-stable under illumination through a combination of fast hole extraction and minimized nonradiative recombination at the hole-selective interface. These features were made possible by a self-assembled, methyl-substituted carbazole monolayer as the hole-selective layer in the perovskite cell. The accelerated hole extraction was linked to a low ideality factor of 1.26 and single-junction fill factors of up to 84%, while enabling a tandem open-circuit voltage of as high as 1.92 volts. In air, without encapsulation, a tandem retained 95% of its initial efficiency after 300 hours of operation.
876 citations
TL;DR: This method reliably and reproducibly achieve rectification ratios in excess of 200 at voltages as low as 370 mV using a symmetric oligomer of thiophene-1,1-dioxide and provides a general route for tuning nonlinear nanoscale device phenomena.
Abstract: Single-molecule diodes with rectification ratios over 200 at low voltages can be obtained with symmetric molecules by creating an environmental asymmetry using electric double-layers.
346 citations
TL;DR: This review reviews charge transport across molecular monolayers, which is central to molecular electronics (MolEl), using large-area junctions (NmJ), and points to creation of a built-in electric field as a key to achieve functionality, including nonlinear current-voltage characteristics that originate in the molecules or their contacts to the electrodes.
Abstract: We review charge transport across molecular monolayers, which is central to molecular electronics (MolEl), using large-area junctions (NmJ). We strive to provide a wide conceptual overview of three main subtopics. First, a broad introduction places NmJ in perspective to related fields of research and to single-molecule junctions (1mJ) in addition to a brief historical account. As charge transport presents an ultrasensitive probe for the electronic perfection of interfaces, in the second part ways to form both the monolayer and the contacts are described to construct reliable, defect-free interfaces. The last part is dedicated to understanding and analyses of current–voltage (I–V) traces across molecular junctions. Notwithstanding the original motivation of MolEl, I–V traces are often not very sensitive to molecular details and then provide a poor probe for chemical information. Instead, we focus on how to analyze the net electrical performance of molecular junctions, from a functional device perspective. ...
208 citations
TL;DR: Analysis of rates of tunneling across self-assembled monolayers (SAMs) of n-alkanethiolates SCn and comparisons of conical Ga2O3/EGaIn tips with the characteristics of other top-electrodes suggests that the EGaIn-based electrodes provide a particularly attractive technology for physical-organic studies of charge transport across SAMs.
Abstract: Analysis of rates of tunneling across self-assembled monolayers (SAMs) of n-alkanethiolates SCn (with n = number of carbon atoms) incorporated in junctions having structure AgTS-SAM//Ga2O3/EGaIn leads to a value for the injection tunnel current density J0 (i.e., the current flowing through an ideal junction with n = 0) of 103.6±0.3 A·cm–2 (V = +0.5 V). This estimation of J0 does not involve an extrapolation in length, because it was possible to measure current densities across SAMs over the range of lengths n = 1–18. This value of J0 is estimated under the assumption that values of the geometrical contact area equal the values of the effective electrical contact area. Detailed experimental analysis, however, indicates that the roughness of the Ga2O3 layer, and that of the AgTS-SAM, determine values of the effective electrical contact area that are ∼10–4 the corresponding values of the geometrical contact area. Conversion of the values of geometrical contact area into the corresponding values of effective ...
208 citations
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2,713 citations
TL;DR: Amine link groups are used to form single-molecule junctions with more reproducible current–voltage characteristics and it is found that the conductance for the series decreases with increasing twist angle, consistent with a cosine-squared relation predicted for transport through π-conjugated biphenyl systems.
Abstract: Since it was first suggested1 that a single molecule might function as an active electronic component, a number of techniques have been developed to measure the charge transport properties of single molecules2,3,4,5,6,7,8,9,10,11,12. Although scanning tunnelling microscopy observations under high vacuum conditions can allow stable measurements of electron transport, most measurements of a single molecule bonded in a metal–molecule–metal junction exhibit relatively large variations in conductance. As a result, even simple predictions about how molecules behave in such junctions have still not been rigorously tested. For instance, it is well known13,14 that the tunnelling current passing through a molecule depends on its conformation; but although some experiments have verified this effect15,16,17,18, a comprehensive mapping of how junction conductance changes with molecular conformation is not yet available. In the simple case of a biphenyl—a molecule with two phenyl rings linked by a single C–C bond—conductance is expected to change with the relative twist angle between the two rings, with the planar conformation having the highest conductance. Here we use amine link groups to form single-molecule junctions with more reproducible current–voltage characteristics19. This allows us to extract average conductance values from thousands of individual measurements on a series of seven biphenyl molecules with different ring substitutions that alter the twist angle of the molecules. We find that the conductance for the series decreases with increasing twist angle, consistent with a cosine-squared relation predicted for transport through π-conjugated biphenyl systems13.
1,266 citations
TL;DR: In this article, the impact of interchain interactions within an organic layer on the transport and optical properties is discussed, from a theoretical standpoint, the electronic structure characteristics and interfacial properties that are of importance in all these areas.
Abstract: The pioneering work of Heeger, MacDiarmid, and Shirakawa, rewarded by the 2000 Nobel Prize in Chemistry, has paved the way for the development of the fields of plastic electronics and photonics. Functional organic molecular materials and conjugated oligomers or polymers now allow the low-cost fabrication of thin films for insertion into new generations of electronic and optoelectronic devices. The performance of these devices relies on the understanding and optimization of several complementary processes (see sketch). Our goal is to discuss, from a theoretical standpoint, the electronic structure characteristics and interfacial properties that are of importance in all these areas. The concept of interface should be taken here in the microscopic sense, i.e., molecular interactions among two or several chains/molecules (of the same or of a different nature). Specifically, we will address the impact of interchain interactions within an organic layer on the transport and optical properties. These issues will therefore be more directly related to transistor and light-emitting diode applications; however, in all instances, the aspects related to interfacial charge or energy transfer processes will dictate the ultimate performance of a material in a given device.
908 citations
TL;DR: It is shown how molecular-scale engineering of these interlayers to form stepped and graded electronic profiles can lead to remarkably efficient single-layer polymer LEDs that exhibit nearly balanced injection, near-perfect recombination, and greatly reduced pre-turn-on leakage currents.
Abstract: Achieving balanced electron-hole injection and perfect recombination of the charge carriers is central to the design of efficient polymer light-emitting diodes (LEDs). A number of approaches have focused on modification of the injection contacts, for example by incorporating an additional conducting-polymer layer at the indium-tin oxide (ITO) anode. Recently, the layer-by-layer polyelectrolyte deposition route has been developed for the fabrication of ultrathin polymer layers. Using this route, we previously incorporated ultrathin (<100 A) charge-injection interfacial layers in polymer LEDs. Here we show how molecular-scale engineering of these interlayers to form stepped and graded electronic profiles can lead to remarkably efficient single-layer polymer LEDs. These devices exhibit nearly balanced injection, near-perfect recombination, and greatly reduced pre-turn-on leakage currents. A green-emitting LED comprising a poly(p-phenylene vinylene) derivative sandwiched between a calcium cathode and the modified ITO anode yields an external forward efficiency of 6.0 per cent (estimated internal efficiency, 15-20 per cent) at a luminance of 1,600 candelas per m2 at 5 V.
740 citations