Electronic Polarization at Pentacene/Polymer Dielectric Interfaces: Imaging Surface Potentials and Contact Potential Differences as a Function of Substrate Type, Growth Temperature, and Pentacene Microstructure

Interfaces between organic semiconductors and dielectrics may exhibit interfacial electronic polarization, which is equivalently quantified as a contact potential difference (CPD), an interface dipole, or a vacuum level shift. Here we report quantitative measurements by scanning Kelvin probe microscopy (SKPM) of surface potentials and CPDs across ultrathin (1–2 monolayer) crystalline islands of the benchmark semiconductor pentacene thermally deposited on a variety of polymer dielectrics (e.g., poly­(methyl methacrylate), polystyrene). The CPDs between the pentacene islands and the polymer substrates are in the range of −10 to +50 mV, they depend strongly on the polymer type and deposition temperature, and the CPD magnitude is correlated with the dipole moment of the characteristic monomers. Surface potential variations within 2 monolayer (3 nm) thick pentacene islands are ∼15 mV and may be ascribed to microstructure (epitaxial) differences. Overall, the microscopy results reveal both strong variations in interfacial polarization and lateral electrostatic heterogeneity; these factors ultimately should affect the performance of these interfaces in devices

Measuring the Thickness and Potential Profiles of the Space-Charge Layer at Organic/Organic Interfaces under Illumination and in the Dark by Scanning Kelvin Probe Microscopy

Scanning Kelvin probe microscopy was used to measure band-bending at the model donor/acceptor heterojunction poly­(3-hexylthiophene) (P3HT)/fullerene (C₆₀). Specifically, we measured the variation in the surface potential of C₆₀ films with increasing thicknesses grown on P3HT to produce a surface potential profile normal to the substrate both in the dark and under illumination. The results confirm a space-charge carrier region with a thickness of 10 nm, consistent with previous observations. We discuss the possibility that the domain size in bulk heterojunction organic solar cells, which is comparable to the space-charge layer thickness, is actually partly responsible for less than expected electron/hole recombination rates

Experimental and Theoretical Analysis of Nanotransport in Oligophenylene Dithiol Junctions as a Function of Molecular Length and Contact Work Function

We report the results of an extensive investigation of metal–molecule–metal tunnel junctions based on oligophenylene dithiols (OPDs) bound to several types of electrodes (M₁–S–(C₆H₄)<i>_n</i>–S–M₂, with 1 ≤ <i>n</i> ≤ 4 and M_1,2 = Ag, Au, Pt) to examine the impact of molecular length (<i>n</i>) and metal work function (Φ) on junction properties. Our investigation includes (1) measurements by scanning Kelvin probe microscopy of electrode work function changes (ΔΦ = Φ_SAM – Φ) caused by chemisorption of OPD self-assembled monolayers (SAMs), (2) measurements of junction current–voltage (<i>I</i>–<i>V</i>) characteristics by conducting probe atomic force microscopy in the linear and nonlinear bias ranges, and (3) direct quantitative analysis of the full <i>I</i>–<i>V</i> curves. Further, we employ transition voltage spectroscopy (TVS) to estimate the energetic alignment ε_h = <i>E</i>_F – <i>E</i>_HOMO of the dominant molecular orbital (HOMO) relative to the Fermi energy <i>E</i>_F of the junction. Where photoelectron spectroscopy data are available, the ε_h values agree very well with those determined by TVS. Using a single-level model, which we justify <i>via ab initio</i> quantum chemical calculations at post-density functional theory level and additional UV–visible absorption measurements, we are able to quantitatively reproduce the <i>I</i>–<i>V</i> measurements in the whole bias range investigated (∼1.0–1.5 V) and to understand the behavior of ε_h and Γ (contact coupling strength) extracted from experiment. We find that Fermi level pinning induced by the strong dipole of the metal–S bond causes a significant shift of the HOMO energy of an adsorbed molecule, resulting in ε_h exhibiting a weak dependence with the work function Φ. Both of these parameters play a key role in determining the tunneling attenuation factor (β) and junction resistance (<i>R</i>). Correlation among Φ, ΔΦ, <i>R</i>, transition voltage (<i>V</i>_t), and ε_h and accurate simulation provide a remarkably complete picture of tunneling transport in these prototypical molecular junctions

Exceptionally Small Statistical Variations in the Transport Properties of Metal–Molecule–Metal Junctions Composed of 80 Oligo­phenylene Dithiol Molecules

Strong stochastic fluctuations witnessed as very broad resistance (<i>R</i>) histograms with widths comparable to or even larger than the most probable values characterize many measurements in the field of molecular electronics, particularly those measurements based on single molecule junctions at room temperature. Here we show that molecular junctions containing 80 oligophenylene dithiol molecules (OPDn, 1 ≤ <i>n</i> ≤ 4) connected in parallel display small relative statistical deviationsδ<i>R</i>/<i>R</i> ≈ 25% after only ∼200 independent measurementsand we analyze the sources of these deviations quantitatively. The junctions are made by conducting probe atomic force microscopy (CP-AFM) in which an Au-coated tip contacts a self-assembled monolayer (SAM) of OPDs on Au. Using contact mechanics and direct measurements of the molecular surface coverage, the tip radius, tip-SAM adhesion force (<i>F</i>), and sample elastic modulus (<i>E</i>), we find that the tip-SAM contact area is approximately 25 nm², corresponding to about 80 molecules in the junction. Supplementing this information with <i>I–V</i> data and an analytic transport model, we are able to quantitatively describe the sources of deviations <i>δR</i> in <i>R</i>: namely, <i>δN</i> (deviations in the number of molecules in the junction), <i>δε</i> (deviations in energetic position of the dominant molecular orbital), and <i>δΓ</i> (deviations in molecule-electrode coupling). Our main results are (1) direct determination of <i>N</i>; (2) demonstration that <i>δN</i>/<i>N</i> for CP-AFM junctions is remarkably small (≤2%) and that the largest contributions to <i>δR</i> are <i>δε</i> and <i>δΓ</i>; (3) demonstration that δ<i>R</i>/<i>R</i> after only ∼200 measurements is substantially smaller than most reports based on >1000 measurements for single molecule break junctions. Overall, these results highlight the excellent reproducibility of junctions composed of tens of parallel molecules, which may be important for continued efforts to build robust molecular devices

High-Mobility Transistors Based on Single Crystals of Isotopically Substituted Rubrene‑<i>d</i>₂₈

We have performed a comprehensive study of chemical synthesis, crystal growth, crystal quality, and electrical transport properties of isotopically substituted rubrene-<i>d</i>₂₈ single crystals (D-rubrene, C₄₂D₂₈). Using a modified synthetic route for protonated-rubrene (H-rubrene, C₄₂H₂₈), we have obtained multigram quantities of rubrene with deuterium incorporation approaching 100%. We found that the vapor-grown D-rubrene single crystals, whose high qualities were confirmed by X-ray diffraction and atomic force microscopy, maintained the remarkable transport properties originally manifested by H-rubrene crystals. Specifically, field-effect hole mobility above 10 cm² V^–1 s^–1 was consistently achieved in the vacuum-gap transistor architecture at room temperature, with an intrinsic band-like transport behavior observed over a broad temperature range; maximum hole mobility reached 45 cm² V^–1 s^–1 near 100 K. Theoretical analysis provided estimates of the density and characteristic energy of shallow and deep traps presented in D-rubrene crystals. Overall, the successful synthesis and characterization of rubrene-<i>d</i>₂₈ paves an important pathway for future spin-transport experiments in which the H/D isotope effect on spin lifetime can be examined in the testbed of rubrene

An ADMET Route to Low-Band-Gap Poly(3-hexadecylthienylene vinylene): A Systematic Study of Molecular Weight on Photovoltaic Performance

The effect of molecular weight on organic photovoltaic device performance is investigated for a series of low-band-gap (ca. 1.65 eV) poly­(3-hexadecylthienylene vinylene)­s (C16-PTVs) prepared by acyclic diene metathesis (ADMET) polymerization. By utilizing monomers of varying cis:trans (<i>Z</i>:<i>E</i>) content, seven C16-PTVs were prepared with a number-average molecular weight range of 6–30 kg/mol. Polymers were characterized by size-exclusion chromatography, ¹H NMR spectroscopy, ultraviolet–visible spectroscopy, thermogravimetric analysis, wide-angle X-ray scattering, and differential scanning calorimetry. C16-PTVs were integrated into bulk-heterojunction (BHJ) solar cells with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), and conversion efficiency was found to increase with increasing molecular weight. This observation is attributable to an increase in polymer aggregation in the solid state and a corresponding increase in hole mobility. Finally, phase behavior and morphology of the C16-PTV:PCBM active layers were investigated by differential scanning calorimetry and atomic force microscopy, respectively

Rubrene-Based Single-Crystal Organic Semiconductors: Synthesis, Electronic Structure, and Charge-Transport Properties

Correlations among the molecular structure, crystal structure, electronic structure, and charge-carrier transport phenomena have been derived from six congeners (<b>2</b>–<b>7</b>) of rubrene (<b>1</b>). The congeners were synthesized via a three-step route from known 6,11-dichloro-5,12-tetracenedione. After crystallization, their packing structures were solved using single-crystal X-ray diffraction. Rubrenes <b>5</b>–<b>7</b> maintain the orthorhombic features of the parent rubrene (<b>1</b>) in their solid-state packing structures. Control of the packing structure in <b>5</b>–<b>7</b> provided the first series of systematically manipulated rubrenes that preserve the π-stacking motif of <b>1</b>. Density functional theory calculations were performed at the B3LYP/6-31G­(d,p) level of theory to evaluate the geometric and electronic structure of each derivative and reveal that key properties of rubrene (<b>1</b>) have been maintained. Intermolecular electronic couplings (transfer integrals) were calculated for each derivative to determine the propensity for charge-carrier transport. For rubrenes <b>5</b>–<b>7</b>, evaluations of the transfer integrals and periodic electronic structures suggest these derivatives should exhibit transport characteristics equivalent to, or in some cases improved on, those of the parent rubrene (<b>1</b>), as well as the potential for ambipolar behavior. Single-crystal field-effect transistors were fabricated for <b>5</b>–<b>7</b>, and these derivatives show ambipolar transport as predicted. Although device architecture has yet to be fully optimized, maximum hole (electron) mobilities of 1.54 (0.28) cm² V^–1 s^–1 were measured for rubrene <b>5</b>. This work lays a foundation to improve our understanding of charge-carrier transport phenomena in organic single-crystal semiconductors through the correlation of designed molecular and crystallographic changes to electronic and transport properties

Rubrene-Based Single-Crystal Organic Semiconductors: Synthesis, Electronic Structure, and Charge-Transport Properties

Correlations among the molecular structure, crystal structure, electronic structure, and charge-carrier transport phenomena have been derived from six congeners (<b>2</b>–<b>7</b>) of rubrene (<b>1</b>). The congeners were synthesized via a three-step route from known 6,11-dichloro-5,12-tetracenedione. After crystallization, their packing structures were solved using single-crystal X-ray diffraction. Rubrenes <b>5</b>–<b>7</b> maintain the orthorhombic features of the parent rubrene (<b>1</b>) in their solid-state packing structures. Control of the packing structure in <b>5</b>–<b>7</b> provided the first series of systematically manipulated rubrenes that preserve the π-stacking motif of <b>1</b>. Density functional theory calculations were performed at the B3LYP/6-31G­(d,p) level of theory to evaluate the geometric and electronic structure of each derivative and reveal that key properties of rubrene (<b>1</b>) have been maintained. Intermolecular electronic couplings (transfer integrals) were calculated for each derivative to determine the propensity for charge-carrier transport. For rubrenes <b>5</b>–<b>7</b>, evaluations of the transfer integrals and periodic electronic structures suggest these derivatives should exhibit transport characteristics equivalent to, or in some cases improved on, those of the parent rubrene (<b>1</b>), as well as the potential for ambipolar behavior. Single-crystal field-effect transistors were fabricated for <b>5</b>–<b>7</b>, and these derivatives show ambipolar transport as predicted. Although device architecture has yet to be fully optimized, maximum hole (electron) mobilities of 1.54 (0.28) cm² V^–1 s^–1 were measured for rubrene <b>5</b>. This work lays a foundation to improve our understanding of charge-carrier transport phenomena in organic single-crystal semiconductors through the correlation of designed molecular and crystallographic changes to electronic and transport properties

Controllable Spin–Orbit Torque Induced by Interfacial Ion Absorption in Ta/CoFeB/MgO Multilayers with Canted Magnetizations

Electrically generated spin–orbit torque (SOT) has emerged as a powerful pathway to control magnetization for spintronic applications including memory, logic, and neurocomputing. However, the requirement of external magnetic fields, together with the ultrahigh current density, is the main obstacle for practical SOT devices. In this paper, we report that the field-free SOT-driven magnetization switching can be successfully realized by interfacial ion absorption in perpendicular Ta/CoFeB/MgO multilayers. Besides, the tunable SOT efficiency exhibits a strong dependence on interfacial Ti insertion thicknesses. Polarized neutron reflection measurements demonstrate the existence of canted magnetization with Ti inserted, which leads to deterministic magnetization switching. In addition, interfacial characterization and first-principles calculations reveal that B absorption by the Ti layer is the main cause behind the enhanced interfacial transparency, which determines the tunable SOT efficiency. Our findings highlight an attractive scheme to a purely electric control spin configuration, enabling innovative designs for SOT-based spintronics via interfacial engineering