Theory for nonlinear conductivity switching in semiconducting organic ferroelectrics.

In this work, the ferroelectric and semiconducting properties of the organic semiconducting ferroelectric benzotrithiophene tricarboxamide (BTTTA), and especially their nonlinear coupling, are theoretically investigated. BTTTA is an exponent of a small class of semiconducting organic ferroelectrics for which experiments have established a surprising polarization direction dependence of the bulk conductivity at finite fields. First, molecular dynamics (MD) simulations are used to investigate the occurrence and, under the influence of an external electric field, the inversion of the macroscopic electric dipole that forms along the axis of supramolecular columns of BTTTA. The MD results are consistent with the experimentally observed ferroelectric behavior of the material. Building on the MD results, a QM/MM scheme is used to investigate the charge carrier mobility in the quasi-1D BTTTA stacks in the linear and non-linear regimes. Indeed, at finite electric fields, a clear resistance switching effect was observed in the form of a hole mobility that is a factor ∼2 larger for antiparallel orientations of the polarization and field than for a parallel orientation. This phenomenon can be understood as a microscopic ratchet that is based on the non-equilibrium interaction between the (oriented) dipoles and the (direction of the) charge transport.

Ferro- and ferrielectricity and negative piezoelectricity in thioamide-based supramolecular organic discotics.

Amide-based discotic supramolecular organic materials are of interest for fundamental understanding of cooperative self-assembly and collective dipole switching mechanisms as well as for practically relevant ferroelectric and piezoelectric properties. Here, we show how replacing amides (dipole moment of ∼3.5 D) with thioamides (∼5.1 D) as dipolar moieties in the archetypal C3-symmetric discotic molecule BTA leads to ferroelectric materials with a higher remnant polarization and lower coercive field. The thioamide-based materials also demonstrate a rare negative piezoelectricity and a previously predicted, yet never experimentally observed, polarization reversal via asymmetric intermediate states, that is, ferrielectric switching.

Nonequilibrium site distribution governs charge-transfer electroluminescence at disordered organic heterointerfaces.

The interface between electron-donating (D) and electron-accepting (A) materials in organic photovoltaic (OPV) devices is commonly probed by charge-transfer (CT) electroluminescence (EL) measurements to estimate the CT energy, which critically relates to device open-circuit voltage. It is generally assumed that during CT-EL injected charges recombine at close-to-equilibrium energies in their respective density of states (DOS). Here, we explicitly quantify that CT-EL instead originates from higher-energy DOS site distributions significantly above DOS equilibrium energies. To demonstrate this, we have developed a quantitative and experimentally calibrated model for CT-EL at organic D/A heterointerfaces, which simultaneously accounts for the charge transport physics in an energetically disordered DOS and the Franck-Condon broadening. The 0-0 CT-EL transition lineshape is numerically calculated using measured energetic disorder values as input to 3-dimensional kinetic Monte Carlo simulations. We account for vibrational CT-EL overtones by selectively measuring the dominant vibrational phonon-mode energy governing CT luminescence at the D/A interface using fluorescence line-narrowing spectroscopy. Our model numerically reproduces the measured CT-EL spectra and their bias dependence and reveals the higher-lying manifold of DOS sites responsible for CT-EL. Lowest-energy CT states are situated ∼180 to 570 meV below the 0-0 CT-EL transition, enabling photogenerated carrier thermalization to these low-lying DOS sites when the OPV device is operated as a solar cell rather than as a light-emitting diode. Nonequilibrium site distribution rationalizes the experimentally observed weak current-density dependence of CT-EL and poses fundamental questions on reciprocity relations relating light emission to photovoltaic action and regarding minimal attainable photovoltaic energy conversion losses in OPV devices.

Ground-state electron transfer in all-polymer donor-acceptor heterojunctions.

Doping of organic semiconductors is crucial for the operation of organic (opto)electronic and electrochemical devices. Typically, this is achieved by adding heterogeneous dopant molecules to the polymer bulk, often resulting in poor stability and performance due to dopant sublimation or aggregation. In small-molecule donor-acceptor systems, charge transfer can yield high and stable electrical conductivities, an approach not yet explored in all-conjugated polymer systems. Here, we report ground-state electron transfer in all-polymer donor-acceptor heterojunctions. Combining low-ionization-energy polymers with high-electron-affinity counterparts yields conducting interfaces with resistivity values five to six orders of magnitude lower than the separate single-layer polymers. The large decrease in resistivity originates from two parallel quasi-two-dimensional electron and hole distributions reaching a concentration of ∼1013 cm-2. Furthermore, we transfer the concept to three-dimensional bulk heterojunctions, displaying exceptional thermal stability due to the absence of molecular dopants. Our findings hold promise for electro-active composites of potential use in, for example, thermoelectrics and wearable electronics.

General rule for the energy of water-induced traps in organic semiconductors.

Charge carrier traps are generally highly detrimental for the performance of semiconductor devices. Unlike the situation for inorganic semiconductors, detailed knowledge about the characteristics and causes of traps in organic semiconductors is still very limited. Here, we accurately determine hole and electron trap energies for a wide range of organic semiconductors in thin-film form. We find that electron and hole trap energies follow a similar empirical rule and lie ~0.3-0.4 eV above the highest occupied molecular orbital and below the lowest unoccupied molecular orbital, respectively. Combining experimental and theoretical methods, the origin of the traps is shown to be a dielectric effect of water penetrating nanovoids in the organic semiconductor thin film. We also propose a solvent-annealing method to remove water-related traps from the materials investigated, irrespective of their energy levels. These findings represent a step towards the realization of trap-free organic semiconductor thin films.

Double doping of conjugated polymers with monomer molecular dopants.

Molecular doping is a crucial tool for controlling the charge-carrier concentration in organic semiconductors. Each dopant molecule is commonly thought to give rise to only one polaron, leading to a maximum of one donor:acceptor charge-transfer complex and hence an ionization efficiency of 100%. However, this theoretical limit is rarely achieved because of incomplete charge transfer and the presence of unreacted dopant. Here, we establish that common p-dopants can in fact accept two electrons per molecule from conjugated polymers with a low ionization energy. Each dopant molecule participates in two charge-transfer events, leading to the formation of dopant dianions and an ionization efficiency of up to 200%. Furthermore, we show that the resulting integer charge-transfer complex can dissociate with an efficiency of up to 170%. The concept of double doping introduced here may allow the dopant fraction required to optimize charge conduction to be halved.

Kinetic Monte Carlo simulations of organic ferroelectrics.

Ferroelectrics find broad applications, e.g. in non-volatile memories, but the switching kinetics in real, disordered, materials is still incompletely understood. Here, we develop an electrostatic model to study ferroelectric switching using 3D Monte Carlo simulations. We apply this model to the prototypical small molecular ferroelectric trialkylbenzene-1,3,5-tricarboxamide (BTA) and find good agreement between the Monte Carlo simulations, experiments, and molecular dynamics studies. Since the model lacks any explicit steric effects, we conclude that these are of minor importance. While the material is shown to have a frustrated antiferroelectric ground state, it behaves as a normal ferroelectric under practical conditions due to the large energy barrier for switching that prevents the material from reaching its ground state after poling. We find that field-driven polarization reversal and spontaneous depolarization have orders of magnitude different switching kinetics. For the former, which determines the coercive field and is relevant for data writing, nucleation occurs at the electrodes, whereas for the latter, which governs data retention, nucleation occurs at disorder-induced defects. As a result, by reducing the disorder in the system, the polarization retention time can be increased dramatically while the coercive field remains unchanged.

Suppressing depolarization by tail substitution in an organic supramolecular ferroelectric.

Despite being very well established in the field of electro-optics, ferroelectric liquid crystals so far lacked interest from a ferroelectric device perspective due to a typically high operating temperature, a modest remnant polarization and/or poor polarization retention. Here, we experimentally demonstrate how simple structural modification of a prototypical ferroelectric liquid-crystal benzene-1,3,5-trisamide (BTA) - introduction of branched-tail substituents - results in materials with a wide operating temperature range and a data retention time of more than 10 years in thin-film solution-processed capacitor devices at room temperature. The observed differences between linear- and branched-tail compounds are analyzed using density functional theory (DFT) and molecular dynamics (MD) simulations. We conclude that morphological factors like improved packing quality and reduced disorder, rather than electrostatic interactions or intra/inter-columnar steric hindrance, underlay the superior properties of the branched-tailed BTAs. Synergistic effects upon blending of compounds with branched and linear side-chains can be used to further improve the materials' characteristics.

A Versatile Method for the Preparation of Ferroelectric Supramolecular Materials via Radical End-Functionalization of Vinylidene Fluoride Oligomers.

A synthetic method for the end-functionalization of vinylidene fluoride oligomers (OVDF) via a radical reaction between terminal olefins and I-OVDF is described. The method shows a wide substrate scope and excellent conversions, and permits the preparation of different disc-shaped cores such as benzene-1,3,5-tricarboxamides (BTAs), perylenes bisimide (PBI), and phthalocyanines (Pc) bearing three to eight ferroelectric oligomers at their periphery. The formation, purity, OVDF conformation, and morphology of the final adducts has been assessed by a combination of techniques, such as NMR, size exclusion chromatography, differential scanning calorimetry, polarized optical microscopy, and atomic force microscopy. Finally, PBI-OVDF and Pc-OVDF materials show ferroelectric hysteresis behavior together with high remnant polarizations, with values as high as Pr ≈ 37 mC/m(2) for Pc-OVDF. This work demonstrates the potential of preparing a new set of ferroelectric materials simply by attaching OVDF oligomers to different small molecules. The use of carefully chosen small molecules paves the way to new functional materials in which ferroelectricity and electrical conductivity or light-harvesting properties coexist in a single compound.

Spontaneous Modulation Doping in Semi-Crystalline Conjugated Polymers Leads to High Conductivity at Low Doping Concentration.

The possibility to control the charge carrier density through doping is one of the defining properties of semiconductors. For organic semiconductors, the doping process is known to come with several problems associated with the dopant compromising the charge carrier mobility by deteriorating the host morphology and/or introducing Coulomb traps. While for inorganic semiconductors these factors can be mitigated through (top-down) modulation doping, this concept has not been employed in organics. Here, this work shows that properly chosen host/dopant combinations can give rise to spontaneous, bottom-up modulation doping, in which the dopants preferentially sit in an amorphous phase, while the actual charge transport occurs predominantly in a crystalline phase with an unaltered microstructure, spatially separating dopants and mobile charges. Combining experiments and numerical simulations, this work shows that this leads to exceptionally high conductivities at relatively low dopant concentrations.

Universal transients in polymer and ionic transition metal complex light-emitting electrochemical cells.

Two types of light-emitting electrochemical cells (LECs) are commonly distinguished, the polymer-based LEC (p-LEC) and the ionic transition metal complex-based LEC (iTMC-LEC). Apart from marked differences in the active layer constituents, these LEC types typically show operational time scales that can differ by many orders of magnitude at room temperature. Here, we demonstrate that despite these differences p-LECs and iTMC-LECs show current, light output, and efficacy transients that follow a universal shape. Moreover, we conclude that the turn-on time of both LEC types is dominated by the ion conductivity because the turn-on time exhibits the same activation energy as the ion conductivity in the off-state. These results demonstrate that both types of LECs are really two extremes of one class of electroluminescent devices. They also implicate that no fundamental difference exists between charge transport in small molecular weight or polymeric mixed ionic and electronic conductive materials. Additionally, it follows that the ionic conductivity is responsible for the dynamic properties of devices and systems using them. This likely extends to mixed ionic and electronic conductive materials used in organic solar cells and in a variety of biological systems.

Charge trapping by self-assembled monolayers as the origin of the threshold voltage shift in organic field-effect transistors.

The threshold voltage is an important property of organic field-effect transistors. By applying a self-assembled monolayer (SAM) on the gate dielectric, the value can be tuned. After electrical characterization, the semiconductor is delaminated. The surface potentials of the revealed SAM perfectly agree with the threshold voltages, which demonstrate that the shift is not due to the dipolar contribution, but due to charge trapping by the SAM.

Organic electronic ratchets doing work.

The possibility to extract work from periodic, undirected forces has intrigued scientists for over a century­in particular, the rectification of undirected motion of particles by ratchet potentials, which are periodic but asymmetric functions. Introduced by Smoluchowski and Feynman to study the (dis)ability to generate motion from an equilibrium situation, ratchets operate out of equilibrium, where the second law of thermodynamics no longer applies. Although ratchet systems have been both identified in nature and used in the laboratory for the directed motion of microscopic objects, electronic ratchets have been of limited use, as they typically operate at cryogenic temperatures and generate subnanoampere currents and submillivolt voltages. Here, we present organic electronic ratchets that operate up to radio frequencies at room temperature and generate currents and voltages that are orders of magnitude larger. This enables their use as a d.c. power source. We integrated the ratchets into logic circuits, in which they act as the d.c. equivalent of the a.c. transformer, and generate enough power to drive the circuitry. Our findings show that electronic ratchets may be of actual use.

The application of x-rays in radiology: from difficult and dangerous to simple and safe.

OBJECTIVE: This article will provide an assessment of the application of x-rays in the early days of radiology, which is an excellent way to come to value the convenience and safety of modern x-ray systems. CONCLUSION: The gas tubes that were originally applied for x-ray production were very unstable because of variations in the tube's vacuum. In an effort to understand some of the problems of these tubes and the high occupational exposure that was indirectly caused by the tubes' erratic behavior, we measured x-ray output rates as a function of the gas pressure inside the tube. The pressure range for the optimal production of x-rays, using an original Ruhmkorff inductor as a high-voltage generator, was found to be narrow. With the vacuum changing over time, this might explain the many photographs from the first years of radiology with operators watching their unshielded tube, either with bare eyes or with a fluoroscope, and their own hand as a test object. This practice often led to severe damage of the hands and to many early deaths due to cancer. Today, after a century of technologic development of x-ray tubes and associated equipment, the total average effective dose of workers in radiology can be close to natural background levels.

Can Organic Solar Cells Beat the Near-Equilibrium Thermodynamic Limit?

Despite an impressive increase over the past decade, experimentally determined power conversion efficiencies of organic photovoltaic cells still fall considerably below the theoretical upper bound for near-equilibrium solar cells. Even in otherwise optimized devices, a prominent yet incompletely understood loss channel is the thermalization of photogenerated charge carriers in the density of states that is broadened by energetic disorder. Here, we demonstrate by extensive numerical modeling how this loss channel can be mitigated in carefully designed morphologies. Specifically, we show how funnel-shaped donor- and acceptor-rich domains in the phase-separated morphology that are characteristic of organic bulk heterojunction solar cells can promote directed transport of positive and negative charge carriers toward the anode and cathode, respectively. We demonstrate that in optimized funnel morphologies this kinetic, nonequilibrium effect, which is boosted by the slow thermalization of photogenerated charges, allows one to surpass the near-equilibrium limit for the same material in the absence of gradients.

Characteristics of a first-generation x-ray system.

PURPOSE: To compare the antiquated x-ray system of Hoffmans and van Kleef (circa 1896) with modern x-ray equipment in terms of radiation dose, x-ray beam properties, image quality, and electrical parameters. MATERIALS AND METHODS: The antiquated x-ray system consisted of a Ruhmkorff inductor, battery, and Crookes tube. The radiation dose rate, x-ray beam properties, and electrical characteristics of this system were determined. A modern computed radiography plate was used to compare images of a hand specimen obtained by using the antiquated system with images obtained by using the modern system. RESULTS: A peak voltage of 73 kV was obtained with an 8-V battery. With Crookes tube number 9, the half-value layer of the generated x-rays was 0.56 mm Al. Pinhole images showed that the x-rays originated from an extended area of the glass wall, causing image blurring. When measured on the skin of a hand specimen, the radiation dose of the antiquated system was about 10 times greater than that of the modern system for the same detector signal. The estimated skin dose was about 74 mGy for the antiquated system and 0.05 mGy for the modern system. The corresponding exposure times were 90 minutes and 21 msec. CONCLUSION: Radiation dose and exposure time of the antiquated system were greater than those of the modern system by about three and five orders of magnitude, respectively. Images of the hand specimen obtained with the antiquated system were severely blurred but were still awe inspiring, considering the simplicity of the system. SUPPLEMENTAL MATERIAL: http://radiology.rsna.org/lookup/suppl/doi:10.1148/radiol.11101899/-/DC1.

Slow Relaxation of Photogenerated Charge Carriers Boosts Open-Circuit Voltage of Organic Solar Cells.

Among the parameters determining the efficiency of an organic solar cell, the open-circuit voltage (VOC) is the one with most room for improvement. Existing models for the description of VOC assume that photogenerated charge carriers are thermalized. Here, we demonstrate that quasi-equilibrium concepts cannot fully describe VOC of disordered organic devices. For two representative donor:acceptor blends, it is shown that VOC is actually 0.1-0.2 V higher than it would be if the system was in thermodynamic equilibrium. Extensive numerical modeling reveals that the excess energy is mainly due to incomplete relaxation in the disorder-broadened density of states. These findings indicate that organic solar cells work as nonequilibrium devices, in which part of the photon excess energy is harvested in the form of an enhanced VOC.

A unifying model for the operation of light-emitting electrochemical cells.

The application of doping in semiconductors plays a major role in the high performances achieved to date in inorganic devices. In contrast, doping has yet to make such an impact in organic electronics. One organic device that does make extensive use of doping is the light-emitting electrochemical cell (LEC), where the presence of mobile ions enables dynamic doping, which enhances carrier injection and facilitates relatively large current densities. The mechanism and effects of doping in LECs are, however, still far from being fully understood, as evidenced by the existence of two competing models that seem physically distinct: the electrochemical doping model and the electrodynamic model. Both models are supported by experimental data and numerical modeling. Here, we show that these models are essentially limits of one master model, separated by different rates of carrier injection. For ohmic nonlimited injection, a dynamic p-n junction is formed, which is absent in injection-limited devices. This unification is demonstrated by both numerical calculations and measured surface potentials as well as light emission and doping profiles in operational devices. An analytical analysis yields an upper limit for the ratio of drift and diffusion currents, having major consequences on the maximum current density through this type of device.

Remnant polarization in thin films from a columnar liquid crystal.

Ferroelectric switching is demonstrated in a hydrogen bonded columnar liquid crystalline (LC) material. Polar order induced in the LC phase can be frozen by crystallization of the alkyl chains in the periphery of the columns yielding thin films with remnant polarization and an unprecedented high surface potential as shown by scanning Kelvin probe microscopy.

The dynamic organic p-n junction.

Static p-n junctions in inorganic semiconductors are exploited in a wide range of today's electronic appliances. Here, we demonstrate the in situ formation of a dynamic p-n junction structure within an organic semiconductor through electrochemistry. Specifically, we use scanning kelvin probe microscopy and optical probing on planar light-emitting electrochemical cells (LECs) with a mixture of a conjugated polymer and an electrolyte connecting two electrodes separated by 120 microm. We find that a significant portion of the potential drop between the electrodes coincides with the location of a thin and distinct light-emission zone positioned >30 microm away from the negative electrode. These results are relevant in the context of a long-standing scientific debate, as they prove that electrochemical doping can take place in LECs. Moreover, a study on the doping formation and dissipation kinetics provides interesting detail regarding the electronic structure and stability of the dynamic organic p-n junction, which may be useful in future dynamic p-n junction-based devices.