Simulating Charge Injection and Dynamics in Microscale Organic Field-Effect Transistors

Monte Carlo simulations were used to investigate the carrier dynamics in realistic, finite-sized, small-molecule, organic field-effect transistors (OFETs) within the first few nanoseconds of device turn-on as well as when the system equilibrates. The results show that the device current exhibits large magnitude oscillations (64 ± 27 nA) during device turn-on if the initial configuration assumed no carriers in the device (i.e., carriers only arrive through injection from the source electrode). After equilibration (125 ns), the current continues to oscillate, however, at lower magnitude (64 ± 2 nA), even if the initial configuration assumed randomly placed charges. Fourier transforms of device current as a function of simulation time show that these oscillations occur at well-defined device geometry-dependent frequencies, independent of initial configuration of the system. Examination of the carrier lifetimes and path lengths, which were found to vary nonlinearly with device length, are used to argue that the oscillations are the result of the charge injection procedure, which assumed a constant probability event. The results suggest that carriers travel in waves in realistically finite-sized devices and that carrier lifetime and path length vary nonlinearly by device geometry. Alternating current studies of OFETs may be useful in confirming these findings

Piezoelectric Effects of Applied Electric Fields on Hydrogen-Bond Interactions: First-Principles Electronic Structure Investigation of Weak Electrostatic Interactions

The piezoelectric properties of 2-methyl-4-nitroaniline crystals were explored qualitatively and quantitatively using an electrostatically embedded many-body (EE-MB) expansion scheme for the correlation energies of a system of monomers within the crystal. The results demonstrate that hydrogen bonding is an inherently piezoelectric interaction, deforming in response to the electrostatic environment. We obtain piezo-coefficients in excellent agreement with the experimental values. This approach reduces computational cost and reproduces the total resolution of the identity (RI)-Møller–Plesset second-order perturbation theory (RI-MP2) energy for the system to within 1.3 × 10^–5%. Furthermore, the results suggest novel ways to self-assemble piezoelectric solids and suggest that accurate treatment of hydrogen bonds requires precise electrostatic evaluation. Considering the ubiquity of hydrogen bonds across chemistry, materials, and biology, a new electromechanical view of these interactions is required

Charge Transport in Imperfect Organic Field Effect Transistors: Effects of Charge Traps

Monte Carlo simulations were used to study the effects of explicit charge traps on charge transport in small-molecule organic field effect transistors. The results show that the source-drain current decreases as the trap/barrier concentration increases, reaches a minimum around 30/70%, and increases as the concentration reaches 100%, regardless of the trap/barrier distribution. Greater current is predicted for heterogeneous trap distributions than for homogeneous trap distributions, due to wider conduction pathways that allow for more charge carriers to reach the drain electrode. Also, the distributions of distances and potential energy between charge carriers and trap sites were shown to depend on the heterogeneity of the traps and device geometry and, in most cases, are non-Gaussian in shape, due to electrostatic effects between charged traps, unlike previous assumptions. For some ranges of heterogeneity, these densities of states exhibit exponential tails. These results suggest that more experimental work is needed to gain insight into the energetic density of states under operating conditions in electronic devices made from mixed films of organic semiconductors, such as solar cells

Charge Transport in Imperfect Organic Field Effect Transistors: Effects of Charge Traps

Monte Carlo simulations were used to study the effects of explicit charge traps on charge transport in small-molecule organic field effect transistors. The results show that the source-drain current decreases as the trap/barrier concentration increases, reaches a minimum around 30/70%, and increases as the concentration reaches 100%, regardless of the trap/barrier distribution. Greater current is predicted for heterogeneous trap distributions than for homogeneous trap distributions, due to wider conduction pathways that allow for more charge carriers to reach the drain electrode. Also, the distributions of distances and potential energy between charge carriers and trap sites were shown to depend on the heterogeneity of the traps and device geometry and, in most cases, are non-Gaussian in shape, due to electrostatic effects between charged traps, unlike previous assumptions. For some ranges of heterogeneity, these densities of states exhibit exponential tails. These results suggest that more experimental work is needed to gain insight into the energetic density of states under operating conditions in electronic devices made from mixed films of organic semiconductors, such as solar cells

Piezoelectric Effects of Applied Electric Fields on Hydrogen-Bond Interactions: First-Principles Electronic Structure Investigation of Weak Electrostatic Interactions

The piezoelectric properties of 2-methyl-4-nitroaniline crystals were explored qualitatively and quantitatively using an electrostatically embedded many-body (EE-MB) expansion scheme for the correlation energies of a system of monomers within the crystal. The results demonstrate that hydrogen bonding is an inherently piezoelectric interaction, deforming in response to the electrostatic environment. We obtain piezo-coefficients in excellent agreement with the experimental values. This approach reduces computational cost and reproduces the total resolution of the identity (RI)-Møller–Plesset second-order perturbation theory (RI-MP2) energy for the system to within 1.3 × 10^–5%. Furthermore, the results suggest novel ways to self-assemble piezoelectric solids and suggest that accurate treatment of hydrogen bonds requires precise electrostatic evaluation. Considering the ubiquity of hydrogen bonds across chemistry, materials, and biology, a new electromechanical view of these interactions is required

Charge Transport in Imperfect Organic Field Effect Transistors: Effects of Charge Traps

Monte Carlo simulations were used to study the effects of explicit charge traps on charge transport in small-molecule organic field effect transistors. The results show that the source-drain current decreases as the trap/barrier concentration increases, reaches a minimum around 30/70%, and increases as the concentration reaches 100%, regardless of the trap/barrier distribution. Greater current is predicted for heterogeneous trap distributions than for homogeneous trap distributions, due to wider conduction pathways that allow for more charge carriers to reach the drain electrode. Also, the distributions of distances and potential energy between charge carriers and trap sites were shown to depend on the heterogeneity of the traps and device geometry and, in most cases, are non-Gaussian in shape, due to electrostatic effects between charged traps, unlike previous assumptions. For some ranges of heterogeneity, these densities of states exhibit exponential tails. These results suggest that more experimental work is needed to gain insight into the energetic density of states under operating conditions in electronic devices made from mixed films of organic semiconductors, such as solar cells

Charge Transport in Imperfect Organic Field Effect Transistors: Effects of Charge Traps

Monte Carlo simulations were used to study the effects of explicit charge traps on charge transport in small-molecule organic field effect transistors. The results show that the source-drain current decreases as the trap/barrier concentration increases, reaches a minimum around 30/70%, and increases as the concentration reaches 100%, regardless of the trap/barrier distribution. Greater current is predicted for heterogeneous trap distributions than for homogeneous trap distributions, due to wider conduction pathways that allow for more charge carriers to reach the drain electrode. Also, the distributions of distances and potential energy between charge carriers and trap sites were shown to depend on the heterogeneity of the traps and device geometry and, in most cases, are non-Gaussian in shape, due to electrostatic effects between charged traps, unlike previous assumptions. For some ranges of heterogeneity, these densities of states exhibit exponential tails. These results suggest that more experimental work is needed to gain insight into the energetic density of states under operating conditions in electronic devices made from mixed films of organic semiconductors, such as solar cells

Comparing the Experiences of Highly Skilled Labor Migrants in Sweden and Japan : Barriers and Doors to Long-term Settlement

As labor markets become increasingly global, competition among industrialized nations to attract highly skilled workers from abroad has intensified. Spurred by concerns over future economic needs caused by the demographic challenges of an aging population, both Japan and Sweden have joined this global competition. This article examines Japanese and Swedish immigration policies for highly skilled migrants and compares the highly skilled migrants' experiences in the two countries through interviews with these migrants. Despite Japan and Sweden's completely different approaches to immigration itself, both countries' policies, as well as the experiences of the skilled migrants, are strikingly similar. Highly skilled migrants experience language barriers and prejudice in both countries, making it difficult to build social networks with natives. Career development seems to be perceived as a common problem, although less so in Sweden, where labor markets are more flexible. Overall, these issues reduce both Japan's and Sweden's ability to retain skilled migrants. While they share similarities, Sweden's famed work-life balance and gender equality give it an edge in the competition for skilled migrants, which Japan does not share. This comparison identifies which social conditions facilitate or impede skilled migrant settlement

Single-Molecule Piezoelectric Deformation: Rational Design from First-Principles Calculations

Conventional piezoelectric materials change shape in response to an applied external electric field, frequently deforming at grain boundaries in addition to intrinsic unit cell changes. We detail a computational investigation, using density functional theory (DFT) calculations of single-molecule piezoelectrics. Rather than deforming along covalent bond lengths or angles, these molecular springs, derivatives of [6]­helicene and phenanthrene, change conformation in response to the applied field, up to 15% of the molecular length. A substituted [6]­helicene has a predicted piezoelectric coefficient of 48.8 pm/V, and one of the phenanthrenes yields a piezoelectric coefficient of up to 54.3 pm/V, which is significantly higher than polymers such as polyvinylidine difluoride (PVDF) and comparable to conventional inorganic materials such as zinc oxide (ZnO). We discuss structural properties that are found to yield large piezoresponse and hypothetical target molecules with up to 64% length change and a predicted piezoelectric coefficient of 272 pm/V. Based on these findings, we believe a new class of highly responsive piezoelectric materials may be created from the “bottom up”, yielding immense electromechanical response

Piezoelectric Hydrogen Bonding: Computational Screening for a Design Rationale

Organic piezoelectric materials are promising targets in applications such as energy harvesting or mechanical sensors and actuators. In a recent paper (Werling, K. A.; et al. <i>J. Phys. Chem. Lett.</i> <b>2013</b>, <i>4</i>, 1365–1370), we have shown that hydrogen bonding gives rise to a significant piezoelectric response. In this article, we aim to find organic hydrogen bonded systems with increased piezo-response by investigating different hydrogen bonding motifs and by tailoring the hydrogen bond strength via functionalization. The largest piezo-coefficient of 23 pm/V is found for the nitrobenzene–aniline dimer. We develop a simple, yet surprisingly accurate rationale to predict piezo-coefficients based on the zero-field compliance matrix and dipole derivatives. This rationale increases the speed of first-principles piezo-coefficient calculations by an order of magnitude. At the same time, it suggests how to understand and further increase the piezo-response. Our rationale also explains the remarkably large piezo-response of 150 pm/V and more for another class of systems, the “molecular springs” (Marvin, C.; et al. <i>J. Phys. Chem. C</i> <b>2013</b>, <i>117</i>, 16783–16790.)