Representative Organic Semiconductors Research Articles

Nonadiabatic Simulation of Exciton Dynamics in Organic Semiconductors Using Neural Network-Based Frenkel Hamiltonian and Gradients.

In this study, we present a multiscale method to simulate the propagation of Frenkel singlet excitons in organic semiconductors (OSCs). The approach uses neural network models to train a Frenkel-type Hamiltonian and its gradient, obtained by the long-range correction version of density functional tight-binding with self-consistent charges. Our models accurately predict site energies, excitonic couplings, and corresponding gradients, essential for the nonadiabatic molecular dynamics simulations. Combined with the fewest switches surface hopping algorithm, the method was applied to four representative OSCs: anthracene, pentacene, perylenediimide, and diindenoperylene. The simulated exciton diffusion constants align well with experimental and reported theoretical values and offer valuable insights into exciton dynamics in OSCs.

Far- and mid-infrared FT-IR analysis of the single-crystal pentacene using a linearly polarized synchrotron radiation light source

Understanding and control of molecular vibrations are essential aspects for both fundamental and application considerations of organic semiconductor electronics. The reason is that the organic electronic devices are driven by diverse electronic processes in molecular solids, such as charge carrier transport and excitonic progression, that are strongly influenced by coupling with vibrations. In the present study, molecular vibrations of single-crystals of pentacene, a representative organic semiconductor material, were examined in the far- to mid-infrared range by means of Fourier transform infrared (FT-IR) spectroscopy using a linearly polarized synchrotron radiation light source. The IR absorption spectra exhibited significant modulation depending on the crystalline in-plane azimuthasl angle of c*-oriented single-crystal pentacene.

Molecular Arrangement Control of [1]Benzothieno[3,2-b][1]benzothiophene (BTBT) via Charge-Assisted Hydrogen Bond

Abstract Organic semiconductors have π-conjugation in the constituent molecules and exhibit optical and electrical properties. Since these properties are significantly affected by the overlap of π-orbitals between adjacent molecules, not only their molecular structures but also their molecular arrangement has been well known as critical; however, control of the molecular arrangement without modifying the electronic character of the constituent molecule has been difficult. In the current work, we report organic salts composed of disulfonic acid with a moiety of a representative organic semiconductor molecule, [1]benzothieno[3,2-b][1]benzothiophene (BTBT), as a functional component, and different types of alkylamines as an arrangement-controlling component via charge-assisted hydrogen bonds. We successfully controlled the molecular arrangement of BTBT moiety by changing alkylamines, without changing the structure of disulfonic acid with the BTBT functional moiety. Depending on the bulkiness of alkylamines, the molecular arrangement of these organic salts changed from an edge-to-face herringbone-type arrangement, where CH/π interactions were dominant similar to the common crystal structure of BTBT, to a novel one-dimensional (1D) slipped parallel-type arrangement for BTBT, without changing the molecular structure of disulfonic acid. In addition, we revealed that the dimensionality of the electronic state and properties of the organic salts also changed according to the molecular arrangement of BTBT moiety.

Efficient Surface Hopping Approach for Modeling Charge Transport in Organic Semiconductors.

The trajectory surface hopping (TSH) method is nowadays widely applied to study the charge/exciton transport process in organic semiconductors (OSCs). In the present study, we systematically examine the performance of two approximations in the fewest switched surface hopping (FSSH) simulations for charge transport (CT) in several representative OSCs. These approximations include (i) the substitution of the nuclear velocity scaling along the nonadiabatic coupling vector (NCV) by rescaling the hopping probability with the Boltzmann factor (Boltzmann correction (BC)) and (ii) a phenomenological approach to treat the quantum feedback from the electronic system to the nuclear system (implicit charge relaxation (IR)) in the OSCs. We find that charge mobilities computed by FSSH-BC-IR are in very good agreement with the mobilities obtained by standard FSSH simulations with explicit charge relaxation (FSSH-ER), however, at reduced computational cost. A key parameter determining the charge carrier mobility is the reorganization energy, which is sensitively dependent on DFT functionals applied. By employing the IR approximation, the FSSH method allows systematic investigation of the effect of the reorganization energies obtained by different DFT functionals like B3LYP or ωB97XD on CT in OSCs. In comparison to the experiments, FSSH-BC-IR using ωB97XD reorganization energy underestimates mobilities in the low-coupling regime, which may indicate the lack of nuclear quantum effects (e.g., zero point energy (ZPE)) in the simulations. The mobilities obtained by FSSH-BC-IR using the B3LYP reorganization energy agree well with experimental values in 3 orders of magnitude. The accidental agreement may be the consequence of the underestimation of the reorganization energy by the B3LYP functional, which compensates for the neglect of nuclear ZPE in the simulations.

Impact of gamma-ray irradiation on the electronic structures of PCBM and P3HT organic semiconductor films

Gamma-ray irradiation alters the material properties of organic semiconductors, especially the electronic structure. These changes due to ionization can be useful in dosimetry. Although there are some device-based reports, the effect of gamma-ray irradiation on the electronic structure of organic semiconductors is still unclear. In this study, we investigated the electronic structures of representative organic semiconductor films, namely, n-type [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and p-type poly(3-hexylthiophene-2,5-diyl) (P3HT), after gamma-ray irradiation generated by a Cs-137 source. The X-ray and ultraviolet photoelectron spectra of the PCBM and P3HT films were measured for various gamma-ray doses. In both PCBM and P3HT, chemical interaction with atmospheric oxygen, assisted by high-energy photons, led to significant oxidation. However, the degree of oxidation of PCBM was considerably higher than that of P3HT. The oxidation also affects the valence electronic structures. The possible chemical structures of the oxidized PCBM and P3HT are estimated using density functional theory calculations.

Protein and Organic-Molecular Crystallography With 300kV Electrons on a Direct Electron Detector.

Electron 3D crystallography can reveal the atomic structure from undersized crystals of various samples owing to the strong scattering power of electrons. Here, a direct electron detector DE64 was tested for small and thin crystals of protein and an organic molecule using a JEOL CRYO ARM 300 electron microscope. The microscope is equipped with a cold-field emission gun operated at an accelerating voltage of 300 kV, quad condenser lenses for parallel illumination, an in-column energy filter, and a stable rotational goniometer stage. Rotational diffraction data were collected in an unsupervised manner from crystals of a heme-binding enzyme catalase and a representative organic semiconductor material Ph-BTBT-C10. The structures were determined by molecular replacement for catalase and by the direct method for Ph-BTBT-C10. The analyses demonstrate that the system works well for electron 3D crystallography of these molecules with less damaging, a smaller point spread, and less noise than using the conventional scintillator-coupled camera.

Organic 2D Optoelectronic Crystals: Charge Transport, Emerging Functions, and Their Design Perspective.

2D organic semiconductor crystals are emerging as a fascinating platform with regard to their applications in organic field-effect transistors (OFETs), attributed to their enhanced charge transport efficiency and their new optoelectronic functions, based on their unique morphological features. Advances in material processing techniques have not only enabled easy fabrication of few-monolayered 2D nanostructures but also facilitated exploration of the interesting properties induced by characteristic 2D morphologies. However, to date, only a limited number of representative organic semiconductors have been utilized in organic 2D optoelectronics. Therefore, in order to further spur this research, an intuitive crystal engineering principle for realizing organic 2D crystals is required. In this regard, here, not only the important implications of applying 2D structures to OFET devices are discussed but also a crystal engineering protocol is provided that first predicts molecular arrangements depending on the molecular factors, which is followed by realizing 2D supramolecular synthon networks for different molecular packing motifs. It is expected that 2D organic semiconductor crystals developed by this approach will pave a promising way toward next-generation organic 2D optoelectronics.

Metal-organic semiconductor interfacial barrier height determination from internal photoemission signal in spectral response measurements

Accurate and convenient evaluation methods of the interfacial barrier ϕb for charge carriers in metal semiconductor (MS) junctions are important for designing and building better opto-electronic devices. This becomes more critical for organic semiconductor devices where a plethora of molecules are in use and standardised models applicable to myriads of material combinations for the different devices may have limited applicability. In this paper, internal photoemission (IPE) from spectral response (SR) in the ultra-violet to near infra-red range of different MS junctions of metal-organic semiconductor-metal (MSM) test structures is used to determine more realistic MS ϕb values. The representative organic semiconductor considered is [6, 6]-phenyl C61 butyric acid methyl ester, and the metals considered are Al and Au. The IPE signals in the SR measurement of the MSM device are identified and separated before it is analysed to estimate ϕb for the MS junction. The analysis of IPE signals under different bias conditions allows the evaluation of ϕb for both the front and back junctions, as well as for symmetric MSM devices.

ChemInform Abstract: Tailoring Crystal Polymorphs of Organic Semiconductors Towards High‐Performance Field‐Effect Transistors

AbstractReview: 78 refs.

Tailoring crystal polymorphs of organic semiconductors towards high-performance field-effect transistors

Abstract As a quite ubiquitous phenomenon, crystal polymorph is one of the key issues in the field of organic semiconductors. This review gives a brief summary to the advances on polymorph control of thin film and single crystal of representative organic semiconductors towards high-performance field-effect transistors. Particularly, the relationship between crystal polymporh and charge transport behaviour has been discussed to shed light on the rational preparation of outstanding organic semiconducting materials with desired crystal polymorph.

Impact of Molecular Orientation and Packing Density on Electronic Polarization in the Bulk and at Surfaces of Organic Semiconductors.

The polarizable environment surrounding charge carriers in organic semiconductors impacts the efficiency of the charge transport process. Here, we consider two representative organic semiconductors, tetracene and rubrene, and evaluate their polarization energies in the bulk and at the organic-vacuum interface using a polarizable force field that accounts for induced-dipole and quadrupole interactions. Though both oligoacenes pack in a herringbone motif, the tetraphenyl substituents on the tetracene backbone of rubrene alter greatly the nature of the packing. The resulting change in relative orientations of neighboring molecules is found to reduce the bulk polarization energy of holes in rubrene by some 0.3 eV when compared to tetracene. The consideration of model organic-vacuum interfaces highlights the significant variation in the electrostatic environment for a charge carrier at a surface although the net change in polarization energy is small; interestingly, the environment of a charge even just one layer removed from the surface can be viewed already as representative of the bulk. Overall, it is found that in these herringbone-type layered crystals the polarization energy has a much stronger dependence on the intralayer packing density than interlayer packing density.

P-109: Reduced Contact Resistance with MoOxInjection Layer for Thin Film Transistors Based on Organic Semiconductors with Deep HOMO Level

We investigated the contact resistance of the OTFTs based on dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]-thiophene (DNTT) as a representative organic semiconductors with deep HOMO level. We confirmed a dramatic decrease in the contact resistance with the MoOx injection layer by transmission line method and y-function method.

Fluctuation analysis of an organic semiconductor–insulator interface

The space-charge region of an organic semiconductor (OS)-insulator interface is probed by analyzing the spontaneous, thermally driven drain current fluctuations of a field-effect transistor in which the OS forms the gate electrode. This so called "excess drain current noise" is the outcome of local fluctuations of the Fermi level, resulting from stochastic exchange of electrons between traps near the Fermi level. The power spectral density of this noise is characteristic of a Lorentzian process with a distribution of time constants, which is attributed to the disorder in the OS film. Furthermore, this disorder leads to local inhomogeneity of the work function in the film and a finite correlation length of the work function fluctuations. The measurement of work function noise is only possible within a correlation length of the OS-insulator interface. Through systematic variation of gate voltage, primary doping and secondary doping levels, the correlation length, disorder, and the trapping/de-trapping time constant are examined on polyaniline as a representative OS. A model is proposed for local work function variations and spontaneous charge-carrier fluctuations within polyaniline films with consequences for organic electronics using organic semiconductors.

Solution-Deposited Organic–Inorganic Hybrid Multilayer Gate Dielectrics. Design, Synthesis, Microstructures, and Electrical Properties with Thin-Film Transistors

We report here on the rational synthesis, processing, and dielectric properties of novel layer-by-layer organic/inorganic hybrid multilayer dielectric films enabled by polarizable π-electron phosphonic acid building blocks and ultrathin ZrO(2) layers. These new zirconia-based self-assembled nanodielectric (Zr-SAND) films (5-12 nm thick) are readily fabricated via solution processes under ambient atmosphere. Attractive Zr-SAND properties include amenability to accurate control of film thickness, large-area uniformity, well-defined nanostructure, exceptionally large electrical capacitance (up to 750 nF/cm(2)), excellent insulating properties (leakage current densities as low as 10(-7) A/cm(2)), and excellent thermal stability. Thin-film transistors (TFTs) fabricated with pentacene and PDIF-CN(2) as representative organic semiconductors and zinc-tin-oxide (Zn-Sn-O) as a representative inorganic semiconductor function well at low voltages (<±4.0 V). Furthermore, the TFT performance parameters of representative organic semiconductors deposited on Zr-SAND films, functionalized on the surface with various alkylphosphonic acid self-assembled monolayers, are investigated and shown to correlate closely with the alkylphosphonic acid chain dimensions.

ELECTRICAL TRANSPORT IN ORGANIC SEMICONDUCTORS

Organic semiconductors have processing and performance advantages for low cost and/or large area applications that have led to their rapid commercialization. Organic semiconductors are π conjugated materials, either small molecules or polymers. Their electrical transport properties are fundamentally distinct from those of inorganic semiconductors. Organic semiconductor thin films are amorphous or polycrystalline and their electronic structures consist of a distribution of localized electronic states with different energies. The localized sites are either individual molecules or isolated conjugated segments of a polymer chain. Electrical transport results from carrier hopping between neighboring sites. At room temperature, equilibration between neighboring sites of different energy is fast enough that carrier transport can be described using a mobility picture. Hopping transport in these disordered systems leads to a mobility that can depend strongly on both the electric field and carrier density. This article presents experimental measurements and theoretical analysis of the electrical transport properties of representative organic semiconductors.

Electrical Transport in Organic Semiconductors

Organic semiconductors have processing and performance advantages for low cost and/or large area applications that have led to their rapid commercialization. Organic semiconductors are π conjugated materials, either small molecules or polymers. Their electrical transport properties are fundamentally distinct from those of inorganic semiconductors. Organic semiconductor thin films are amorphous or polycrystalline and their electronic structures consist of a distribution of localized electronic states with different energies. The localized sites are either individual molecules or isolated conjugated segments of a polymer chain. Electrical transport results from carrier hopping between neighboring sites. At room temperature, equilibration between neighboring sites of different energy is fast enough that carrier transport can be described using a mobility picture. Hopping transport in these disordered systems leads to a mobility that can depend strongly on both the electric field and carrier density. This article presents experimental measurements and theoretical analysis of the electrical transport properties of representative organic semiconductors.