Charge Transport In Organic Transistors Research Articles

Antiambipolar, ambipolar, and unipolar charge transport in organic transistors based on a single vertical P–N heterointerface

Our studies demonstrate organic heterojunction transistors with three charge transport behaviours including antiambipolar, ambipolar, and unipolar modes by introduced a single vertical heterointerface.

Enhancement of NO2 gas sensing ability through strong binding energy by modification of interface characteristics

Conjugated polymers have strong potential as sensing materials because they are flexible, simple to fabricate, and lightweight. Organic gas sensors based on field-effect transistors have attracted interest because of their high resolution through signal amplification. The main charge transport in organic transistors occurs in the vicinity of the interface between the semiconductor and the gate dielectric. Thus, controlling this interface characteristic also offers an efficient approach to enhancing the current level and sensing properties of these devices. Herein, we fabricated a sensitive NO 2 sensor based on an organic transistor by introducing various self-assembled monolayers (SAMs) as a gas-interactive modifier. SiO 2 surfaces were modified using four different SAMs, and the effect of the surface characteristics on the performance of the transistor-based sensors was investigated under various NO 2 concentrations. Molecular dynamics simulations revealed that the SAM with N-containing functional groups strongly interacts with the target NO 2 molecules, and the trend showed good agreement with NO 2 sensitivity results. • Various SAMs were introduced as a gas-interactive modifier at the channel region. • P3HT gas sensor modified with SAMs were examined as analyte channel materials. • The nitrogen-containing functional groups showed a high responsivity for NO 2 gas. • MD simulations revealed that NO 2 gas is tightly adsorbed onto the N2 surface.

Ultraviolet irradiation creates morphological order via conformational changes in polythiophene films

Here, we developed a facile post-treatment method using ultraviolet irradiation that produced crystalline polymer nanofibrils in the solid film state. Ultraviolet irradiation over a few minutes effectively transformed the polymer chain conformational structure and promoted polymer chain extension and association in the film state. Brief ultraviolet irradiation of a thin film fabricated using a high boiling point solvent produced well-ordered molecular structures among the extended P3HT chains, whereas ultraviolet irradiation over longer times led to breaks in the chemical structures of the P3HT, resulting in shortened conjugation lengths. Conformational changes in the polymer main chain and resulting nanofibril morphologies induced by ultraviolet irradiation facilitated charge transport in organic transistors prepared using these films The relationship between the molecular structural order and the electrical characteristics of the films was used to determine the optimum ultraviolet irradiation time.

Ultrasonication-Mediated Self-Assembly in Polythiophene Films via Control of Residual Solvent Evaporation

We have developed a facile ultrasonication method for the preparation of ordered polymer films. This method was tested by determining the structural, morphological, and electrical characteristics of poly(3-hexylthiophene) films ultrasonicated for various periods under various solvent vapor pressures. Ultrasonic irradiation for short periods under solvent vapor conditions intensifies the interactions between the polymer molecules and the solvent, which results in decreased polymer chain entanglement and in turn promotes polymer association and reorganization. The resulting improved molecular order and favorable morphologies were found to enhance the charge transport in organic transistors prepared with these films. The correlation between the molecular structural order and the electrical characteristics of the films was used to determine the appropriate ultrasonication time and solvent vapor pressure.

Crossover from band-like to thermally activated charge transport in organic transistors due to strain-induced traps

The temperature dependence of the charge-carrier mobility provides essential insight into the charge transport mechanisms in organic semiconductors. Such knowledge imparts critical understanding of the electrical properties of these materials, leading to better design of high-performance materials for consumer applications. Here, we present experimental results that suggest that the inhomogeneous strain induced in organic semiconductor layers by the mismatch between the coefficients of thermal expansion (CTE) of the consecutive device layers of field-effect transistors generates trapping states that localize charge carriers. We observe a universal scaling between the activation energy of the transistors and the interfacial thermal expansion mismatch, in which band-like transport is observed for similar CTEs, and activated transport otherwise. Our results provide evidence that a high-quality semiconductor layer is necessary, but not sufficient, to obtain efficient charge-carrier transport in devices, and underline the importance of holistic device design to achieve the intrinsic performance limits of a given organic semiconductor. We go on to show that insertion of an ultrathin CTE buffer layer mitigates this problem and can help achieve band-like transport on a wide range of substrate platforms.

ChemInform Abstract: Principles that Govern Electronic Transport in Organic Conductors and Transistors

AbstractReview: 128 refs + subrefs

Principles that Govern Electronic Transport in Organic Conductors and Transistors

Abstract Energy bands of organic conductors are calculated on the basis of the estimation of intermolecular overlap integrals and the tight-binding approximation. The resulting Fermi surface has been investigated by the measurements of low-temperature magnetoresistance in detail. However, we have to take electron correlation into account to explain the variation of the metal-insulator transition temperatures and the universal phase diagram. In particular, intermolecular Coulomb repulsion gives a variety of charge-order patterns, in which non-stripe charge order is important in a triangular network of organic conductors. Non-stripe charge order is an origin of flat resistivity, nonlinear conductivity, and potentially Dirac fermions. The estimation of intermolecular interaction is extended to the πd-systems, where the magnetic interactions J between the π-electrons and metal spins make a network. To discuss the charge transport in organic transistors, energy levels of the molecules are important. However, since the energy levels are considerably modified at the metal/organic interface, it is useful to use chemical doping and organic charge-transfer salts in the conducting parts of organic transistors. Temperature dependence of an organic transistor comes from the midgap trap states, but eliminating the traps in a single-crystal transistor, we can achieve band-like transport maintained down to low temperatures.

Charge Transport in Organic Transistors Accounting for a Wide Distribution of Carrier Energies—Part II: TFT Modeling

Charge Transport in Organic Transistors Accounting for a Wide Distribution of Carrier Energies—Part II: TFT Modeling

Charge Transport in Organic Transistors Accounting for a Wide Distribution of Carrier Energies—Part I: Theory

An extended theory of carrier hopping transport in organic transistors is proposed. According to many experimental studies, the density of localized states in organic thin-film transistors can be described by a double-exponential function. In this work, using a percolation model of hopping, the analytical expressions of conductivity and mobility as functions of temperature and charge concentration are obtained. The conductivity depends only on the tail states, while the mobility is determined by the total charge carriers in the semiconductor.

Influence of Contact Resistance on Effective Mobility in Organic Thin Film Transistor

We investigated the charge transport in organic thin film transistors based on the variable range hopping theory, including the contribution of contact resistance. The impact of contact resistance on the temperature dependence of the extracted mobility is extensively analyzed. It was found that contact resistance is responsible for the experimentally observed no-Arrhenius behavior of charge transport in organic transistors in the low temperature region. The developed model quantitatively explains the temperature dependence of mobility observed in conjugated polymer thin film transistor.

Trap-limited transport in rubrene transistors

The charge carrier mobility in the transport channel of an organic transistor is estimated within the framework of a trap-and-release model. The model accounts for the observed dependence of the mobility on the dielectric constant ε of the gate insulator. This dependence is attributed to both the effective mass of the carrier and the energetic depth of transport traps due to interface defects being functions of ε. These results are used to describe the critical role of the interface between the organic semiconductor and the dielectric material in governing charge transport in organic transistors.

Direct Observation of Field-Induced Carrier Dynamics in Pentacene Thin-Film Transistors by Electron Spin Resonance Spectroscopy

We report our recent studies on interfacial charge transport in organic transistors on the basis of electron spin resonance (ESR) spectroscopy of field-induced carriers. We successfully observed motional narrowing effects in field-induced ESR spectra for relatively high-mobility (∼0.6 cm2 V-1 s-1) pentacene thin-film transistors (TFTs); Lorentz-type resonance spectra become narrower as the carrier motion becomes more active with increasing temperature or gate voltage. The variation of spectral linewidth is within the range of 30–200 µT, from which we conclude that the charge transport is dominated by the traps with trap residence time of 4 ns. Meanwhile, it was also reported for lower-mobility (∼0.01 cm2 V-1 s-1) pentacene TFTs that ESR spectra do not show motional narrowing and that the linewidth remains unchanged by gate voltage or temperature at about 200 µT. On the basis of these examinations, we discuss the microscopic nature of field-induced carrier dynamics in organic TFTs associated with the origin of linewidth in ESR spectra.

Unified description of potential profiles and electrical transport in unipolar and ambipolar organic field-effect transistors

Validation of models for charge transport in organic transistors is fundamentally important for their technological use. Usually current-voltage measurements are performed to investigate organic transistors. In situ scanning Kelvin probe microscopy measurements provide a powerful complementary technique to distinguish between models based on band and hopping transports. We perform combined current-voltage and Kelvin probe microscopy measurements on unipolar and ambipolar organic field-effect transistors. We demonstrate that by this combination we can stringently test these two different transport models and come up with a unified description of charge transport in disordered organic semiconductors.