Layer In Organic Field-effect Transistors Research Articles

Solution processable benzo[b]thiophene derivatives as small molecular organic semiconductors for organic field effect transistors

In this study, four new organic semiconductors based on benzo[b]thiophene derivatives were synthesized and characterized as the active layer of organic field-effect transistors (OFETs) to investigate the effect of different isomers and end-group moieties on the electrical performance. The top-contact/bottom-gate OFETs were fabricated via solution-shearing methods and exhibited p-channel activity. In particular, the thin film with a single BT moiety attached via the 6-position showed the highest mobility up to 0.055 cm2/Vs and current on/off ratio of 2.5 × 107. The microstructure and surface morphology of the thin films were analyzed by X-ray diffraction (XRD) and atomic force microscopy (AFM) to investigate the factors contributing to the difference in electrical performance of the devices.

In-depth theoretical analysis of the influence of an external electric field on charge transport parameters.

It is important to develop materials with environmental stability and long device shelf life for use in organic field-effect transistors (OFETs). The microscopic, molecular-level nature of the organic layer in OFETs is not yet well understood. The stability of geometric and electronic structures and the regulation of the external electric field (EEF) on the charge transport properties of four typical homogeneous organic semiconductors (OSCs) were investigated by density functional theory (DFT). The results showed that under the EEF, the structural changes in single-bond linked oligomers were more sensitive and complex than those of condensed molecules, and there were non-monotonic changes in their reorganization energy (λ) during charge transport under an EEF consisting of decreases and then increases (Series D). The change in λ under an EEF can be preliminarily and qualitatively determined by the change in the frontier molecular orbitals (FMOs) - the number of C-atoms with nonbonding characteristics. For single-bonded molecules, the transfer integral is basically unchanged under a low EEF, but it will greatly change at a high EEF. Because the structure and properties of the molecule will greatly change under different EEFs, the effect of an EEF should be fully considered when determining the intrinsic mobility of OSCs, which could cause a deviation 0.3-20 times in mobility. According to detailed calculations, one heterogeneous oligomer, TH-BTz, was designed. Its λ can be greatly reduced under an EEF, and the change in the energy level of FMOs can be adjusted to different degrees. This study provides a reasonable idea for verification of the experimental mobility value and also provides guidance for the directional design of stable high-mobility OSCs.

Ferroelectric Organic Semiconductor: [1]Benzothieno[3,2-b][1]benzothiophene-Bearing Hydrogen-Bonding -CONHC14H29 Chain.

An alkylamide-substituted [1]benzothieno[3,2-b][1]benzothiophene (BTBT) derivative of BTBT-CONHC14H29 (1) and C8H17-BTBT-CONHC14H29 (2) were prepared to design the multifunctional organic materials, which can show both ferroelectric and semiconducting properties. Single-crystal X-ray structural analyses of short-chain (-CONHC3H7) derivatives revealed the coexistence of two-dimensional (2D) electronic band structures brought from a herringbone arrangement of the BTBT π core and the one-dimensional (1D) hydrogen-bonding chains of -CONHC3H7 chains. The thin films of 1 and 2 fabricated on the Si/SiO2 substrate surface have monolayer and bilayer structures, respectively, resulting in conducting layers parallel to the substrate surface, which is suitable for a channel layer of organic field-effect transistors (OFETs). The thin film of 1 indicated a hole mobility μFET = 2.4 × 10-5 cm2 V-1 s-1 and threshold voltage VTh = - 29 V, whereas that of 2 showed a μFET = 2.1 × 10-2 cm2 V-1 s-1 and threshold voltage VTh = -9.7 V. Both 1 and 2 formed the smectic E (SmE) phase above 410 and 369 K, respectively, where the existence of a hole transport pathway was confirmed in the SmE phase. The ferroelectric hysteresis behavior was observed in bulk 1 and 2 in the polarization-electric field (P-E) curves at the SmE phase. 1 showed the remanent polarization Pr = 2.3 μC cm-2 and coercive electric field Ec = 5.2 V μm-1, whereas the Pr and Ec of 2 were 3.4 μC cm-2 and 7.0 V μm-1 at the conditions of 453 K and 1 Hz. Introduction of alkylamide units into the BTBT π core has the potential to develop the external stimulus-responsive organic semiconductors brought from both ferroelectricity and semiconducting properties.

Surface-modified polydimethylsiloxane with soft-plasma as dielectric layer for flexible artificial synaptic transistors

The conductance of field-effect transistors (FETs) can be effectively stimulated by electrical or optical pulses, which could be potentially applied as artificial synapses. The combination of solution processable elastomers with polymer semiconductors has attracted extensive interest in all-polymer flexible synaptic electronics, due to the flexible and light weight property. In this work, a glow-discharge soft-plasma is utilized to treat the widely-used elastomer polydimethylsiloxane (PDMS) layer. Subsequently, the surface energy of PDMS is increased, from the contact angle of 100° to fully spread state and induce high charge trapping density (Δn, about 1.9 × 1012 cm−2 under 525 nm green light irradiation and positive gate-bias stress) on the surface. The PDMS surface are homogenously etched by soft plasma without damaging underneath materials, which can be used as dielectric layer in organic FETs. Thus, prepared organic FETs warrant quick charge transfer dynamics between PDMS surface and semiconductor layer. Upon which, flexible synaptic FETs are demonstrated, corresponding synaptic functions like excitatory postsynaptic current (EPSC), paired pulse facilitation (PPF), short-term plasticity (STP) are all realized. Furthermore, the handwritten digits with an accuracy of 86.2% are realized utilizing prepared synaptic devices.

Nonlinear Optical Imaging of Carrier Transport at the Semiconductor-Insulator Interface in Organic Field-Effect Transistors

Organic field-effect transistors (FETs) are witnessing a revolution in their performance, in part, due to a reduction in contact resistance, and through controlled morphology of the organic semiconducting film. Another major consideration is the role of the dielectric layer in dictating transport properties. The transient electric-field-induced second-harmonic generation (EFISHG) technique allows visualization of carrier dynamics in the channel region of a FET by capturing the movement of the induced dielectric polarization. A microscopic imaging system using a broadband femtosecond laser and a pulse compensation arrangement is constructed for imaging transient EFISHG from organic FETs by synchronized laser and voltage pulses. By varying the dielectric layer in organic FETs comprising pentacene and a donor-acceptor conjugated polymer as semiconducting layers, we show that EFISHG images provide an accurate estimate of the carrier mobility along with providing insights into channel formation and the electric field distribution within the channel region. We further correlate the interface charging effects observed in EFISHG to the interface trap density obtained by capacitance-voltage and conductance-voltage from metal-insulator-semiconductor diodes.

Characterization of [1]Benzothieno[3,2-b]benzothiophene (BTBT) Derivatives with End-Capping Groups as Solution-Processable Organic Semiconductors for Organic Field-Effect Transistors

Solution-processable [1]benzothieno[3,2-b]benzothiophene (BTBT) derivatives with various end-capping groups, 2-(phenylethynyl)benzo[b]benzo[4,5]thieno[2,3-d]thiophene (Compound 1), 2-octyl-7-(5-(phenylethynyl)thiophen-2-yl)benzo[b]benzo[4,5]thieno[2,3-d]thiophene (Compound 2), and triisopropyl((5-(7-octylbenzo[b]benzo[4,5]thieno[2,3-d]thiophen-2-yl)thiophen-2-yl)ethynyl)silane (Compound 3), have been synthesized and characterized as active layers for organic field-effect transistors (OFETs). Thermal, optical, and electrochemical properties of the newly synthesized compounds were characterized using thermogravimetric analysis (TGA), a differential scanning calorimeter (DSC), UV–vis spectroscopy, and cyclic voltammetry (CV). Thin films of each compound were formed using the solution-shearing method and the thin film surface morphology and texture of the corresponding films were characterized using atomic force microscopy (AFM) and θ–2θ X-ray diffraction (XRD). All semiconductors exhibited p-channel characteristics in ambient and Compound 1 showed the highest electrical performance with a carrier mobility of ~0.03 cm2/Vs and current on/off ratio of ~106.

Influence of the cooling rate in annealing post-treatment on small molecule with siloxane side chains for solution-processed organic field-effect transistors

In this study, a donor-acceptor-type small molecule possessing a narrow bandgap with siloxane-terminated side chains, namely, S-LBG, was synthesized and characterized to be used as a semiconducting layer in organic field-effect transistors (OFETs). The diketopyrrolopyrrole (DPP)-based small molecule contains silaindacenodithiophene as an electron-donor core and 2-(1,1-dicyanomethylene)rhodanine as an electron-accepting end group. S-LBG films show highly crystalline structures after thermal annealing treatment owing to the strong intermolecular interactions between its siloxane side chains; furthermore, a smooth-surface film with a large polycrystalline grain size can be fabricated by controlling the cooling rate after annealing. Solution-processed OFETs using cooled S-LBG films at a rate of 1 °C/min and exhibits a high hole mobility of 0.102 cm2 V−1 s−1 with a π–π stacking distance of 0.659 nm.

Isoindigo dyes functionalized with terminal electron-withdrawing groups: Computational, optical and electrical characterization

Eight novel isoindigo (iI) based small molecules have been successfully synthesized. Their molecular structure consists of an electron acceptor iI core symmetrically linked to two furan (F-series) or thiophene (T-series) rings and end-functionalized with four auxiliary electron withdrawing groups (EWGs) of different strength. The optical properties of the dyes in chloroform solution are uniformly modulated by the terminal EWGs so that absorption maxima wavelengths move to higher values as the EWG's strength increases. A computational (DFT level) analysis provides useful information on the electronic structure of the dyes: upon photoexcitation, the electron density moves away from iI core or towards it according to the different EWG considered. Optical analysis is performed on dyes' thin films as well and a general broadening and red shift of the absorption is observed as compared to the behaviour in solution; all the dye's thin films are characterized by narrow bandgaps (<1.60 eV) and diffused absorption of most of the visible light. From XRD diffraction analysis performed on drop casted films of the dyes, it is possible to observe a lamellar organization in the solid phase with lamellae width clearly linked to the nature of the terminal EWG. HOMO and LUMO energies of the dyes, determined by cyclic voltammetry analysis performed on dyes' thin films, show very stable LUMO and HOMO energy levels, suggesting, respectively, a tendency to act as n-type semiconductors and a very good thermo-oxidative stability. The dyes are finally employed as active layers in organic field-effect transistors to study their charge transport properties: all of them display unipolar n-type charge transport with the presence of the electron accumulation phenomenon under the application of positive gate voltages. For one of the dye, mobility (μ) up to 10−2 cm2/V∙s was measured, whereas values around 10−3 cm2/V∙s were found for the others.

Modification of alkyl side chain on thiophene-containing benzothieno[3,2-b]benzothiophene-based organic semiconductors for organic field-effect transistors

In this study, three new organic semiconductors based on benzothieno[3,2-b]benzothiophene with double thiophene rings and different types of alkyl side chains were synthesized and characterized as active layers for organic field-effect transistors. Physicochemical properties of the newly synthesized organic compounds including thermal, optical, and electrochemical properties were characterized through thermogravimetric analysis (TGA), differential scanning calorimeter (DSC), UV–vis spectroscopy, and cyclic voltammetry (CV). The thin film surface morphology and texture of all three compounds fabricated by solution-shearing process were analyzed via atomic force microscopy (AFM) and θ-2θ X-ray diffraction (XRD). All semiconductor thin films showed p-channel activity and the compound with monoalkylated 2-ethylhexyl group as a substituent exhibited the highest electrical performance with carrier mobility of 0.033 cm2/Vs and current on/off ratio of ∼106.

Effects of Ionic Liquid, 1-Ethyl-3-methylimidazolium Chloride ([EMIM]Cl), on the Material and Electrical Characteristics of Asphaltene Thin Films.

Asphaltene is a component of crude oil that has remained relatively unexplored for organic electronic applications. In this study, we report on its extraction technique from crude oil tank bottom sludge (COTBS) and its thin-film characteristics when 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl) ionic liquid (IL) was introduced as dopants. The extraction technique yielded asphaltene with more than 80% carbon content. The IL resulted in asphaltene thin films with a typical root-mean-square surface roughness of 4 nm, suitable for organic electronic applications. The thin films each showed an optical band gap of 3.8 eV and a sheet resistance as low as 105 Ω/□. When the film was used as a conductive layer in organic field-effect transistors (OFET), it exhibited hole and electron conduction with hole (µh) and electron (µe) mobilities in the order of 10−8 and 10−6 cm2/Vs, respectively. These characteristics are just preliminary in nature. With the right IL, asphaltene thin films may become a good alternative for a transport layer in organic electronic applications.

Electrohydrodynamic-Jet-Printed Phthalimide-Derived Conjugated Polymers for Organic Field-Effect Transistors and Logic Gates.

A π-conjugated polymer semiconductor, PBDTTTffPI, was synthesized for use as an organic semiconductor suitable for electrohydrodynamic (EHD) jet printing technology. Bulky alkylation of the polymer gave PBDTTTffPI good solubility in several organic solvents. EHD jet printing using PBDTTTffPI ink produced direct patterns of polymer semiconductors while maintaining smooth surface morphologies and crystal structures similar to those of spin-coated PBDTTTffPI films. EHD-jet-printed PBDTTTffPI was appropriate for use as a semiconductor layer in organic field-effect transistors (OFETs) and logic gates. OFETs that used EHD-jet-printed PBDTTTffPI had better electrical characteristics than devices that used spin-coated semiconductor films. When a dielectric material (Al2O3) with a high dielectric constant was introduced, the jet-printed PBDTTTffPI operated well at low voltages. Integrated devices such as inverters, NAND gates, and NOR gates were fabricated by printing PBDTTTffPI patterns and showed good switching behaviors. Therefore, the use of printable PBDTTTffPI provides an advance toward fabrication of practical integrated arrays in next-generation devices.

Organic Field-Effect Transistors Based on a Lithium Phthalocyanine Stable Radical Compound

Abstract The stable organic radical compound, lithium phthalocyanine (LiPc), is employed as the active layer in organic field-effect transistors (OFETs). Using crystalline α-LiPc and x-LiPc thin film, OFETs exhibiting n-type semiconducting behavior are achieved. The α-LiPc OFET exposed to air for 7 weeks preserves its n-type semiconducting behavior.

Retraction Note: Recent progress on rubrene as active layer in organic field-effect transistors

Organic field-effect transistor (OFET) is kind of organic electronic devices, which consists of gate insulator layer, an active layer, and 3 electrodes (source, gate electrodes, and drain). Among them, the active layer as a key part has been widely concerned by scientific researchers. Rubrene, as a member of the star molecules, has been widely studied. Rubrene exhibits attractive properties, for instance, having one of the utmost reported transistor mobilities at room temperature, and the crystal growth mode is different in different solvents and so on. This paper summarized several methods for producing high-performance single-crystal transistors. The objective of this problem is to offer an extensive overview of rubrene as active layer in OFET.

Isomers of B←N-Fused Dibenzo-azaacenes: How B←N Affects Opto-electronic Properties and Device Behaviors?

The B←N unit has a large dipole and it is isoelectronic to C-C moiety with no dipole. Incorporating B←N units into π-conjugated system is a powerful strategy to design organic small molecules and polymers with intriguing opto-electronic properties and excellent opto-electronic device performance. However, it is unclear how the B←N unit affects electronic structures and opto-electronic properties of large π-conjugated molecules. In this work, to address this question, we developed three dibenzo-azaacene molecules in which two B←N units were introduced at different positions. Although the dibenzo-azaacene skeleton is fully π-conjugated, the effect of B←N unit on the electronic structures of the adjacent rings is much stronger than that of the distant rings. As a result, the three molecules with isomerized B←N incorporation patterns possess different electronic structures and exhibit tunable opto-electronic properties. Among the three molecules, the centrosymmetrical molecule exhibits higher LUMO/HOMO energy levels than those of the two axisymmetrical molecules. When used as the active layer in organic field-effect transistors (OFETs), while the two axisymmetrical molecules show unipolar electron transporting property, the centrosymmetrical molecule exhibits ambipolar hole and electron transporting behavior. This work not only deepens our understanding on organoboron π-conjugated molecules, but also indicates a new strategy to tune opto-electronic properties of organic semiconductors for excellent device performance.

Interface engineering of moisture-induced ionic albumen dielectric layers through self-crosslinking of cysteine amino acids for low voltage, high-performance organic field-effect transistors.

The interface roughness between the semiconducting and dielectric layers of organic field-effect transistors (OFETs) plays a crucial role in the charge transport mechanism through the device. Here we report the interface engineering of a moisture induced ionic albumen material through systematic control of the temperature-dependent self-crosslinking of cysteine amino acids in the dielectric layer. The evolution of the surface morphologies of albumen and pentacene semiconducting films has been studied to achieve a smooth interface for enhanced charge transport. A structural transition of pentacene films from crystalline dendrite to amorphous was induced by the higher surface roughness of the albumen film. The devices showed a high transconductance of 11.68 μS at a lower threshold voltage of -0.9 V.

RETRACTED ARTICLE:Recent progress on rubrene as active layer in organic field-effect transistors

Organic field-effect transistor (OFET) is kind of organic electronic devices, which consists of gate insulator layer, an active layer, and 3 electrodes (source, gate electrodes, and drain). Among them, the active layer as a key part has been widely concerned by scientific researchers. Rubrene, as a member of the star molecules, has been widely studied. Rubrene exhibits attractive properties, for instance, having one of the utmost reported transistor mobilities at room temperature, and the crystal growth mode is different in different solvents and so on. This paper summarized several methods for producing high-performance single-crystal transistors. The objective of this problem is to offer an extensive overview of rubrene as active layer in OFET.

Tuning Charge Transport in PVDF-Based Organic Ferroelectric Transistors: Status and Outlook.

The use of polymer ferroelectric dielectrics in organic field-effect transistors (FETs) for nonvolatile memory application was demonstrated more than 15 years ago. The ferroelectric dielectric polyvinylidene fluoride (PVDF) and its copolymers are most widely used for such applications. In addition to memory applications, polymer ferroelectrics as a dielectric layer in organic FETs yield insights into interfacial transport properties. Advantages of polymer ferroelectric dielectrics are their high dielectric constant compared to other polymer dielectrics and their tunable dielectric constant with temperature. Further, the polarization strength may also be tuned by externally poling the ferroelectric dielectric layer. Thus, PVDF and its copolymers provide a unique testbed not just for investigating polarization induced transport in organic FETs, but also enhancing device performance. This article discusses recent developments of PVDF-based ferroelectric organic FETs and capacitors with a focus on tuning transport properties. It is shown that FET carrier mobilities exhibit a weak temperature dependence as long as the dielectric is in the ferroelectric phase, which is attributed to a polarization fluctuation driven process. The low carrier mobilities in PVDF-based FETs can be enhanced by tuning the poling condition of the dielectric. In particular, by using solution-processed small molecule semiconductors and other donor-acceptor copolymers, it is shown that selective poling of the PVDF-based dielectric layer dramatically improves FET properties. Finally, the prospects of further improvement in organic ferroelectric FETs and their challenges are provided.

Preparation of Nanocomposite-based High Performance Organic Field Effect Transistor via Solution Floating Method and Mechanical Property Evaluation.

We demonstrate that using nanocomposite thin films consisting of semiconducting polymer, poly(3-hexylthiophene) (P3HT), and electrochemically exfoliated graphene (EEG) for the active channel layer of organic field-effect transistors (OFETs) improves both device performances and mechanical properties. The nanocomposite film was developed by directly blending P3HT solution with a dispersion of EEG at various weight proportions and simply transferring to an Si/SiO2 substrate by the solution floating method. The OFET based on P3HT/EEG nanocomposite film showed approximately twice higher field-effect mobility of 0.0391 cm2·V−1·s−1 and one order of magnitude greater on/off ratio of ~104 compared with the OFET based on pristine P3HT. We also measured the mechanical properties of P3HT/EEG nanocomposite film via film-on-elastomer methods, which confirms that the P3HT/EEG nanocomposite film exhibited approximately 2.4 times higher modulus (3.29 GPa) than that of the P3HT film (1.38 GPa), while maintaining the good bending flexibility and durability over 10.0% of bending strain and bending cycles (1000 cycles). It was proved that the polymer hybridization technique, which involves adding EEG to a conjugated polymer, is a powerful route for enhancing both device performances and mechanical properties while maintaining the flexible characteristics of OFET devices.

Synthetic strategy for thienothiophene-benzotriazole-based polymers with high backbone planarity and solubility for field-effect transistor applications

We report on a synthetic strategy for a series of thienothiophene-benzotriazole-based polymers—octyl-PTTBTz-F, Si-PTTBTz, and Si-PTTBTz-F—wherein conformational locks are introduced to induce intra- and intermolecular interactions (F···S, F···H–C, and C–F··· πF) for high backbone planarity and siloxane-terminated side chains are introduced to increase solubility. The three polymers were utilized as active layers in organic field-effect transistors, and the effects of thermal annealing on the polymer crystallinity and device performances were studied. Although the fluorine-atom-substituted octyl-PTTBTz-F showed a moderate hole mobility of up to 4.2×10−3cm2V−1s−1 with enhanced molecular orientation because of conformational locks, it had poor solubility in common organic solvents. The Si-PTTBTz polymer with the siloxane-terminated side chains showed good solubility but inferior device performance with a hole mobility of up to 2.2×10−4cm2V−1s−1. The rationally designed Si-PTTBTz-F polymer, which contains both fluorine atoms in the backbone and siloxane-terminated groups in the side chains, showed an excellent hole mobility of up to 0.11cm2V−1s−1 and good solubility in common organic solvents.

3,4-Ethylenedioxythiophene-Based Isomer-Free Quinoidal Building Block and Conjugated Polymers for Organic Field-Effect Transistors

Since quinoidal molecules have double-bond linkages between aromatic rings, they have many advantages for efficient charge transport resulting from high planarity and extended π-conjugation length. However, they unavoidably generate some isomers, which cause difficulty in purification and characterization. In this study, sulfur–oxygen conformation locking and steric repulsion approach is introduced to manipulate syn- and anti-isomerization of a quinoidal building block (bis-QEDOT). As a result, isomer-free bis-QEDOT is synthesized by introducing the 3,4-ethylenedioxy group, and the geometrical structure of bis-QEDOT is identified by thin-layer chromatography, 1H NMR, and density functional theory calculation. Furthermore, thiophene (T), bithiophene (2T), and thienylene vinylene (TV) as π-conjugated building blocks are polymerized with bis-QEDOT. Due to the quinoid structure, PQEDOT-T, PQEDOT-2T, and PQEDOT-TV show an intensified near-IR absorption and a low band gap around ∼1.16 eV. Grazing incidence wide-angle X-ray diffraction reveals that three quinoidal polymers show in the (h00) diffraction peaks up to third order after thermal annealing at 250 °C, demonstrating high crystallinity of the films. Finally, the electrical properties of the three polymers are investigated as an active layer in organic field-effect transistors showing hole mobilities of 4.3 × 10–2 (PQEDOT-T), 1.8 × 10–2 (PQEDOT-2T), and 7.8 × 10–3 cm2 V–1 s–1 (PQEDOT-TV).