Performance Of Organic Field-effect Transistors Research Articles

Novel functionalized indigo derivatives for organic electronics

A series of nine novel indigo derivatives, including diiodoindigo, octahalogenated indigoids and compounds with extended π-conjugated system, were synthesized, characterized and investigated as semiconductor materials in organic field-effect transistors (OFETs). Among them, 6,6′-diiodoindigo demonstrated the ambipolar behavior with balanced p-type and n-type mobilities. The complete substitution of hydrogens at the indigo core with halogen atoms led to low electron mobilities in OFETs. An extension of the conjugated system through the introduction of small aromatic substituents (thiophene and phenyl) resulted in predominant p-type behavior. Fusion of aromatic rings resulted in z-shaped dibenzoindigo, which showed poor charge transport properties due to the non-optimal arrangement of molecules along each other in the crystal lattice. The acquired data fulfilled the previously reported model based on the relationship between the chemical nature of substituents and their positions at the indigo core, optoelectronic properties of materials and their performance in OFETs. The results of this study will be useful for rational design of a new generation of the indigo-based semiconductors for biocompatible organic electronics.

Polymer Electret Improves the Performance of the Oxygen-Doped Organic Field-Effect Transistors

Chemical doping is widely used in the electronic devices. In p-type semiconductor thin films, oxygen doping fills the hole traps and increases hole concentrations, improving the performance of the organic field-effect transistors (OFETs). Due to the low ionization potential for p-type semiconductors, the superfluous holes induced by the oxygen doping degrades the OFETs off-state leakage performance. On the other hand, for p-type semiconductors with high ionization potential (up to 5.5-6.0 eV), the limited oxidation of oxygen is hard to achieve satisfactory doping concentrations to fill the trap states. This refers to the well-known intrinsic incompatibility between the oxygen doping and high-performance OFETs. Herein, a novel strategy is introduced to overcome the incompatibility and achieve high-performance OFETs by using the structural polymer electret. That is, moderate hole concentrations induced by low-pressure (30 Pa) oxygen plasma fill the hole traps within semiconductor. And the built-in field resulted from polymer electret accumulates the holes inside semiconductor near the semiconductor/electret interface, thus improving the OFETs performance. Using a model organic semiconductor with high ionization potential-2,7-didodecyl[1]benzothieno [3,2-b][1]benzothiophene (C12-BTBT) as an example, the high-performance OFETs with field-effect mobility ( $\mu _{FET}$ ) of 3.5 cm2V−1 s−1, subthreshold-swing ( SS ) of 110 mV decade−1, on-off ratio of 104, and widely-tunable threshold voltage ( ${V} _{t}$ ) are realized at a low voltage below 2 V in the open air.

Parylene-based polymeric dielectric top-gate organic field-effect transistors exposed to a UV/ozone environment

Abstract Organic field-effect transistors (OFETs) structured with a top-gate configuration have several advantages, such as better charge-carrier injection, easy gate line designing, and encapsulation or protection of devices, over those configured with a bottom-gate architecture. In particular, Poly (para-xylylene) (Parylene) -based polymer and its derivatives are considered suitable insulating materials for these transistors, due to thin film formation through a low-temperature vapor deposition process, superior barrier properties, and excellent dielectric characteristics. In this study, we analyzed two types of Parylene dielectrics (Parylene-C and Parylene -AF) considering their thin film formation process, surface, insulating, and OFET operation characteristics. Furthermore, we investigated the durability of the top-gate based OFETs composed of two polymer dielectrics while it was exposed to a UV/ozone environment. Also, we could fabricate UV/ozone durable top-gate OFETs composed by a bilayer with both Parylene dielectrics. We expect, therefore, that these results will help the selection and use of versatile dielectric layers for top-gate based OFETs.

Relationship between the surface structure of the gate insulator and the performance of organic thin-film transistors

To investigate the impact on the device performance of different surface structures of the self-assembled monolayer (SAM) used to form the gate insulator of organic field effect transistors (OFETs), sum frequency generation spectroscopy and contact angle measurements were performed on SAM insulators with different surface structures. Furthermore, the surface structure of the SAM was varied with changing its dipping or spin coating formation method. Moreover, the electrical properties of OFETs were examined when only the surface structure of the gate insulator had changed. The results showed that compared with the dipping method, the disorder of alkyl chains (gauche defects) increased on the octadecylphosphonic acid (ODPA) SAM surface, deposited through the spin coating method. Contact angle measurements revealed that the surface free energy of ODPA-SAM increased in tandem with an increase in gauche defects on the SAM surface. Contrary to expectations, the device performance of OFETs with the SAM insulator with gauche defects improved, which is most probably due to the smaller surface free energy difference between ODPA-SAM and poly(3-hexylthiophene) (P3HT), which improved the adhesion at the ODPA-SAM/P3HT interface.

New strategy for enhancing performance of P3HT-based organic field-effect transistor via regulating the precipitated speed of solute in volatile solvents

In this work, a new strategy is proposed to improve the performance of poly (3-hexylthiophene-2,5-diyl) (P3HT) based organic field-effect transistor (OFET). The high orientation of P3HT chain is obtained via utilizing precipitation characteristics of P3HT in volatile CH2Cl2 with the temperature decrease of solution in the hot spin-coating process. Meanwhile, a small amount of 1,2-dichlorobenzene (ODCB) with high boiling point is introduced into CH2Cl2 to make the inner part of the film stay in liquid state for a long time, and prolong the self-organization time of P3HT chain. Compared with control device prepared in pure CH2Cl2, the mobility of optimized device obtained by using blended solvent increases about 20-fold, the on/off ratio enhances about three orders of magnitude, and the operation time window of spin-coating up to 30 min (control device < 30 s). With this new strategy, the film quality can be easily controlled, which provides a new method for preparation of high-performance polymer optoelectronic devices.

π-Conjugated oligomers based on aminobenzodifuranone and diketopyrrolopyrrole

A new donor-acceptor type oligomer, named as O1 with aminobenzodifuranone (ABDF) capped at both ends of thiophene DPP, was synthesized and characterized. The amino groups (NH) in this oligomer react with the carbonyl units (CO) from the neighboring molecules forming hydrogen bonding (NH…OC), which results in hydrogen bonded oligomer O1. The thermal, optical, electrochemical, thin film properties as well as the density functional theory computational simulation of O1 were investigated. O1 thin film presented not only strong aggregation, but also hydrogen bonding association. This is beneficial for the charges to be carried across the individual molecules. The high HOMO levels of O1 (−5.31 eV) facilitate charge injection and ohmic contacts with electrodes. Apart from that, the good planarity backbone combined with strong D-A system for the π-conjugation of O1 is favorable for the intracharge transport. These properties provided the O1 thin film based organic field-effect transistors (OFET) with hole mobility (μh) of 0.28 cm2/V·s, and after annealing, the μh increased up to 0.39 cm2/V·s. Such hydrogen-bonding-mediated enhancement of π – π stacking exhibits potential for improving the performance of solution processable organic electronic materials. This work shows that the hydrogen bonding association is a simple and useful strategy to design π-conjugation oligomers for high performance OFETs.

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.

Synthesis of Phenanthro[9,10‐d]imidazoles and their Potential Applications in Solution Processable Bottom‐Gated OFETs

AbstractPhenanthro[9,10‐d]imidazole is synthesized by employing Radziszewski method and further functionalized through palladium catalyzed Suzuki cross‐coupling reaction. Solution processable organic field‐effect transistors (OFETs) are assembled in bottom gate‐top contact architecture. A binary solvent system comprising of chloroform/toluene in the ratio of 1/1 is preferred for the deposition of active semiconducting layer. The fabricated devices demonstrated excellent p‐type characteristics with a good mobility, high ON/OFF ratio of 109 and a minimum threshold voltage of −4.5 V. Computational investigations indicted proper charge separation of the energy levels for the phenanthro[9,10‐d]imidazole containing nitro group. Frontier molecular orbitals and the packing patterns obtained from computational studies suggest an efficient band gap for better device operation. The ordered morphologies of the molecules obtained from SEM analysis prove to be beneficial in the OFET performance. The photophysical, electrochemical and thermal properties of the molecules are studied in detail. The research work majorly emphasized on structural designing of the phenanthro[9,10‐d]imidazoles for potential applications in organic devices.

Tuning the Ambipolar Character of Copolymers with Substituents: A Density Functional Theory Study.

Ambipolar copolymers play a vital role in reducing the cost and simplifying the fabrication of ambipolar organic field-effect transistors (OFETs). However, the development of them lags behind that of p-type and n-type copolymers. Herein, we aim to obtain ambipolar copolymers by introducing appropriate substituents into DPP-TVT [diketopyrrolopyrrole-(E)-1,2-di(2-thienyl)ethane]. We study the effects of substituents (-NH2, -OCH3, -F, -CF3, and -CN) on backbone planarity, electronic structures, and charge carrier mobility by density functional simulations. Electronic structure analyses show that the CN-substituted DPP-TVT lies in the trap-free energy window and has the narrowest band gap, suggesting promising ambipolar character. This was further confirmed by the results of its charge carrier mobility along both intra- and interchain directions. Besides the -CN group, the -NH2 group also proved to be effective in turning DPP-TVT into the ambipolar copolymer. Our study provides insights into modulating the performances of OFETs by introducing appropriate substituents into the copolymers to achieve ambipolar character.

Non-lithographic direct patterning of carbon nanomaterial electrodes via electrohydrodynamic-printed wettability patterns by polymer brush for fabrication of organic field-effect transistor

In this study, we report a new approach for patterning of carbon nanomaterials via the solution route through wettability patterning using electrohydrodynamic (EHD) printing. Differences in wettability were successfully obtained by treating dimethylchlorosilane-terminated polystyrene (PS-brush) with the micro-dripping mode of EHD printing up to 6-μm line width and scaled fine patterns with various shapes at 6-μm printing conditions. We obtained sub-micrometer scaled carbon nanomaterial patterns, which were utilized as source and drain (S/D) electrodes for organic field-effect transistors (OFETs), without using any lithographic process. In addition, the PS-brush treatment modified the surface hydrophobicity of the substrates that served as the PS-brush and SiO2 bilayer dielectrics to guarantee high stability during the OFET operation. As a result, the OFETs employing S/D electrodes based on carbon nanomaterial patterns exhibited the electrical performance of typical p-type OFETs with negligible hysteresis. Additionally, we confirmed that a solution processing organic semiconductor forms a more suitable crystal structure than a vacuum process based organic semiconductor by several tools like CPOM, AFM, and 2D-GIXD.

Interface Engineering in Organic Field-Effect Transistors: Principles, Applications, and Perspectives.

Heterogeneous interfaces that are ubiquitous in optoelectronic devices play a key role in the device performance and have led to the prosperity of today's microelectronics. Interface engineering provides an effective and promising approach to enhancing the device performance of organic field-effect transistors (OFETs) and even developing new functions. In fact, researchers from different disciplines have devoted considerable attention to this concept, which has started to evolve from simple improvement of the device performance to sophisticated construction of novel functionalities, indicating great potential for further applications in broad areas ranging from integrated circuits and energy conversion to catalysis and chemical/biological sensors. In this review article, we provide a timely and comprehensive overview of current efficient approaches developed for building various delicate functional interfaces in OFETs, including interfaces within the semiconductor layers, semiconductor/electrode interfaces, semiconductor/dielectric interfaces, and semiconductor/environment interfaces. We also highlight the major contributions and new concepts of integrating molecular functionalities into electrical circuits, which have been neglected in most previous reviews. This review will provide a fundamental understanding of the interplay between the molecular structure, assembly, and emergent functions at the molecular level and consequently offer novel insights into designing a new generation of multifunctional integrated circuits and sensors toward practical applications.

Megahertz operation of vertical organic transistors for ultra-high resolution active-matrix display

Organic field-effect transistors (OFETs) are the technology of choice for flexible electronic devices such as active-matrix (AM) displays. However, despite the continuous improvement of charge carrier mobility in organic semiconductors, the performance of conventional OFETs is too poor for demanding electronic applications. Furthermore, hero-devices reported in literature often make use of processes (shadow mask fabrication, large channel width devices) which cannot be adapted in production lines, e.g. for AM displays. Here we present an OFET with a novel vertical device structure. It has static and dynamic transistor performance superior over conventional lateral organic transistors with regard to application in AM displays. We show that these vertical transistors can be integrated using processes well-established in the micro-electronic industry and thus offer seamless transfer into production lines. We discuss that these transistors obey scaling laws for footprint and capacitance which make them superior over other planar transistor devices. In combination with excellent device stability and uniformity, vertical OFET might enable ultra-high resolution flexible displays of the future.

Low-power-consumption organic field-effect transistors

At present, the electrical performance of organic field-effect transistors (OFETs) has reached the level of commercial amorphous silicon. OFETs show considerable application potential in artificial intelligence, deep learning algorithms, and artificial skin sensors. The devices which can operate with high performance and low power consumption are needed for these applications. The recent energy-related improvement to realize low-power consumption OFETs were reviewed, including minimizing operating voltage, reducing subthreshold swing, and decreasing contact resistance. In this review, we demonstrate breakthroughs in materials and methods to decrease power consumption, providing a promising avenue toward low-power consumption organic electronics.

Understanding molecular surface doping of large bandgap organic semiconductors and overcoming the contact/access resistance in organic field-effect transistors.

The contact resistance (Rc) and the effective carrier mobility (μeff) are considered as the important indicators of the performance of organic field-effect transistors (OFETs). Conventionally, the contact resistance is regarded as the interface effect between the metal electrodes and the organic semiconductors, while the carrier mobility is correlated to the crystallinity and π-π stacking of the organic molecules. In the staggered OFETs, Rc is actually closely correlated to μeff through the channel sheet resistance. Besides, the accuracy of the carrier mobility directly extracted from the non-ideal transfer curves with significant contact effect is always questionable. Herein, a diffusion-lead surface doping approach is employed to improve the contact resistance and mobility issues simultaneously. By suppressing the trap states in the sublimated 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) organic semiconductor with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ), we observed a 3-fold increase in the carrier mobility from 0.5 to 1.6 cm2 V-1 s-1, and the Rc also drops remarkably from 25.7 kΩ cm to 5.2 kΩ cm. Moreover, the threshold voltage (VTH), subthreshold swing (SS) and the bias stability of the OFETs are also significantly improved. Based on the detailed characterization of the C8-BTBT film upon surface doping, including X-ray diffraction (XRD) for the film crystallinity, Kelvin probe force microscopy (KPFM) for the surface potential, trap state investigation by density of states (DOS) measurement and electrical circuit modeling for partial doping analysis, we confirmed that the spontaneous charge transfer process due to the diffusion of the F4-TCNQ dopants in the C8-BTBT matrix can lead to an effective trap filling. This technique and findings can be potentially developed into a general approach for the improvement of different performance parameters of OFETs.

Hysteresis-Free, High-Performance Polymer-Dielectric Organic Field-Effect Transistors Enabled by Supercritical Fluid.

Organic field-effect transistors (OFETs) are of the core units in organic electronic circuits, and the performance of OFETs replies critically on the properties of their dielectric layers. Owing to the intrinsic flexibility and natural compatibility with other organic components, organic polymers, such as poly(vinyl alcohol) (PVA), have emerged as highly interesting dielectric materials for OFETs. However, unsatisfactory issues, such as hysteresis, high subthreshold swing, and low effective carrier mobility, still considerably limit the practical applications of the polymer-dielectric OFETs for high-speed, low-voltage flexible organic circuits. This work develops a new approach of using supercritical CO2 fluid (SCCO2) treatment on PVA dielectrics to achieve remarkably high-performance polymer-dielectric OFETs. The SCCO2 treatment is able to completely eliminate the hysteresis in the transfer characteristics of OFETs, and it can also significantly reduce the device subthreshold slope to 0.25 V/dec and enhance the saturation regime carrier mobility to 30.2 cm2 V−1 s−1, of which both the numbers are remarkable for flexible polymer-dielectric OFETs. It is further demonstrated that, coupling with an organic light-emitting diode (OLED), the SCCO2-treated OFET is able to function very well under fast switching speed, which indicates that an excellent switching behavior of polymer-dielectric OFETs can be enabled by this SCCO2 approach. Considering the broad and essential applications of OFETs, we envision that this SCCO2 technology will have a very broad spectrum of applications for organic electronics, especially for high refresh rate and low-voltage flexible display devices.

Structural Transition and Interdigitation of Alkyl Side Chains in the Conjugated Polymer Poly(3-hexylthiophene) and Their Effects on the Device Performance of the Associated Organic Field-Effect Transistor.

Direct grazing-angle X-ray scattering evidence of the order-disorder transition and interdigitation of side chains in a conjugated polymer poly(3-hexylthiophene) (P3HT) is presented. The free methyl ends of the side chains exhibit closest packing, as in n-alkane crystallization, and cause a structural mismatch due to the difference between their packing density and the areal density of the attached ends. This mismatch is resolved by increases in the tilt angle of the side chains and local interdigitation. In situ X-ray scattering and electrical measurements show that the structural transition and interdigitation of these side chains strongly affect its surface morphology as well as the charge transport properties of the resulting P3HT-based organic field-effect transistor. Since most conjugated polymers have side chains, the results of this study provide a deeper understanding of the effects of side chains on the structural and electrical properties of conjugated backbones. These results also provide a new perspective on the formation of a metastable polymorph consisting of interdigitated P3HT.

Low-Voltage Organic Nonvolatile Memory Transistors with Water-Soluble Polymers Containing Thermally Induced Radical Dipoles.

A water-soluble acidic polymer, poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPSA), was applied as a gate-insulating layer for organic field-effect transistors (OFETs). Before depositing the poly(3-hexylthiophene) (P3HT) channel layers, the PAMPSA layers were subjected to thermal treatment at various temperatures from 140 to 230 °C. The OFET performance was greatly enhanced by thermal treatment between 140 and 170 °C, whereas it became very poor at higher temperatures (200-230 °C). In particular, the transfer curves showed pronounced hysteresis phenomena at 170 °C. Various measurements including thermogravimetric analysis and X-ray photoelectron spectroscopy disclosed that the PAMPSA chains underwent thermal degradation from ca. 160 °C and could generate carbon radicals leading to the formation of dipoles with the nitrogen lone pair electrons. The carbon-nitrogen dipoles delivered hysteresis phenomena to the OFETs with the PAMPSA layers treated at 170 °C, which exhibited excellent memory retention characteristics up to 10 000 cycles even at -1 V. Hence, it is expected that the thermally treated PAMPSA layers can be used as one of the viable gate-insulating memory materials for low-voltage transistor-type organic memory devices (TOMDs).

Interface modification of DNTT-based organic field effect transistors using boronic acid derivatives

The dielectric/semiconductor interface in organic field effect transistors (OFETs) is critical to their performance. Modification of this interface with functional molecules provides a wide range of possibilities for their applications as sensors. In this work, boronic acid molecules were used to modify the SiO2 dielectric surface in dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene based OFETs. The device parameters, including most notably the threshold voltage, were significantly improved. The dielectric/semiconductor interface was analyzed using various measurement techniques, such as contact angle and atomic force microscopy. Our work provides evidence that easily functionable boronic acid derivatives improve the device performance of OFETs, which lays the foundation for further studies of such interface modified OFETs for use in sensing applications.

Developments of Diketopyrrolopyrrole‐Dye‐Based Organic Semiconductors for a Wide Range of Applications in Electronics

In recent times, fused aromatic diketopyrrolopyrrole (DPP)-based functional semiconductors have attracted considerable attention in the developing field of organic electronics. Over the past few years, DPP-based semiconductors have demonstrated remarkable improvements in the performance of both organic field-effect transistor (OFET) and organic photovoltaic (OPV) devices due to the favorable features of the DPP unit, such as excellent planarity and better electron-withdrawing ability. Driven by this success, DPP-based materials are now being exploited in various other electronic devices including complementary circuits, memory devices, chemical sensors, photodetectors, perovskite solar cells, organic light-emitting diodes, and more. Recent developments in the use of DPP-based materials for a wide range of electronic devices are summarized, focusing on OFET, OPV, and newly developed devices with a discussion of device performance in terms of molecular engineering. Useful guidance for the design of future DPP-based materials and the exploration of more advanced applications is provided.

Improving device performance of p-type organic field-effect transistor using butterfly like triarylamines

Facile and economic strategy to create a controlled self-structured architecture of semiconducting molecules is very crucial in the accomplishment of carbon electronic devices. A series of new unsymmetrically functionalized butterfly type triarylamines with electron donating/withdrawing substituents were synthesized. Simple and efficient solution based method was used as a successful strategy for OFET with improved characteristics. Binary solvent system with chloroform and toluene (7:3) has improved inter/intra molecular communication in the active layer. The morphology of the films was analyzed by SEM reveals the uniform packing. In addition post thermal annealing has enhanced the crystallinity with minimal grain boundaries. HOMO values -5.1 to -5.4 eV obtained from cyclic voltammetry ensure that these molecules can efficiently transport holes. OFET devices were fabricated in bottom gate top contact (BGTC) architecture by spin coating. Molecules exhibited typical p -channel transistor behavior with mobility up to 1 cm 2 V −1 s −1 with 10 6 I on/off current ratio. Density functional theory (DFT) visualized the packing and interaction of molecules and supported the efficient mobility. Our investigation gives insight into the role of solvents to control the film architecture in solution processing OFET device construction. • New functionalized triarylamines are designed, synthesized and scrutinised for OFET applications. • HOMO level (-5.2–-5.4 eV) ensures efficient hole injection and transport. • Band gap tuned with respect to the electron donating and withdrawing substituents. • OFETs with bottom gate top contact architecture is fabricated by spin coating from a binary solvent mixture. • Exhibited higher melting points ensures the device durability with higher mobility up to 1 cm 2 V −1 s −1 with 10 6 I on/off current ratio.