Integrated Materials Design of Organic Semiconductors for Field-Effect Transistors

Linear acenes are a widely studied class of materials in the field of Organic Electronics. Their aromatic system and the strong interaction of the π-electrons of neighbouring molecules in the solid state allow an efficient charge transport in these materials. The defined molecular structure of these small molecules and the possibility to tune their optical and electronic properties as well as the solid state packing through careful chemical design and synthesis have resulted in numerous applications of acenes in organic transistors and optoelectronic devices.
\nThis work focuses on the application of N-Heteroacenes and non-conjugated pentacene-based polymers as semiconductors in solution-processed organic field-effect transistors. These devices are used to evaluate the charge transport properties of the materials and derive structure-function relationships for the various compounds. To draw structure-function relationships from the studies described in this thesis, a myriad of characterisation techniques was employed to obtain an insight into the optical, electronic, electrical and morphological properties of each material. The effects of order, energetics and processing of the semiconductor on the transistor performance are all investigated.
\nWhile non-conjugated pentacene-based polymers offer an ease of processability, their amorphous nature inhibits efficient hole transport, resulting in a relatively poor transistor performance. The fashion in which the pentacene systems are connected to the polymer backbone changes their flexibility and therefore affecting the injection behaviour and charge transport properties.
\nThe nitrogen substitution in N-Heteroacenes results in an energetic stabilisation of the frontier molecular orbitals, allowing for an enhanced electron injection into these materials. For the symmetrical tetraazapentacene and two halogenated phenazine derivatives relatively high electron mobilities were achieved demonstrating their potential for future application as n-type semiconductors in organic field-effect transistors.
\nThe use of N-heteroacenes is not limited to electron transport only. Their N,N’-dihydro forms are electron rich compounds that exhibit good hole transport. This is demonstrated for differently substituted tetraazapentacenes as well as for a N,N’-dihydro diazahexacene and -heptacene. For these materials it is shown that the processing conditions not only affect the macroscopic transistor performance, but also influence the formation of polymorphs in thin films.
\nThe solid state packing of functionalised acenes is typically determined by their solubilising side chains. A norbornadienyl substitution at the side chain of the well-known 6,13-bis(triisopropylsiliylethynyl)pentacene and its tetraaza derivative was shown to result in an enhancement of the charge transport properties for the p-type derivatives and deterioration of the performance of the n-type transistors. These observations are related to changes in the charge transfer integrals, the film microstructure and the solid state packing.
\nIn conclusion, this work contributes to the development of guidelines for the design and synthesis of next generation N-heteroacenes to be applied in the future in state of the art organic electronic devices.

Recent intensive research and development of organic field-effect transistors (FETs) [1–6] have been motivated by a new class of applications that cannot be easily realized by conventional electronics based on inorganic semiconductors. Organic transistors are mechanically flexible, thin, lightweight, and shock-resistant, because organic devices are manufactured on plastic films at low (ambient) temperature. Furthermore, manufacturing costs of organic transistor circuits would be inexpensive, even for large areas, when they are fabricated using printing technologies and/or roll-to-roll processes. There are two major applications for organic transistors. The first one is a flexible display. This new display includes a paper-like display or an electronic paper, where electric inks, electroluminescent (EL) devices, and liquid crystals or other mediums are powered by organic transistor active matrices [4, 7]. The second one is a radio frequency identification (RFID) tag [8, 9]. An organic transistor-based RFID tag may be printed on packages of products, resulting in an inexpensive and robust electronics. As another application of organic transistors, we demonstrated large-area flexible sensors. The first organic transistor-based large-area sensors are flexible pressure sensor matrices; organic transistor active matrices are used to read out pressure distributions over a large area from a 2-D array of pressure sensor cells. The new pressure sensor can be ideal for electronic artificial skin (e-skin) applications for future generations of robots. The mobility of pentacene that is known as a high-mobility low-molecular weight semiconductor is typically 1 cm2/Vs. This value is about two or three orders of magnitude lower than that of polyor single-crystalline silicon, respectively. Although flexible displays and/or RFID tags require usually high-electronic performance, the

Synthesis and characterization of benzo[b]thieno[2,3-d]thiophene (BTT) derivatives as solution-processable organic semiconductors for organic field-effect transistors

We have fabricated the first example of totally flexible field effect device for chemical detection based on an organic field effect transistor (OFET) made by pentacene films grown on flexible plastic structures. The ion sensitivity is achieved by employing a thin Mylar foil as gate dielectric. A sensitivity of the device to the pH of the electrolyte solution has been observed. A similar structure can be used also for detecting mechanical deformations on flexible surfaces. Thanks to the flexibility of the substrate and the low cost of the employed technology, these devices open the way for the production of flexible chemical and strain gauge sensors that can be employed in a variety of innovative applications such as wearable electronics, e-textiles, new man-machine interfaces.

Modeling of bias-induced changes of organic field-effect transistor characteristics

Columnar, smectic and lamellar polymeric liquid crystals are widely recognized as very promising charge-transporting organic semiconductors due to their ability to spontaneously self-assemble into highly ordered domains in uniform thin films over large areas. The transport properties of smectic and columnar liquid crystals are discussed in Chaps. 2 and 3. Here we examine their application to organic field-effect transistors (OFETs): after a short introduction in Sect. 9.1 we introduce the OFET configuration and show how the mobility is measured in Sect. 9.2. Section 9.3 discusses polymeric liquid crystalline semiconductors in OFETs. We review research that shows that annealing of polymers in a fluid mesophase gives a more ordered microcrystalline morphology on cooling than that kinetically determined by solution processing of the thin film. We also demonstrate the benefits of monodomain alignment and show the application of liquid crystals in light-emitting field-effect transistors. Some columnar and smectic phases are highly ordered with short intermolecular separation to give large π-π coupling. We discuss their use in OFETs in Sects. 9.4, and 9.5 respectively. Section 9.6 summarises the conclusions of the chapter.

Organic field-effect transistors (OFETs) are attracting more and more attention due to their potential applications in low-cost, large-area and flexible electronic products. Organic semiconductors (OSCs) are the key components of OFETs and basically determine the device performance. The past five years have witnessed great progress of OSCs. OSCs used for OFETs have made rapid progress, with field-effect mobility much larger than that of amorphous silicon (0.5-1.0 cm2/(V s)) and of up to 10 cm2/(V s) or even higher. In this review, we demonstrate the latest progress of OSCs for OFETs, where more than 50 representative OSCs are highlighted and analyzed to give some valuable insights for this important but challenging field.

Interest in organic electronic materials, and in particular their potential for low-cost fabrication over large areas, led to the development of organic field-effect transistors (OFETs). The potential of OFETs has been demonstrated in a variety of applications, including pixel drivers for displays, bionic skin, wearable electronics and sensitive chemical sensors that can operate in aqueous environments. OFETs use conjugated, semiconducting small molecules and polymers and offer an alternative to inorganic devices for applications in which facile processing on different substrates and tunable electronic properties are required. The flexibility requirement implies either performance stability towards deformation, or conversely, detectable response to the deformation itself. The knowledge of the electromechanical response of organic semiconductors to external stresses is therefore not only interesting from a fundamental point of view, but also necessary for the development of real world applications. To this end, this presentation highlights the importance of the choice of functional materials (organic semiconductors and dielectrics) as well as the relationship structure/properties in transistors based sensors. Organic semiconductors (OSCs) are promising transducer materials when applied in OFETs taking advantage of their electrical properties that highly depend on the morphology of the semiconducting film. The effects of a high-performance p-type organic semiconductor, namely dinaphtho [2,3-b:2,3-f] thieno [3,2–b] thiophene (DNTT) thickness on its piezoresistive sensitivity are presented. A critical thickness corresponding to the appearance of charge carriers percolation paths in the material can tune the gauge factors (GFs) by a factor 10. In addition, single crystal OSC are regarded as promising electroactive materials for strain sensing application. Herein this talk, we will present how strain induces simultaneous mobility changes along all three axes, and that in some cases the response is higher along directions orthogonal to the mechanical deformation. These variations cannot be explained by the modulation of intermolecular distances, but only by a more complex molecular reorganisation, which is particularly enhanced, in terms of response, by p-stacking and herringbone stacking. This microscopic knowledge of the relation between structural and mobility variations is essential for the interpretation of electromechanical measurements for crystalline organic semiconductors, and for the rational design of electronic devices. Alternatively, this talk will highlight how the use of an active gate dielectric layer such as poly(vinylidenefluoride/trifluoroethylene) (P(VDF-TrFE)) piezoelectric polymer can lead to highly efficient electro-mechanical sensitivity. In such case, the sensing mechanism of the electro-mechanical transducer originates from the piezoelectric material itself, which affects the electrical behavior of the transistor as signature of a mechanical event. The second part of this talk will focus on another kind of TFT based sensor, namely the organic electrochemical transistors (OECTs) which have found recently applications in chemical and biological sensing and interfacing and neuromorphic computing. OECT rely on ions that are injected from the electrolyte into polymer-based mixed conductors, thereby changing its doping state and hence its conductivity. While great progress has been achieved, organic mixed conductors frequently experience significant volumetric changes during ion uptake/rejection, i.e., during doping/ de-doping and charging/discharging. Although ion dynamics may be enhanced in expanded networks, these volumetric changes can have undesirable consequences, e.g., negatively affecting hole/electron conduction and severely shortening device lifetime. New materials able to transport ions and electrons/holes and that exhibits low swelling will be presented, expanding the materials-design toolbox for the creation of low-swelling soft mixed conductors with tailored properties and applications in bioelectronics and beyond.

A nonplanar but conjugated heteroacene, biindeno[2,1-b]thiophenylidene (BTP), is employed to design and synthesize solution-processable polymer semiconductors for organic field-effect transistors (OFETs) applications first. By copolymerizing with isoindigo (IDG), diketopyrrolopyrrole (DPP), and naphthalenediimide (NDI) derivatives, three novel BTP-based copolymers (PBTP-IDG, PBTP-DPP, and PBTP-NDI) have been synthesized and characterized successfully. The results indicate that three BTP-based polymers exhibit broad absorption spectra and good solubility in most common solvents. Because of the dominantly electron-deficient contributions to the whole polymer backbones, the energy levels of the lowest unoccupied molecular orbitals are decreased to ca. −4.0 eV for all these polymers, thus exhibiting good electron affinities. Moreover, the deep-lying energy levels of the highest occupied molecular orbitals (HOMO) have been demonstrated for three BTP-based polymers, with the HOMO values ranging from −5.48 to −5.80 eV. Investigation of the OFETs performance indicates that three BTP-based polymers exhibit well hole transport properties in ambient air and excellent ambipolar performance in a N2 glovebox. Compared with PBTP-IDG and PBTP-NDI, the uniform morphological structure, interconnected polycrystalline grain, and close π–π stacking distance endow PBTP-DPP with higher hole mobility of 1.43 cm2 V–1 s–1. Particularly, the well-balanced hole and electron mobilities of 0.68 and 0.13 cm2 V–1 s–1 have been demonstrated for the PBTP-DPP-based OFETs in a N2 atmosphere, respectively. The results suggest that the nonplanar BTP unit and its derivatives are promising π-conjugated building blocks for the design and synthesis of solution-processable polymer semiconductors with high charge-transporting performance.

In the past three decades, organic semiconductor field-effect transistors (OFETs) have drawn intense attentions as promising candidates for drive circuits of flat panel display, radio frequency identifications, chemical/bio-sensors and other devices. Generally, the key parameters of OFETs, carrier mobility, threshold voltage and on/off current ratio, are closely related to the degree of order and surface/interface electronic structure of organic semiconductor (OSC) films. The ordering of the films is crucially determined by the molecule-substrate interactions. On inert substrates (such as SiO2) OSC films can hardly reach high order of degree without growth templates, while traditional single crystal surfaces usually force the OSC molecules deviate their favorite assemble manner resulting in an unstable structure. Recently, the rise of two-dimensional materials (2D) provides a possible solution. The in-plane lattice of 2D materials can offer possible epitaxy templates for OSCs while the weak van der Waals (vdWs) interaction between OSC and 2D layers allows more flexibility to realize the epitaxy growth of OSCs with their favored assemble manner. In addition, the various band structures tuned by layer numbers of 2D materials encourage widely modified OSC electronic structures by interface doping between the OSC and 2D layers, which benefits to obtain high-performance OFETs. In this review, we emphasize and discuss the recent advances of OSC-2D hybrid OFETs. The OSC-2D heterostructures not only promote the OFET device performances by film morphology/structure optimization and channel electronic structure modification, but also offer platforms for basic organic solids physics investigations and further functional optoelectronic devices.

Transient phenomena that impact the speed of electron devices fabricated from organic semiconductor materials are of critical importance for the development of this new technology. Here we investigate theoretically and experimentally the effects of traps on the establishment and the depletion of the conducting channel in organic semiconductor field effect transistors (OFETs). The device structures explored resemble typical organic thin-film transistors with one of the channel contacts removed. The channel length is varied, and generally is longer than in typical OFETs, in order to vary the carrier transit time. By measuring the displacement current associated with charging and discharging of the channel in these capacitors, transient effects on the carrier transport in organic semiconductors may be studied. When carriers are injected into the device, a conducting channel is established with traps that are initially empty. Gradual filling of the traps then modifies the transport characteristics of the injected charge carriers. In contrast, DC experiments as they are typically performed to characterize the transport properties of organic semiconductor channels investigate a steady state with traps partially filled. Numerical and approximate analytical models of the formation of the conducting channel and the resulting displacement currents are presented. Two simple scenarios are considered first: the spatially-averaged carrier density model neglects carrier dynamics but shows the salient trapping/detrapping effects. Second: a simple geometric model neglects traps but elucidates the effect of carrier dynamics and is used to define an effective mobility that can be extracted from experiments. Finally, numerical simulations are used to study the dynamical and trap effects together. Comparing the average trapping time with the time scale of the current transient sheds light on the influence of traps on the effective mobility. If the average trapping time is very long the effect of traps is negligible and the mobility is largely unaffected by traps on the timescale of the experiment. If the average trapping time is short the effective mobility is reduced but it is time-independent. However, the effective mobility changes with time if the average trapping time is comparable to the current transient interval, eventually reaching a constant value once the trapping and detrapping events balance. Furthermore, the temperature dependence of the effective mobility arising from the temperature dependence of the trap emission rate is explored. Experimental data for pentacene based devices of different channel length and obtained over in the temperature range 120K < T < 300K support the concepts of the physical model.

In this perspective article, we discuss the dynamic instability of charge carrier transport in a range of popular organic semiconductors. We observe that in many cases field-effect mobility, an important parameter used to characterize the performance of organic field-effect transistors (OFETs), strongly depends on the rate of the gate voltage sweep during the measurement. Some molecular systems are so dynamic that their nominal mobility can vary by more than one order of magnitude, depending on how fast the measurements are performed, making an assignment of a single mobility value to devices meaningless. It appears that dispersive transport in OFETs based on disordered semiconductors, those with a high density of localized trap states distributed over a wide energy range, is responsible for the gate voltage sweep rate dependence of nominal mobility. We compare such rate dependence in different materials and across different device architectures, including pristine and trap-dominated single-crystal OFETs, as well as solution-processed polycrystalline thin-film OFETs. The paramount significance given to a single mobility value in the organic electronics community and the practical importance of OFETs for applications thus suggest that such an issue, previously either overlooked or ignored, is in fact a very important point to consider when engaging in fundamental studies of charge carrier mobility in organic semiconductors or designing applied circuits with organic semiconductors.

Organic molecular semiconductors have been paid great attention due to their advantages of low-temperature processability, low fabrication cost, good flexibility, and excellent electronic properties. As a typical example of five-ring-fused organic semiconductors, a single crystal of pentacene shows a high mobility of up to 40 cm2 V-1 s-1, indicating its potential application in organic electronics. However, the photo- and optical instabilities of pentacene make it unsuitable for commercial applications. But, molecular engineering, for both the five-ring-fused building block and side chains, has been performed to improve the stability of materials as well as maintain high mobility. Here, several groups (thiophenes, pyrroles, furans, etc.) are introduced to design and replace one or more benzene rings of pentacene and construct novel five-ring-fused organic semiconductors. In this review article, ∼500 five-ring-fused organic prototype molecules and their derivatives are summarized to provide a general understanding of this catalogue material for application in organic field-effect transistors. The results indicate that many five-ring-fused organic semiconductors can achieve high mobilities of more than 1 cm2 V-1 s-1, and a hole mobility of up to 18.9 cm2 V-1 s-1 can be obtained, while an electron mobility of 27.8 cm2 V-1 s-1 can be achieved in five-ring-fused organic semiconductors. The HOMO-LUMO levels, the synthesis process, the molecular packing, and the side-chain engineering of five-ring-fused organic semiconductors are analyzed. The current problems, conclusions, and perspectives are also provided.

Electron donors and acceptors such as tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ), which have been long known as components of organic metals, act as p- and n-channel organic semiconductors in organic field-effect transistors. High performance, good thin-film properties, and long-term stability have been achieved on the basis of appropriate molecular design. In addition, their charge-transfer complexes work as the active layers as well as conducting electrode materials showing low contact resistance.

Side chain engineering of [1]benzothieno[3,2-b]benzothiophene (BTBT)-based semiconductors for organic field-effect transistors