Mobility Of Organic Thin-film Transistors Research Articles

Development of degradable networked-organic semiconductors and effects on charge carrier mobility in organic thin-film transistors

Controlling the degradation rates in degradable polymers depends on the type of crosslinker used for transient OTFTs.

Blends of Conjugated and Adhesive Polymers for Sticky Organic Thin‐Film Transistors

AbstractHere, a polymer blend active layer that exhibits both electronic and adhesive properties is introduced. Various conjugated polymers are blended with a catechol‐based polymer that shows high adhesion, such that blends serve as the active layer of multifunctional sticky organic thin‐film transistors (OTFTs). Blend films maintain relatively constant field‐effect charge carrier mobility in OTFTs regardless of composition. Lap shear adhesion strength tests show that all blend films exhibit adhesive properties with adhesion values ranging from 0.05 to 4.30 MPa. With relatively consistent mobility and the presence of adhesive properties at different compositions, blends of conjugated and adhesive polymers can lead to next‐generation organic transistors for stable 3D stacking and waterproof adhesive sensors.

Development of Alkylthiazole-Based Novel Thermoelectric Conjugated Polymers for Facile Organic Doping.

In this study, we developed two novel conjugated polymers that can easily be doped with F4TCNQ organic dopants using a sequential doping method and then studied their organic thermoelectric (OTE) properties. In particular, to promote the intermolecular ordering of OTE polymers in the presence of the F4TCNQ dopant, alkylthiazole-based conjugated building blocks with highly planar backbone structures were synthesized and copolymerized. All polymers showed strong molecular ordering and edge-on orientation in the film state, even in the presence of the F4TCNQ organic dopant. Thus, the sequential doping process barely changed the molecular ordering of the polymer films while making efficient molecular doping. In addition, the doping efficiency was improved in the more π-extended polymer backbones with thienothiophene units due to the emptier space in the polymer lamellar structure to locate ionized F4TCNQ. Moreover, the study of organic thin-film transistors (OTFTs) revealed that higher hole mobility in OTFTs was the key to increasing the electrical conductivity of OTE devices fabricated using the sequential doping method.

Poly(3-hexylthiophene)-Based Organic Thin-Film Transistors with Virgin Graphene Oxide as an Interfacial Layer.

We fabricated and characterized poly(3-hexylthiophene-2, 5-diyl) (P3HT)-based Organic thin-film transistors (OTFTs) containing an interfacial layer made from virgin Graphene Oxide (GO). Previously chemically modified GO and reduced GO (RGO) were used to modify OTFT interfaces. However, to our knowledge, there are no published reports where virgin GO was employed for this purpose. For the sake of comparison, OTFTs without modification were also manufactured. The structure of the devices was based on the Bottom Gate Bottom Contact (BGBC) OTFT. We show that the presence of the GO monolayer on the surface of the OTFT's SiO2 dielectric and Au electrode surface noticeably improves their performance. Namely, the drain current and the field-effect mobility of OTFTs are considerably increased by modifying the interfaces with the virgin GO deposition. It is suggested that the observed enhancement is connected to a decrease in the contact resistance of GO-covered Au electrodes and the particular structure of the P3HT layer on the dielectric surface. Namely, we found a specific morphology of the organic semiconductor P3HT layer, where larger interconnecting polymer grains are formed on the surface of the GO-modified SiO2. It is proposed that this specific morphology is formed due to the increased mobility of the P3HT segments near the solid boundary, which was confirmed via Differential Scanning Calorimetry measurements.

(Invited) Side-Chain Engineering of Small-Molecular Organic Semiconductors for Organic Electronic Devices

Molecular orientation and packing structure of small molecular organic semiconductors are very important for the charge carrier mobility of organic thin-film transistors (OTFT). Furthermore, the miscibility of p-and n-type materials and the grain size of each material in bulk heterojunction are critical for the performance of organic photovoltaics (OPVs). Benzoporphyrins (BPs) are one of the outstanding p-type organic semiconductors and have been studied for OTFT and OPV materials [1]. For practical use of organic electronic devices, solution process is desirable but BPs are not soluble in organic solvents.To achieve the solution process of BPs, two strategies have been studied in the last two decades. One is a precursor approach: as-spun films of soluble precursors of BPs are in-situ converted into BP thin-films by retro-Diels–Alder reaction. With this method, the film of intact BP is available but the mixed film of edge-on and face-on grains are obtained because of the too-good crystallinity of BPs. The other is a substituent engineering: proper substituents induced to the BP framework can improve the solubility and processability of the film as well as the morphology and orientation of the molecules. Recently we were successful to obtain edge-on dominant film of BP derivatives by a simple dip-coating. 5,15-triisopropylsilyl-ethynyl(TIPS)-BP could make brickwork-type packing to achieve 1.1 cm2 V-1 s-1 FET charge carrier mobility (m FET) [2].This side-chain engineering was also effective for the control of packing structure of bulk-hetero junction OPV. The alkyl chain length drastically influenced the miscibility of p- and n-materials and the short-circuit current density [3]. We also found open-circuit-voltage shift of over 0.5 V in organic photovoltaic cells induced by a minor structural difference in alkyl substituents of acene-type p-materials [4]. In this presentation, the relation between control of packing structure and the OPV performance will be discussed.

(Invited) Parametrization of Gaussian Disordered Model with Fully Spatial and Energetic Disorders for Organic Thin Film Transistors

So far, the most widely addressed hopping transport model for organic disordered semiconductors (ODSs) is the so-called extended Gaussian disordered model (EGDM) [1] based on numerical simulations within the cubic lattice without spatial disorder. As known since 1973 [2] and confirmed recently [3], the EGDM is based on the parametrization of transport coefficients in the framework of the irrelevant set of parameters. Parameters responsible for the effects are not in the equations, while parameters in the equations are not responsible for the effects [3]. The deficiencies of the EGDM are particularly pronounced for systems with spatial disorder. Therefore, the study of Gaussian disordered model (GDM) with spatial disorder is necessary to provide a simulation tool for transport coefficients of charge carriers in real organic thin film transistors (OTFTs) and organic light-emitting diodes (OLEDs) based on materials with spatial disorder.In this study, we examine the GDM which contains both spatial and energetic disorders and propose a physical parametrization for OTFTs. We determine all the parameters by adjusting the localization length and the attempt-to-escape frequency to satisfy experimentally measured mobility. For the localization length, the concept of effective localization length [4] is adopted which has the order of a molecular diameter, because assumption of the very low localization length (commonly order of 10-8 cm) induces very low mobility due to the strong localization of the each state. The large effective localization length can alleviate the strong localization and so it can predict the similar scale of mobility with experimentally measured one. In addition, the proper value of the attempt-to-escape frequency is estimated from comparison between a direct calculation of Miller-Abrahams (MA) transition rate and theoretical estimations of the hopping transition rate based on MA and Marcus theories. We show that its value should be increased by more than two orders of magnitude than conventional value to simulate high mobility device.Then, we validate our parametrization of GDM through comparison between experimentally measured and numerically calculated transfer curves and charge carrier mobility of OTFTs at various temperature (Fig. 1). The staggered pentacene-based OTFT device was fabricated and measured at different temperatures T = 329, 293, 258 and 233 K. The charge carrier mobility was extracted by the transmission line method (TLM). For numerical simulation, we implemented the GDM by Baranovskii [3] into the commercial TCAD simulation tool, Silvaco ATLAS. Then, the numerical solver calculated Poisson’s, continuity and drift-diffusion equations in a self-consistent manner with characteristics of ODSs such as Gaussian density-of-states, generalized Einstein relation and GDM by Baranovskii. Numerical simulation with a single set of parameters makes an accurate fit with experimentally measured transfer curves (Fig. 1a) and charge carrier mobility (Fig. 1b) at every temperature condition.In conclusion, we propose physically based parametrization of GDM with fully spatial and energetical disorder for OTFTs. It is confirmed that the localization length and the attempt-to-escape frequency are key parameters for the practical fitting with experimental result. In addition, we can expect that the electronic design automate (EDA) industry can be promoted by providing rigorous transport model and its parametrization for OTFTs within commercial simulation software.[1] W. F. Pasveer, J. Cottaar, C. Tanase, R. Coehoorn, P. A. Bobbert, P. W. M. Blom, M. De Leeuw, and M. A. J. Michels, Phys. Rev. Lett. 94, 206601 (2005).[2] B.I. Shklovskii Sov. Phys.-Semicond. 6, 1964 (1973).[3] S. D. Baranovskii, Phys. Status Solidi Appl. Mater. Sci. 215, 1700676 (2018).[4] A. V Nenashev, J. O. Oelerich, and S. D. Baranovskii, J. Phys. Condens. Matter 27, 093201 (2015). Figure 1

Organic Thin Film Transistors Fabricated by a Solution Process Using Direct Patterned Single-Layer Graphene Electrodes.

We propose the direct transfer method of single-layer graphene (SLG) from metal catalyst Cu-foil to a polymeric insulator and the direct patterning method of the SLG for electrodes of organic thin-film transistors (OTFTs) without contamination using soft-lithography. Through soft-lithography, SLG can be formed in various patterns relatively easily in comparison with the conventional photolithography method that has multiple complex process steps to make graphene patterns. Furthermore, the 6,13-bis(triisopropylsilylethynyl) pentacene OTFTs are fabricated in solution with SLG source and drain electrodes. As a result, the field-effect mobility of OTFTs based on SLG electrodes was enhanced about 4 times in comparison with that of OTFTs using typical metal electrodes due to the decrease in contact resistance.

Macromodel for AC and Transient Simulations of Organic Thin-Film Transistor Circuits Including Nonquasistatic Effects

A charge-based macromodel for small-signal ac and transient analyses of organic thin-film transistors (TFTs) is presented. Due to the comparatively small charge-carrier mobility in organic TFTs, the dynamic behavior of the gate-field-induced carrier channel is greatly affected by the frequency of the applied gate–source and drain–source voltages. The model presented here is, therefore, based on the transmission-line model and shown to reproduce the results of frequency-dependent admittance measurements and numerical simulations of the transient switching behavior of organic TFTs fabricated in the staggered architecture with good accuracy. The model has been implemented in the hardware description language Verilog-A.

Modification of Indacenodithiophene-Based Polymers and Its Impact on Charge Carrier Mobility in Organic Thin-Film Transistors.

The polymer indacenodithiophene-co-benzothiadiazole (IDT-BT) has been thoroughly studied for its use in p-type organic thin-film transistors over the course of the past decade. While a variety of modifications have been made to its structure, few analogues have been able to match or surpass the hole mobility that can be obtained by IDT-BT. Here, we discuss the rationale behind the chemical modifications that have been utilized and suggest design principles toward high-mobility indacenodithiophene-based polymers. It is clear that planarizing intramolecular interactions, which exist between the peripheral thiophene of the IDT unit and the benzothiadiazole, are imperative for achieving high hole mobilities in this relatively amorphous polymer. Moreover, despite the less ordered backbones of the extended fused-ring cores that have recently been utilized (TIF-BT and TBIDT-BT), high mobilities were still attained in these polymers owing to additional interchain charge transfer. Thus, maintaining the beneficial thiophene-benzothiadiazole intramolecular interactions, while further extending the IDT core to promote interchain charge transfer, is a logical strategy toward high-mobility p-type polymers.

Contact line curvature-induced molecular misorientation of a surface energy patterned organic semiconductor in meniscus-guided coating

The printing of organic semiconductors (OSCs) by means of meniscus guided coating (MGC) has great potential for the fabrication of high-performance, uniform, and large area flexible electronics. Furthermore, surface energy patterning allows low-cost, large area manufacturing, and is a requirement for device isolation to achieve accurate drive and to minimize power consumption. We investigated the morphological and electrical characteristics of surface energy patterned OSCs coated using the MGC method. A unique phenomenon, contact line curvature-induced molecular misorientation effect, which occurs in the patterning of OSCs with the MGC method, is demonstrated. This effect is highly dependent on the pattern width which decided contact line curvature. It influences the charge transport properties and hence the field-effect mobility of organic thin-film transistors (OTFTs). OTFTs in widths ranging from 50 to 500 µm were fabricated. The highest misorientation angle was induced in the 50 µm pattern width and the corresponding OTFTs exhibited the highest mobility.

Highly-ordered Triptycene Modifier Layer Based on Blade Coating for Ultraflexible Organic Transistors

We present a highly ordered surface modification layer for polymers based on ambient solution-processed triptycene (Trip) derivatives for high-mobility organic thin-film transistors (OTFTs). The nested packing of Trip molecules results in the formation of 2D hexagonal arrays, which stack one-dimensionally on the surface of polymer dielectrics without anchoring groups. The Trip surface was previously shown to be preferable for the growth of organic semiconductors (OSCs), and hence for enhancing the mobility of OTFTs. However, although the Trip modifier layer has been realized by thermal evaporation in a high-vacuum environment (TVE), it still has grain-boundary disorders that hinder the optimal growth of OSCs. To fabricate OTFTs with higher mobility, a disorder-free Trip layer is needed. We developed highly ordered Trip layers on polymer dielectrics via blade coating. In addition, we clarified that the highly ordered Trip modifier layer enhances the mobility of the OTFTs by more than 40%, relative to the disordered Trip layer prepared by TVE. Finally, we realized a ring oscillator composed of OTFTs with a highly ordered Trip layer.

Head-to-Head Linked Dialkylbifuran-Based Polymer Semiconductors for High-Performance Organic Thin-Film Transistors with Tunable Charge Carrier Polarity

A planar backbone conformation is essential for enabling polymer semiconductors with high charge carrier mobility in organic thin-film transistors. Benefiting from the smaller van der Waals radius ...

Effect of the length and branching point of alkyl side chains on DPP-thieno[3,2-b]thiophene copolymers for organic thin-film transistors

Solid-state packing and polymer orientation are critical for achieving high charge carrier mobility in organic thin-film transistors. Four side chain substituents, 2-octyldodecyl (−20), 4-octyltetradecyl (−22), 2-decyltetradecyl (−24), and 4-decylhexadecyl (−26), were synthesized to study the effect of side chains on the molecular crystallinity and charge transport properties of diketopyrrolopyrrole–thieno[3,2-b]thiophene copolymers (DPPTTs). Achieving high-performance DPP-based polymers not only depends on crystallinity, but also on the appropriate proportions of polymer chain orientations. DPPTT22 and DPPTT26 have more coplanarity of the backbone structure and a shorter π–π stacking distance, but the charge mobilities are limited. The objective of this study is to determine the influence of different side chains on the layer distance and π–π stacking regarding 3D carrier channel formation. However, a favorable morphology is also important to improve the charge mobility. DPPTT24 shows improved mobility due to its suitable edge-on-oriented crystallinity and well-interconnected domains with small grain boundaries.

Investigating the Thermal Stability of Organic Thin-Film Transistors and Phototransistors Based on [1]-Benzothieno-[3,2-b]-[1]-benzothiophene Dimeric Derivatives.

Two new highly thermally stable [1]benzothieno[3,2-b][1]benzothiophene (BTBT) dimeric derivatives, namely 1,4-bis([1]benzothieno[3,2-b][1]benzothiophene-2-yl)benzene (BTBT-Ph-BTBT) and 4,4'-bis([1]benzothieno[3,2-b][1]benzothiophene-2-yl)-1,1'-biphenyl (BTBT-DPh-BTBT), were synthesized by combining two simple fragment structures. Compared to the monomer compound 2-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT, μmax =3.4×10-2 cm2 V-1 s-1 ), the organic thin-film transistors (OTFTs) based on BTBT-Ph-BTBT and BTBT-DPh-BTBT showed significantly higher mobility (up to 2.5 and 3.6 cm2 V-1 s-1 for BTBT-Ph-BTBT and BTBT-DPh-BTBT, respectively). The mobility of OTFTs based on BTBT-Ph-BTBT was kept at a high value (2.4×10-1 cm2 V-1 s-1 ) after the devices were thermally annealed at 350 °C. Furthermore, the organic phototransistors (OPTs) based on BTBT-Ph-BTBT and BTBT-DPh-BTBT displayed high photosensitivities in a range of 250-400 nm with a low intensity, making these materials potentially applicable for sensitive optoelectronic devices.

Solution-processable liquid crystalline chrysene semiconductors with wide band gap: Self-organization and carrier transport properties

Abstract High-performance solution-processable organic semiconductors (OSCs) with wide bang gap (≥3.3eV) are highly required for fabricating transparent organic thin-film transistors (OTFTs) with the inexpensive and low-temperature printed technique. Herein, in this study, we reported the newly synthesized chrysene-based OSCs (di-CR-C12, mono-CR-C12) with considerably large band gap of 3.70 eV, superior liquid crystallinity and solution-processed OTFTs mobility as high as 0.5 cm2/V. Such large band gap resulted in complete optical transparency of thin film in the visible-light region and good photostability. Highly ordered smectic E (SmE) phase of mono-CR-C12 was the first reported case of the monoalkylated single core structure demonstrating advantageous capacity of CR backbone in stablizing Sm mesophase. Remarkably, high hole mobility exceeding 0.1 cm2/V in the bulk phase was determined by time-of-flight (TOF) technique intrinsically revealing their satisfactory carrier transport characteristics. Good solubility of mono-CR-C12 enables solution-processed OTFTs with Au nanoparticles deposited at room-temperature and under ambient conditions, exhibiting good device performance with mobility up to 0.5 cm2/V. All results demonstrate that they are very promising candidates as active component for transparent electronics based on OTFTs.

Recent Progress in High-Mobility Organic Transistors: A Reality Check.

Over the past three decades, significant research efforts have focused on improving the charge carrier mobility of organic thin-film transistors (OTFTs). In recent years, a commonly observed nonlinearity in OTFT current-voltage characteristics, known as the "kink" or "double slope," has led to widespread mobility overestimations, contaminating the relevant literature. Here, published data from the past 30 years is reviewed to uncover the extent of the field-effect mobility hype and identify the progress that has actually been achieved in the field of OTFTs. Present carrier-mobility-related challenges are identified, finding that reliable hole and electron mobility values of 20 and 10 cm2 V-1 s-1 , respectively, have yet to be achieved. Based on the analysis, the literature is then reviewed to summarize the concepts behind the success of high-performance p-type polymers, along with the latest understanding of the design criteria that will enable further mobility enhancement in n-type polymers and small molecules, and the reasons why high carrier mobility values have been consistently produced from small molecule/polymer blend semiconductors. Overall, this review brings together important information that aids reliable OTFT data analysis, while providing guidelines for the development of next-generation organic semiconductors.

Temperature-dependent field-effect carrier mobility in organic thin-film transistors with a gate SiO2 dielectric modified by H2O2 treatment

The effect of the modification of a gate SiO2 dielectric using an H2O2 solution on the temperature-dependent behavior of carrier transport for pentacene-based organic thin-film transistors (OTFTs) is studied. H2O2 treatment leads to the formation of Si(–OH)x (i.e., the formation of a hydroxylated layer) on the SiO2 surface that serves to reduce the SiO2 capacitance and weaken the pentacene–SiO2 interaction, thus increasing the field-effect carrier mobility (µ) in OTFTs. The temperature-dependent behavior of carrier transport is dominated by the multiple trapping model. Note that H2O2 treatment leads to a reduction in the activation energy. The increased value of µ is also attributed to the weakening of the interactions of the charge carriers with the SiO2 dielectric that serves to reduce the activation energy.

Temperature-dependent hole transport for pentacene thin-film transistors with a SiO2 gate dielectric modified by (NH4)2Sx treatment

The effect of a SiO2 gate dielectric modified by (NH4)2Sx treatment on the temperature-dependent hole transport behavior for pentacene-based organic thin-film transistors (OTFTs) is studied. (NH4)2Sx treatment leads to the formation of SSi bonds (i.e., the formation of a sulfurated layer) on the SiO2 surface that serves to reduce the SiO2 capacitance, increasing the value of the hole mobility in OTFTs. The temperature-dependent hole transport is dominated by the multiple trapping model. It is shown that (NH4)2Sx treatment leads to a reduction in the activation energy, resulting from the formation of a sulfurated layer at the pentacene/SiO2 interface that serves to suppress the pentacene-SiO2 interaction. (NH4)2Sx treatment provides an opportunity to realize the stable and reliable carrier conduction behavior for OTFTs.

Influence of Surface Energy and Roughness on Hole Mobility in Solution-Processed Hybrid Organic Thin Film Transistors

We investigate the influence of the surface energy and roughness on the hole mobility in organic thin-film transistors where a poly(3,3”’-dialkylquarterthiophene) (PQT-12) hybridized with an octyltrichlorosilane self-assembled monolayer is employed as the semiconducting layer on a silicon nitride (SiN x ) gate insulator. Here, these surface properties are modified with varying the duration of oxygen plasma treatments on the SiN x surface, eluding to a different surface roughness and energy. From our analysis coupled with the experimental results, it is found that the surface roughness ( $R _{a}$ ) controls the degree of surface roughness scattering ( $\chi _{\mathrm{ SR}}$ ) while the surface energy ( $E_{S}$ ) determines the reference mobility ( $\mu _{b}$ ), yielding the effective hole mobility expressed as a product of two terms associated with $\mu _{b}$ and $\chi _{\mathrm{ SR}}$ , respectively. It is found that $\mu _{b}$ follows a power law as a function of $E_{S}$ , and $\chi _{\mathrm{ SR}}$ grows exponentially with increasing $R_{a}$ . In addition, a characteristics scattering length ( $\lambda _{c}$ ) appears in the mobility expression, which turns out to be a demarcation, suggesting that $R_{a}$ is required to be at least smaller than $\lambda _{c}$ to minimize $\chi _{\mathrm{ SR}}$ .

Space Environment Effects on Flexible, Low-Voltage Organic Thin-Film Transistors.

Organic electronic devices fabricated on flexible substrates are promising candidates for applications in environments where flexible, lightweight, and radiation hard materials are required. In this work, device parameters such as threshold voltage, charge mobility, and trap density of 13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene)-based organic thin-film transistors (OTFTs) have been monitored for performing electrical measurements before and after irradiation by high-energy protons. The observed reduction of charge carrier mobility following irradiation can be only partially ascribed to the increased trap density. Indeed, we used other techniques to identify additional effects induced by proton irradiation in such devices. Atomic force microscopy reveals morphological defects occurring in the organic dielectric layer induced by the impinging protons, which, in turn, induce a strain on the TIPS-pentacene crystallites lying above. The effects of this strain are investigated by density functional theory simulations of two model structures, which describe the TIPS-pentacene crystalline films at equilibrium and under strain. The two different density of states distributions in the valence band have been correlated with the photocurrent spectra acquired before and after proton irradiation. We conclude that the degradation of the dielectric layer and the organic semiconductor sensitivity to strain are the two main phenomena responsible for the reduction of OTFT mobility after proton irradiation.