Mobility In Organic Transistors Research Articles

Understanding Effects of Ion Diffusion on Charge Carrier Mobility of Electrolyte‐Gated Organic Transistor Using Ionic Liquid‐Embedded Poly(3‐hexylthiophene)

In electrolyte‐gated organic electronics, including electrochromic devices, organic field‐effect transistors, and organic electrochemical transistors, the underlying working principle is determined by the permeability of the electrochemically active ions within the electrolyte dielectric into the organic semiconductor layer; as such, the carrier mobility of organic semiconductors in electrolyte‐gated devices remains unclear because of the different degrees of ion penetration depending on the fabrication process and device architecture. Here, ion‐embedded organic semiconductors are developed by precisely incorporating ionic liquid (IL) in poly(3‐hexylthiophene) (P3HT), and then the effects on the charge carrier mobility in organic transistors using poly(methyl methacrylate) (PMMA), poly(vinylidene fluoride‐co‐hexafluoropropylene) (P(VDF‐HFP)) and electrolyte dielectrics are systematically investigated. Neat P3HT transistors show saturation mobility of 0.080 ± 0.003, 0.287 ± 0.025, and 5.04 ± 0.16 cm2 V−1 s−1 using PMMA, P(VDF‐HFP), and polymer electrolyte dielectrics, respectively. Compared with control neat P3HT devices, nonproportional normalized saturation mobilities of 0.46 (0.30), 1.65 (1.35), and 0.74 (0.89) are observed for P3HT:IL of 99.5:0.5(98:2) v/v% devices using PMMA, P(VDF‐HFP), and polymer electrolyte dielectrics, respectively. In addition, it is found that the ion penetration into P3HT can influence the metal/semiconductor contact and interfacial charge trapping at the dielectric/semiconductor, which can disrupt the efficient charge carrier transport.

Synergistic Use of Pyridine and Selenophene in a Diketopyrrolopyrrole‐Based Conjugated Polymer Enhances the Electron Mobility in Organic Transistors

AbstractTo achieve semiconducting materials with high electron mobility in organic field‐effect transistors (OFETs), low‐lying energy levels (the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)) and favorable molecular packing and ordering are two crucial factors. Here, it is reported that the incorporation of pyridine and selenophene into the backbone of a diketopyrrolopyrrole (DPP)‐based copolymer produces a high‐electron‐mobility semiconductor, PDPPy‐Se. Compared with analogous polymers based on other DPP derivatives and selenophene, PDPPy‐Se features a lower LUMO that can decrease the electron transfer barrier for more effective electron injection, and simultaneously a lower HOMO that, however, can increase the hole transfer barrier to suppress the hole injection. Combined with thermal annealing at 240 °C for thin film morphology optimization to achieve large‐scale crystallite domains with tight molecular packing for effective charge transport along the conducting channel, OFET devices fabricated with PDPPy‐Se exhibit an n‐type‐dominant performance with an electron mobility (μe) as high as 2.22 cm2 V−1 s−1 and a hole/electron mobility ratio (μh/μe) of 0.26. Overall, this study demonstrates a simple yet effective approach to boost the electron mobility in organic transistors by synergistic use of pyridine and selenophene in the backbone of a DPP‐based copolymer.

Impact of p-type doping on charge transport in blade-coated small-molecule:polymer blend transistors

Blade-coating is used to fabricate high hole mobility organic transistors based on a p-doped small-molecule:polymer blend semiconductor.

Macroscopic Interfacial Property as a Determining Parameter for Reliable Prediction of Charge Mobility in Organic Transistors

Physical or chemical properties characterizing a surface of gate dielectric have a huge impact on the electrical properties of organic field-effect transistors. Here, we applied various organic interlayers between an organic semiconductor and a gate dielectric to describe field-effect mobilities being a function of a certain macroscopic parameter associated with the surface energy of gate dielectric. The organic interlayers with various chemical moieties, that is, hydroxyl, methyl, octadecyl, polystyrene, and polymethylmetacrylate, are obtained using diverse organosilane compounds and hydroxyl-end-terminated polymer brushes. Two prototypical vapor-deposited p-type organic small molecules, dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene and pentacene, are used as semiconducting layers. We separate the surface energy of the organic interlayers into two terms, that is polar and dispersive terms, and define three parameters consisting of these two terms, so-called surface energy ratio, polar ratio, and polarity. The three parameters are plotted with the field-effect mobilities and it becomes apparent that the field-effect mobility is a function of polar ratio and polarity regardless of the semiconducting material as well as its morphology and crystallinity. In particular, the polarity that is the polar energy term divided by the total surface energy showed a clear exponential relationship, allowing a reliable prediction of field-effect mobilities.

Precise Characterisation of Molecular Orientation in a Single Crystal Field-Effect Transistor Using Polarised Raman Spectroscopy.

Charge transport in organic semiconductors is strongly dependent on the molecular orientation and packing, such that manipulation of this molecular packing is a proven technique for enhancing the charge mobility in organic transistors. However, quantitative measurements of molecular orientation in micrometre-scale structures are experimentally challenging. Several research groups have suggested polarised Raman spectroscopy as a suitable technique for these measurements and have been able to partially characterise molecular orientations using one or two orientation parameters. Here we demonstrate a new approach that allows quantitative measurements of molecular orientations in terms of three parameters, offering the complete characterisation of a three-dimensional orientation. We apply this new method to organic semiconductor molecules in a single crystal field-effect transistor in order to correlate the measured orientation with charge carrier mobility measurements. This approach offers the opportunity for micrometre resolution (diffraction limited) spatial mapping of molecular orientation using bench-top apparatus, enabling a rational approach towards controlling this orientation to achieve optimum device performance.

Tailoring the Dielectric Layer Structure for Enhanced Carrier Mobility in Organic Transistors: The Use of Hybrid Inorganic/Organic Multilayer Dielectrics

Hybrid (organic-inorganic) multilayers are used as gate dielectric layers on thin-film transistors. Larger charge-carrier mobility and better operational stability are observed in comparison with transistors using simpler gate dielectrics. The interface roughness is identified as the major factor determining the carrier mobility of the devices. Effects of polarization and charge trapping arising from the interface roughness are discussed. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

플라즈마 표면처리에 따른 유기트랜지스터 특성

In this paper, we fabricated insulator thin films by plasma polymerization method for organic thin film transistor's insulator layer. For improving the electrical characteristics of organic transistor, we treated the semiconductor thin film with plasma. As results, the surface energy of organic transistor was increased from to and the mobility of organic transistor was increased , that is increased 29% average ratio. Therefore, we have known that oragnic transistor's mobility can improve with plasma treatment of semiconductor thin film's surface.

A highly conducting organic metal derived from an organic-transistor material: benzothienobenzothiophene

BTBT ([1]benzothieno[3,2-b][1]benzothiophene) is an organic semiconductor that realizes high mobility in organic transistors. Here we report that the charge-transfer (CT) salt, (BTBT)2PF6, shows a high room-temperature conductivity of 1500 S cm(-1). This compound exhibits a resistivity jump around 150 K, but when it is covered with Apiezon N grease the resistivity jump is suppressed, and the metallic conductivity is maintained down to 60 K. Owing to the very high conductivity, the ESR signal shows a significantly asymmetric Dysonian lineshape (A/B ≅ 3) even at room temperature. Since most organic conductors are based on strong electron donors, it is remarkable that such a weak electron donor as BTBT realizes a stable and highly conducting organic metal.

The impact of tetrahedral capping groups and device processing conditions on the crystal packing, thin film features and OFET hole mobility of 7,14-bis(ethynyl)dibenzo[b,def]chrysenes

A series of tetrahedrally capped acetylene dibenzochrysene derivatives were modelled and then synthesised resulting in compounds with very similar electronic properties, but differing physical properties depending on the capping group. By varying the size of the tetrahedral capping group we have undertaken a study on the relationship between the molecular structure and charge mobility in organic transistors. Thin film characterisation techniques (near-edge X-ray absorption fine structure spectroscopy, scanning electron microscopy, atomic force microscopy and X-ray diffraction) were coupled with organic field effect transistor (OFET) hole mobility data to explore the interplay between molecular structure, molecular packing, substrate dielectrics and transistor hole mobility. Using this strategy we identified a lead compound, TMS-DBC 2b, that has a one-dimensional slipped stack packing motif with a high degree of π–π overlap that produces hole mobilities as high as 0.012 cm2 V−1 s−1 when deposited on SiO2-ODTS substrates. Further refinement of the substrate temperature and deposition rate results in TMS-DBC 2b achieving hole mobilities as high as 1.17 cm2 V−1 s−1. These results show that subtle modification of the tetrahedral capping groups can alter the crystal packing and thin film microstructure of these angular dibenzochrysene materials, which dramatically influences the hole mobility in transistors. Further refinement of the device processing conditions on the lead compound results in a significant increase in transistor mobility.

Carrier mobility in organic field-effect transistors

A study of carrier transport in top-gate and bottom-contact TIPS-pentacene organic field-effect transistors (OFETs) based on mobility is presented. Among three mobilities extracted by different methods, the low-field mobility obtained by the Y function exhibits the best reliability and ease for use, whereas the widely applied field-effect mobility is not reliable, particularly in short-channel transistors and at low temperatures. A detailed study of contact transport reveals its strong impact on short-channel transistors, suggesting that a more intrinsic transport analysis is better implemented in relatively longer-channel devices. The observed temperature dependences of mobility are well explained by a transport model with Gaussian-like diffusivity band tails, different from diffusion in localized states band tails. This model explicitly interprets the non-zero constant mobility at low temperatures and clearly demonstrates the effects of disorder and hopping transport on temperature and carrier density dependences of mobility in organic transistors.

Theoretical analysis of carrier mobility in organic field-effect transistors

A theoretical analysis of the carrier mobility in organic transistors is presented. We noticed that the assumption of zero potential at open/quasi-free surface may cause a large deviation of the areal charge density in the organic film, greater in thinner-film transistors. Taking into account this effect, the effective mobility is obtained using the Kubo–Greenwood integral, which provides the total conductivity in the band and thus in the whole organic film. The mobility is studied with respect to gate voltage and temperature, for various disorder and transport diffusivity levels, enabling a better insight of the carrier mobility in organic transistors.

Direct evaluation of low-field mobility and access resistance in pentacene field-effect transistors

Organic field-effect transistors (OFETs) suffer from limitations such as low mobility of charge carriers and high access resistance. Direct and accurate evaluation of these quantities becomes crucial for understanding the OFETs properties. We introduce the Y function method (YFM) to pentacene OFETs. This method allows us to evaluate the low-field mobility without the access or contact resistance influence. The low-field mobility is shown to be more appropriate than the currently applied field-effect mobility for the OFETs’ performance evaluation. Its unique advantage is to directly suppress the contact resistance influence in individual transistors, although such contact resistance is a constant as compared to the widely accepted variable one with respect to the gate voltage. After a comparison in detail with the transmission-line method, the YFM proved to be a fast and precise alternative method for the contact resistance evaluation. At the same time, how the contact resistance affects the effective mobility and the field-effect mobility in organic transistors is also addressed.

The morphology of integrated self-assembled monolayers and their impact on devices – A computational and experimental approach

We report on a combined experimental and computational study on self-assembled monolayers (SAMs). The dielectric properties of SAMs based on n-alkyl phosphonic acids in large area thin film devices (organic field-effect transistors with good hole mobilities up to 0.3 cm 2 V −1 s −1 at −1 V and capacitors) depend on their chain length, but not consistently to theoretical pictures of tunneling through saturated n-alkanes. An unexpected saturation of current density with increasing chain length was obtained in devices, what impact on the leakage current in transistors and current density in capacitors. This is attributed to different self-assembled monolayer morphology ranging from an amorphous state for short alkyl chains to a quasi-crystalline state for longer alkyl chains. Molecular dynamics (MD) simulations provide a deeper insight into the nature of the three-dimensional intermolecular interactions and support the proposed SAM morphology. The change in morphology leads to a reduced effective SAM thickness in devices, which could be described by a Simmons approach quantitatively. The morphological aspect of self-assembled molecules is of enormous importance beyond the applications of SAMs in low-voltage, high mobility organic transistors because of it’s relevance for the hole field of molecular scale electronics. It demonstrates that even small changes in the molecular design can change the molecular interactions and monolayer assembly.

Growth front nucleation of rubrene thin films for high mobility organic transistors

We demonstrate a mode of thin film growth in which amorphous islands crystallize into highly oriented platelets. A cascade of crystallization is observed, in which platelets growing outward from a central nucleation point impinge on neighboring amorphous islands and provide a seed for further nucleation. Through control of growth parameters, it is possible to produce high quality thin films which are well suited to the formation of organic transistors. We demonstrate this through the fabrication of rubrene thin film transistors with high carrier mobility.

Photocurable Organic Gate Insulator for the Fabrication of High‐Field Effect Mobility Organic Transistors by Low Temperature and Solution Processing

A new organic gate insulator in OTFT devices, which satisfies the requirements for the fabrication of a practical TFT array, such as a good chemical resistance, a low moisture uptake, a low temperature process, and a good film surface smoothness, is introduced. Since this gate insulator film is formed by photo-curing after spincasting on the top of the gate electrode, the process does not require high temperatures.

Low-voltage and high-field-effect mobility organic transistors with a polymer insulator

A key issue in research into organic thin-film transistors (OTFTs) is low-voltage operation. In this study, we fabricated a pentacene thin-film transistor with an ultrathin layer of polyvinyl alcohol (9nm) as a gate insulator, and obtained a device with excellent electrical characteristics at low operating voltages (below 2V). This device was found to have a field-effect mobility of 1.1cm2∕Vs, a threshold voltage of −0.98V, an exceptionally low subthreshold slope of 180mV/decade, and an on/off current ratio of 106. This favorable combination of properties means that such OTFTs can be operated successfully at voltages below 2V.

High mobility organic transistors fabricated from single pentacene microcrystals grown on a polymer film

Isolated multilayer, bilayer, and monolayer crystals of pentacene are grown on polymer thin films. Source and drain contacts are formed to individual crystallites and transistor operation, based on the formation of an inversion layer at the pentacene/polymer interface is demonstrated for multilayers, trilayers, and bilayers. Hole inversion layers are formed with mobilities up to 1.2 cm2/V s for multilayer crystals, while a reduced mobility is observed for trilayers and bilayers.