High-mobility Organic Semiconductors Research Articles

High mobility organic semiconductors for field-effect transistors

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.

Optical conductivity and optical effective mass in a high-mobility organic semiconductor: Implications for the nature of charge transport

We present a multiscale modeling of the infrared optical properties of the rubrene crystal. The results are in very good agreement with the experimental data that point to nonmonotonic features in the optical conductivity spectrum and small optical effective masses. We find that, in the static-disorder approximation, the nonlocal electron-phonon interactions stemming from low-frequency lattice vibrations can decrease the optical effective masses and lead to lighter quasiparticles. On the other hand, the charge-transport and infrared optical properties of the rubrene crystal at room temperature are demonstrated to be governed by localized carriers driven by inherent thermal disorders. Our findings underline that the presence of apparently light carriers in high-mobility organic semiconductors does not necessarily imply bandlike transport.

Megahertz-class printed high mobility organic thin-film transistors and inverters on plastic using attoliter-scale high-speed gravure-printed sub-5 μm gate electrodes

High performance organic thin-film transistors and inverters operating at MHz frequencies at 10V are fabricated on plastic with high throughput (printing speed up to 1.0m/s) using attoliter-scale high-speed gravure printing. The high performance of the devices is achieved using highly scaled gravure printed features (smaller than 5μm) and optimizing a high mobility organic semiconductor for the short-channel gravure printed devices (>0.5cm2/Vs) to realize record performance levels.

32.3:Invited Paper: Demonstration of High Mobility Un‐isolated Corbino OTFT's with Improved On/Off Ratios Suitable for use in Flexible Displays

AbstractPresented are OTFT arrays fabricated from a high mobility organic semiconductor ink wherein the preferred TFTs are corbino‐type devices. As a direct consequence of the corbino design, the resulting TFTs have very high on/off ratios, typically > 107coupled with very low off currents. This corbino approach demonstrates that high mobility OTFTs with superior switching ratios can be obtained without the need for isolation of the organic semiconductor. Hansen solubility parameters and mobility data are reported. This also indicates potential for the commercial use of organic semiconductor materials in the industrial production of OTFT backplanes for flexible AMOLED displays.

Improving charge injection in high-mobility rubrene crystals: From contact-limited to channel-dominated transistors

With progressively improving charge carrier mobility in organic semiconductors and miniaturization of organic field-effect transistors (OFETs), low contact resistances become ever more important. To study the capabilities of metal electrodes in OFETs and to explore the transition from contact-limited to channel-dominated transistor operation, we used flip-crystal FETs with gold electrodes having different contact resistances Rc to high-quality rubrene crystals. 4-terminal transfer and output measurements reveal that Rc decreases from 105–106 Ω cm for 15 min air exposure to 3 × 103 Ωcm for at least 5 h air exposure of the gold electrodes before the flip-crystal FET is assembled. We conclude the reduction of Rc to be caused by a growing contamination layer on the gold electrodes that weakens the electrostatic coupling between rubrene crystal and gold electrode, and lowers the Schottky contact diode parameter V0. In channel-dominated (low Rc) FETs, the mobility is in the range of 10–17 cm2/(Vs); in contrast, in contact-limited (high Rc) FETs, the apparent mobility decreases significantly with increasing contact resistance. The apparent μ – Rc dependence is not intrinsic, but rather the result of incorrect assumptions of the potential and the charge carrier density in the channel region. Thus, the development of high-mobility organic semiconductors requires further efforts to improve contacts beyond traditional metal electrodes.

High Performance Organic Transistors Using Small Molecule Semiconductors and High Permittivity Semiconducting Polymers

High mobility organic semiconductor formulations with excellent uniformity across large area substrates are prepared via the use of formulations containing small molecule and high permittivity semiconducting oligomers. The use of these high-k (k > 3.3) oligomers allows control of the wetting via the manipulation of the surface energy of the substrate being coated. Organic thin film transistors results with mobilities of up to 5 cm2 V-1 s-1, standard deviation <10 %, on/off ratios of 109 are presented.

In-Plane Orientation Control of 2,7-Diphenyl[1]benzothieno[3,2-b][1]benzothiophene Monolayer on Bismuth-Terminated Si(111) Vicinal Surfaces with Wettability Optimization

We report the in-plane orientation control of a high-mobility organic semiconductor: 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene (DPh-BTBT). As previously reported for the monolayer pentacene, it was revealed that bunched steps on the vicinal Si(111) with a bismuth termination break the surface 3-fold symmetry and reduce the multiple orientation of the DPh-BTBT grains into a quasi-single orientation. Interestingly, the critical step height necessary for the orientation control was higher than that of pentacene. We examined several mechanisms of orientation control and concluded that the facet nanostructure fabricated by step bunching works as an anisotropic template for the nucleation. We also show the wettability optimization of the bismuth-terminated silicon surface and show that the growth mode of DPh-BTBT is dependent on the surface nanostructure of Bi–Si.

High-Mobility Pyrene-Based Semiconductor for Organic Thin-Film Transistors

Numerous conjugated oligoacenes and polythiophenes are being heavily studied in the search for high-mobility organic semiconductors. Although many researchers have designed fused aromatic compounds as organic semiconductors for organic thin-film transistors (OTFTs), pyrene-based organic semiconductors with high mobilities and on-off current ratios have not yet been reported. Here, we introduce a new pyrene-based p-type organic semiconductor showing liquid crystal behavior. The thin film characteristics of this material are investigated by varying the substrate temperature during the deposition and the gate dielectric condition using the surface modification with a self-assembled monolayer, and systematically studied in correlation with the performances of transistor devices with this compound. OTFT fabricated under the optimum deposition conditions of this compound, namely, 1,6-bis(5'-octyl-2,2'-bithiophen-5-yl)pyrene (BOBTP) shows a high-performance transistor behavior with a field-effect mobility of 2.1 cm(2) V(-1) s(-1) and an on-off current ratio of 7.6 × 10(6) and enhanced long-term stability compared to the pentacene thin-film transistor.

Ultraviolet photoelectron spectra of 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene and dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene

Abstract High air stability is the key issue for the organic thin film transistors (OTFTs). Recently developed high-mobility organic semiconductors, 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene (DPh-BTBT) and dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT), are much more stable than pentacene. We measured the ultraviolet photoelectron spectra (UPS) of DPh-BTBT and DNTT and revealed that their HOMOs are situated at deeper energy levels than that of pentacene, which contributes to their high stability. The UPS of DNTT is well reproduced by the DFT calculation, while that of DPh-BTBT is not fully reproduced. This difference is discussed in terms of dihedral angle between the phenyl groups and the BTBT ring.

Impact of Materials versus Geometric Parameters on the Contact Resistance in Organic Thin‐Film Transistors

AbstractThe contact resistance is known to severely hamper the performance of organic thin‐film transistors, especially when dealing with large injection barriers, high mobility organic semiconductors, or short channel lengths. Here, the relative significance of how it is affected by materials‐parameters (mobility and interfacial level‐offsets) and geometric factors (bottom‐contact vs top‐contact geometries) is assessed. This is done using drift‐diffusion‐based simulations on idealized device structures aiming at a characterization of the “intrinsic” situation in the absence of traps, differences in the film morphology, or metal‐atoms diffusing into the organic semiconductor. It is found that, in contrast to common wisdom, in such a situation the top‐contact devices do not always outperform the bottom‐contact ones. In fact, the observed ratio between the contact resistances of the two device structures changes by up to two orders of magnitude depending on the assumed materials parameters. The contact resistance is also shown to be strongly dependent on the hole mobility in the organic semiconductor and influenced by the chosen point of operation of the device.

Thermally Dried Ink-Jet Process for 6,13-Bis(triisopropylsilylethynyl)-Pentacene for High Mobility and High Uniformity on a Large Area Substrate

In this study we developed a simple ink-jet process for 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene), which is known as a high-mobility soluble organic semiconductor, to achieve relatively high-mobility and high-uniformity performance for large-area applications. We analyzed the behavior of fluorescent particles in droplets and applied the results to determining a method of controlling the behavior of TIPS-pentacene molecules. The grain morphology of TIPS-pentacene varied depending on the temperature applied to the droplets during drying. We were able to obtain large and uniform grains at 46 °C without any “coffee stain”. The process was applied to a large-size organic thin-film transistor (OTFT) backplane for an electrophoretic display panel containing 192×150 pixels on a 6-in.-sized substrate. The average of mobilities of 36 OTFTs, which were taken from different locations of the backplane, was 0.44±0.08 cm2·V-1·s-1, with a small deviation of 20%, over a 6-in.-size area comprising 28,800 OTFTs. This process providing high mobility and high uniformity can be achieved by simply maintaining the whole area of the substrate at a specific temperature (46 °C in this case) during drying of the droplets.

Charge Injection in Solution‐Processed Organic Field‐Effect Transistors: Physics, Models and Characterization Methods

A high-mobility organic semiconductor employed as the active material in a field-effect transistor does not guarantee per se that expectations of high performance are fulfilled. This is even truer if a downscaled, short channel is adopted. Only if contacts are able to provide the device with as much charge as it needs, with a negligible voltage drop across them, then high expectations can turn into high performances. It is a fact that this is not always the case in the field of organic electronics. In this review, we aim to offer a comprehensive overview on the subject of current injection in organic thin film transistors: physical principles concerning energy level (mis)alignment at interfaces, models describing charge injection, technologies for interface tuning, and techniques for characterizing devices. Finally, a survey of the most recent accomplishments in the field is given. Principles are described in general, but the technologies and survey emphasis is on solution processed transistors, because it is our opinion that scalable, roll-to-roll printing processing is one, if not the brightest, possible scenario for the future of organic electronics. With the exception of electrolyte-gated organic transistors, where impressively low width normalized resistances were reported (in the range of 10 Ω·cm), to date the lowest values reported for devices where the semiconductor is solution-processed and where the most common architectures are adopted, are ∼10 kΩ·cm for transistors with a field effect mobility in the 0.1-1 cm(2)/Vs range. Although these values represent the best case, they still pose a severe limitation for downscaling the channel lengths below a few micrometers, necessary for increasing the device switching speed. Moreover, techniques to lower contact resistances have been often developed on a case-by-case basis, depending on the materials, architecture and processing techniques. The lack of a standard strategy has hampered the progress of the field for a long time. Only recently, as the understanding of the rather complex physical processes at the metal/semiconductor interfaces has improved, more general approaches, with a validity that extends to several materials, are being proposed and successfully tested in the literature. Only a combined scientific and technological effort, on the one side to fully understand contact phenomena and on the other to completely master the tailoring of interfaces, will enable the development of advanced organic electronics applications and their widespread adoption in low-cost, large-area printed circuits.

The application of a high-k polymer in flexible low-voltage organic thin-film transistors

Although much progress has been made in the study of high-mobility organic semiconductors in recent years, one major challenge for organic thin film transistors (OTFTs) in real applications is the relatively high operating voltage, which is mainly related to the gate dielectric of the OTFT. Here we show the application of a high-k poly(vinylidene fluoride-trifluoroethylene-chlorofloroethylene) (P(VDF-TrFE-CFE)) as the gate dielectric of flexible low-voltage pentacene OTFTs for the first time. The performance of the OTFTs is optimized by modifying the surface of P(VDF-TrFE-CFE) films with various thin polymer films by a solution process. The flexible OTFTs show excellent performance at the operating voltage of 4 V and good stability after 1000 bending tests. It is expected that P(VDF-TrFE-CFE) is a suitable gate dielectric for low-voltage OTFTs based on various small-molecule organic semiconductors because P(VDF-TrFE-CFE) terpolymer films can be easily modified with different thin polymer films by a solution process.

Organic transistors with high thermal stability for medical applications

The excellent mechanical flexibility of organic electronic devices is expected to open up a range of new application opportunities in electronics, such as flexible displays, robotic sensors, and biological and medical electronic applications. However, one of the major remaining issues for organic devices is their instability, especially their thermal instability, because low melting temperatures and large thermal expansion coefficients of organic materials cause thermal degradation. Here we demonstrate the fabrication of flexible thin-film transistors with excellent thermal stability and their viability for biomedical sterilization processes. The organic thin-film transistors comprise a high-mobility organic semiconductor, dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene, and thin gate dielectrics comprising a 2-nm-thick self-assembled monolayer and a 4-nm-thick aluminium oxide layer. The transistors exhibit a mobility of 1.2 cm(2) V(-1)s(-1) within a 2 V operation and are stable even after exposure to conditions typically used for medical sterilization.

Low-voltage organic transistor with subfemtoliter inkjet source-drain contacts

Abstract

Novel liquid crystalline conjugated oligomers based on phenanthrene for organic thin film transistors

Three thiophene/phenanthrene hybrid oligomers were synthesized and their liquid crystalline (LC) properties were characterized by differential scanning calorimetry (DSC), polarized optical microscopy (POM), temperature-dependent X-ray diffraction (XRD) and electron diffraction. Highly ordered LC phases with a broad temperature range were observed for all three oligomers probably owing to the absence of symmetry against the long axis of phenanthrene unit. Highly ordered smectic LC thin films were prepared by vacuum deposition and their organic thin film transistors (OTFTs) exhibited high mobilities up to 0.34 cm2 V−1s−1. Solution processed films showed self-healing behaviors upon thermal annealing, and OTFTs prepared by solution drop casting showed mobilities up to 0.05 cm2 V−1s−1. These results indicate that thiophene/phenanthrene hybrid oligomers are promising high mobility organic semiconductors for OTFTs.

Theoretical Characterization of the Air-Stable, High-Mobility Dinaphtho[2,3-b:2′3′-f]thieno[3,2-b]-thiophene Organic Semiconductor

Recently, an optimum mobility of 8.3 cm2/(Vs) has been measured for single-crystal organic field-effect transistors based on the dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]-thiophene (DNTT) molecule. Here, on the basis of quantum chemistry calculations and molecular dynamics simulations, we investigate the microscopic charge transport parameters of the DNTT molecule and crystal. Our findings confirm that the moderate anisotropy of the hole mobility in DNTT is highly dependent on the presence of in-plane herringbonelike intermolecular interactions with large electronic coupling (transfer integral) values (ca. 0.1 eV). Also, we demonstrate that the π-extended heteroaromatic structure leads to strong electronic coupling interactions among neighboring molecules and to a decrease of the intramolecular reorganization energy. In DNTT, thermal modulations of the electronic couplings at 300 K remain small when compared to those exhibited by the pentacene single crystal. This theoretical study suggests that heteroacenes are a promising route toward high-mobility organic semiconductor materials. Charge transport is discussed in the framework of both band and hopping models.

Preparation of highly oriented copper phthalocyanine film by molecular templating effects for organic field-effect transistor

Highly oriented copper phthalocyanine (CuPc) film consisting of many crystal domains was prepared by introducing titanyl-phthalocyanine (TiOPc) as molecular template at high substrate temperature. CuPc molecules stand up on the top of TiOPc domains exhibiting herringbone structure and high-order arrangement investigated by atomic force microscopy and X-ray diffraction. Then organic transistors were fabricated that exhibited improved device performances, a 0.12 cm 2/V s of mobility and a 10 5 of on/off ratio. Furthermore, template thickness dependence on device performance was disclosed. These results indicated that molecular template is a very effective method to prepare high-mobility organic semiconductor film.

Experimental Study and Statistical Analysis of Solution-Shearing Processed Organic Transistors Based on an Asymmetric Small-Molecule Semiconductor

Solution processed organic field-effect transistors (SPOFETs) are crucial for realizing low-cost large-area/ubiquitous flexible electronics. Currently, both soluble high-mobility organic semiconductors and efficient solution processes are in demand. In this paper, we report the systematic experimental study and statistical modeling/analysis for the SPOFETs based on an asymmetric small-molecule organic semiconductor, trimethyl-[2, 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">'</sup> ; 5 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">'</sup> , 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">''</sup> ; 5 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">''</sup> , 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">'''</sup> ] quarter-thiophen-5-yl-silane (4 T-TMS), which was deposited as the active layer through a recently developed low-temperature solution-shearing process. Three-dimensional statistical modeling and analysis bas ed on 46 different processing conditions was used to comprehensively study the solution-shearing process control and optimization for fabricating high-performance 4T-TMS SPOFETs. Various effects including solution concentration effect, shearing speed effect, and deposition temperature effect were investigated and discussed. Under optimized processing conditions, well-oriented crystalline 4 T-TMS thin films were deposited for the SPOFETs, which showed remarkable effective field-effect mobility up to 0.3 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V middots in the saturation region and current on/off ratios over 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> . Gaussian fitted uniformity and good air stability of these devices stored and tested under ambient conditions for six months suggest that 4 T-TMS SPOFETs based on the optimized solution-shearing process are promising for applications in organic electronic circuits and displays. Importantly, the systematic experiment design and the corresponding statistical modeling/analysis presented here provide a general guideline for process optimization for fabricating high-performance SPOFETs.

Electronic functionalization of solid-to-liquid interfaces between organic semiconductors and ionic liquids: Realization of very high performance organic single-crystal transistors

High-performance electronic function of current amplification is realized with the use of solid-to-liquid interfaces between organic semiconductors and ionic liquid. To hold in place the ionic liquid of 1-ethyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide known for low viscosity and high ionic conductivity, an elastomeric well structure is fabricated with polydimethylsiloxane on which organic single crystals of rubrene are electrostatically attached. As the result of rapid formation of electric double layers in the ionic liquid interfacing, the high-mobility organic semiconductor crystals’ fast-switching transistor function is demonstrated with the application of gate voltage, realizing the highest sheet transconductance, namely, amplifying performance, ever achieved.