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Heteroacenes containing a dihydropyrrolo[3,2‐b]pyrrole core such as 5,10‐dihydroindolo[3,2‐b]indole (DBPP) and dinaphtho [2,3‐b:2',3'‐f]pyrrolo[3,2‐b]pyrrole(DNPP) are potential p‐type semiconductors for organic field‐effect transistors (OFETs) owing to their electron‐rich and easy‐to‐modify core structure. Here we report that how the structural variations such as ring fusion pattern, heteroatom introduction and chlorination influence the crystallinity, orientation and morphology of this type of heteroacenes hence their OFET performance. It is revealed that angular fused C8‐iso‐DNPP has a higher HOMO energy level with matches better with the work‐function of Au and shows better crystallinity and more uniformed morphology compared to the linearly fused isomer C8‐DNPP. Thus it exhibits very similar hole mobility to C8‐DNPP but much smaller threshold voltages. On the other hand, the introduction of extra N atoms into DNPP affords an analogue DQPP and greatly alters the orientation of the crystalline film from "edge‐on" to "face‐on", which results in a two‐order decrease of the hole mobility. Our result also shows that heavily chlorinated DBPP (TClDBPP) leads to the completely loss of the hole transport capability due to the large π‐π distance imposed by the large chlorine atoms. [ABSTRACT FROM AUTHOR]

We report diketopyrrolopyrrole (DPP) based copolymers, poly (diketopyrrolopyrrole-cyclopentadithiophene) (PDPPTT-CPDT) and poly(diketopyrrolopyrrolebenzo[1,2-b:4,5-b’]dithiophene) (PDPPTT-BDT) to investigate donor unit structure effects on organic field-effect transistor (OFET) performance. The highest occupied molecular orbital (HOMO) level and copolymer band gap were tuned by introducing different donor units on the acceptor DPP backbone. PDPPTT-BDT OFETs show 10 fold higher hole field-effect mobility, up to 0.08 cm 2  V −1  s −1 , than PDPPTT-CPDT OFETs due to better crystallinity and higher HOMO energetic levels. PDPPTT-CPDT and PDPPTT-BDT were operationally stable with negligible threshold voltage shift after cycling. [ABSTRACT FROM AUTHOR]

A new and exciting pattern transfer method is presented to enhance the photoconductivity of Ag/CSA-PANI/PMMA/ITO, organic field effect transistor (OFET) by employing soft-lithography based grating over the channel region. The light is diffracted by the 265 nm gratings to enhance the UV (365 nm) activity of the channel material. The electrical characteristics of the grating-based OFET exhibited p-channel properties with the saturated hole mobility and the threshold voltage of ∼9.5 × 10 −5 cm 2 /V-s and ∼ −1.72 V respectively. The photo-generated charge carriers strongly influence the drain current in comparison to the dark condition. The External Quantum Efficiency of ∼2.45 × 10 5 establishes the photo-multiplication as a dominant factor for enhancement of the drain current. Moreover, the fabricated grating based OFET exhibited high photo-sensitivity and photo-responsivity of ∼1.16 × 10 2 and ∼7.33 × 10 4 A/W, respectively. Thus, the proposed soft-lithography grating based Ag/CSA-PANI/PMMA/ITO, OFET is an exceptional candidate for phototransistor applications. [ABSTRACT FROM AUTHOR]

High-quality rubrene crystals were directly grown on a polymeric gate dielectric-coated ITO/glass substrate by a simple solution process. Organic field-effect transistors were fabricated based on these rubrene crystals, and their electrical performance was investigated. Atomic force micrographs of the rubrene crystal reveal that its surface has a conventional terrace structure, confirming the well-ordered rubrene molecules. The organic transistor with the rubrene crystal shows the highest charge carrier hole mobility of 3.74 cm 2 /V·s that can be attributed to the strong π-π overlap between the adjacent rubrene molecules. The device shows a stable static and dynamic electrical stress response under ambient conditions. For evaluating the applicability of the transistor in a logic circuit, a simple load-type inverter is fabricated by combining the rubrene transistor and a 10 MΩ load-resistor, to achieve a clear dynamic switching response up to 100 Hz. [ABSTRACT FROM AUTHOR]

The performance of organic field-effect transistors (OFETs) can be greatly limited due to the inefficient charge injection caused by the large interfacial barrier at the metal/organic semiconductor interface. To improve this, two-dimensional graphene films have been suggested as alternative electrode materials; however, a comparative study of OFET performances using different types of graphene electrodes has not been systematically investigated. Here, we present a comparative study on the performance of pentacene OFETs using chemical vapor deposition (CVD) grown graphene and reduced graphene oxide (RGO) as electrodes. The large area electrodes were patterned using a simple and environmentally benign patterning technique. Although both the CVD graphene and RGO electrodes showed enhanced device performance compared to metal electrodes, we found the maximum performance enhancement from CVD grown graphene electrodes. Our study suggests that, in addition to the strong p-p interaction at the graphene/organic interface, the higher conductivity of the electrodes also plays an important role in the performance of OFETs. [ABSTRACT FROM AUTHOR]

In this paper, organic light-emitting field effect transistors (OLEFETs) operating at low voltages | V op | below 10 V were first produced and characterized. n -Dodecylphosphonic acid (PA-C12)-self-assembled monolayer (SAM)-passivated HfO x dielectrics prepared using sol-gel chemistry decreased the device-operating voltages. Crystalline organic active layers of pentacene and N,N ′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (P13) acted as p -and n -type materials, respectively. 4-(dicyanomethylene)-2-methyl-6-( p -dimethylaminostyryl)-4H-pyran doped onto a tris(8-hydroxyquinoline) aluminum (Alq 3 :DCM) layer was used as a light-emitting medium to enhance the luminescence efficiency. Based on a tri-layered structure with balanced ambipolar transports and the combined high capacitance-low leakage current of the PA-C12-SAM-treated HfO x dielectrics, the OLEFETs demonstrated good device performance with light emission at | V op | < 10 V. The transistor characteristics and operating light-emission mechanisms were discussed based on the EL images acquired using a charge-coupled device (CCD) camera under ambient conditions. [ABSTRACT FROM AUTHOR]

We have succeeded in preparation of novel paramagnetic Cu(II) complexes composed of tetrathiafulvalene (TTF)-based ligands as electrical conducting units, [Cu II (Bz-OMe- t Bu-sae-TTF) 2 ], [Cu II (EDT-bissae-TTF)] and [Cu II (Bz-bis(OMe- t Bu-sae)-TTF)]. Electrochemical measurements of these neutral complexes revealed that they exhibited two pairs of redox waves attributed to the oxidation of TTF moieties. Among the newly synthesized TTF complexes, only [Cu II (EDT-bissae-TTF)] gave a radical salt by electrochemical crystallization, but its resistive behavior was insulating. Field-effect transistors (FET) were fabricated with spin-coated films of [Cu II (Bz-OMe- t Bu-sae-TTF) 2 ] and [Cu II (EDT-bissae-TTF)], and their FET performances were very low compared with the conventional organic thin film transistors (OTFT) based on TTF derivatives. Atomic force microscopy (AFM) images of the spin-coated films indicated the non-uniform film surfaces with many small grains leading to the large grain boundaries, which probably causes the low carrier mobility of the FET devices. [ABSTRACT FROM AUTHOR]

Diketopyrrolopyrrole based donor-acceptor-donor conjugated small molecules using ethylene dioxythiophene as a donor was synthesized. Electron deficient diketopyrrolopyrrole unit was substituted with thermocleavable ( tert -butyl acetate) side chains. The thermal treatment of the molecules at 160 °C eliminated the tert -butyl ester group results in the formation of corresponding acid. Optical and theoretical studies revealed that the molecules adopted a change in molecular arrangement after thermolysis. The conjugated small molecules possessed p-channel charge transport characteristics in organic field effect transistors. The charge carrier mobility was increased after thermolysis of tert -butyl ester group to 5.07 × 10 −5 cm 2 /V s. [ABSTRACT FROM AUTHOR]

Organic thin-film transistors (OTFTs) with high crystallization copper phthalocyanine (CuPc) active layers were fabricated. The performance of CuPc OTFTs was studied without and with treatment by Solvent Vapor Annealing on CuPc film. The values of the threshold voltage without and with solvent-vapor annealing are −17 V and −10.5 V respectively. The field-effect mobility values in saturation region of CuPc thin-film transistors without and with Solvent Vapor Annealing are 0.00027 cm 2 /V s and 0.0025 cm 2 /V s respectively. Meanwhile, the high crystallization of the CuPc film with a larger grain size and less grain boundaries can be observed by investigating the morphology of the CuPc active layer through scanning electron microscopy and X-ray diffraction. The experimental results showed the decreased of the resistance of the conducting channel, that led to a performance improvement of the OTFTs. [ABSTRACT FROM AUTHOR]

In organic field-effect transistors (OFETs), the quality of charge-transport pathway, controlled by crystal structures of organic semiconductors (OSCs), strongly affects the performance of the device. To achieve higher charge mobility, solution-processed single-crystal (SPSC) techniques have been used to decrease crystal defects by aligning the crystals of OSCs in the in-plane direction. Nonetheless, through SPSC techniques, whether the crystalline lattices are well-aligned in the out-of-plane direction and how the out-of-plane lattice misorientaion affects OFET performances remain unclear. Here, a characterization protocol based on polarized optical microscope, X-ray diffraction, and electron diffraction is established to identify the lattice structure, the in-plane and out-of-plane lattice alignment in the crystal array of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-PEN). Regardless of the solvents used in the PDMS-assisted crystallization, the characterization protocol confirms that all the crystal arrays share the same lattice structure (form I phase), and have similar in-plane lattice alignment. However, TIPS-PEN molecules have sufficient time to unify their out-of-plane orientation and prevent the formation of low angle grain boundary (LAGB) during crystal growth if high boiling temperature solvents are used. The improved out-of-plane lattice alignment increases the hole mobility and decreases the performance fluctuations of devices. The results confirm that the out-of-plane lattice alignment significantly impacts the performance of the devices and the reproducibility of the solution-processed TIPS-PEN OFETs. [ABSTRACT FROM AUTHOR]

Electrolyte-gated organic transistors offer low bias operation facilitated by direct contact of the transistor channel with an electrolyte. Their operation mode is generally defined by the dimensionality of charge transport, where a field-effect transistor allows for electrostatic charge accumulation at the electrolyte/semiconductor interface, whereas an organic electrochemical transistor (OECT) facilitates penetration of ions into the bulk of the channel, considered a slow process, leading to volumetric doping and electronic transport. Conducting polymer OECTs allow for fast switching and high currents through incorporation of excess, hygroscopic ionic phases, but operate in depletion mode. Here, we show that the use of glycolated side chains on a thiophene backbone can result in accumulation mode OECTs with high currents, transconductance, and sharp subthreshold switching, while maintaining fast switching speeds. Compared with alkylated analogs of the same backbone, the triethylene glycol side chains shift the mode of operation of aqueous electrolyte-gated transistors from interfacial to bulk doping/transport and show complete and reversible electrochromism and high volumetric capacitance at low operating biases. We propose that the glycol side chains facilitate hydration and ion penetration, without compromising electronic mobility, and suggest that this synthetic approach can be used to guide the design of organic mixed conductors. [ABSTRACT FROM AUTHOR]

Environmental stability is one of the most important parameters for high-performance organic field-effect transistors (OFETs). Hydrophobic insulators usually possess much better air stability than some conventional inorganic and hydrophilic organic insulators. However, fabrication of devices with high integration by photolithography method cannot be directly performed on hydrophobic insulators due to their photoresist-unwettable properties. In this work, a simple yet efficient metal-assisted photolithography method is developed to achieve large-scale fabrication of highly integrated organic electronic devices on photoresist-unwettable insulators. By using copper (Cu) as sacrificial layer, photolithography can be performed on these insulators with nearly the same resolution, uniformity, and reproducibility as the conventional photolithography. This method shows excellent flexibility and is capable of fabricating high-integrated devices on a variety of hydrophobic insulators including hydrophobic amorphous fluoropolymer (CYTOP), poly(dimethylsiloxane) (PDMS), and octadecyltrichlorosilane (OTS)-modified SiO 2 . OFETs based on 6,13-dichloropentacene (DCP) microwires (MWs) with CYTOP as the insulator layer were fabricated, which exhibited excellent device performance. Much improved device stability with very low mobility degradation (less than 9%) was observed after 24 days. While devices fabricated on hydrophilic insulators of poly-(vinyl phenol) (PVP) and bare SiO 2 experienced dramatic performances decay within 24 days. Furthermore, by using metal-assisted photolithography method, flexible OFETs arrays could be further fabricated on polyethylene naphthalate (PEN) substrate, which showed excellent mechanical bending flexibility and stability. This work unveils the great potential of metal-assisted photolithography method for long-term stable high-integration organic electronic devices. [ABSTRACT FROM AUTHOR]

Flexible organic field-effect transistors (OFETs) with TIPS-pentacene: polystyrene (PS) blend are demonstrated to exhibit enhanced mobility and significantly improved electrical stability compared to neat TIPS-pentacene on poly(4-vinylphenol) (PVP) dielectric (bi-layer OFETs), along with high mechanical stability. Due to merit of high quality dielectric-semiconductor interface, pristine TIPS-pentacene: PS blend OFETs exhibited maximum mobility of 0.93 cm 2 V −1 s −1 with average of 0.44(±0.25) cm 2 V −1 s −1 compared to 0.14(±0.10) cm 2 V −1 s −1 for bi-layer OFETs with high current on-off ratios on the order 10 5 for both. Both types of devices exhibited high electrical stability upon bending with increasing magnitude of strain or its duration up to 5 days. However, significant differences in electrical stability of devices were observed upon applying constant bias-stress for 40 min to 1 h. Pristine blend devices exhibited outstanding electrical stability with very low drain current decay of <5% compared to ∼30% for bi-layer devices. Even upon bias-stress after 5 days of bending, the drain current decay levels were only changed to <10% and ∼50% for blend and bi-layer devices respectively. [ABSTRACT FROM AUTHOR]

Organic molecular crystals (OMCs) are of significant interest due to their potential use in transistors, photovoltaic devices, light emitting diodes, and other applications. However, conventional vacuum-based methods of growing crystalline OMC films are costly and provide limited control over crystal growth. In this study, we present a new method for preparing high performance single-crystal tetracene field-effect transistors under near-ambient conditions using organic vapor-liquid-solid (OVLS) deposition. We find that the mobility of OVLS-grown tetracene is comparable to high quality crystalline films prepared by physical vapor deposition. These results establish OVLS deposition as a relatively low cost, low substrate temperature, and ambient pressure method for growing high quality OMC films for device applications. [ABSTRACT FROM AUTHOR]

In this paper, we used the low and high density porous structure of polymethylmethacrylate (PMMA) film as tunneling dielectric layer in the floating-gate organic field-effect transistor (OFET) memory devices. Compared to the thin/thick nonporous structure of PMMA tunneling layer, the porous structure of PMMA tunneling layer had positive impacts on the device performance of the floating-gate OFET memory devices. Moreover, it was found that the memory performance was also increased as pore density of PMMA film increased. The atomic force microscopy (AFM) results of both porous structure of PMMA film and pentacene film on porous structure of PMMA film revealed that high density porous structure of PMMA tunneling layer can produce larger tunneling area and more electron transfer paths between pentacene film and PMMA film, which resulted in high electron capture and release efficiency of the floating-gate OFET memory devices with porous structure of PMMA tunneling layer. In addition, our porous structure of PMMA tunneling layer as well as nonporous PMMA film has high electrical insulating property due to their semi-hollow structure film, which is favourable to maintain stable retention property. Eventually, the floating-gate OFET memory devices with high density porous structure of PMMA tunneling layer showed good nonvolatile memory properties with a large memory window of about 43 V, a high ON/OFF current ratio of about 10 4 , and stable endurance and retention properties. Our results provided a new strategy to achieve the high performance floating-gate OFET memory devices. [ABSTRACT FROM AUTHOR]

One important figure of merit for the commercial usability of organic transistors (OFETs) is their electrical stability. With the aim of identifying the microscopic location of degradation sites within a transistor channel, we have investigated the bias stress stability of OFETs by electrical measurements as well as by conductive atomic force microscopy. Air-stable n-channel FETs based on a N,N′-bis(2-ethylhexyl)-1,7(1,6)-dicyano-perylene[3,4:9,10]bis (dicarboximide) were fabricated to understand the relation between the thin-film morphology, the substrate temperature during the vacuum de position with the aim to fabricate transistors with a mobility not dominated by interface traps. The devices showed a maximum carrier mobility of (0.12 ± 0.01) cm2/V s and an on/off ratio up to 107. The electrical performance as well as the bias stress behavior of the semiconductor thin-films is significantly influenced by grain boundaries. For example, the grain boundary resistance was found to increase upon electrical stress by more than 150% (from 2 ± 0.2 GΩ to 5 ± 1.5 GΩ), while the resistance within the grains remains unchanged. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

The development of air-stable and high-performance organic field-effect transistors (FETs) is highly important for practical applications. In present research we studied how the domain size and boundary influence the electrical properties and the stability of CuPc thin films fabricated by the weak epitaxial growth (WEG) method. The electrical properties of fresh CuPc devices have a strong dependence on the domain size, which has been demonstrated based on the analysis of film morphology, electrical properties, and kelvin probe force microscopy (KPFM) measurements. The field-effect mobility of fresh CuPc devices increased with the domain sizes, and the mobility as high as of 0.18 cm 2 /Vs was obtained for a large crystalline domain size of about 60 μm 2 . Furthermore, the CuPc/ p -6P FETs with large domains of ordered crystallites show excellent stability after being exposed to ambient conditions for 20 days. In contrast, it was found that the device with randomly aligned crystallites stored in atmosphere with the same temperature and humidity for 20 days exhibited large changes in the electric characteristics including positive-shifted threshold voltage, much lower I on /I off and mobility. X-ray photoelectron spectroscopy (XPS) results proved that the oxygen content in ordered, large-domain CuPc thin film is much less than that in randomly aligned CuPc thin film with small-size crystallites as stored in the same conditions. It results from the diffusion of more oxygen and water into the randomly aligned CuPc thin film with small-size crystallites, which has more boundaries and higher degree of misorientation than the one with ordered, large-domain crystallites. The investigation reveals the importance of domain boundaries in the device stability, and provides a guide for rational optimization of film morphology for air-stable, high-performance organic FETs. [ABSTRACT FROM AUTHOR]

The key parameters of conjugated polymers are lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels. Few approaches can simultaneously lower LUMO and HOMO energy levels of conjugated polymers to a large extent (>0.5 eV). Disclosed herein is a novel strategy to decrease both LUMO and HOMO energy levels of conjugated polymers by about 0.6 eV through replacement of a CC unit by a B←N unit. The replacement makes the resulting polymer transform from an electron donor into an electron acceptor, and is proven by fluorescence quenching experiments and the photovoltaic response. This work not only provides an effective approach to tune the LUMO/HOMO energy levels of conjugated polymers, but also uses organic boron chemistry as a new toolbox to develop conjugated polymers with high electron affinity for polymer optoelectronic devices. [ABSTRACT FROM AUTHOR]

A novel benzoannulated pentathienoacene derivative dihexyl-[1]benzothieno[2′′,3′′:4′,5′]thieno[2′′,3′′:4,5;5′′,4′′:4′,5′]bisthieno[3,2-b:3′,2′-b′][1]benzothiophene was designed and synthesized. UV-vis spectra, electrochemistry and thermogravimetric analysis results reveal that this material has a large energy bandgap, low-lying highest occupied molecular orbital level and good thermal stability. Atomic force microscopy demonstrates that the thin films show high surface roughness and consisted of interconnected sheet-like crystalline grains. A highly ordered aggregation structure in grains was evidenced by X-ray diffraction measurements. Mobility up to 0.04 cm2 V−1 s−1 and an on/off ratio over 105 were achieved for the thin film field-effect transistors. These results suggest that this new benzoannulated pentathienoacene derivative is an important member of the thienoacene family and will contribute to a comprehensive analysis of the structure–property relationship of the thienoacene analogue. [ABSTRACT FROM AUTHOR]

Highlights: [•] Aligned organic field-effect transistors were fabricated on flexible substrate. [•] Flexible OFETs exhibited mobility up to 2.0cm2 V−1 s−1. [•] The flexible OFETs showed excellent bend stability with bending radius of 3mm. [Copyright &y& Elsevier]

Highlights: [•] We investigated these effects of the interface on the top channel established by Cytop™ and poly(3-hexylthiophene) (P3HT). [•] Controlling roughness of P3HT film renders the transistor to have the mobility of 0.12cm2/Vs in top-gate device. [•] The control of channel roughness and interfacial properties is a critical key in organic transistors. [Copyright &y& Elsevier]

Low-voltage-operating pentacene-based organic field-effect transistors (OFETs) with HfO2 gate insulators have been fabricated at room temperature. Despite its thin capacitance equivalent thickness of 3.6 nm, the room-temperature-processed HfO2 gate insulator shows a low leakage current density of 1.2\times 10^-7~A/cm^2 at a gate voltage of -2~V. Pentacene films grown on the HfO2 gate insulators have a large grain size and a highly ordered molecular structure due to the appropriate surface properties of the HfO2 gate insulators. The as-fabricated pentacene-based OFET (W/L=1300~\mum/100~\mum) with a HfO2 gate insulator has a low subthreshold swing of 0.13 V/decade, a large ON/OFF current ratio of 9.8\times 10^\rm\bf 4, and a high hole mobility of 0.34 cm^2V^-1s^-1 at an operating voltage of -2~V. Furthermore, the environmental stability in the room-temperature-fabricated pentacene-based OFETs with HfO2 gate insulators was investigated. [ABSTRACT FROM AUTHOR]

We have produced stable organic field-effect transistors (OFETs) with an ultra-thin HfO2 gate insulator deposited directly on top of rubrene single crystals by atomic layer deposition (ALD). We find that ALD is a gentle deposition process to grow thin films without damaging rubrene single crystals, as results these devices have a negligibly small threshold voltage and are very stable against gate-bias-stress, and the mobility exceeds 1 cm2/V s. Moreover, the devices show very little degradation even when kept in air for more than 2 months. These results demonstrate thin HfO2 layers deposited by ALD to be well suited as high capacitance gate dielectrics in OFETs operating at small gate voltage. In addition, the dielectric layer acts as an effective passivation layer to protect the organic semiconductor. [ABSTRACT FROM AUTHOR]

We report the use of silk fibroin as the gate dielectric material in solution-processed organic field-effect transistors (OFETs) with poly(3-hexylthiophene) (P3HT) as the semiconducting layer. Such OFETs exhibit a low threshold of -0.77V and a low-operating voltage (0 to -3 V) compatible with the voltage level commonly-used in current electronic industry. The carrier mobility of such OFETs is as high as 0.21 cm² V-1 s-1 in the saturation regime, comparable to the best value of P3HT-based OFETs with dielectric layer that is not solution-processed. The high-performance of this kind of OFET is related with the high content of β strands in fibroin dielectric which leads to an array of fibers in a highly ordered structure, thus reducing the trapping sites at the semiconductor/dielectric interface. [ABSTRACT FROM AUTHOR]

Abstract: The performance of n-type organic field-effect transistors (OFETs) based on C60 active layer was investigated by focusing on the role of 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene (TPBi), bathocuproine (BCP) and bathophenanthroline (Bphen) as buffer layers. It was found that the OFETs with the organic buffer layers exhibited significant improved electrical characteristics, such as saturation current, threshold voltage, field-effect mobility and current on/off ratio. Moreover, by optimizing the film thicknesses of these buffer layers, we also found that the buffer layers with appropriate film thickness can effectively improve charge carrier injection from Ag source/drain electrodes to C60 films. Also, the interface properties and the contact resistance between Ag source/drain electrodes and C60 films have been analyzed. [Copyright &y& Elsevier]

We present a vertical organic transistor with high operating frequencies and high current densities based on the organic semiconductor C60. In this vertical organic triode (VOT), doped layers can easily be inserted to efficiently reduce the contact resistance without the need for additional structuring. Combined with annealing at elevated temperatures (150°C for 2 h), we achieve a current density of 1 A/cm2 at a driving voltage of 3 V together with an extremely high transconductance of 30 mS. The transistor retains a voltage gain above 1 up to 1.5 MHz in a simple inverter circuit. [ABSTRACT FROM AUTHOR]

A method for patterning organic single-crystal transistor arrays via surface selective deposition is presented. Solvent-vapor annealing is applied for the formation of organic crystals under ambient conditions. The devices are fabricated with a dual-gate structure, which allows the investigation of device performances of bottom-gate/top-contact (BG/TC) and top-gate/top-contact (TG/TC) structures based on the same crystals. The resulting BG/TC devices exhibit the field-effect mobility (μFET) up to 3.5 cm2/Vs. And μ FET is influenced by the contact resistance, resulting in large variation in device performance. On the other hand, TG/TC structure exhibits better performance with smaller variation. The highest μ FET is 7.4 cm2/Vs. [ABSTRACT FROM PUBLISHER]

Two donor-acceptor copolymers, P1 and P2, containing the novel donor component benzo[2,1-b:3,4-b′:5,6-c″]trithiophene were synthesized. Both polymers show small π-stacking distances (0.35 nm for P1 and 0.37 nm for P2) due to the use of the disklike-shaped donor unit. However, they exhibit remarkable differences in supramolecular organization, film microstructure, and transistor performance. Indeed, P1 reveals a distinct supramolecular organization in the bulk in comparison to conventional conjugated polymers, including P2. Interestingly, no charge carrier transport was observed for P1 in field-effect transistors, while P2 exhibited a hole mobility of up to 0.04 cm2 V-1 s-1. This variation in device behavior is attributed to the evidently different degree of curvature in the polymer backbone induced by the introduction of two additional thiophene units in P2. [ABSTRACT FROM AUTHOR]

Due to the rapidity of morphological development during deposition, solution-processed organic semiconductor thin films exist in semicrystalline or polycrystalline states, incorporating a high degree of local variations in molecular orientation compared to their single-crystal counterparts. Spherulites, a common crystalline superstructure found in these systems, for example, incorporate a large distribution of molecular orientations about the radial axis to maintain their space-filling growth habit. Here, we aim to determine how this distribution of molecular orientations influences charge transport by fabricating arrays of devices on single spherulites. Given that the orientation distribution that is present about the radial axis mandates the presence of low-angle grain boundaries within single spherulites, we find intraspherulitic charge transport to be independent of the general direction of π-stacking; organic field-effect transistors exhibit comparable mobilities regardless of how their channels are oriented with respect to the general π-stacking direction. [ABSTRACT FROM AUTHOR]

Review: 29 refs. [ABSTRACT FROM AUTHOR]

Solution-processed organic field-effect transistors (OFETs) are essential for developing organic electronics. The encouraging development in solution-processed OFETs has attracted research interest because of their potential in low-cost devices with performance comparable to polycrystalline-silicon-based transistors. In recent years, unidirectional coating technology, featuring thin-film coating along only one direction and involving specific materials as well as solution-assisted fabrication methods, has attracted intensive interest. Transistors with organic semiconductor layers, which are deposited via unidirectional coating methods, have achieved high performance. In particular, carrier mobility has been greatly enhanced to values much higher than 10 cm2 V−1 s−1. Such significant improvement is mainly attributed to better control in morphology and molecular packing arrangement of organic thin film. In this review, typical materials that are being used in OFETs are discussed, and demonstrations of unidirectional coating methods are surveyed. [ABSTRACT FROM AUTHOR]

Organic field-effect transistors (OFETs) with a top-contact structure were fabricated by applying deoxyribonucleic acid (DNA) as a hole injection layer using a simple spray-coating process. Compared with that of the bare OFET, the OFETs incorporated with a DNA hole injection layer exhibited a remarkable enhancement of field-effect mobility from 0.02 to 0.104 cm2 V−1 s−1. By analysing the electrical characteristics of OFETs and the surface morphology of pentacene film, the results showed that the dipole formation effect of the DNA interlayer effectively reduced the contact resistance between gold electrodes and pentacene. Consequently, improved hole injection was obtained along with enhanced electrical characteristics. This effect was also observed in OFETs based on an α-hexathiophene organic semiconductor and OFETs based on silver electrodes. In addition, a guanine base-pair, a major constituent of DNA, was selected to be further analysed and confirmed the existence of the dipole formation effect. [ABSTRACT FROM AUTHOR]

Two different solution processable anthradithiophenes were used as semiconductor material in top gate organic field effect transistors (OFETs) on flexible substrates. We observed good crystallization of the material in thin layers and determined charge carrier mobilities of up to 0.6 cm2/Vs. This value is only half of the mobility of the today best material triisopropylsilylethynyl-pentacene. Furthermore our OFETs show very good air stability over more than 20 months without significant changes of the mobility or on/off-ratio. [ABSTRACT FROM AUTHOR]