N-channel Organic Field-effect Transistors Research Articles

Ambient stable solution-processed organic field effect transistors from electron deficient planar aromatics: effect of end-groups on ambient stability.

Ambient stable solution processed n-channel organic field effect transistors (OFETs) are essential for next-generation low-cost organic electronic devices. Several molecular features, such as suitable orbital energy levels, easy synthetic steps, etc., must be considered while designing efficient active layer materials. Here, we report a case of improved ambient stability of solution-processed n-type OFETs upon suitable end-groups substitution of the active layer materials. A pair of core-substituted napthalenediimide (NDIFCN2 and EHNDICN2) derivatives with alkyl and perfluorinated end groups are considered. The transistor devices made out of these two derivatives exhibited largely different ambient stability behavior. The superior device stability (more than 25 days under ambient conditions) of one of the derivatives (NDIFCN2) was ascribed to the presence of fluorinated end groups that function as hydrophobic guard units inhibiting moisture infiltration into the active layer, thereby achieving ambient stability under humid conditions (>65% relative atmospheric humidity). Molecular level optical and electrochemical properties, thermal stability, and the solution-processed (spin coat and drop cast active layers) device characteristics are described in detail. Our findings highlight the requirement of hydrophobic end groups or sidechains for ambient stability of active layer materials, along with deep LUMO levels for ambient stability.

The Hansen solubility approach towards green solvent processing: n-channel organic field-effect transistors under ambient conditions

This paper demonstrates a rational Hansen solubility approach for the development of green solvent processing in ambient-stable n-channel OFETs. Unique thermodynamic findings are revealed for molecular semiconductors.

Complementary inverter and reward-modulated spike timing dependent plasticity circuit based on organic transistors

Organic materials are the key to the next generation of electric memory elements because of their low cost, high flexibility and high scalability. Solution-processed n-channel and p-channel organic field effect transistors (FETs) by spinning coating technique was demonstrated. We use based-poly (vinylcarbazole) (PVK) p-type transistor and based-6-phenyl-C61 butyric acid methyl ester (PCBM) n-type transistor as the channel of bottom gate structure FET, respectively, to fabricate the complementary inverter. The maximum voltage gain of the electrically isolated FET complementary inverter is up to 22, the power consumption is less than 3 μW, and the transition voltage is −4.2 V. In addition, our devices are able to implement “AND”, “NAND”, “OR”, and “NOT” gate logic well. But more than that, p-type transistor based on PVK and n-type transistor based on PCBM were combined with the memristor, reward-modulated spike timing dependent plasticity was implemented by introduced reward/punishment signals to guide synaptic weight updates. It shows great potential in the electronic application system with low cost and high energy efficiency.

Solution-Processable Indenofluorenes on Polymer Brush Interlayer: Remarkable N-Channel Field-Effect Transistor Characteristics under Ambient Conditions.

The development of solution-processable n-type molecular semiconductors that exhibit high electron mobility (μe ≥ 0.5 cm2/(V·s)) under ambient conditions, along with high current modulation (Ion/Ioff ≥ 106-107) and near-zero turn on voltage (Von) characteristics, has lagged behind that of other semiconductors in organic field-effect transistors (OFETs). Here, we report the design, synthesis, physicochemical and optoelectronic characterizations, and OFET performances of a library of solution-processable, low-LUMO (-4.20 eV) 2,2'-(2,8-bis(3-alkylthiophen-2-yl)indeno[1,2-b]fluorene-6,12-diylidene)dimalononitrile small molecules, β,β'-Cn-TIFDMTs, having varied alkyl chain lengths (n = 8, 12, 16). An intriguing correlation is identified between the solid-isotropic liquid transition enthalpies and the solubilities, indicating that cohesive energetics, which are tuned by alkyl chains, play a pivotal role in determining solubility. The semiconductors were spin-coated under ambient conditions on densely packed (grafting densities of 0.19-0.45 chains/nm2) ultrathin (∼3.6-6.6 nm) polystyrene-brush surfaces. It is demonstrated that, on this polymer interlayer, thermally induced dispersive interactions occurring over a large number of methylene units between flexible alkyl chains (i.e., zipper effect) are critical to achieve a favorable thin-film crystallization with a proper microstructure and morphology for efficient charge transport. While C8 and C16 chains show a minimal zipper effect upon thermal annealing, C12 chains undergo an extended interdigitation involving ∼6 methylene units. This results in the formation of large crystallites having lamellar stacking ((100) coherence length ∼30 nm) in the out-of-plane direction and highly favorable in-plane π-interactions in a slipped-stacked arrangement. Uninterrupted microstructural integrity (i.e., no face-on (010)-oriented crystallites) was found to be critical to achieving high mobilities. The excellent crystallinity of the C12-substituted semiconductor thin film was also evident in the observed crystal lattice vibrations (phonons) at 58 cm-1 in low-frequency Raman scattering. Two-dimensional micrometer-sized (∼1-3 μm), sharp-edged plate-like grains lying parallel with the substrate plane were observed. OFETs fabricated by the current small molecules showed excellent n-channel behavior in ambient with μe values reaching ∼0.9 cm2/(V·s), Ion/Ioff ∼ 107-108, and Von ≈ 0 V. Our study not only demonstrates one of the highest performing n-channel OFET devices reported under ambient conditions via solution processing but also elucidates significant relationships among chemical structures, molecular properties, self-assembly from solution into a thin film, and semiconducting thin-film properties. The design rationales presented herein may open up new avenues for the development of high-electron-mobility novel electron-deficient indenofluorene and short-axis substituted donor-acceptor π-architectures via alkyl chain engineering and interface engineering.

Enhancement of Stability in n-Channel OFETs by Modulating Polymeric Dielectric.

In this study, a high-K material, aluminum oxide (AlOx), as the dielectric of organic field-effect transistors (OFETs) was used to reduce the threshold and operating voltages, while focusing on achieving high-electrical-stability OFETs and retention in OFET-based memory devices. To achieve this, we modified the gate dielectric of OFETs using polyimide (PI) with different solid contents to tune the properties and reduce the trap state density of the gate dielectric, leading to controllable stability in the N, N'-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13)-based OFETs. Thus, gate field-induced stress can be compensated for by the carriers accumulated due to the dipole field created by electric dipoles within the PI layer, thereby improving the OFET's performance and stability. Moreover, if the OFET is modified by PI with different solid contents, it can operate more stably under fixed gate bias stress over time than the device with AlOx as the dielectric layer only can. Furthermore, the OFET-based memory devices with PI film showed good memory retention and durability. In summary, we successfully fabricated a low-voltage operating and stable OFET and an organic memory device in which the memory window has potential for industrial production.

Thermal Sensing Characteristics of Low-Voltage n-Channel Organic Field-Effect Transistors With Triple Layers of Naphthalenediimide-Containing Conjugated Polymer and Gate-Insulating Polymers

Organic field-effect transistors (OFETs) were fabricated with n-type naphthalenediimide-based conjugated polymer (N2200) as a channel layer and poly(methyl methacrylate) (PMMA)/poly(vinyl alcohol) (PVA) as a gate-insulating layer. The OFETs with the N2200/PMMA/PVA triple layers were operated with an n-channel mode at low voltages (≤5 V) and their electron mobility reached ca. 0.63 cm2/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{V}\cdot \text{s}$ </tex-math></inline-formula> at a drain voltage of 5 V. The drain current of OFETs was gradually increased as the temperature of channel region increased from 25 °C to 80 °C, whereas no shift in threshold voltage was measured upon the temperature variation. The relative thermal sensitivity of devices was almost linearly increased with the temperature, while a two-stage behavior by a border of ca. 35 °C was measured for specific thermal sensitivity. The present n-channel OFETs exhibited excellent thermal sensing performances upon repeated approaching/retracting tests using a heat source.

Benzothiadiazole versus Thiazolobenzotriazole: A Structural Study of Electron Acceptors in Solution-Processable Organic Semiconductors.

Despite the rapid progress of organic electronics, developing high-performance n-type organic semiconductors is still challenging. Donor-acceptor (D-A) type conjugated structures have been an effective molecular design strategy to achieve chemically-stable semiconductors and the appropriate choice of the acceptor units determines the electronic properties and device performances. We have now synthesized two types of A1 -D-A2 -D-A1 type conjugated molecules, namely, NDI-BTT-NDI and NDI-TBZT-NDI, with different central acceptor units. In order to investigate the effects of the central acceptor units on the charge-transporting properties, organic field-effect transistors (OFETs) were fabricated. NDI-TBZT-NDI had shallower HOMO and deeper LUMO levels than NDI-BTT-NDI. Hence, the facilitated charge injection resulted in ambipolar transistor performances with the optimized hole and electron mobilities of 0.00134 and 0.151 cm2 V-1 s-1 , respectively. In contrast, NDI-BTT-NDI displayed only an n-channel OFET performance with the electron mobility of 0.0288 cm2 V-1 s-1 . In addition, the device based on NDI-TBZT-NDI showed a superior air stability to that based on NDI-BTT-NDI. The difference in these OFET performances was reasonably explained by the contact resistance and film morphology. Overall, this study demonstrated that the TBZ acceptor is a promising building block to create n-type organic semiconductors.

Significant Performance Improvement in n-Channel Organic Field-Effect Transistors with C60 :C70 Co-Crystals Induced by Poly(2-ethyl-2-oxazoline) Nanodots.

Solution-processed organic field-effect transistors (OFETs) have attracted great interest due to their potential as logic devices for bendable and flexible electronics. In relation to n-channel structures, soluble fullerene semiconductors have been widely studied. However, they have not yet met the essential requirements for commercialization, primarily because of low charge carrier mobility, immature large-scale fabrication processes, and insufficient long-term operational stability. Interfacial engineering of the carrier-injecting source/drain (S/D) electrodes has been proposed as an effective approach to improve charge injection, leading also to overall improved device characteristics. Here, it is demonstrated that a non-conjugated neutral dipolar polymer, poly(2-ethyl-2-oxazoline) (PEOz), formed as a nanodot structure on the S/D electrodes, enhances electron mobility in n-channel OFETs using a range of soluble fullerenes. Overall performance is especially notable for (C60 -Ih )[5,6]fullerene (C60 ) and (C70 -D5h(6) )[5,6]fullerene (C70 ) blend films, with an increase from 0.1 to 2.1 cm2 V-1 s-1 . The high relative mobility and eighteen-fold improvement are attributed not only to the anticipated reduction in S/D electrode work function but also to the beneficial effects of PEOz on the formation of a face-centered-cubic C60 :C70 co-crystal structure within the blend films.

Low-voltage complementary inverters using solution-processed, high-mobility organic single-crystal transistors fabricated by polymer-blend printing

Organic field-effect transistors (OFETs) have attracted great attention as key elements in Internet-of-Thing (IoT) devices due to their advantages of low cost and mass producibility made possible by printing technology. Such devices require organic semiconductors (OSCs) that intrinsically possess high carrier mobility and air stability. In addition, the demand for low-voltage operation and low power consumption has been increasing because the potential power sources for actual devices are implementable energy harvesters that supply low power and low voltages. Based on recently developed high-performance single-crystal p-type and n-type OSCs, this work demonstrated air-stable, high-mobility OFETs with low-voltage operation by using an insulating polymer-blend printing method. By comparing two acrylic polymers poly(methyl methacrylate) and poly(adamantyl methacrylate) (PADMA), having remarkably different thermal properties, we found that PADMA showing a high glass transition temperature &amp;gt;200 °C was suitable for device fabrication, enhancing the flexibility of OSC materials. Also, PADMA spontaneously produced good charge-transport interfaces with the OSC single crystals, leading to high carrier mobilities of 6.6 and 2.2 cm2 V−1 s−1 in p-channel and n-channel OFETs at ≤1.5 V, respectively. The current electron mobility was the highest among low voltage-operation OFETs reported so far. These high-mobility OFETs were integrated into a complementary inverter, for which a low static power consumption of 6.6 pW was confirmed. Therefore, this study reports an advantage of polymer-blend printing for OFETs with enhanced processability and performance suitable for IoT applications.

Reduction Treatment of Molecular-Doped Polymer Semiconductors for High-Performance N-Channel Organic Field-Effect Transistors

Here, we study an efficient reduction treatment for improving the electron transport properties in n-type organic field-effect transistors (OFETs) based on solution-processed polymer semiconductors. If complementary-type printed electronic circuits are to be enabled, equivalently high-performance p-type and n-type organic semiconductors have to develop. Most organic semiconductors have p-type performance superior to the n-type performance. Therefore, high-performance n-type organic semiconductors and their doping process are essential. Reduced-state viologen is known to be a powerful molecular dopant via donating electrons to the conjugated polymer semiconductors. However, the reduced-state viologen readily converts to its initial ionic species under ambient conditions, which leads to insufficient n-doping performance, especially for low electron affinity organic semiconductors. N-doped polymer semiconductors using pristine viologen molecular dopants show inadequate n-channel OFET behavior. For enhancing the electron transfer doping process, solution-based reduction treatment is used to improve remarkably the performance of n-channel OFETs based on initially p-type dominant donor-acceptor-type ambipolar polymer semiconductors. This solution-processed organic active channel layer can be produced at low cost and can be applied to flexible electronic and optoelectronic devices by using various graphic art printing techniques.

Arylacetylene end capped naphthodithiophene diimide (NDTI)-based semiconductors for air-stable, solution-processed n-channel organic field-effect transistors: Effect of terminal aryl groups on charge transport

Solution-processable n-channel organic field-effect transistors (OFETs) are crucial for realizing low-cost, large-area, and all organic logic/complementary circuits. Herein, we present the synthesis, characterization and OFET characteristics of a series of arylacetylene end capped NDTI-based n-channel small molecule semiconductors with four different arylacetylene groups, i.e., thiophenylethynyl (TE, NDTI-BTE), phenylethynyl (PE, NDTI-BPE), 2-pyridinylethynyl (2PdE, NDTI-B2PdE), 4-pyridinylethynyl (4PdE, NDTI-B4PdE). All four NDTIs exhibit excellent solubility behavior in common organic solvents. Electrochemical and optical studies reveal that the introduction of the arylacetylene groups at the thiophene α-positions of NDTI endowed these molecules have sufficiently low-lying LUMO energy levels from −4.12 to −4.24 eV, suitable for better air-stable electron transport. Under ambient atmosphere, we found that solution-processed bottom-gate top-contact (BGTC) transistors based on 120 °C-annealed thin-films of these NDTIs exhibit the best performance with the maximum electron mobilities of 0.049, 0.023, 0.011 and 0.033 cm2 V−1 s−1 for NDTI-BTE, NDTI-BPE, NDTI-B2PdE and NDTI-B4PdE, respectively, due to better thin film microstructures. AFM measurements reveal that the spin-coated films of these materials exhibited quite smooth and uniform morphology although these are small molecules forming crystalline structures. These results clearly demonstrate that arylacetylene groups are potential functional moieties for developing ambient-stable solution-processable small molecular electron-transporting materials for printable organic electronics.

Effects of ladder structure on the electronic properties and field-effect transistor performance of Poly(benzobisimidazobenzophenanthroline)

Abstract A comparative study of the semi-ladder (BBB) and ladder (BBL) forms of poly(benzobisimidazobenzophenanthroline) and their binary blends shows that the ladder structure endows a smaller optical bandgap, greater electron delocalization and substantially enhanced electron transport. n-Channel organic field-effect transistors (OFETs) fabricated from the semi-ladder BBB were found to have an electron mobility of 1.5 × 10−3 cm2/V whereas similar ladder BBL OFETs had an electron mobility of 2.2 × 10−2 cm2/V, which is a 15-fold enhancement in electron transport in favor of the full ladder polymer analogue. The BBB and BBL OFETs had similar good stability in ambient air while their electrical characteristics were successfully simulated by using a SPICE model, revealing dramatically reduced source/drain contact resistance in the BBL devices. Compared to its ladder analogue BBL, semi-ladder BBB has been rarely studied in semiconductor devices, and hence the present observation of high electron mobility in BBB thin films with good stability in air suggest that it is also promising for developing various electronic and energy conversion/storage devices.

The role of surface morphology in a performance of top-gate OFETs prepared from a solution processable derivative of perylene bisimide

A commercially available, soluble in chlorinated hydrocarbons perylene derivative, N,N-bis(1-ethylpropyl)-perylene-3,4:9,10-bis(dicarboximide) (PTCDI-C5(3)), was used to construct n-channel organic field effect transistors (OFETs). The transistors had the top-gate-bottom-contacts (TGBC) architecture, with a spin-coated layer of the active material. This material was subsequently coated with Parylene C gate dielectric layer which, together with a silver gate electrode deposited on the top of that layer, provided a protection of the device against the water and oxygen molecules. In order to investigate the effect of the morphology of semiconductor/gate dielectric interface on the device parameters, a series of OFETs with PTCDI-C5(3) layers spin-coated at different spinning rates were prepared. Three chlorinated hydrocarbon solvents with different boiling points were used: chloroform, chlorobenzene and 1,2-dichlorobenzene. The resulting polycrystalline layers of perylene derivative differed with regard to their thickness and to their roughness as well as exhibited different grains size and shape. Surprisingly however, no direct correlation between the transistor parameters and the thickness of the semiconductor layer or the roughness of the semiconductor/dielectric interface was found – in fact the worst performance was recorded for the OFETs with PTCDI-C5(3) layers of the lowest roughness, spin-coated from chloroform.

π-Extension of electron-accepting dithiarubicene with a cyano-substituted electron-withdrawing group and application in air-stable n-channel organic field effect transistors

New π-extended dithiarubicene derivatives were synthesized for application in n-type organic semiconductors.

Top dielectric induced ambipolarity in an n-channel dual-gated organic field effect transistor

The realization of both p-type and n-type operations in a single organic field effect transistor (OFET) is critical for simplifying the design of complex organic electronic circuits.

Halogenated Tetraazapentacenes with Electron Mobility as High as 27.8 cm2 V-1 s-1 in Solution-Processed n-Channel Organic Thin-Film Transistors.

Molecular engineering of tetraazapentacene with different numbers of fluorine and chlorine substituents fine-tunes the frontier molecular orbitals, molecular vibrations, and π-π stacking for n-type organic semiconductors. Among the six halogenated tetraazapentacenes studied herein, the tetrachloro derivative (4Cl-TAP) in solution-processed thin-film transistors exhibits electron mobility of 14.9 ± 4.9 cm2 V-1 s-1 with a maximum value of 27.8 cm2 V-1 s-1 , which sets a new record for n-channel organic field-effect transistors. Computational studies on the basis of crystal structures shed light on the structure-property relationships for organic semiconductors. First, chlorine substituents slightly decrease the reorganization energy of the tetraazapentacene whereas fluorine substituents increase the reorganization energy as a result of fine-tuning molecular vibrations. Second, the electron transfer integral is very sensitive to subtle changes in the 2D π-stacking with brickwork arrangement. The unprecedentedly high electron mobility of 4Cl-TAP is attributed to the reduced reorganization energy and enhanced electron transfer integral as a result of modification of tetraazapentacene with four chlorine substituents.

Sequential Fluorination on Napthaleneamide-Based Conjugated Polymers and Their Impact on Charge Transport Properties

Improving π-conjugated polymer electron transport and injection efficiency are important to realize high performance n-channel organic field-effect transistors (OFETs). This paper reports a series of naphthalene diimide (NDI-) based n-type π-conjugated polymers, NDI-T-1FP-T, NDI-T-2FP-T, and NDI-T-4FP-T, with varioulis amounts of fluorine atoms (1F, 2F, and 4F) in their backbones. We found notable differences in energy levels, ground and excited state dipole moments (Δμge), exciton lifetimes, dihedral angles, and charge transport properties by varying the fluorine content: Δμge and charge transport properties reduced with increasing fluorine content. Electron mobility of optimized top gate bottom contact OFETs using the n-type π-conjugated polymers were 0.35, 0.18, and 0.16 cm2 V–1 s–1, respectively. OFET performances were further improved further by low concentration polyethylenimine (PEI) doping, significantly improving field-effect mobility to (maximum) 0.51, 0.34, and 0.26 cm2 V–1 s–1, respectively. D...

Pronounced Side Chain Effects in Triple Bond-Conjugated Polymers Containing Naphthalene Diimides for n-Channel Organic Field-Effect Transistors.

Three triple bond-conjugated naphthalene diimide (NDI) copolymers, poly{[ N, N'-bis(2-R1)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]- alt-[(2,5-bis(2-R2)-1,4-phenylene)bis(ethyn-2,1-diyl)]} (PNDIR1-R2), were synthesized via Sonogashira coupling polymerization with varying alkyl side chains at the nitrogen atoms of the imide ring and 2,5-positions of the 1,4-diethynylbenzene moiety. Considering their identical polymer backbone structures, the side chains were found to have a strong influence on the surface morphology/nanostructure, thus playing a critical role in charge-transporting properties of the three NDI-based copolymers. Among the polymers, the one with an octyldodecyl (OD) chain at the nitrogen atoms of imide ring and a hexadecyloxy (HO) chain at the 2,5-positions of 1,4-diethynylbenzene, P(NDIOD-HO), exhibited the highest electron mobility of 0.016 cm2 V-1 s-1, as compared to NDI-based copolymers with an ethylhexyl chain at the 2,5-positions of 1,4-diethynylbenzene. The enhanced charge mobility in the P(NDIOD-HO) layers is attributed to the well-aligned nano-fiber-like surface morphology and highly ordered packing structure with a dominant edge-on orientation, thus enabling efficient in-plane charge transport. Our results on the molecular structure-charge transport property relationship in these materials may provide an insight into novel design of n-type conjugated polymers for applications in the organic electronics of the future.

Synthesis of Naphthalenediimide-Based Co-Polymers for n-Channel Organic Field-Effect Transistors

Synthesis of Naphthalenediimide-Based Co-Polymers for<i> n</i>-Channel Organic Field-Effect Transistors

Performance Enhancement in N-Channel Organic Field-Effect Transistors Using Ferroelectric Material as a Gate Dielectric

Organic field-effect transistors (OFETs) were elaborated using N,N’-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C $_{13}$ ) as transport electron material and two polymeric insulators were used as gate dielectric: polymethyl methacrylate (PMMA), a low-k material widely used in OFETs, and poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)), a ferroelectric material that has a high relative permittivity. Several configurations using both dielectrics were studied. We report a threshold and operating voltage reduction by 3 and 2, respectively, with P(VDF-TrFE)/PMMA gate dielectric compared to PMMA reference OFETs due to a higher capacitance value. It exhibits good performances with a mobility of 0.165 cm $^{2}$ /V $\cdot$ s, a threshold voltage of 2.9 V, and an on–off current ratio $>3\; \times \;10^{4}$ .