Organic Thin-film Field-effect Transistors Research Articles

Solution-processable quinoidal compounds containing heterocycle for air-stable n‑type organic field-effect transistors and gas sensors

Organic field effect transistors (OFETs) have potential applications in gas sensing area, but the performance and diversity of n-type OFETs are still far behind that of p-type OFETs due to the poor stability in air atmosphere. Herein, a series of novel dicyanomethylene-terminated quinoids compounds were synthesized and characterized by using electron withdrawing aromatic heterocycles as the core structural units. The introduction of alkyl branch chain greatly improves the solubility, which is helpful to the preparation of thin film OFETs devices by low-cost solution method. These OFETs show typical n-channel FET characteristics with mobility ranging from 10−3∼10−1 cm2V−1s−1. Among them, the 3-T-TZCN based OFETs exhibit the best stability, in which the mobility decreased no more than 18 % after 90 days stored in air atmosphere, this is an outstanding performance compared with the previous researches. We further investigated the dimethyl methylphosphonate (DMMP, which is a substitute for detecting sarin) gas sensing performance of each material, the experimental results indicated that the sensitivity, recovery and stability of 3-T-TZCN are better than other materials, and the limit of detection can be lower to 10 ppb level. This general approach could offer new solution for constructing n-type OFET-based gas sensors with good stability in air atmosphere.

Mechanically robust stretchable semiconductor metallization for skin-inspired organic transistors.

Despite recent remarkable advances in stretchable organic thin-film field-effect transistors (OTFTs), the development of stretchable metallization remains a challenge. Here, we report a highly stretchable and robust metallization on an elastomeric semiconductor film based on metal-elastic semiconductor intermixing. We found that vaporized silver (Ag) atom with higher diffusivity than other noble metals (Au and Cu) forms a continuous intermixing layer during thermal evaporation, enabling highly stretchable metallization. The Ag metallization maintains a high conductivity (>104 S/cm) even under 100% strain and successfully preserves its conductivity without delamination even after 10,000 stretching cycles at 100% strain and several adhesive tape tests. Moreover, a native silver oxide layer formed on the intermixed Ag clusters facilitates efficient hole injection into the elastomeric semiconductor, which transcends previously reported stretchable source and drain electrodes for OTFTs.

Mixed-Orbital Charge Transport in N-Shaped Benzene- and Pyrazine-Fused Organic Semiconductors.

The hole-carrier transport of organic semiconductors is widely known to occur via intermolecular orbital overlaps of the highest occupied molecular orbitals (HOMO), though the effect of other occupied molecular orbitals on charge transport is rarely investigated. In this work, we first demonstrate evidence of a mixed-orbital charge transport concept in the high-performance N-shaped decyl-dinaphtho[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (C10–DNBDT–NW), where electronic couplings of the second HOMO (SHOMO) and third HOMO (THOMO) also contribute to the charge transport. We then present the molecular design of an N-shaped bis(naphtho[2′,3′:4,5]thieno)[2,3-b:2′,3′-e]pyrazine (BNTP) π-electron system to induce more pronounced mixed-orbital charge transport by incorporating the pyrazine moiety. An effective synthetic strategy for the pyrazine-fused extended π-electron system is developed. With substituent engineering, the favorable two-dimensional herringbone assembly can be obtained with BNTP, and the decylphenyl-substituted BNTP (C10Ph–BNTP) demonstrates large electronic couplings involving the HOMO, SHOMO, and THOMO in the herringbone assembly. C10Ph–BNTP further shows enhanced mixed-orbital charge transport when the electronic couplings of all three occupied molecular orbitals are taken into consideration, which results in a high hole mobility up to 9.6 cm2 V–1 s–1 in single-crystal thin-film organic field-effect transistors. The present study provides insights into the contribution of HOMO, SHOMO, and THOMO to the mixed-orbital charge transport of organic semiconductors.

Evaluation of Effective Field-Effect Mobility in Thin-Film and Single-Crystal Transistors for Revisiting Various Phenacene-Type Molecules.

The magnitude of the field-effect mobility μ of organic thin-film and single-crystal field-effect transistors (FETs) has been overestimated in certain recent studies. These reports set alarm bells ringing in the research field of organic electronics. Herein, we report a precise evaluation of the μ values using the effective field-effect mobility, μeff, a new indicator that is recently designed to prevent the FET performance of thin-film and single-crystal FETs based on various phenacene molecules from being overestimated. The transfer curves of a range of FETs based on phenacene are carefully categorized on the basis of a previous report. The exact evaluation of the value of μeff depends on the exact classification of each transfer curve. The transfer curves of all our phenacene FETs could be successfully classified based on the method indicated in the aforementioned report, which made it possible to evaluate the exact value of μeff for each FET. The FET performance based on the values of μeff obtained in this study is discussed in detail. In particular, the μeff values of single-crystal FETs are almost consistent with the μ values that were reported previously, but the μeff values of thin-film FETs were much lower than those previously reported for μ, owing to a high absolute threshold voltage, |Vth|. The increase in the field-effect mobility as a function of the number of benzene rings, which was previously demonstrated based on the μ values of single-crystal FETs with phenacene molecules, is well reproduced from the μeff values. The FET performance is discussed based on the newly evaluated μeff values, and the future prospects of using phenacene molecules in FET devices are demonstrated.

Charge transfer complexes of a benzothienobenzothiophene derivative and their implementation as active layer in solution-processed thin film organic field-effect transistors

Single crystals of novel (C8O-BTBT-OC8)(FxTCNQ) charge complexes (CT) are prepared and fully characterised. Solution processed films of the (C8O-BTBT-OC8)(F4TCNQ) CT are implemented in organic field-effect transistors giving an n-type behaviour.

Approaching isotropic charge transport of n-type organic semiconductors with bulky substituents

Benzo[de]isoquinolino[1,8-gh]quinolinetetracarboxylic diimide (BQQDI) is an n-type organic semiconductor that has shown unique multi-fold intermolecular hydrogen-bonding interactions, leading to aggregated structures with excellent charge transports and electron mobility properties. However, the strong intermolecular anchoring of BQQDI presents challenges for fine-tuning the molecular assembly and improving the semiconducting properties. Herein, we report the design and synthesis of two BQQDI derivatives with phenyl- and cyclohexyl substituents (Ph–BQQDI and Cy6–BQQDI), where the two organic semiconductors show distinct molecular assemblies and degrees of intermolecular orbital overlaps. In addition, the difference in their packing motifs leads to strikingly different band structures that give rise to contrasting charge-transport capabilities. More specifically, Cy6–BQQDI bearing bulky substituents exhibits isotropic intermolecular orbital overlaps resulting in equal averaged transfer integrals in both π-π stacking directions, even when dynamic disorders are taken into account; whereas Ph–BQQDI exhibits anisotropic averaged transfer integrals in these directions. As a result, Cy6–BQQDI shows excellent device performances in both single-crystalline and polycrystalline thin-film organic field-effect transistors up to 2.3 and 1.0 cm2 V−1 s−1, respectively.

Effect of molecular geometry and extended conjugation on the performance of hydrogen-bonded semiconductors in organic thin-film field-effect transistors

Hydrogen bonding has been used to direct the solid state packing of four polyheteroaromatic molecules. The correlation between structure and charge mobility has been evaluated in OFETs.

Fluorinated Thiophene-Phenylene Co-Oligomers for Optoelectronic Devices.

Organic optoelectronics requires materials combining bright luminescence and efficient ambipolar charge transport. Thiophene-phenylene co-oligomers (TPCOs) are promising highly emissive materials with decent charge-carrier mobility; however, they typically show poor electron injection in devices, which is usually assigned to high energies of their lowest unoccupied molecular orbitals (LUMOs). A widely used approach to lower the frontier orbitals energy levels of a conjugated molecule is its fluorination. In this study, we synthesized three new fluorinated derivatives of one of the most popular TPCOs, 2,2'-(1,4-phenylene)bis[5-phenylthiophene] (PTPTP) and studied them by cyclic voltammetry, absorption, photoluminescence, and Raman spectroscopies. The obtained data reveal a positive effect of fluorination on the optoelectronic properties of PTPTP: LUMO levels are finely tuned, and photoluminescence quantum yield and absorbance are increased. We then grew crystals from fluorinated PTPTPs, resolved their structures, and showed that fluorination dramatically affects the packing motif and facilitates π-stacking. Finally, we fabricated thin-film organic field-effect transistors (OFETs) and demonstrated a strong impact of fluorination on charge injection/transport for both types of charge carriers, namely, electrons and holes. Specifically, balanced ambipolar charge transport and electroluminescence were observed only in the OFET active channel based on the partially fluorinated PTPTP. The obtained results can be extended to other families of conjugated oligomers and highlight the efficiency of fluorination for rational design of organic semiconductors for optoelectronic devices.

Relieving the Photosensitivity of Organic Field-Effect Transistors.

It is generally believed that the photoresponse behavior of organic field-effect transistors (OFETs) reflects the intrinsic property of organic semiconductors. However, this photoresponse hinders the application of OFETs in transparent displays as driven circuits due to the current instability resulting from the threshold voltage shift under light illumination. It is necessary to relieve the photosensitivity of OFETs to keep the devices stable. 2,6-diphenyl anthracene thin-film and single-crystal OFETs are fabricated on different substrates, and it is found that the degree of molecular order in the conducting channels and the defects at the dielectric/semiconductor interface play important roles in determining the phototransistor performance. When highly ordered single-crystal OFETs are fabricated on polymeric substrates with low defects, the photosensitivity (P) decreases by more than 105 times and the threshold voltage shift (ΔVT ) is almost eliminated compared with the corresponding thin-film OFETs. This phenomenon is further verified by using another three organic semiconductors for similar characterizations. The decreased P and ΔVT of OFETs ensure a good current stability for OFETs to drive organic light-emitting diodes efficiently, which is essential to the application of OFETs in flexible and transparent displays.

Excellent Semiconductors Based on Tetracenotetracene and Pentacenopentacene: From Stable Closed-Shell to Singlet Open-Shell.

Designing stable open-shell organic materials through the modifications of the π-topology of molecular organic semiconductors has recently attracted considerable attention. However, their uses as an active layer in organic field-effect transistors (OFETs) are very limited, and the obtained hole and electron charge mobilities are around 10-3 cm2 V-1 s-1. Herein, we disclose the synthesis of two peri-fusedmaterials, so-called tetracenotetracene (TT) and pentacenopentacene (PP), which have low band gaps of 1.79 and 1.42 eV, respectively. Their ground state natures have been investigated by different experiments including steady state absorption, electron spin resonance, superconducting quantum interfering device, and variable-temperature NMR along with DFT calculations. TT and PP have closed-shell and singlet open-shell structures in their ground state, respectively, and possess high stability. Their biradical characteristics were found to be 0.50 and 0.64. The origin of the open-shell character of PP is related to the concomitant opening of two tetracenes with the recovering of two extra aromatic sextets and a small HOMO-LUMO energy gap (gap <1.5 eV). Thanks to the high stability, thin film OFET devices could be fabricated. In TG-BC configuration PP shows a remarkably high hole mobility of 1.4 cm2 V-1 s-1, while TT exhibits a hole mobility of 0.77 cm2 V-1 s-1. In the configuration of BG-TC, ambipolar behaviors for both were obtained with hole and electron mobilities of 0.21 and 0.01 cm2 V-1 s-1 for PP and 0.14 and 0.006 cm2 V-1 s-1 for TT.

Solution processed copper tetrabenzotriazaporphyrin films for organic field effect transistors

Experimental characteristics of bottom-gate-contact organic thin film field-effect transistors (OTFTs) with 70 nm thick films of solution processed non-peripherally octahexyl-substituted copper tetrabenzotriazaporphyrin (6CuTBTAP) novel asymmetric molecules as active layers on silicon substrates are analyzed by both linear and non-linear parameter extraction methods in order to examine the field-dependent mobility and contact effects. Both linear and saturation field effect mobilities of OTFTs have been computationally determined as a function of gate voltage in the presence of the contact resistances. The Poole-Frankel mechanism is found to be responsible for charge transport during the saturation regime, giving the highest mobility of 6.9 × 10−2 cm2 V−1 s−1 at the gate-source voltage (VG) of 50 V. The on-off ratio is found to be 106. The contact resistance at the interface between gold electrodes and the 6CuTBTAP active layer decreases with VG sharply from 75 MΩ corresponding to VG = 0 V to 10 MΩ once the transistor is turned on at the threshold voltage.

Polymorphic Behavior of Perylene and Its Influences on OFET Performances

Among the polycyclic aromatic hydrocarbons, although perylene is commercially available and possesses higher solubility and stability than the others, its thin-film structures and organic field-effect transistor (OFET) performances have been rarely explored. To understand its potential as an active material in OFETs, the polymorphic behaviors, packing structures, and OFET characteristics of perylene were carefully examined. The well-oriented crystal arrays of perylene prepared via droplet-pinned crystallization delivered the highest hole mobility among the reported perylene OFETs. Fluorescence microscope, electron diffraction, and lattice modeling results confirm the polymorphic behavior of perylene in the solution-processed crystal arrays and its influences on the OFET performances. The concentration-sensitive and temperature-sensitive polymorphic behavior of perylene make processing conditions crucial in the preparation of pure-phase crystal arrays. The results show the great potential of perylene as an...

Solution-crystallized n-type organic thin-film transistors: An impact of branched alkyl chain on high electron mobility and thermal durability

Abstract Single-crystal organic thin-film field-effect transistors (OTFTs) have been actively studied because they are regarded as an ideal platform for charge carrier transport phenomena in small-molecule organic materials. Moreover, recently developed solution-based techniques to grow single-crystalline thin films have attracted industrial attention beyond fundamental physics. In this work, one of the solution-based techniques, namely, the edgecasting method, was employed to study single-crystal OTFTs of n-type organic semiconductors, dimerized naphtho[2,3-b]thiophene diimides, with linear and branched alkyl chains. A high electron mobility of >1 cm2 V−1 s−1 and an excellent thermal durability were observed in the branched alkyl-chain derivative, while the OTFT based on linear alkyl-substituted one showed clear degradation of mobility after thermal annealing. Therefore, this work will demonstrate a potential availability of branched alkyl chains for improving carrier mobility and thermal durability with large-scalable printing methods.

Flexible, high-sensitive, and wearable strain sensor based on organic crystal for human motion detection

Abstract A flexible high-sensitive wearable strain sensor has been fabricated based on flexible rubrene single crystal as sensing material. Different from the previously extensively reported thin-film organic field-effect transistor (OFET) based strain sensors, the present sensing system is based on the modulation of the intermolecular distance of organic crystal under strain, and applied the two-terminal device configuration. This new device designed with the simple device configuration exhibits the comparable sensing performance with the conventional thin-film OFET based strain sensors, including high sensitivity (gauge factor = 279), wide detection range, and high cycle stability. Furthermore, the rubrene single-crystal strain sensors can accurately detect the motions of the body, such as swallowing saliva of throat, inside-bending and outside-bending of finger, and multi-angle bending of arm. Such a new-type device design opens a new powerful route to realize ultra-sensitive strain detection for promising applications in artificial intelligence and healthcare systems.

Influences of Conjugation Length on Organic Field-Effect Transistor Performances and Thin Film Structures of Diketopyrrolopyrrole-Oligomers.

Here, two diketopyrrolopyrrole (DPP)-based oligomers, DPP-4T and DPP-6T, are studied to reveal the influences of conjugation length on thin-film morphology and organic field-effect transistor (OFET) performances. PDMS-assisted crystallization in a solvent-annealing chamber is applied to prepare crystal arrays of DPP-4T and DPP-6T to optimize the quality of charge channels for OFET characterizations. To deliver insights into microstructure and morphology of thin films, a characterization procedure for determining molecular packing in thin film and crystallinity of the crystal arrays is presented via grazing incidence wide-angle X-ray scattering, electron diffraction, and lattice simulation software package (Cerius2). With the lattice parameters derived from analyses of grazing incidence wide-angle X-ray scattering (GIWAXS) and electron diffraction (ED), the lattice modeling results indicate that the inferior organic field-effect transistor (OFET) performances of DPP-6T are attributed to longer π-stacking distance. Also, less-ordered molecular arrangement and lower continuity of crystalline domains, both of which are revealed from crystallinity results, lead to lower mobility of DPP-6T. In this case, longer conjugated backbones with more conformational degrees of freedom thus cause inherent crystal defects during the crystal growth process, despite the potential to enhance intermolecular π-orbital overlap. Therefore, to achieve better OFET performance, suitable backbone length makes conjugated oligomers give high intermolecular π-orbital overlap and low density of structural disorder, which are the priorities for constructing good charge channel.

Fully transparent conformal organic thin-film transistor array and its application as LED front driving.

A fully transparent conformal organic thin-film field-effect transistor array is demonstrated based on a photolithography-compatible ultrathin metallic grid gate electrode and a solution-processed C8-BTBT film. The resulting organic field-effect transistor array exhibits a high optical transparency of >80% over the visible spectrum, mobility up to 2 cm2 V-1 s-1, on/off ratio of 105-106, switching current of >0.1 mA, and excellent light stability. The transparent conformal transistor array is demonstrated to adhere well to flat and curved LEDs as front driving. These results present promising applications of the solution-processed wide-bandgap organic semiconductor thin films in future large-scale transparent conformal active-matrix displays.

Highly Sensitive Ammonia Gas Sensor Based on Single-Crystal Poly(3-hexylthiophene) (P3HT) Organic Field Effect Transistor.

A highly sensitive organic field-effect transistor (OFET)-based sensor for ammonia in the range of 0.01 to 25 ppm was developed. The sensor was fabricated by employing an array of single-crystal poly(3-hexylthiophene) (P3HT) nanowires as the organic semiconductor (OSC) layer of an OFET with a top-contact geometry. The electrical characteristics (field-effect mobility, on/off current ratio) of the single-crystal P3HT nanowire OFET were about 2 orders of magnitude larger than those of the P3HT thin film OFET with the same geometry. The P3HT nanowire OFET showed excellent sensitivity to ammonia, about 3 times higher than that of the P3HT thin film OFET at 25 ppm ammonia. The ammonia response of the OFET was reversible and was not affected by changes in relative humidity from 45 to 100%. The high ammonia sensitivity of the P3HT nanowire OFET is believed to result from the single crystal nature and high surface/volume ratio of the P3HT nanowire used in the OSC layer.

Rodlike Tetracene Derivatives.

Efficient and versatile synthetic access to rodlike tetracene derivatives was developed by means of Diels-Alder cycloaddition, halogenation, halogen-metal exchange, and transition metal mediated coupling reactions. Herein, the synthesis and structural, electrical, and charge-transport properties of three of the resulting materials, namely, 2-(tetracen-2-yl)tetracene, 1,4-bis(2-tetracenyl)benzene, and 2,5-bis(2-tetracenyl)thiophene, are presented. Good crystallization behavior on SiO2 substrates, narrowing of the bandgap by 0.2 eV, and a decrease of the ionization potential of more than 0.5 eV compared to tetracene were observed. Charge-carrier field-effect mobilities on the order of 10-1 cm2 V-1 s-1 , on/off ratios of 105 , and threshold voltages Vth <15 V were found in thin-film organic field-effect transistors prepared by standard high-vacuum deposition techniques.

Laser Patterning of Optically Reconfigurable Transistor Channels in a Photochromic Diarylethene Layer.

Optical switching organic field-effect transistors (OFETs) provide a new direction for optoelectronics based on photochromic molecules. However, the patterning of OFETs is difficult because conventional fabrication processes, including lithography and ion etching, inevitably cause severe damage to organic molecules. Here, we demonstrate laser patterning of one-dimensional (1D) channels on an OFET with a photochromic diarylethene (DAE) layer. The main findings are (i) a number of 1D channels can be repeatedly written and erased in the DAE layer by scanning focused ultraviolet and visible light laser beams alternately between the source and drain electrodes, (ii) the conductivity (or resistivity) of the 1D channel can be controlled by the illumination conditions, such as the laser power density and the scan speed, and (iii) it is possible to draw an analogue adder circuit by optically writing 1D channels so that a portion of the channels overlaps and to perform optical summing operations by local laser illumination of the respective channels. These findings will open new possibilities for realizing various optically reconfigurable, low-dimensional organic transistor circuits, which are not possible with conventional thin film OFETs.

Intrinsically stretchable and healable semiconducting polymer for organic transistors.

Thin-film field-effect transistors are essential elements of stretchable electronic devices for wearable electronics. All of the materials and components of such transistors need to be stretchable and mechanically robust. Although there has been recent progress towards stretchable conductors, the realization of stretchable semiconductors has focused mainly on strain-accommodating engineering of materials, or blending of nanofibres or nanowires into elastomers. An alternative approach relies on using semiconductors that are intrinsically stretchable, so that they can be fabricated using standard processing methods. Molecular stretchability can be enhanced when conjugated polymers, containing modified side-chains and segmented backbones, are infused with more flexible molecular building blocks. Here we present a design concept for stretchable semiconducting polymers, which involves introducing chemical moieties to promote dynamic non-covalent crosslinking of the conjugated polymers. These non-covalent crosslinking moieties are able to undergo an energy dissipation mechanism through breakage of bonds when strain is applied, while retaining high charge transport abilities. As a result, our polymer is able to recover its high field-effect mobility performance (more than 1 square centimetre per volt per second) even after a hundred cycles at 100 per cent applied strain. Organic thin-film field-effect transistors fabricated from these materials exhibited mobility as high as 1.3 square centimetres per volt per second and a high on/off current ratio exceeding a million. The field-effect mobility remained as high as 1.12 square centimetres per volt per second at 100 per cent strain along the direction perpendicular to the strain. The field-effect mobility of damaged devices can be almost fully recovered after a solvent and thermal healing treatment. Finally, we successfully fabricated a skin-inspired stretchable organic transistor operating under deformations that might be expected in a wearable device.