Properties Of Organic Field-effect Transistors Research Articles

Molecular Engineering on Kinetics‐Driven Self‐Assembled Monolayers Working as Auxiliary Layers on Dielectrics in Organic Field‐Effect Transistors

AbstractSelf‐assembled monolayers (SAMs) are a class of quasi‐2D materials adhesive to the substrate by chemisorption. Due to their transparency, diversity, stability, sensitivity, selectivity, and great potential in surface passivation, SAMs have been extensively investigated and applied in various functional devices, particularly in organic field effect transistors (OFETs). Among all the processing methods, kinetic‐driven spin‐coating is frequently used for the SAM preparation due to its high efficiency and low cost. However, the importance of SAM quality and its relationship to device performance has not been studied in detail, hindering the new SAM development and device optimization. In this study, SAMs prepared by kinetic‐driven spin‐coating are carefully investigated in terms of their surface morphology, density, and regularity, and proposed a correlation model between chemical structure and SAM quality. Additionally, the prepared SAMs are utilized as auxiliary layers on dielectrics and analyzed their effects on OFET properties. Through these investigations, a sequential relationship is established between chemical structure, SAM quality, and device performance, which can provide efficient feedback for system optimization.

Influence of Volatile Organic Compound Adsorption on the Characteristics of Organic Field-Effect Transistors and Rules for Gas-Sensing Measurements.

Owing to the advantages of organic field-effect transistors (OFETs) in the versatility of organic synthesis, multiparameter measurement, and signal amplification, sensors based on OFETs have received increasing attention for detecting volatile organic compounds (VOCs). However, false device operation and gas-sensing measurements often occur to vitiate the advantages of OFETs and even output error gas-sensing signals. In this work, by experimentally and theoretically studying the effects of VOC adsorption on the operational characteristics of the OFET, the proper operations of OFETs in gas-sensing measurements were clarified. The multiparameter measurements of OFETs showed that the source-drain current was the optimized parameter for achieving high responsivity, and other OFET parameters could be used for fingerprint analysis. By operating OFETs in the near-threshold region, the amplification effect was switched to enhance the responsivity by orders of magnitude to VOCs, while in the overthreshold region, the OFETs had a low signal-to-noise ratio. Besides, a counteraction effect and an uncertainty effect were discovered, leading to error gas-sensing signals. A theoretical study was carried out to reveal the dependency of the gas-sensing properties of OFETs on VOC adsorption. A series of rules were proposed for guiding the measurements of OFET sensors by taking full advantage of transistors in gas-sensing applications.

Pyrrolopyrrole-Based Aza-BODIPY Small Molecules for Organic Field-Effect Transistors.

Diketopyrrolopyrrole (DPP), due to its good planarity, π-conjugate structure, thermal stability, and structural modifiability, has received much attention from the scientific community as an excellent semiconductor material for its applications in the field of optoelectronics, such as organic solar cells, organic photovoltaics, and organic field effect transistors. In this study, a new small molecule, pyrrolopyrrole aza-BODIPY (PPAB), based on the thiophene-substituted DPP structure was developed using the Schiff-base formation reaction of DPP and heteroaromatic amines. Absorption spectroscopy, electrochemistry, X-ray diffraction, molecular theoretical simulation calculation were performed, and organic field-effect transistor properties based on PPAB were investigated. It was found that PPAB exhibits a broad absorption range in the visible and near-infrared regions, which is attributed to its long-range conjugate structure. In addition, it is worth noting that PPAB has multiple F atoms resulting in the low LUMO level, which is conducive to the injection and transportation of charge carriers between the semiconductor layer and the electrode. Meanwhile, its hole carrier mobility is up to 1.3 × 10−3 cm2 V−1 s−1 due to its large conjugate structure, good intramolecular charge transfer effect, and high degree of coplanarity. In this study, a new chromophore with electron-deficient ability for designing high-performance semiconductors was successfully synthesized.

Dedicated to the memory of V.D. Aleksandrov Effect of annealing in an inert atmosphere on the electrical properties of crystalline pentacene films

The results of a study of the effect of annealing at 150^oC in an inert atmosphere (Ar + 5% H2) on the electrical properties of organic field-effect transistors based on pentacene are presented. Crystalline pentacene films with a thickness of 95±5 nm were obtained using thermal vacuum deposition. The transfer and output characteristics of field-effect transistors before and after annealing for 15 hours are investigated. It was found that as a result of heat treatment, the hole mobility in the saturation regime increased by an average of 30%, and the threshold voltage decreased approximately two times. According to the data of atomic force microscopy, annealing led to a more than twofold decrease in the surface roughness of pentacene films, as well as to a noticeable enlargement of grains, which led to a decrease in the concentration of traps for hole electric transport in the channel of the field-effect transistor. Keywords: pentacene, vacuum thermal deposition, crystalline films, organic field-effect transistors, hole mobility, annealing in an inert atmosphere.

Morphology and Charge Transport Properties of P(NDI2OD-T2)/Polystyrene Blends

While the strategy of blending commodity insulating polymers with conjugated polymers has been previously applied to modify the properties of organic field-effect transistors (OFETs), comprehensive studies on the evolution of morphology and OFET performance with film composition are scarce. In this contribution, films comprising the n-type semiconducting polymer P(NDI2OD-T2) and the insulating polymer polystyrene (PS) are explored in detail. In particular, the electrical properties and film morphology of blends with a P(NDI2OD-T2) concentration ranging from 0.125 to 100 wt % are studied. Two regimes characterized by distinct dependencies of device mobility as a function of P(NDI2OD-T2) content are observed: Above 12.5 wt % P(NDI2OD-T2), the OFET mobility exhibits a weak dependence on P(NDI2OD-T2) concentration despite the presence of strong lateral phase separation, while below 12.5%, the device mobility is observed to decrease strongly with decreasing P(NDI2OD-T2) concentration attributed to disrupted surface connectivity of the P(NDI2OD-T2) phase. UV–vis absorption spectroscopy and grazing incidence wide-angle X-ray scattering (GIWAXS) measurements indicate that the aggregation and crystalline packing are not strongly affected by the presence of PS. Atomic force microscopy and scanning transmission X-ray spectro-microscopy reveal lateral phase separation for blends with compositions of 87.5 to 25 wt % P(NDI2OD-T2). Enrichment of P(NDI2OD-T2) at the film surface is confirmed by near-edge X-ray absorption fine structure spectroscopy, with a high P(NDI2OD-T2) surface composition (>80 wt %) maintained down to an overall blend composition of 12.5 wt % P(NDI2OD-T2). The strong decrease in OFET mobility for P(NDI2OD-T2) concentration less than 25 wt % is attributed to a loss of P(NDI2OD-T2) surface connectivity at low P(NDI2OD-T2) loading. We also observe a transition from face-on to edge-on dominated configuration at low P(NDI2OD-T2) content with GIWAXS, consistent with an edge-on surface layer that persists down to monolayer surface coverage.

Electron-to Hole Transport Change Induced by Solvent Vapor Annealing of Naphthalene Diimide Doped with Poly(3-Hexylthiophene).

Herein we report on fabrication and properties of organic field-effect transistors (OFETs) based on the spray-coated films of N,N′-dioctyl naphthalene diimide (NDIC8) doped with 2.4 wt% of poly (3-hexylthiophene) (P3HT). OFETs with the untreated NDIC8:P3HT films revealed electron conductivity [μe* = 5 × 10–4 cm2×(Vs)−1]. After the annealing in chloroform vapor the NDIC8:P3HT films revealed the hole transport only [μh* = 0.9 × 10–4 cm2×(Vs)−1]. Due to the chemical nature and energy levels, the hole transport was not expected for NDIC8-based system. Polarized optical- and scanning electron microscopies indicated that the solvent vapor annealing of the NDIC8:P3HT films caused a transition of their fine-grained morphology to the network of branched, dendritic crystallites. Grazing incidence wide-angle X-ray scattering studies indicated that the above transition was accompanied by a change in the crystal structure of NDIC8. The isotropic crystal structure of NDIC8 in the untreated film was identical to the known crystal structure of the bulk NDIC8. After the solvent annealing the crystal structure of NDIC8 changed to a not-yet-reported polymorph, that, unlike in the untreated film, was partially oriented with respect to the OFET substrate.

Controlling Structural and Energetic Disorder in High-Mobility Polymer Semiconductors via Doping with Nitroaromatics

Molecular doping has emerged as a powerful strategy to tune the charge transport properties of organic field-effect transistors (OFETs). However, the limited tool-box of molecular dopants and unres...

Посвящается памяти В.Д. Александрова Влияние отжига в инертной атмосфере на электрические свойства кристаллических пленок пентацена

The results of a study of the effect of annealing at 150 °C in an inert atmosphere (Ar + 5% H2) on the electrical properties of organic field-effect transistors based on pentacene are presented. Crystalline pentacene films with a thickness of 95 ± 5 nm were obtained using thermal vacuum deposition. The transfer and output characteristics of field-effect transistors before and after annealing for 15 hours are investigated. It was found that as a result of heat treatment, the hole mobility in the saturation regime increased by an average of 30%, and the threshold voltage decreased approximately two times. According to the data of atomic force microscopy, annealing led to a more than twofold decrease in the surface roughness of pentacene films, as well as to a noticeable enlargement of grains, which led to a decrease in the concentration of traps for hole electric transport in the channel of the field-effect transistor.

Relationship between the surface structure of the gate insulator and the performance of organic thin-film transistors

To investigate the impact on the device performance of different surface structures of the self-assembled monolayer (SAM) used to form the gate insulator of organic field effect transistors (OFETs), sum frequency generation spectroscopy and contact angle measurements were performed on SAM insulators with different surface structures. Furthermore, the surface structure of the SAM was varied with changing its dipping or spin coating formation method. Moreover, the electrical properties of OFETs were examined when only the surface structure of the gate insulator had changed. The results showed that compared with the dipping method, the disorder of alkyl chains (gauche defects) increased on the octadecylphosphonic acid (ODPA) SAM surface, deposited through the spin coating method. Contact angle measurements revealed that the surface free energy of ODPA-SAM increased in tandem with an increase in gauche defects on the SAM surface. Contrary to expectations, the device performance of OFETs with the SAM insulator with gauche defects improved, which is most probably due to the smaller surface free energy difference between ODPA-SAM and poly(3-hexylthiophene) (P3HT), which improved the adhesion at the ODPA-SAM/P3HT interface.

Phenothiazine derivatives, diketopyrrolopyrrole-based conjugated polymers: synthesis, optical and organic field effect transistor properties

Novel donor–acceptor (D-A) conjugated polymers containing phenothiazine and diketopyrrolopyrrole derivatives were successfully synthesized via direct arylation polymerization using the palladium catalyst system. The C-N coupling reaction was performed for the synthesis of 4-(10H-phenothiazin-10-yl)-N,N-diphenylaniline (PDA) and 10-(pyren-1-yl)-10H-phenothiazine (PYP) as donor monomers. The D-A conjugated polymers synthesized by polymerization of PDA/PY with diketopyrrolopyrrole have been characterized via GPC, 1H NMR, FTIR, XRD, DSC, PL and UV-Vis spectroscopy, and used for fabrication of organic field effect transistors.

Enhanced Gas Sensing Performance of Organic Field‐Effect Transistors by Modulating the Dimensions of Triethylsilylethynyl‐Anthradithiophene Microcrystal Arrays

AbstractThis paper systematically compares the gas sensing properties of organic field‐effect transistors (OFETs) based on patterned 5,11‐bis(triethylsilylethynyl)anthradithiophene (TES‐ADT) films, by adopting TES‐ADT crystal arrays of various shapes and dimensions. The patterning and crystallization of spin‐cast TES‐ADT layers are achieved by the use of a solvent‐containing engraved polydimethylsiloxane (PDMS) mold. Decreasing width of the TES‐ADT pattern enhances gas sensing performance, as well as field‐effect mobility of OFETs. The decreased grain boundary density at narrower line width contributes to the increase of field‐effect mobility. On the other hand, the increased sensing performance is mainly due to the increased area of crystal edges, which provides a diffusion pathway for gas molecules to arrive at the semiconductor‐dielectric interface. This study provides new perspectives on the diffusion pathway of gas molecules in OFET‐based gas sensor, and will be useful for the design of active channel to boost the gas sensing properties of OFETs.

Effect of thiophene/furan substitution on organic field effect transistor properties of arylthiadiazole based organic semiconductors

This study determines that furan could display comparable charge transport properties to its thiophene analogue. The OFET device employing furan-substituted benzothiadiazole as the channel layer showed a mobility (μmax) of 0.0122 cm2 V−1 s−1.

The effect of thickness on the optoelectronic properties of organic field-effect transistors: towards molecular crystals at monolayer limit

This manuscript reviews recent progress in the ultrathin monolayer molecular crystals (MMCs) for high performance optoelectronic devices.

Controlled alignment of polymer chains near the semiconductor-dielectric interface

Abstract Alignment of semiconducting polymer chains is crucial for achieving desired charge transport in organic field effect transistors (OFETs). Although the transistor performance is known to be critically affected by the charge accumulation layer of the semiconducting polymer film next to the semiconductor-gate dielectric interface, it is challenging to investigate the effect of polymer chain alignment specifically at the interface due to experimental limitations. In this work, the unique capability of multilayer formation and polymer chain alignment of the floating film transfer method (FTM) is utilized to study the alignment effect on the charge transport properties of OFETs. Discrete modulation of polymer chain alignment for the top half and bottom half layers of the films was achieved in a multilayered structure. When the bottom layer, which is close to the interface with the gate dielectric, has a parallel orientation of the polymer chains and the chains in the top layer are oriented perpendicular to the source-drain direction in the OFET devices, the average hole mobility is increased by a factor of 3.3 compared to that in the opposite case where the bottom layer has a perpendicular orientation and a parallel orientation in the top layer, even though the two cases have very similar bulk polymer chain alignments. In addition, no matter which polymer chain orientation is chosen for the top layer, the polymer chain orientation direction of the bottom layer governs the overall device performance. These experimental findings evidently support that the polymer chain orientation adjacent to the semiconductor/dielectric interface is a decisive factor in charge transport of OFETs.

Precise Extraction of Charge Carrier Mobility for Organic Transistors

AbstractUnreliable mobility values, and particularly greatly overestimated values and severely distorted temperature dependences, have recently hampered the development of the organic transistor field. Given that organic field‐effect transistors (OFETs) have been routinely used to evaluate mobility, precise parameter extraction using the electrical properties of OFETs is thus of primary importance. This review examines the origins of the various mobilities that must be determined for OFET applications, the relevant extraction methods, and the data selection limitations, which help in avoiding conceptual errors during mobility extraction. For increased precision, the review also discusses device fabrication considerations, calibration of both the specific gate‐dielectric capacitance and the threshold voltage, the contact effects, and the bias and temperature dependences, which must actually be handled with great care but have mostly been overlooked to date. This review serves as a systematic overview of the OFET mobility extraction process to ensure high precision and will also aid in improving future research.

Understanding, Optimizing, and Utilizing Nonideal Transistors Based on Organic or Organic Hybrid Semiconductors

AbstractMany advanced materials have been developed for organic field‐effect transistors (OFETs) or thin‐film transistors (TFTs) based on organic and organic hybrid materials. However, although many new OFETs exhibit superior characteristic parameters (such as high mobility), most of them show nonideal performances that have strongly limited progress in the design of molecules, the understanding of transport mechanisms, and the circuit applications of OFETs. In this review, the device physics of ideal and nonideal OFETs is discussed first to understand the factors that limit effective mobility in semiconducting channels, distort the potential distribution, or reduce the drift electric field. Then, recent advances in optimizing the material combinations, device structures, and fabrications of OFETs toward ideal transistors are discussed. Based on the good control of materials and interfaces, some new and novel concepts to utilize the nonideal properties of OFETs to build low‐power circuits and integrated sensors are also discussed.

Gamma-ray irradiation effects on the electrical properties of organic field-effect transistors

We investigated the changes in the organic transistor characteristics of poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) thin films upon high-energy gamma-ray irradiation. Devices based on both organic semiconductors showed a lower field-effect mobility and electrical current levels after gamma-ray irradiation. The threshold voltage of P3HT transistors shifted negative, while that of PCBM-based devices shifted positive. These changes in electrical performance of the devices can be utilized as a gamma-ray sensing elements.

Low power organic field effect transistors with copper phthalocyanine as active layer

Bottom gate, top contact Organic Field Effect Transistors (OFETs) were fabricated using copper phthalocyanine (CuPc) as an active layer. The electrical properties of OFETs fabricated with CuPc annealed at different annealing temperatures and different channel length to width (L/W) ratios were studied. The transfer characteristics of the devices appear to improve with annealing temperature of CuPc and increasing L/W ratios of the devices. Upon annealing, the field effect mobility increased from 0.03 ± 0.004 cm2/V to 1.3 ± 0.02 cm2/V. Similarly, the interface state density reduced from 5.14 ± 0.39 × 1011 cm−2eV−1 for the device fabricated using as deposited CuPc, to 2.41 ± 0.05 × 1011 cm−2eV−1 for the device with CuPc annealed at 80 °C. The on/off current ratio increased from 102 for the as-deposited device, to 105 for the device with CuPc annealed at 80 °C. The dependence of the subthreshold swing on the L/W ratio was also investigated.

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

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

Understanding hydroscopic properties of silk fibroin and its use as a gate-dielectric in organic field-effect transistors

Abstract Silk fibroin (SF) has attracted great interest as gate dielectric in organic field-effect transistors (OFETs), owing to outstanding characteristics such as high dielectric constant, transparency, flexibility, and solution processability. In this report, we investigated the relationship between the structural properties of SF films and their performance as SF gate dielectrics in OFETs. Solvent vapor treatment with water or methanol altered the structural properties of the SF films, which adopted a β-sheet structure; accordingly, both the surface energy and areal capacitance of the SF films were reduced. Notably, atmospheric water contributed to the increased capacitance of the SF film, especially before the solvent vapor treatment. The growth characteristics of pentacene on the SF films were determined by the surface conditions of the films; in particular, Stranski-Krastanov (layer-plus-island) growth mode with mixed standing-up/lying-down orientation of pentacene on the SF film was observed before solvent vapor treatment, whereas Volmer-Weber mode with standing-up orientation dominated after solvent vapor treatment. Pentacene OFETs based on the untreated SF film exhibited a higher on-current compared with the devices based on the solvent vapor-treated SF film. The hydroscopic characteristics of the untreated SF film enhanced its capacitance, thereby inducing accumulation of hole carriers. The present results show that the structural characteristics of SF films have a marked impact on the electrical properties of OFETs based on SF gate dielectrics. In particular, the water uptake capability of SF is a key factor in the electrical properties of organic electronic devices using SF materials.