Materials For Organic Field-effect Transistors Research Articles

Shellac as dielectric materials in organic field-effect transistors: from silicon to paper substrates

Recent advances in the design and preparation of electroactive materials, particularly semiconducting and conductive polymers, have resulted in the creation of novel organic electronics with advanced functionality and performance competitive with that of devices made of silicon. With an increasing number of organic and printed electronics being engineered and produced at a larger scale, the environmental cost of the final organic electronic devices (life cycle, environmental impact, etc) needs to be considered. While e-waste is already a growing global problem, improving the sustainability of emerging electronics through a careful materials selection is highly desirable. In this work, we explore the use of shellac as a sustainable greener dielectric material in organic field-effect transistors. A careful examination of shellac in combination with diketopyrrolopyrrole-based semiconducting polymers was performed on rigid substrates through atomic force microscopy (AFM) and the fabrication of thin film transistors. All devices made from this green dielectric showed good performance and device characteristics. Building from this investigation, shellac was further integrated with paper substrates to fabricate paper-based thin film transistors. Thin film samples based on shellac on both silicon wafer and paper substrates were characterized by AFM to investigate solid-state morphology of shellac and selected semiconducting materials. Through careful optimization of the device architecture and processing time, device characteristics and performances on paper substrates (average charge mobilities and on/off current ratios) were comparable to those of devices prepared on silicon wafers, confirming that shellac, in combination with organic semiconducting polymers, can be an advantageous dielectric material to be used for the fabrication of greener and sustainable thin film electronics from renewable feedstocks and components.

Computational studies on nitrogen (N)-substituted 2,6-diphenylanthracene: a novel precursor of organic field effect transistor materials

The investigation shows that nitrogen (N)-substituted π-conjugated semiconductor materials have improved optical and electronic performance and work efficiently in organic field-effect transistors (OFETs).

Effect of Chlorine or Ester Incorporation on Diketopyrrolopyrrole -Based Polymeric Semiconductors

Organic photovoltaic materials incorporating either chlorine atoms or ester groups have been studied as replacement strategy for fluorinated compounds. However, the characteristics of such materials when used in organic field-effect transistors (OFETs) remain unclear. The present work synthesized three polymers via the copolymerization of diketopyrrolopyrrole (DPP) with monothiophene units having no substituents or incorporating either chlorine atoms or ester groups. All three materials were found to have similar photophysical and electrochemical properties even though their repeating units had different substituents. The maximum hole mobility values for the polymer specimens were determined to be 0.37 cm 2 V −1 s −1 . The introduction of chlorine into the DPP-based polymer generated an amorphous semiconductor material that makes it a very promising candidate for commercial OFETs since its properties are independent of fabrication conditions. The new polymeric semiconductors demonstrated herein suggest the viability of such materials with regard to incorporation in organic field-effect transistors. •Both ester groups and chlorine atoms were successfully introduced into DPP-based polymers. •Chlorination and esterification applied to polymeric OFET materials •The introduction of chlorine into the DPP-based polymer generated an amorphous semiconductor material.

Packing Polymorphism Affecting the Optoelectronic Properties of a π-Conjugated Organic Compound

[1]Benzothieno[3,2-b]benzothiophene (BTBT) derivatives are widely employed as hole transport materials in organic field-effect transistors. The electronic properties of these materials depend critically on the crystal packing, which in turn depends on the choice of the attached functional group. With symmetrically attached alcohol chains, the structure may display different packing modes depending on the number of CH2 groups in the alkyl chain. The dipentanol BTBT derivative has two polymorphs I and II, crystallizing from different solvents, with distinct packing modes and consequently different electronic properties. Whereas the conformational changes are very small between I and II and the hydrogen bonding networks in the structures are identical, the adjacent BTBT cores are differently shifted and oriented with respect to each other. It is shown by density functional theory that polymorph I having unfavorable electronic properties is slightly more stable than polymorph II. This is caused by the much more attractive cross stacking between BTBT cores and C5OH chains in I than in II. The stacking in II originates rather from electrostatic interactions between the BTBT cores. The differences and similarities with the packing modes of the dibutanol and dihexanol derivatives are discussed. Both polymorphic forms I and II display negative uniaxial thermal expansion, but in different directions with respect to the packing of the molecules. Different scanning calorimetry measurements suggest that more than two polymorphs exist.

Diketopyrrolopyrrole Based Organic Semiconductor Materials for Field-Effect Transistors.

Over the past several decades, organic conjugated materials as semiconductors in organic field effect transistors (OFETs) have attracted more and more attention from the scientific community due to their intriguing properties of mechanical flexibility and solution processability. However, the device fabrication technique, design, and synthesis of novel organic semiconductor materials with high charge carrier mobility is crucial for the development of high-performance OFETs. In the past few years, more and more novel materials were designed and tested in the OFETs. Among which, diketopyrrolopyrrole (DPP) and its derivatives, as the electron acceptors to build donor-acceptor (D-A) typed materials, are the perspective. In this article, recently reported molecules regarding the DPP and its derivatives for OFETs application are reviewed. In addition, the relationship between the chemical structures and the performance of the device are discussed. Furthermore, an outlook of DPP-based materials in OFETs with a future design concept and the development trend are provided.

Novel functionalized indigo derivatives for organic electronics

A series of nine novel indigo derivatives, including diiodoindigo, octahalogenated indigoids and compounds with extended π-conjugated system, were synthesized, characterized and investigated as semiconductor materials in organic field-effect transistors (OFETs). Among them, 6,6′-diiodoindigo demonstrated the ambipolar behavior with balanced p-type and n-type mobilities. The complete substitution of hydrogens at the indigo core with halogen atoms led to low electron mobilities in OFETs. An extension of the conjugated system through the introduction of small aromatic substituents (thiophene and phenyl) resulted in predominant p-type behavior. Fusion of aromatic rings resulted in z-shaped dibenzoindigo, which showed poor charge transport properties due to the non-optimal arrangement of molecules along each other in the crystal lattice. The acquired data fulfilled the previously reported model based on the relationship between the chemical nature of substituents and their positions at the indigo core, optoelectronic properties of materials and their performance in OFETs. The results of this study will be useful for rational design of a new generation of the indigo-based semiconductors for biocompatible organic electronics.

Random copolymerization of regiorandom polythiophene to improve planarity, aggregation and hole-transport

Charge transport in semiconducting conjugated polymers mainly devolves on the solid-state ordering of polymer chains with respect to its packing and orientation. Therefore, controlling the inter- and intra-molecular interactions of polymer chains is vital for achieving high performance conjugated polymers for electronic applications. Herein, by a macromolecular design scheme, we optimized the microstructure of a regiorandom poly (thiophene) copolymer for efficient charge transport. By introducing side chain-free phenyl and pyridyl units into the backbone, we achieved regiorandom polymers possessing improved inter- and intra-molecular interactions with a much closer π-π molecular stacking and a larger size of crystallites. Improved charge carrier motion and transport in both vertical and horizontal directions were revealed when the resulting phenyl and pyridyl substituted polymers were applied as hole-transporting materials in organic field-effect transistors, diode-like space-charge-limited current devices, and perovskite solar cells. Particularly, the pyridyl substituted polymer exhibited more than two times higher charge carrier mobilities in the above mentioned device configurations compared with the pristine regiorandom poly (thiophene).

Nanodielectrics approaches to low-voltage organic transistors and circuits

In this review, advances in nanoscale dielectric materials for organic field-effect transistors (OFETs) are summarized. OFETs are highly promising device units for ultra-thin, light-weight, flexible, and wearable electronics systems, while the operating voltages of the reported devices are in many cases much higher than what is relevant to modern technological applications. Key aspects behind this issue are clarified in terms of basic transistor device physics, which translate into the important motivations for realizing nanodielectric-based low-voltage OFETs. Different possibilities of a device design are explained in detail by introducing important recent publications on each material class. Finally, several forward-looking remarks on the integration of nanodielectrics into next-generation OFETs are provided.

Organic transistors based on oxidized single-walled carbon nanotube electrodes patterned via simple spray coating

We developed a facile spray-printing method to fabricate oxidized single-walled carbon nanotube (Oxy-SWCNT) source-drain electrodes for use in organic field-effect transistors (OFETs). Surface oxidized SWCNT bundles enables the solution processability of Oxy-SWCNT without dispersant. The spray printing of an Oxy-SWCNT solution via a shadow mask led to electrode patterns with excellent transparency and electrical conductivity. Pattern resolution was optimized by adjusting printing volume and substrate temperature. The performance of the pentacene FETs with Oxy-SWCNT electrodes was compared with that of conventional Au electrodes. The FETs with Oxy-SWCNT electrodes exhibited higher charge carrier mobilities, on/off current ratios, and optical transmittances than those of FETs with Au electrodes. The increased device performance of the FETs with Oxy-SWCNT electrodes is due to the low contact resistance resulting from the efficient charge injection and transport across the electrode-channel interface. High work function (−5.1 eV) of the Oxy-SWCNT facilitated the charge injection. Our study demonstrates that Oxy-SWCNTs exhibit exceptional potential as a printable electrode material for OFETs and promise a better replacement for electrodes in transparent and flexible electronics.

Carbonyl-Terminated Quinoidal Oligothiophenes as p-Type Organic Semiconductors.

A series of quinoidal oligothiophenes terminated with carbonyl groups (nTDs, n = 2–4) are studied as p-type organic semiconductors for the active materials in organic field-effect transistors (OFETs) both by the theoretical and experimental approaches. The theoretical calculations clearly show their high-lying highest occupied molecular orbital (HOMO) energy levels (EHOMOs), small reorganization energies for hole transport (λholes), and large contribution of sulfur atoms to HOMOs, all of which are desirable for p-type organic semiconductors. Thus, we synthesized nTDs from the corresponding aromatic oligothiophene precursors and then evaluated their physicochemical properties and structural properties. These experimental evaluations of nTDs nicely proved the theoretical predictions, and the largest 4TDs in the series (4,4′′′-dihexyl- and 3′,4,4″,4′′′-tetrahexyl-5H,5′′′H-[2,2′:5′,2″:5″,2′′′-quaterthiophene]-5,5′′′-dione) can afford solution-processed OFETs showing unipolar p-type behaviors and hole mobility as high as 0.026 cm2 V−1 s−1.

Symmetric Mixed Sulfur-Selenium Fused Ring Systems as Potential Materials for Organic Field-Effect Transistors.

A reliable synthetic protocol toward a series of fused chalcogenopheno[1]benzochalcogenophene (CBC) building blocks was developed based on a Fiesselmann reaction. The obtained CBC units were applied in McMurry and Stille coupling reactions toward symmetric regioisomeric ene-linked dimers. These π-conjugated compounds were characterized regarding their photophysical and electrochemical properties and proved to be materials with reduced HOMO-LUMO gaps compared to their sulfur-based analogues. Single-crystal X-ray diffraction experiments revealed strong intermolecular selenium-selenium and selenium-carbon interactions depending on the position and number of incorporated selenium atoms. Good field-effect transistor performance with charge carrier mobilities up to 4×10-3 cm2 V-1 s-1 and high on/off ratios could be observed.

Hysteresis-Free, High-Performance Polymer-Dielectric Organic Field-Effect Transistors Enabled by Supercritical Fluid.

Organic field-effect transistors (OFETs) are of the core units in organic electronic circuits, and the performance of OFETs replies critically on the properties of their dielectric layers. Owing to the intrinsic flexibility and natural compatibility with other organic components, organic polymers, such as poly(vinyl alcohol) (PVA), have emerged as highly interesting dielectric materials for OFETs. However, unsatisfactory issues, such as hysteresis, high subthreshold swing, and low effective carrier mobility, still considerably limit the practical applications of the polymer-dielectric OFETs for high-speed, low-voltage flexible organic circuits. This work develops a new approach of using supercritical CO2 fluid (SCCO2) treatment on PVA dielectrics to achieve remarkably high-performance polymer-dielectric OFETs. The SCCO2 treatment is able to completely eliminate the hysteresis in the transfer characteristics of OFETs, and it can also significantly reduce the device subthreshold slope to 0.25 V/dec and enhance the saturation regime carrier mobility to 30.2 cm2 V−1 s−1, of which both the numbers are remarkable for flexible polymer-dielectric OFETs. It is further demonstrated that, coupling with an organic light-emitting diode (OLED), the SCCO2-treated OFET is able to function very well under fast switching speed, which indicates that an excellent switching behavior of polymer-dielectric OFETs can be enabled by this SCCO2 approach. Considering the broad and essential applications of OFETs, we envision that this SCCO2 technology will have a very broad spectrum of applications for organic electronics, especially for high refresh rate and low-voltage flexible display devices.

Synthesis and Performance of (E)-3-Phenyl-2-(thiophen-2-yl)acrylonitrile-Based Small-Molecule Semiconductors

Based on diketopyrrolopyrrole (DPP) and (E)-3-phenyl-2-(thiophen-2-yl)acrylonitrile (BVCNT)-linked conjugated backbones, three donor–acceptor type conjugated organic small-molecule compounds DPP-BVCNT, DPP-2FBVCNT, and DPP-3FBVCNT were designed and synthesized. Among them, the 2-decyltetradecyl side chain on the DPP acceptor unit was used to ensure the solubility of the material. The fluorine (F) atoms combined with the nitrile on the BVCNT donor unit were used to adjust electronic structures and charge carrier transport properties of the conjugated system. All the three small molecules exhibited good solution dispersibility and thermal stability, providing an important guarantee for the solution processing and annealing optimization of organic field-effect transistors (OFETs). The top-gate-bottom-contact OFET devices based on these compounds showed good ambipolar or p-type performances. The relationship between molecular structures and OFET performances indicated that the F-substitution and its position significantly affected their charge carrier transport properties. The F-substitution could remarkably change the performance from p-type to ambipolar especially for the outer-side-F-substituted compound DPP-2FBVCNT, which showed the best OFET performances with the maximum hole/electron mobilities of 0.023/0.220 cm2 V−1 s−1. These results provided a promising idea for developing small-molecule OFET materials with good solution processability, good thermal stability, and high ambipolar performances.

Sulfur-Containing Bent N-Heteroacenes.

A series of novel sulfur-containing bent N-heteroacenes were constructed and characterized by NMR and UV/Vis spectroscopy, cyclic voltammetry, and single-crystal X-ray diffraction. By introducing sulfur-containing groups (thio, sulfinyl, and sulfonyl) into bent azaacenes, their electronic delocalization was improved and frontier energy levels were modulated. The target products displayed tunable optical and electronic properties through altering the valence of sulfur and fused length of the azaacenes. For the first time, typical products were utilized as organic field effect transistor materials, affording promising results.

High-Performance Organic Field-Effect Transistors Fabricated with High-k Composite Polymer Gel Dielectrics

This study presents a composite gel-like dielectric material for organic field-effect transistors (OFET) applications. Poly(methyl methacrylate) (PMMA) gelled with propylene carbonate was used as gel dielectric material. Copper phthalocyanine was used as active layer in the OFET structures. In order to enhance the performance of the PMMA-gel dielectric material, silicium dioxide (SiO2) was used as an additive material. Various ratios of SiO2 were added to the gel dielectric and the effect of SiO2 on the OFET performance was investigated. It was clearly observed that SiO2 enhanced the performance and source-drain current of the fabricated OFETs. SiO2 was added to the PMMA-gel with different doping ratios of 0%, 10%, 30%, 50% and 100% by using a solution-processing method. The dielectric properties of the PMMA-gel:SiO2 composite materials were analyzed with impedance spectroscopy in terms of their effective capacitance ©I), tangent factor (tan(δ)), phase angle and complex dielectric constant (e′ and e″). The hole mobility of the OFETs was enhanced by 50% SiO2 nanoparticles in PMMA-gel dielectric materials from 6.83 × 10−1 cm2 V−1 s−1 to 4.66 × 100 cm2 V−1 s−1 (at VDS = − 0.5 V). The time-dependent IDS curves were analyzed for OFETs fabricated with PMMA-gel:SiO2 composite dielectric layers. It was found that all the devices worked stably under bias stress and gave fast responses for all gate voltages.

A Quinacridone-Diphenylquinoxaline-Based Copolymer for Organic Field-Effect Transistors.

In this work, we characterized poly(quinacridone-diphenylquinoxaline) (PQCTQx). PQCTQx was synthesized by a Suzuki coupling reaction and the synthesized PQCTQx was used as a polymeric semiconducting material in organic field-effect transistors (OFETs) to research the potential of using quinacridone derivatives. The measured field-effect mobility of the pristine PQCTQx film was 6.1 × 10−3 cm2/(V·s). A PQCTQx film heat-treated at 150 °C exhibited good field-effect performances with a hole mobility of 1.2 × 10−2 cm2/(V·s). The improved OFET behaviors resulting from the mild thermal treatment was attributed to improved packing of the molecules in the film, as determined using X-ray diffraction, and to decreased channel resistance.

Polymer binder assisted, solution processed cyanophenyl functionalized diketopyrrolopyrrole microwire for n-channel field-effect transistors

A cyanophenyl functionalized diketopyrrolopyrrole molecular semiconductor, DPP-PhCN, is used as an n-channel active material for organic field-effect transistors (FETs) with a polymer gate dielectric. Two types of DPP-PhCN active layer are prepared for the formation of the FET devices; 1) thin-film formed through a thermal vacuum deposition at different substrate temperatures and 2) microwire (single and multiple), prepared through a polymer binder assisted solution process. Both types of devices show electron dominant charge transport behavior. The device based on the polymer assisted solution processed microwire exhibit higher performance than the device with a thin-film. The highest electron mobility of 0.23 cm2/Vs in ambient condition is achieved from a device with only one microwire of DPP-PhCN, and is 30 times higher than that of the device with DPP-PhCN thin-film (0.008 cm2/Vs).

Synthesis, characterization, and hole-transporting properties of benzotriazatruxene derivatives

Four derivatives of benzotriazatruxene are synthesized and tested as hole-transporting materials in organic field-effect transistors and organic light-emitting diodes.

New π-extended catecholato complexes of Pt(ii) and Pd(ii) containing a benzothienobenzothiophene (BTBT) moiety: synthesis, electrochemical behavior and charge transfer properties.

Benzothienobenzothiophene (BTBT) and derivatives have received increasing attention as organic field-effect transistor materials and molecular conductors. This report presents the first synthesis of metal complexes involving a BTBT moiety, which was achieved by complexation of 2,2'-bipyridyl complexes of Pt(ii) and Pd(ii) with dihydroxy-substituted BTBT (1) as a new π-extended catecholato ligand (tBu2Bpy = 4,4'-di-tert-butyl-2,2'-dipyridyl). The resulting complexes M(tBu2Bpy)(O2BTBT) (M = Pt (3Pt) and Pd (3Pd)) were characterized by UV-vis spectroscopy, density functional theory (DFT) calculations, and cyclic voltammetry. The electron donating ability of BTBT was substantially enhanced upon including two oxygen substituents followed by metal coordination. This enabled chemical oxidation of 3Pt and 3Pd with a mild chemical oxidant (ferrocenium hexafluorophosphate) and formation of the one-electron-oxidized state. While 3Pt and 3Pd exhibited an absorption band originating from a catecholate → Bpy ligand-to-ligand charge transfer transition typical of this class of catecholato complexes, the radical cations exhibited a unique π-π* intramolecular charge transfer (ICT) transition absorption in which the π and π* orbitals were the newly incorporated benzothienothiophene-based donor and semiquinonato-based acceptor, respectively. The BTBT+ skeleton was electronically divided into two sites by the present chemical modification. The ICT properties of the complexes were found to be modulated by varying the metal ions. These findings offer a new approach to molecular design for (semi)conducting materials using optical properties.

Fine Structural Tuning of Thieno[3,2-b] Pyrrole Donor for Designing Banana-Shaped Semiconductors Relevant to Organic Field Effect Transistors

On the basis of the newly synthesized banana-shaped thieno[3,2- b] pyrrole building block [Bulumulla, C.; Gunawardhana, R.; Kularatne, R. N.; Hill, M. E.; McCandless, G. T.; Biewer, M. C.; Stefan, M. C. Thieno[3,2- b] pyrrole-Benzothiadiazole Banana-Shaped Small Molecules for Organic Field Effect Transistors. ACS Appl. Mater. Interfaces 2018, 10, 11818-11825], several small molecules that can be used as organic semiconducting materials were theoretically designed. We have shown that these novel molecules with the donor-π conjugated bridge-acceptor-π conjugated bridge-donor (D-π-A-π-D) building block exhibit superior charge transport properties in organic field-effect transistors (OFETs). A variety of donors, π-bridges, and acceptors are examined, and the structural, electronic, optical, and charge transport properties of designed semiconductors are systematically investigated. The results highlight the impact of the core acceptor in improving the transport properties of the designed molecules. In particular, this work points toward the benzo-bis(1,2,5-thiadiazole) as the most promising acceptor that can be combined with thiophene π-bridge and flanked benzo-thiadiazole terminal units to produce a reasonable candidate for synthesis and for incorporating into OFET materials. For the suggested semiconductor, the small electron reorganization energy and large intramolecular coupling originating from dense π-stacking gave rise to enhanced electron mobility. This strategy can be helpful for further improving the performance of curved small molecules in field-effect devices.