Materials For Organic Field-effect Transistors Research Articles

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.

Effect of space charge limited current on performance of organic field-effect transistors

The crucial parameter that determines the performance of a semiconductor material in organic field-effect transistors (OFETs) is the charge carrier mobility. The conventional method of its determination based on Shockley’s equations can lead to incorrect mobility evaluation due to contact effects. Particularly, in the common staggered OFET architecture (top-contact bottom-gate or bottom-contact top-gate), the space-charge limited current (SCLC) effect in the active layer under/above the source and drain contacts decreases the source-drain current. In this work, we model the effect of SCLC under/above the source and drain electrodes on the OFET apparent mobility (i.e., calculated from the device characteristics) and apparent threshold voltage for different active layer thickness and intrinsic mobility anisotropy. For the saturation regime, we derived simple analytical expressions for transfer characteristics and apparent mobility. Our modeling shows that the apparent OFET mobility is more than five times lower than the intrinsic one for the active layer thicker than 100 nm with mobility anisotropy (along vs across the active layer) higher than 100. While the SCLC effect does not change the apparent threshold voltage, it reveals itself as a kink at near zero voltage in the output characteristics. The proposed model gives analytical expressions for the transfer characteristics and apparent mobility as explicit functions of the intrinsic mobility and the device parameters in the saturation regime and as implicit functions in the linear regime. Our findings provide guidelines for accurate evaluation of the intrinsic mobility in OFETs fabricated in the staggered architecture and for further improvement of OFET performance.

Novel naphthalene-diimide-based small molecule with a bithiophene linker for use in organic field-effect transistors

Abstract We report on the synthesis and characterization of a novel naphthalene diimide (NDI)-based small molecule with a bithiophene linker unit, NDI-BT-NDI-EH, for application as a solution-processable n-channel active material in organic field-effect transistors (OFETs). NDI-BT-NDI-EH was synthesized via the Stille coupling reaction and the bithiophene linker unit was introduced as a rotatable linker between two NDI cores to improve the solubility of NDI-BT-NDI-EH while maintaining its high field-effect mobility. We found that the crystallinity and molecular ordering of NDI-BT-NDI-EH thin films could be improved by thermal annealing; a moderate annealing temperature of 120 °C appeared to be optimal for promoting self-organization among the NDI-BT-NDI-EH molecules. The NDI-BT-NDI-EH molecules in thermally annealed thin films mainly adopted an edge-on orientation with improved π-π stacking on the surface of octyltrichlorosilane-treated SiO2 substrates, which is favorable for lateral charge transport. As a result, the optimally annealed NDI-BT-NDI-EH OFET exhibited evident n-type OFET characteristics with high field-effect mobilities of up to 0.016 cm2 V−1 s−1 and an on/off ratio of 1.4 × 105.

Control of Concentration of Nonhydrogen-Bonded Hydroxyl Groups in Polymer Dielectrics for Organic Field-Effect Transistors with Operational Stability.

Poly(4-vinylphenol) (PVP) is a promising gate dielectric material for organic field-effect transistors (OFETs) and circuits fabricated on plastic substrates. Thermal cross-linking of PVP with a cross-linker, such as poly(melamine- co-formaldehyde) methylated(PMF), at a high temperature (above 170 °C) is widely considered an effective method to remove residual hydroxyl groups that induce polarization effects in the dielectric bulk. However, the threshold voltage shift in transfer characteristics is still observed for an OFET with a PVP-PMF dielectric when it is operated at a slow gate voltage sweep rate. The present study examines the cause of the undesired hysteresis phenomenon and suggests a route to enable a reliable operation. We systematically investigate the effect of the PVP-PMF weight ratio and their annealing temperature on the transfer characteristics of OFETs. We discover that the size of the hysteresis is closely related to the concentration of nonhydrogen-bonded hydroxyl groups in the dielectric bulk and this is controlled by the weight ratio. At a ratio of 0.5:1, a complete elimination of hysteresis was observed irrespective of the annealing temperature. We finally demonstrate a highly reliable operation of small-molecule-based OFETs fabricated on a plastic substrate at a low temperature.

44.1: Invited Paper: High Performance Organic Field Effect Transistor Materials for Printed Organic Electronics

Organic Field Effect Transistors for display backplanes and circuitry require materials that guarantee high performance and enable robust processes. To meet these needs, BASF is developing solution-processable organic semiconductors and matching photo-patternable dielectrics which are tailored to a variety of coating and printing processes. We present a market-ready material set giving field effect transistors with mobilities of 1 cm2/Vs based on a scalable, highly homogenous process. Next generation materials demonstrate mobilities of up to 4 cm2/Vs. For process integration we showcase a simplified i-line photopatterning process that reduces the number of steps compared to classical photolithography without loss of system performance.

Thiophene-Fused Naphthalene Diimides: New Building Blocks for Electron Deficient π-Functional Materials

Abstract Development of novel π-conjugated building blocks that can be integrated into molecular or macromolecular systems is key to the evolution of new superior organic semiconductors utilized as the active materials in organic electronics devices such as organic field-effect transistors (OFETs), organic photovoltaics (OPVs), and organic thermoelectric (TE) devices. This review affords a brief overview of thiophene-fused naphthalene diimide (NDI), namely naphtho[2,3-b:6,7-b′]dithiophene diimide (NDTI) and naphtho[2,3-b]thiophene diimide (NTI), recently developed as novel electron deficient building blocks for n-type and ambipolar organic semiconductors. These thiophene-fused NDI building blocks had not been known until 2013 owing to their synthetic difficulty; more precisely, the difficulty in attaching fused-thiophene ring(s) on the NDI core. We have successfully established a thiophene-annulation reaction on ethyne-substituted NDI derivatives, which allows us to elaborate various NDTI and NTI derivatives. The key features of these building blocks are low-lying energy levels of lowest unoccupied molecular orbitals (LUMO, 3.8–4.1 eV below the vacuum level) and easy functionalizability of the thiophene α-positions, which allows their derivatives and polymers to conjugate efficiently with additional π- and co-monomer units. These features make the NDTI- and NTI-derivatives and polymers promising n-type and ambipolar materials for OFETs and acceptors for OPVs. In fact, various useful materials have already been derived from the NDTI and NTI building blocks: air-stable n-type small molecules and polymers with high electron mobility (∼0.8 cm2 V−1 s−1), ambipolar oligomers and polymers with well-balanced hole and electron mobilities, doped n-type semiconductors affording bulk conductors applicable to n-type TE materials, and electron acceptor molecules and polymers for OPVs showing promising power conversion efficiencies of up to 9%. These impressive and diversified device performances testify the usefulness of thiophene-fused NDI building blocks in the development of new electron deficient π-functional materials.

Parylene C as a versatile dielectric material for organic field-effect transistors.

An emerging new technology, organic electronics, is approaching the stage of large-scale industrial application. This is due to a remarkable progress in synthesis of a variety of organic semiconductors, allowing one to design and to fabricate, so far on a laboratory scale, different organic electronic devices of satisfactory performance. However, a complete technology requires upgrading of fabrication procedures of all elements of electronic devices and circuits, which not only comprise active layers, but also electrodes, dielectrics, insulators, substrates and protecting/encapsulating coatings. In this review, poly(chloro-para-xylylene) known as Parylene C, which appears to become a versatile supporting material especially suitable for applications in flexible organic electronics, is presented. A synthesis and basic properties of Parylene C are described, followed by several examples of use of parylenes as substrates, dielectrics, insulators, or protecting materials in the construction of organic field-effect transistors.

Application of Azulene in Constructing Organic Optoelectronic Materials: New Tricks for an Old Dog.

Azulene, as an isomer of naphthalene, has received increasing interest due to its unique chemical structure and unusual photophysical properties, including a large dipole moment of 1.08 D, a narrow energy gap between the HOMO and LUMO, and abnormal fluorescence (anti-Kasha's rule) from the second excited state to the ground state. In this Minireview, the general strategies and representative synthetic methods for the preparation and functionalization of azulene and its derivatives are presented, and then the application of azulene-based optoelectronic materials in organic field-effect transistors and solar cells is discussed. Finally, the challenges and outlook on developing azulene-based optoelectronic materials are discussed, together with several key points on molecular design and synthesis.

Design and Functions of Semiconducting Fused Polycyclic Furans for Optoelectronic Applications.

The fused polycyclic furan structure is a ubiquitous motif in naturally occurring organic compounds. However, they had been rarely seen in the literature of organic electronic research until very recently, probably because of the lack of stability of simple furans under conditions that the compounds experience in the active layer of the device. Nonetheless, from the viewpoint of molecular structure, furans look to have potential merits as organic semiconductors such as thiophenes, which are more popular in the organic electronic area. For example, the small atomic radius and large electronegativity of oxygen will increase intermolecular molecular orbital (MO) overlap and hence facilitate charge transporting ability in the solid state. In this Account, we describe the molecular design and optoelectronic applications of fused polycyclic furans, such as benzodifurans (BDFs), naphthodifurans (NDFs), and anthradifurans (ADFs). The molecular design that was exploited in this study crucially depends on the synthetic flexibility of a "modular" synthetic strategy that we purposely developed and reviewed in a separate report. Our synthetic strategy comprises two steps carried out in situ: cyclization of an o-alkynylphenol into a zincio benzofuran and its electrophilic Negishi-type trapping to obtain a range of multisubstituted fused furan compounds. These compounds are found to possess electronic structures resembling those of fused polyaromatic hydrocarbons, such as acenes or phenacenes, rather than oxygen-bridged phenylenevinylene, along with unique characteristics: a wide HOMO-LUMO gap originating from the weak aromaticity of the furan rings, an intense photoluminescent character, and mechanofluorochromism. Semiconducting properties of fused furans are also excellent among organic materials: some BDF derivatives show high hole mobility on the order of 10-3 cm2/(V s) in the amorphous state using time-of-flight (TOF) technique. The p-type BDFs exhibit high performance as hole-transporting material in heterojunction organic light-emitting diodes (OLEDs), while carbazole-substituted BDFs (CZBDFs) are ambipolar with well-balanced high carrier mobility for both hole and electron and serve as host materials for full-color electroluminescence in both hetero- and homojunction architectures. More π-expanded NDFs showed good crystallinity and are effective active materials for organic field-effect transistors (OFETs) with a high hole mobility of up to 3.6 cm2/(V s) using a solution process. These studies have illustrated the high potential of fused polycyclic furans in organic electronics research, which thus far have attracted much less attention than their thiophene congeners.

(Invited) A New Materials Concept for High Performance Organic Thin Film Transistors

We have established a new materials concept for organic field effect transistors (OFETs) solution-processed, i.e., utilization of highly ordered smectic liquid crystals, which exhibit self-organization and solid-like nature, in order to solve the problems such as film uniformity and thermal durability in small molecule OFET materials. 2-Phenyl-7-decyl-benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10) exhibiting SmE phase was designed and synthesized to apply OFETs. Bottom-gate and bottom-contact OFETs fabricated with its polycrystalline thin films exhibited high mobility of over 10cm2/Vs with its small variation from device to device and high thermal durability up to 200oC . In addition, we succeeded in operating the OFETs at a few volt with a low-dielectric gate insulator material of SiO2.

Selenium-Containing Fused Bicyclic Heterocycle Diselenolodiselenole: Field Effect Transistor Study and Structure-Property Relationship.

The first application of the diselenolodiselenole (C4Se4) heterocycle as an active organic field effect transistor materials is demonstrated here. C4Se4 derivatives (2a-2d) were obtained by using a newly developed straightforward diselenocyclization protocol, which includes the reaction of diynes with selenium powder at elevated temperature. C4Se4 derivatives exhibit strong donor characteristics and planar structure (except 2d). The atomic force microscopic analysis and thin-film X-ray diffraction pattern of compounds 2a-2d indicated the formation of distinct crystalline films that contain large domains. A scanning electron microscopy study of compound 2b showed development of symmetrical grains with an average diameter of 150 nm. Interestingly, 2b exhibited superior hole mobility, approaching 0.027 cm(2) V(-1) s(-1) with a transconductance of 9.2 μS. This study correlate the effect of π-stacking, Se···Se intermolecular interaction, and planarity with the charge transport properties and performance in the field effect transistor devices. We have shown that the planarity in C4Se4 derivatives was achieved by varying the end groups attached to the C4Se4 core. In turn, optoelectronic properties can also be tuned for all these derivatives by end-group variation.

64-2: New Organic Semiconductors for Improved Processing - Direct Photo-Patterning and High Mobility Materials for Flexible TFTs

Targeting flexible display backplanes and circuitry, BASF is developing high performance materials for Organic Field-Effect Transistors. Here we present our recent development focusing on directly photo-patternable semiconductors and dielectrics, as well as new semiconductors for solution-processed transistors with mobilities of up to 4 cm2/Vs.

Theoretical investigation on the crystal structures and electron transfer properties of cyanated TTPO and their selenium analogs

We designed a series of 5,6,7-trithiapentacene-13-one (TTPO) derivatives (from 1 to 20) by attaching cyano groups (–CN) and by replacing S atoms with Se. The incoherent charge-hopping model combined with the Marcus–Hush electron transfer theory were used to evaluate the charge mobility of the title compounds as potential n-type organic field-effect transistors (OFETs) candidates. Based on the optimized face-to-face dimers, the crystal structures were predicted. The replacement of S with Se can decrease the electron reorganization energy (λ –). The introduction of −CN can decrease the energy levels of both HOMOs and LUMOs, which increases the stability. According to the electronic affinities (EA) and structures of dimers predicted by the DFT method coupled with dispersion corrections (−D), we selected compounds 2, 14, 15, and 19 as initial well-performance OFETs materials and predicted their crystal structures. Based on the optimized crystal structures, the charge (both electron and hole) transport mobilities are estimated. Electron mobilities of 14 and 15 are as high as 2.32 and 2.48 cm2 V−1 s−1, respectively, indicating that 14 and 15 are the promising n-type OFETs materials. What’s more, the predicted crystals show remarkable anisotropic charge mobilities. The maximum electron mobility appears along the direction of face-to-face π-stacking.

The effect of tuning the microstructure of TIPS-tetraazapentacene on the performance of solution processed thin film transistors

We report a comprehensive study of the symmetrical 6,13-bis(triisopropylsilylethynyl)tetraazapentacene used as an electron transporting material in organic field-effect transistors deposited by spin/zone-casting.

ChemInform Abstract: Single‐Crystal Organic Field‐Effect Transistors of Naphthodifurans

AbstractReview: [23 refs.

Single-Crystal Organic Field-Effect Transistors of Naphthodifurans

Abstract Three types of naphthodifurans (NDFs) have been designed as active organic materials in organic field-effect transistors (OFETs), and one of them, naphtho[2,1-b:6,5-b′]difuran, has been found to show an excellent OFET mobility of up to 3.6 cm2 V−1 s−1. Based on the crystal-packing structure and theoretical analyses, we attribute the high carrier mobility of the NDF derivatives to large intermolecular interactions and small reorganization energies. The NDF compounds show very high thermal and electrochemical stability. Together with some previous studies, the present study indicates that fused furan compounds show a performance that is equal to or better than that of the corresponding thiophene compounds.

Hole mobility in thermally evaporated pentacene: Morphological and directional dependence

Pentacene has been extensively studied as an active material for organic field-effect transistors as it shows very good charge carrier mobility along its preferred transport direction. In this contribution, we investigate the hole transport in pentacene thin films by measurement in conventional lateral organic field-effect transistors (OFETs), which yields the hole mobility along the a-b plane of pentacene, and by the recently published potential mapping (POEM) approach, which allows for direct extraction of the charge carrier mobility perpendicular to the substrate, in this case perpendicular to the a-b plane, without the assumption of a specific transport model. While the mobility along the a-b plane—determined from OFET measurements—is found to be in the region of 0.45 cm2/Vs, transport perpendicular to this plane shows an average mobility at least one order of magnitude lower. Investigating also how these effective mobility values depend on the deposition rate of the pentacene films, we find that the decrease in grain size for increasing deposition rate causes the mobility to decrease both parallel and perpendicular to the substrate due to the increased number of grain boundaries to be overcome. For the out-of-plane transport, this effect is found to saturate for deposition rates higher than 2.5 Å/s.

Antiaromatic characteristic analysis of 1,4-diazapentalene derivatives: a theoretical study.

1,4-Diazapentalene heteroacenes are potential n-type semiconductors that could be used as a new type of material for organic field-effect transistors (OFETs), but their synthesis is still challenging due to their antiaromaticity. The study on their structure-stability relationship should provide useful guidance to the design of stable diazapentalenes. We examined the stability of several types of heteroacenes bearing the 1,4-diazapentalene core using NICS(1)zz calculations. The influence of the fusion pattern, the introduction of substituents, and the incorporation of other heterocycles on the antiaromaticity of the central 1,4-diazapentalene core was systematically studied. It was found that the linear fusion of aromatic rings to the antiaromatic core increases the stability of the heteroacene. The fusion of electron-poor heterocyclic rings also enhances the stability effectively, whereas the fusion of electron-rich heterocyclic rings destabilizes the system. In addition, the combination of the linear fusion pattern or introduction of electron-poor heterocyclic rings to the antiaromatic core reduces the reorganization energy for electron transport, suggesting a way to achieve better n-type semiconductors.

The impact of carbon sp2 fraction of reduced graphene oxide on the performance of reduced graphene oxide contacted organic transistors

One of the major bottlenecks in fabricating high performance organic field effect transistors (OFETs) is a large interfacial contact barrier between metal electrodes and organic semiconductors (OSCs) which makes the charge injection inefficient. Recently, reduced graphene oxide (RGO) has been suggested as an alternative electrode material for OFETs. RGO has tunable electronic properties and its conductivity can be varied by several orders of magnitude by varying the carbon sp2 fraction. However, whether the sp2 fraction of RGO in the electrode affects the performance of the fabricated OFETs is yet to be investigated. In this study, we demonstrate that the performance of OFETs with pentacene as OSC and RGO as electrode can be continuously improved by increasing the carbon sp2 fraction of RGO. When compared to control palladium electrodes, the mobility of the OFETs shows an improvement of ∼200% for 61% sp2 fraction RGO, which further improves to ∼500% for 80% RGO electrode. Similar improvements were also observed in current on-off ratio, on-current, and transconductance. Our study suggests that, in addition to π-π interaction at RGO/pentacene interface, the tunable electronic properties of RGO electrode have a significant role in OFETs performance.