Solution-processed Organic Field-effect Transistors Research Articles

Solution-Processable Field-Effect Transistor Behavior of π-Conjugated Oligothiophene

The effect of substitution pattern of the linear-type π-conjugated oligothiophenes is examined. The material solubility, thermal properties, and charge transport characteristics of two hexyl subsituted quinquethiophenes (DH5T) were determined along with the characteristics of an organic field-effect transistor (OFET). The solubility of 4,7-bis (3’-hexyl-2,2’-bithiophene-5-yl) thiophene (β-DH5T), which featured a lateral substitution, was significantly better than that of α,α’-dihexylquinquethiophene (α-DH5T) by altering the directional position of the alkyl group, which makes it a good candidate for solution-processed OFETs. Despite the well-defined vertical alignment of α-DH5T, the hole mobility of the β-DH5T film (3.02 × 10–3 cm2 V–1 s–1) was one order of magnitude higher than the hole mobility of the α-DH5T film (2.47 × 10–4 cm2 V–1 s–1) prepared by solution processing.

Interplay of Molecular Orientation, Film Formation, and Optoelectronic Properties on Isoindigo- and Thienoisoindigo-Based Copolymers for Organic Field Effect Transistor and Organic Photovoltaic Applications

A systematic study on the effects of heteroarenes on the solid state structure and optoelectronic properties of isoindigo analogues, namely, PBDT-IIG and PBDT-TIIG, used in solution-processed organic field effect transistors (OFETs) and organic photovoltaics (OPVs) is reported. We discover that the optical absorption, frontier orbitals, backbone coplanarity, molecular orientation, solubility, film morphology, charge carrier mobility, and solar cell performance are critically influenced by the heteroarenes in the acceptor subunits. PBDT-IIG exhibits good p-type OFET performance with mobility up to 1.03 × 10–1 cm2 V–1 s–1, whereas PBDT-TIIG displays ambipolar mobilities of μh = 7.06 × 10–2 cm2 V–1 s–1 and μe = 2.81 × 10–4 cm2 V–1 s–1. PBDT-IIG and PBDT-TIIG blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) yield promising power conversion efficiencies (PCEs) of 5.86% and 2.55%, respectively. The excellent mobility of PBDT-IIG can be attributable to the growing fraction of edge-on packing by t...

Isoindigo-Based Small Molecules with Varied Donor Components for Solution-Processable Organic Field Effect Transistor Devices.

Two solution-processable small organic molecules, (E)-6,6′-bis(4-(diphenylamino)phenyl)-1,1′-bis(2-ethylhexyl)-(3,3′-biindolinylidene)-2,2′-dione (coded as S10) and (E)-6,6′-di(9H-carbazol-9-yl)-1,1′-bis(2-ethylhexyl)-(3,3′-biindolinylidene)-2,2′-dione (coded as S11) were successfully designed, synthesized and fully characterized. S10 and S11 are based on a donor-acceptor-donor structural motif and contain a common electron accepting moiety, isoindigo, along with different electron donating functionalities, triphenylamine and carbazole, respectively. Ultraviolet-visible absorption spectra revealed that the use of triphenylamine donor functionality resulted in an enhanced intramolecular charge transfer transition and reduction of optical band gap, when compared with its carbazole analogue. Both of these materials were designed to be donor semiconducting components, exerted excellent solubility in common organic solvents, showed excellent thermal stability, and their promising optoelectronic properties encouraged us to scrutinize charge-carrier mobilities using solution-processable organic field effect transistors. Hole mobilities of the order of 2.2 × 10−4 cm2/Vs and 7.8 × 10−3 cm2/Vs were measured using S10 and S11 as active materials, respectively.

Low-resistance contacts of electroless-plated metals with high-mobility organic semiconductors: Novel organic field-effect transistors with solution-processed electrodes

The establishment of a reliable vacuum-free method for the formation of electrical contacts on high-performance organic semiconductors has become an urgent task due to rapid progress made in the development of solution-processable high-mobility organic field-effect transistors (OFETs). We have recently proposed that electroless plating, a standard technology to mass produce wirings in currently commercialized electronic devices, is suited for high-performance solution-crystallized OFETs. A low contact resistance at the source and drain electrodes is necessary with organic semiconductors for high-speed device operation; therefore, we have evaluated the contact resistance using the transfer line method. A top-contact geometry with sufficient contact area is employed to achieve stable carrier injection, which has enabled contact resistances as low as 1.4 kΩ cm on a polyethylene naphthalate substrate at a gate voltage of −10 V. This marks outstanding performance among the solution-processed metal electrodes reported for OFETs, particularly on plastic substrates. The result indicates that high-quality boundaries with minimized trap densities are realized due to the mild conditions of the electroless plating process at room temperature.

Unidirectional coating technology for organic field-effect transistors: materials and methods

Solution-processed organic field-effect transistors (OFETs) are essential for developing organic electronics. The encouraging development in solution-processed OFETs has attracted research interest because of their potential in low-cost devices with performance comparable to polycrystalline-silicon-based transistors. In recent years, unidirectional coating technology, featuring thin-film coating along only one direction and involving specific materials as well as solution-assisted fabrication methods, has attracted intensive interest. Transistors with organic semiconductor layers, which are deposited via unidirectional coating methods, have achieved high performance. In particular, carrier mobility has been greatly enhanced to values much higher than 10 cm2 V−1 s−1. Such significant improvement is mainly attributed to better control in morphology and molecular packing arrangement of organic thin film. In this review, typical materials that are being used in OFETs are discussed, and demonstrations of unidirectional coating methods are surveyed.

Solution-processed high-LUMO-level polymers in n-type organic field-effect transistors: a comparative study as a semiconducting layer, dielectric layer, or charge injection layer

In solution-processed organic field-effect transistors (OFETs), the polymers with high level of lowest unoccupied molecular orbitals (LUMOs, > −3.5 eV) are especially susceptible to electron-trapping that causes low electron mobility and strong instability in successive operation. However, the role of high-LUMO-level polymers could be different depending on their locations relative to the semiconductor/insulator interface, or could even possibly benefit the device in some cases. We constructed unconventional polymer heterojunction n-type OFETs to control the location of the same polymer with a high LUMO level, to be in, under, or above the accumulation channel. We found that although the devices with the polymer in the channel suffer from dramatic instability, the same polymer causes much less instability when it acts as a dielectric modification layer or charge injection layer. Especially, it may even improve the device performance in the latter case. This result helps to improve our understanding of the electron-trapping and explore the value of these polymers in OFETs.

Solution-processed n-type fullerene field-effect transistors prepared using CVD-grown graphene electrodes: improving performance with thermal annealing.

Solution-processed organic field effect transistors (OFETs), which are amenable to facile large-area processing methods, have generated significant interest as key elements for use in all-organic electronic applications aimed at realizing low-cost, lightweight, and flexible devices. The low performance levels of n-type solution-processed bottom-contact OFETs unfortunately continue to pose a barrier to their commercialization. In this study, we introduced a combination of CVD-grown graphene source/drain (S/D) electrodes and fullerene (C60) in a solution-processable n-type semiconductor toward the fabrication of n-type bottom-contact OFETs. The C60 coating in the channel region was achieved by modifying the surface of the oxide gate dielectric layer with a phenyl group-terminated self-assembled monolayer (SAM). The graphene and phenyl group in the SAMs induced π-π interactions with C60, which facilitated the formation of a C60 coating. We also investigated the effects of thermal annealing on the reorganization properties and field-effect performances of the overlaying solution-processed C60 semiconductors. We found that thermal annealing of the C60 layer on the graphene surface improved the crystallinity of the face-centered cubic (fcc) phase structure, which improved the OFET performance and yielded mobilities of 0.055 cm(2) V(-1) s(-1). This approach enables the realization of solution-processed C60-based FETs using CVD-grown graphene S/D electrodes via inexpensive and solution-process techniques.

Low temperature cross-linked, high performance polymer gate dielectrics for solution-processed organic field-effect transistors

Thermal cross-linking the bi-functional polymer thin-films at low temperature for gate dielectric application in solution processed organic field-effect transistors.

Naphthodithiophenediimide (NDTI)-based triads for high-performance air-stable, solution-processed ambipolar organic field-effect transistors

We report new NDTI-based triad-type ambipolar molecular semiconductors for high-performance air-stable, solution-processed OFETs and complementary-like inverters.

Dielectric surface-polarity tuning and enhanced operation stability of solution-processed organic field-effect transistors

The electrical performance of triethylsilylethynyl anthradithiophene (TES-ADT) organic field-effect transistors (OFETs) was significantly affected by dielectric surface polarity controlled by grafting hexamethyldisilazane and dimethyl chlorosilane-terminated polystyrene (PS-Si(CH3)2Cl) to 300-nm-thick SiO2 dielectrics. On the untreated and treated SiO2 dielectrics, solvent–vapor annealed TES-ADT films contained millimeter-sized crystals with low grain boundaries (GBs). The operation and bias stability of OFETs containing similar crystalline structures of TES-ADT could be significantly increased with a decrease in dielectric surface polarity. Among dielectrics with similar capacitances (10.5–11nFcm−2) and surface roughnesses (0.40–0.44nm), the TES-ADT/PS-grafted dielectric interface contained the fewest trap sites and therefore the OFET produced using it had low-voltage operation and a charge-carrier mobility ∼1.32cm2V−1s−1, on–off current ratio >106, threshold voltage ∼0V, and long-term operation stability under negative bias stress.

Optimized grafting density of end-functionalized polymers to polar dielectric surfaces for solution-processed organic field-effect transistors.

Polystyrene (PS) grafted to silicon oxide (SiO2, referred to as gPS-SiO2) bilayers generated via a polymer grafting method were used as organic-oxide hybrid gate dielectrics to fabricate solution-processed triethylsilylethynyl anthradithiophene (TES-ADT) organic field-effect transistors (OFETs). The dielectric surface properties were significantly altered by the areal grafting densities of different molecular weight (Mw) PS chains with end-functionalized dimethylchlorosilane attached to the SiO2 surfaces. Lesser grafting densities of longer PS chains increased the surface roughness of the treated SiO2 surfaces from 0.2 to 1.5 nm, as well as the water contact angles from 94° to 88°. Below a critical Mw of end-functionalized PS, the gPS chains on the SiO2 surfaces appeared to form a brush-like conformation with an areal density value greater than 0.1 chains nm(-2), but other high-Mw gPS chains formed pancake structures in which the polymeric layers were easily incorporated with solution-processed TES-ADT as a solute. These findings indicate that low-density gPS layers interfered with the self-assembly of TES-ADT in cast films, causing great decreases in crystal grain size and π-conjugated orientation. The presence of compact gPS chains on the SiO2 surface could yield high electrical performance of TES-ADT OFETs with a field-effect mobility of 2.1 cm2 V(-1) s(-1), threshold voltage of -2.0 V, and on/off current ratio of greater than 10(7) when compared to those developed using less-concentrated gPS-SiO2 surfaces.

Organic field-effect transistors with a low driving voltage using albumin as the dielectric layer

Solution-processed organic field-effect transistors (OFETs) were fabricated based on a hygroscopic biomaterial, egg-white albumin, as the gate dielectric and poly(3-hexylthiophene) as the semiconductor. With increasing humidity, such OFETs can show improved performance exhibiting a very low driving voltage, together with enhanced carrier mobility and suppressed off-state current.

Improved Performance of Solution-Processed n-Type Organic Field-Effect Transistors by Regulating the Intermolecular Interactions and Crystalline Domains on Macroscopic Scale

The development of four new n-channel naphthalene diimide (NDI) and perylene diimide (PDI) copolymers (NDI-Ph, NDI-BT, PDI-Ph, and PDI-BT) and their solution processed thin film transistor (TFT) devices are reported. Remarkable enhancements in the electron transport behavior for all the four copolymers were achieved on improving the intermolecular interactions in their thin film structures. These solution processable n-type copolymers having NDI and PDI backbone were synthesized in high yields (83–86%) by palladium catalyzed Suzuki coupling reactions, and their excellent solubility in several organic solvents allowed their deposition in organic thin film transistor (OTFT) devices from solution directly. Since these copolymers possess crystalline domains, annealing their films induced crystalline phases in the thin film structures with a very high degree of enhancement in crystallinity that was more prominent for PDI copolymers as compared to NDI derivatives. This resulted in significant enhancement in the...

Effect of Ozone on the Stability of Solution-Processed Anthradithiophene-Based Organic Field-Effect Transistors

We have investigated the degradation effects of ozone exposure on organic field-effect transistors based on 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene as the organic semiconducting channel layer, as well as on thin films of this widely used, high-mobility, small molecule semiconductor. Electrical I–V measurements showed a loss of transistor characteristic behavior. We present 1H Nuclear Magnetic Resonance (NMR) spectroscopy results as well as X-ray Photoemission Spectroscopy (XPS) and Fourier Transform Infrared (FTIR) spectroscopy measurements showing the oxidation of the parent molecule, from which we suggest various possible reaction paths.

High crystallinity oligo(3-methylthiophenes) for p-channel organic field-effect transistors

A new type of oligo(3-methylthiophenes) with highly regiosymmetric chemical structure was presented for developing high performance and solution-processable organic field-effect transistors (OFETs). The representative molecule Br4MT, tetramer of the oligo(3-methylthiophenes), was synthesized through the oxidative cross-coupling reactions using Cu(II)-catalyst and fully characterized. The Br4MT exhibited remarkably high crystallinity and great ionization potential due to its unique molecular structure, evidenced by differential scanning calorimetry, electrochemical and X-ray diffraction studies. The solution-processed OFETs based on Br4MT achieved high hole mobility of up to 0.24 cm2/V/s with reasonable ambient stability, suggesting that the Br4MT had great potential for application in low-cost OFETs.

Improving solution-processed n-type organic field-effect transistors by transfer-printed metal/semiconductor and semiconductor/semiconductor heterojunctions

Solution-processed n-type organic field effect transistors (OFETs) are in need of proper metal contact for improving injection and mobility, as well as balanced hole mobility for building logic circuit units. We address the two distinct problems by a simple technique of transfer-printing. Transfer-printed Au contacts on a terrylene-based semiconductor (TDI) significantly reduced the inverse subthreshold slope by 5.6V/dec and enhanced the linear mobility by over 5 times compared to evaporated Au contacts. Hence, devices with a high-work-function metal (Au) are comparable with those with low-work-function metals (Al and Ca), indicating a fundamental advantage of transfer-printed electrodes in electron injection. We also transfer-printed a poly(3-hexylthiophene) (P3HT) layer onto TDI to construct a double-channel ambipolar transistor by a solution process for the first time. The transistor exhibits balanced hole and electron mobility (3.0×10−3 and 2.8×10−3cm2V−1s−1) even in a coplanar structure with symmetric Au electrodes. The technique is especially useful for reaching intrinsic mobility of new materials, and enables significant enlargement of the material tanks for solution-processed functional heterojunction OFETs.

Interface control in organic electronics using mixed monolayers of carboranethiol isomers.

We employ mixed self-assembled monolayers of carboranethiols to alter the work function of gold and silver systematically. We use isomers of symmetric carboranethiol cage molecules to vary molecular dipole moments and directions, which enable work function tunability over a wide range with minimal alterations in surface energy. Mixed monolayers of carboranethiol isomers provide an ideal platform for the study and fabrication of solution-processed organic field-effect transistors; improved device performance is demonstrated by interface engineering.

Pyrene and Diketopyrrolopyrrole-Based Oligomers Synthesized via Direct Arylation for OSC Applications

In this report, an atom efficient and facile synthetic strategy for accessing multi-diketopyrrolopyrrole (DPP)-based oligomers used in solution-processed organic field effect transistors (OFETs) and organic solar cells (OSCs) has been developed. The DPP units were successfully installed onto benzene and pyrene cores via palladium-catalyzed dehydrohalogenative coupling of mono-capped DPPs with multi-bromo-benzene or -pyrene (direct arylation), affording four oligomer small molecules (SMs 1-4) containing bis-, tri-, tri-, and tetra-DPP, respectively, in high yields of 78-96%. All the designed linear or branched DPP-based oligomers exhibit broad light absorptions, narrow band-gaps (1.60-1.73 eV), deep highest occupied molecular orbital (HOMO) levels (-5.26∼-5.18 eV), and good thermal stability (Td=390-401 °C). OFETs based on SMs 1-4 showed hole mobilities of 0.0033, 0.0056, 0.0005, and 0.0026 cm2 V(-1) s(-1), respectively. OSCs based on SMs 1-4 under one sun achieved power conversion efficiencies of 3.00%, 3.71%, 2.47%, and 1.86% accordingly, along with high open-circuit voltages of 0.86-0.94 V. For OSC devices of SM 1, SM 3, and SM 4, the solvent CHCl3 was solely employed to the formation of active layers; neither high boiling point additives nor annealing post-treatment was needed. Such a simple process benefits the large-scale production of OSCs via roll to roll technology.

Non-volatile organic memory with sub-millimetre bending radius

High-performance non-volatile memory that can operate under various mechanical deformations such as bending and folding is in great demand for the future smart wearable and foldable electronics. Here we demonstrate non-volatile solution-processed ferroelectric organic field-effect transistor memories operating in p- and n-type dual mode, with excellent mechanical flexibility. Our devices contain a ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) thin insulator layer and use a quinoidal oligothiophene derivative (QQT(CN)4) as organic semiconductor. Our dual-mode field-effect devices are highly reliable with data retention and endurance of >6,000 s and 100 cycles, respectively, even after 1,000 bending cycles at both extreme bending radii as low as 500 μm and with sharp folding involving inelastic deformation of the device. Nano-indentation and nano scratch studies are performed to characterize the mechanical properties of organic layers and understand the crucial role played by QQT(CN)4 on the mechanical flexibility of our devices.

High-performance, low-operating voltage, and solution-processable organic field-effect transistor with silk fibroin as the gate dielectric

We report the use of silk fibroin as the gate dielectric material in solution-processed organic field-effect transistors (OFETs) with poly(3-hexylthiophene) (P3HT) as the semiconducting layer. Such OFETs exhibit a low threshold of −0.77 V and a low-operating voltage (0 to −3 V) compatible with the voltage level commonly-used in current electronic industry. The carrier mobility of such OFETs is as high as 0.21 cm2 V−1 s−1 in the saturation regime, comparable to the best value of P3HT-based OFETs with dielectric layer that is not solution-processed. The high-performance of this kind of OFET is related with the high content of β strands in fibroin dielectric which leads to an array of fibers in a highly ordered structure, thus reducing the trapping sites at the semiconductor/dielectric interface.