Solution-processed Organic Transistors Research Articles

Synergistic polarization engineering of dielectric towards low-voltage high-mobility solution-processed ultraflexible organic transistors

The emerging wearable skin-like electronics require the ultra-flexible organic transistor to operate at low voltage for electrical safety and energy efficiency and simultaneously enable high field-effect mobility to ensure the carrier migration ability and the switching speed of circuits. However, the currently reported low-voltage organic transistors generally present low mobility, originating from the trade-off between molecular polarity and surface polarity of the dielectrics. In this work, the orientation polarization of the dielectric is enhanced by introducing a flexible quaternary ammonium side chain, and the surface polarity is weakened by the shielding effect of the nonpolar methyl groups on the polar nitrogen atom. The resulting antisolvent QPSU dielectric enables the high-dielectric constant up to 18.8 and the low surface polarity with the polar component of surface energy only at 2.09 mJ/m2. Such a synergistic polarization engineering between orientation polarization and surface polarity makes the solution-processed ultraflexible transistors present the ultralow operational voltage down to −3 V, the ultrahigh charge-carrier mobility up to 8.28 cm2 V−1 s−1 at 1 Hz, excellent cyclic operational stability and long-term air stability. These results combined with the ultrathin thickness of transistor as low as 135 nm, the ultralight mass of 0.5 g/m2, the conformal adherence capability on human skin and 1-μm blade edge, and the strong mechanical robustness with stable electrical properties for 30,000 bending cycles, open up an available strategy to successfully realize low-voltage high-mobility solution-processed organic transistor, and presents the potential application of QPSU dielectric for the next-generation wearable imperceptible skin-like electronics.

AI-Assisted Development of Solution-Processed Organic Transistors

AI-Assisted Development of Solution-Processed Organic Transistors

Reduction of current path of solution-processed organic photosynaptic transistors for neuromorphic computing

Reduction of current path of solution-processed organic photosynaptic transistors for neuromorphic computing

Electrically programmable multilevel nonvolatile memories based on solution-processed organic floating-gate transistors

The development of nonvolatile organic field-effect transistor (OFET) memories with a satisfactory solution processability is highly desirable to fabricate the data storage media for flexible and printed electronic devices. In this study, we fabricate top-gate/bottom-contact OFET memories having an organic floating-gate structure by a spin-coating process and investigate their memory characteristics. An ambipolar polymer semiconductor of poly(N-alkyldiketopyrrolo-pyrrole-dithienylthieno[3,2-b]thiophene) (DPP-DTT) was used to fabricate an organic semiconductor layer, on which an organic composite of polystyrene and 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) was deposited to form an organic floating-gate structure through vertical phase separation. The existence of a deep lowest unoccupied molecular orbital (LUMO) level and the excellent electron transport property of the DPP-DTT enables the injection of electrons from the Au source-drain electrodes to the DPP-DTT semiconductor layer and the storage of electrons in the LUMO level of the TIPS-pentacene floating gates by programming under dark conditions. A high work function metal oxide layer of MoO3 was inserted at the Al gate electrode/CYTOP gate insulator interface to tune the energy level difference between the Au source-drain and Al gate electrodes. The DPP-DTT FET memories with MoO3/Al gate electrodes exhibit satisfactory retention characteristics and, because of the ambipolar trapping characteristics, allow the storing of holes in the highest occupied molecular orbital level of the TIPS-pentacene floating gates in the erasing process. Furthermore, the molecular floating-gate OFET memories exhibit a high storage capacity for multi-level data, and four state levels can be recorded with stable retention characteristics.

Solution-processed pseudo-vertical organic transistors based on TIPS-pentacene

Powerful and versatile transistors are indispensable to realize the vision of future flexible electronics, e.g., for wearable active-matrix displays, smart sensors, or radio-frequency identification tags. Organic thin-film transistors are considered to be a perfect match for flexible electronics. However, conventional organic thin-film transistors still do not match the demanding performance targets of many applications. Furthermore, often complex and expensive fabrication steps are employed for the fabrication of hero-devices, which is not compatible with the paradigm of low-cost production of flexible electronics.In this contribution, we present the first solution-processed vertical organic field-effect transistor (VOFET) with good on-state performance comparable to vacuum-processed VOFETs. This approach unites the advantages of a low-temperature, low-cost solution processing with an ultra-short channel transistor concept possibly enabling large-area, low-cost flexible electronics. We examine the influence of different crystal morphologies (spherulitic and ribbons) on the transistor performance by using spin- and shear-coating as solution-based deposition methods. The solution-processed VOFETs reach channel width-normalized transconductances of up to 0.26 μS/μm with a charge carrier mobility of 4.8 cm2/V. S-parameter measurements finally verify that transition frequencies up to 6 MHz are reachable with shear-coated TIPS-pentacene. However, compared to small molecule-based VOFETs, the solution-processed VOFETs show an unfavorably high off-state current and hysteresis, which are explained by background doping and charge carrier trapping. Hence, in order to advance with, further optimization of the semiconductor material and the insulator–semiconductor interface is thus required to qualify solution-processed VOFETs for commercial applications.

Highly Stable Nonhydroxyl Antisolvent Polymer Dielectric: A New Strategy towards High-Performance Low-Temperature Solution-Processed Ultraflexible Organic Transistors for Skin-Inspired Electronics.

Scarcity of the antisolvent polymer dielectrics and their poor stability have significantly prevented solution-processed ultraflexible organic transistors from low-temperature, large-scale production for applications in low-cost skin-inspired electronics. Here, we present a novel low-temperature solution-processed PEI-EP polymer dielectric with dramatically enhanced thermal stability, humidity stability, and frequency stability compared with the conventional PVA/c-PVA and c-PVP dielectrics, by incorporating polyethyleneimine PEI as crosslinking sites in nonhydroxyl epoxy EP. The PEI-EP dielectric requires a very low process temperature as low as 70°C and simultaneously possesses the high initial decomposition temperature (340°C) and glass transition temperature (230°C), humidity-resistant dielectric properties, and frequency-independent capacitance. Integrated into the solution-processed C8-BTBT thin-film transistors, the PEI-EP dielectric enables the device stable operation in air within 2 months and in high-humidity environment from 20 to 100% without significant performance degradation. The PEI-EP dielectric transistor array also presents weak hysteresis transfer characteristics, excellent electrical performance with 100% operation rate, high mobility up to 7.98 cm2 V−1 s−1 (1 Hz) and average mobility as high as 5.3 cm2 V−1 s−1 (1 Hz), excellent flexibility with the normal operation at the bending radius down to 0.003 mm, and foldable and crumpling-resistant capability. These results reveal the great potential of PEI-EP polymer as dielectric of low-temperature solution-processed ultraflexible organic transistors and open a new strategy for the development and applications of next-generation low-cost skin electronics.

Enhancement of Optical Memory Characteristics of Solution-Processed Organic Transistor Memories with Polymer-Small-Molecule Composite Charge Storage Layers

Proceedings of the International Display Workshops Volume 27 (IDW '20),Enhancement of Optical Memory Characteristics of Solution-Processed Organic Transistor Memories with Polymer-Small-Molecule Composite Charge Storage Layers

Direct-writing of organic field-effect transistors on plastic achieving 22 MHz transition frequency

Organic field effect transistors are considered among the key enablers of flexible, wearable, lightweight and portable electronics. The possibility to fabricate polymer-based devices from solution with high throughput processes makes organic electronics suitable for low cost mass-production of new and diverse applications. Nevertheless, the investigation of the dynamic behaviour of these devices, i.e. their operational frequency, is still limited and the reported operational speed remains low, especially on flexible substrates, limiting their applicability in many circuitry applications. In this work, we report downscaled p-type polymer field effect transistors (FETs), fabricated on both rigid and flexible polymer substrates, by a combination of coating and direct-writing techniques, achieving radio-frequency operation. FETs with the shortest channel of 1.2 µm can operate up to 22 MHz at a relatively low bias voltage of −12 V, as measured by scattering parameters. Such a transition frequency (fT) is the highest so far recorded for organic transistors fabricated on flexible substrates. In terms of fT normalized by the applied gate-to-source voltage (fT/VGS), the achieved 1.83 MHz V−1 is the highest for solution-processed organic transistors on plastic. This result paves the way to the implementation of polymer radio-frequency devices with cost-effective fabrication processes on flexible substrates.

Highly Emissive Semi-Ladder-Type Copolymers, Aggregation State, and Solution-Processed Organic Light-Emitting Transistor

An organic light-emitting transistor (OLET) integrates the logic function of a field-effect transistor (FET) with electroluminescence of a light-emitting diode (LED) into a single device. This has ...

Fluorosurfactant-Improved Wettability of Low-Viscosity Organic Semiconductor Solution on Hydrophobic Surfaces for High-Mobility Solution-Processed Organic Transistors

Solution processibility is a unique superiority of organic semiconductors, which provides tremendous potential for the low-cost production of wearable and implantable electronics. However, the solution affinity to the substrate surface remains a serious dilemma due to the difficulty of liquid manipulation on hydrophobic surfaces, but the use of such surfaces is essential to improve device performance. Here, we applied a new-type additive, non-ionic fluorosurfactant, to add into the organic semiconductor solution. It not only can effectively improve the wettability of the low-viscosity solution on the hydrophobic surface, but also enhance the field-effect performance of the fabricated poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b']dithophen-2-yl)-alt-[1, 2, 5]- thiadiazolo[3,4-c]pyridine] organic thin-film transistors (OTFTs) by forming vertical phase separation. Combined with the nanogrooved octadecyltrichlorosilane/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , the obtained OTFTs exhibit excellent field-effect characteristics with mobility as high as 4.02 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , which far surpasses that of modern industrial amorphous silicon. This work provides a simple strategy to prepare continuous and uniform organic semiconductor thin film on the hydrophobic surface, indicating promising potential for future high-performance solution-processed organic electronic devices.

Foldable semi-ladder polymers: novel aggregation behavior and high-performance solution-processed organic light-emitting transistors.

A critical issue in developing high-performance organic light-emitting transistors (OLETs) is to balance the trade-off between charge transport and light emission in a semiconducting material. Although traditional materials for organic light-emitting diodes (OLEDs) or organic field-effect transistors (OFETs) have shown modest performance in OLET devices, design strategies towards high-performance OLET materials and the crucial structure–performance relationship remain unclear. Our research effort in developing cross-conjugated weak acceptor-weak donor copolymers for luminescent properties lead us to an unintentional discovery that these copolymers form coiled foldamers with intramolecular H-aggregation, leading to their exceptional OLET properties. An impressive external quantum efficiency (EQE) of 6.9% in solution-processed multi-layer OLET devices was achieved.

Wettability control with self-assembler patterning for printed electronics

We investigated the lyophobic/lyophilic patterning method of self-assembled monolayers (SAMs) as gate dielectrics for printable electronics. Lyophobic/lyophilic patterning has been previously shown to damage the substrate surface. Therefore, it is mainly used for electrodes, but not semiconductors. We reduced the damage by partially removing the alkyl chains of the lyophobic SAM with oxygen plasma or vacuum ultraviolet treatment and replacing them with lyophilic molecules. This lyophilic area was formed in a room-temperature process, and we obtained a solution-processed organic transistor with a mobility of 2 × 10−2 cm2 V−1 s−1 at 2.5 V operation. The transistor retained 54% of its mobility without the SAM replacement. By integrating a p-type transistor prepared in the same way, we achieved 2 V operation of an organic inverter and ring oscillator with gain of 30 and oscillation frequency of 3.1 Hz.

Solution-Processed Nonvolatile Organic Transistor Memory Based on Semiconductor Blends.

Solution-processed nonvolatile organic transistor memory devices are fabricated by employing semiconductor blends of p-channel 6,13-bis(triisopropylsilylethynyl)pentacene and n-channel poly{[ N, N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]- alt-5,5'-(2,2'-bithiophene)} (P(NDI2OD-2T); N2200) on polystyrene-brush as a polymer electret. Electret-based memory characteristics are significantly changed depending on the frontier molecular orbitals of the active semiconductors because the charge-trapping efficiency is mainly determined by the energy barrier to transfer electrons and holes from the active channel to the electret layer. A semiconductor mixture with an optimized blending ratio results in an efficient programming and erasing process. Thus, we obtained a remarkably high ratio of ON/OFF current (memory ratio) about 107 and a large amount of shifts in the threshold voltage (memory window) between the programmed and erased states of 55 V, while single-component N2200 showed only writing-once-read-many (WORM)-type memory. Especially, the programmed data can be stably retained more than 10 years with a sufficient memory ratio of 103. Furthermore, our semiconductor blend system leads to preferable vertical phase separation, which affords good reliability under a sequential memory operation condition as well as stability in ambient air. It is expected that our memory devices can be applied for versatile data storage in printed and flexible electronic applications.

Optical memory characteristics of solution-processed organic transistors with self-organized organic floating gates for printable multi-level storage devices

Abstract Exploring optical memory functions in nonvolatile organic field-effect transistor (OFET) memories with top-gate/bottom-contact (TG/BC) configurations can offer effective routes for developing printable, high-density organic memory circuits capable of multi-level data storage. Here, we use a solution process to fabricate TG/BC OFET devices with organic floating-gate structures and investigate their memory characteristics under light illumination. A solution-processable organic composite of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) and poly(methylmethacrylate) is employed to self-organize organic floating-gate structures on a solution-processed semiconductor layer composed of poly(3-hexylthiophene) (P3HT). The floating-gate OFET devices programmed with blue, green, and red light exhibit large threshold voltage (Vth) shifts of approximately 30 V and stable charge retention characteristics even under light illumination. The devices also exhibit high sensitivity to incident light during programming, and the degree of Vth shift and the on-state current can be tuned using light and programming voltage to facilitate distinct storage and readout of multi-level data.

Solution-Processed Organic and Halide Perovskite Transistors on Hydrophobic Surfaces.

Solution-processable electronic devices are highly desirable due to their low cost and compatibility with flexible substrates. However, they are often challenging to fabricate due to the hydrophobic nature of the surfaces of the constituent layers. Here, we use a protein solution to modify the surface properties and to improve the wettability of the fluoropolymer dielectric Cytop. The engineered hydrophilic surface is successfully incorporated in bottom-gate solution-deposited organic field-effect transistors (OFETs) and hybrid organic-inorganic trihalide perovskite field-effect transistors (HTP-FETs) fabricated on flexible substrates. Our analysis of the density of trapping states at the semiconductor-dielectric interface suggests that the increase in the trap density as a result of the chemical treatment is minimal. As a result, the devices exhibit good charge carrier mobilities, near-zero threshold voltages, and low electrical hysteresis.

Solution-processed single-crystalline organic transistors and their device applications

Solution-processed single-crystalline organic transistors and their device applications

Graphene as an electrode for solution-processed electron-transporting organic transistors.

Organic field-effect transistors (OFETs) are fundamental building blocks for plastic electronics such as organic photovoltaics or bendable displays with organic light emitting diodes, and radio-frequency identification (RFID) tags. A key part in the performance of OFET is the organic material constituting the channel. OFETs based on solution-processed polymers represent a new class of organic electronic devices. Recent developments in upscale solution-processed polymers have advanced towards high throughput, low-cost, and environmentally friendly materials for high-performance applications. Together with the integration of high performance materials, another enduring challenge in OFET development is the improvement and control of the injection of charge carriers. Graphene, a two-dimensional layer of covalently bonded carbon atoms, is steadily making progress into applications relying on van der Waals heterointerfaces with organic semiconductors. Here, we demonstrate the versatile operation of solution-processed organic transistors both in lateral and vertical geometries by exploiting the weak-screening effect and work function modulation properties of graphene electrodes. Our results demonstrate a general strategy for overcoming traditional noble metal electrodes and to integrate graphene with solution-processed Polyera ActiveInk™ N2200 polymer transistors for high-performance devices suitable for future plastic electronics.

Small Molecule/Polymer Blend Organic Transistors with Hole Mobility Exceeding 13 cm(2) V(-1) s(-1).

A ternary organic semiconducting blend composed of a small-molecule, a conjugated polymer, and a molecular p-dopant is developed and used in solution-processed organic transistors with hole mobility exceeding 13 cm(2) V(-1) s(-1) (see the Figure). It is shown that key to this development is the incorporation of the p-dopant and the formation of a vertically phase-separated film microstructure.

Solvent-assisted reduction in the lateral leakage current in solution-processed organic transistors

We report an effective method for bringing about a significant reduction in the lateral leakage current passing through the non-channel region of an organic semiconductor through an organic solvent vapor treatment. The hydroxyl-containing dimethyl-sulfoxide molecules interrupt the bulk charge transport after undergoing solvent vapor treatment and reduce the lateral leakage current effectively, but the channel current in the solution-processed organic thin-film transistors is not degraded. The lateral leakage current was found to be 20 times lower compared with the case of the non-treated device. A highly improved on/off current ratio was achieved by reducing the lateral leakage current through a solvent vapor treatment.

Control of Threshold Voltage for Top-Gated Ambipolar Field-Effect Transistor by Gate Buffer Layer

The threshold voltage and onset voltage for p-channel and n-channel regimes of solution-processed ambipolar organic transistors with top-gate/bottom-contact (TG/BC) geometry were effectively tuned by gate buffer layers in between the gate electrode and the dielectric. The work function of a pristine Al gate electrode (-4.1 eV) was modified by cesium carbonate and vanadium oxide to -2.1 and -5.1 eV, respectively, which could control the flat-band voltage, leading to a remarkable shift of transfer curves in both negative and positive gate voltage directions without any side effects. One important feature is that the mobility of transistors is not very sensitive to the gate buffer layer. This method is simple but useful for electronic devices where the threshold voltage should be precisely controlled, such as ambipolar circuits, memory devices, and light-emitting device applications.