High-performance Organic Thin-film Transistors Research Articles

A high-performance organic thin-film transistor with Parylene/PMMA bilayer insulation based on P3HT

This work introduces a pioneering approach in the development of organic thin-film transistors (OTFTs),featuring a double-layer dielectric structure that combines poly(para-xylylene)s (Parylene) and poly(methyl methacrylate) (PMMA) to leverage the high insulation properties and high surface polarity of Parylene with the low insulation properties and low surface polarity of PMMA. This combination results in devices that showcase significantly enhanced electrical performance, including superior charge carrier mobility, increased current on/off ratios, and greater transconductance. Utilizing poly(3-hexylthiophene) (P3HT) for the active layer, the study demonstrates the advantage of the dual dielectric layers in minimizing hysteresis in the transfer curve, thereby facilitating the systematic growth of the organic active layer and enhancing electrical conductivity over single-layer alternatives. The superior performance of the Parylene/PMMA double-layer insulating structure opens new avenues for the advancement of organic electronics, presenting methodologies for performance optimization and expanding the application spectrum of OTFTs.

High-performance organic thin-film transistors: principles and strategies

Organic thin film transistors (OTFTs) mark a breakthrough in flexible electronics, offering advantages over traditional inorganic semiconductors through their adaptability, cost efficiency, and low-temperature production.

Photolithographic Rugged Electrode for High-Density Low-Contact-Resistance Coplanar Organic Transistors.

The realization of high-performance photolithographic coplanar organic thin film transistors (OTFTs) is fundamental to boost cosmicallycommercial applications of organic electronics. However, photolithographic coplanar OTFTs generally suffer from poor charge injection and therefore poor filed-effect performance. Here, a simple and effective strategy is developed to fabricate photolithographic rugged electrodes, and successfully achieve high-density low-contact-resistance photolithographic coplanar OTFTs. Based on this versatile electrode, the wafer-scale photolithographic rugged electrode can be easily achieved, and the device density of the coplanar OTFTs is as high as 28000cm-2 . The device shows excellent electrical properties with mobility up to 2.01cm2 V-1 s-1 and Rc as low as 7.8kΩcm, which is superior to all the reported Ag-electrode coplanar OTFTs. This work shows a reliable strategy to reduce the contact resistance of photolithographic coplanar OTFTs and elucidates the effect of injection resistance (Rinj ) and access resistance (Racc ) on coplanar OTFTs.

The Effect of Alkyl Chain Length in Organic Semiconductor and Surface Polarity of Polymer Dielectrics in Organic Thin-Film Transistors (OTFTs).

The interface between dielectric and organic semiconductor is critically important in determining organic thin-film transistor (OTFT) performance. Surface polarity of the dielectric layer can hinder charge transport characteristics, which has restricted utilization of polymeric dielectric materials containing polar functional groups. Herein, the electrical characteristics of OTFTs are analyzed depending on the alkyl chain length of organic semiconductors and surface polarity of polymer dielectrics. High-performance dibenzothiopheno[6,5-b:6',5'-f]thieno[3,2-b]thiophene (DBTTT) and newly synthesized its alkylated derivatives (C6-DBTTT and C10-DBTTT) are utilized as organic semiconductors. As dielectric layers, non-polar poly(1,3,5-trimethyl-1,3,5-trivinylcyclitrisiloxane) (pV3D3) and poly(2-cyanoethyl acrylate-co-diethylene glycol divinyl ether) [p(CEA-co-DEGDVE)] with polar cyanide functionality are utilized. The fabricated OTFTs with pV3D3 commonly exhibit the excellent charge transport characteristics. In addition, the OTFT performance is improved with lengthening the alkyl chain in organic semiconductors, which can be attributed to the molecular orientation of semiconductors. On the other hand, non-alkylated DBTTT OTFTs with polar p(CEA-co-DEGDVE) show relatively poor electrical characteristics, while their performance is drastically enhanced with the alkylated DBTTTs. The ultraviolet photoelectron spectroscopy (UPS) reveals that surface polarity of the dielectric layer can be abated with alkyl chain in organic semiconductors. It is believed that this study can provide a useful insight to optimize dielectric/semiconductor interface to achieve high-performance OTFTs.

Molecular layer modulation of two-dimensional organic ferroelectric transistors

Ferroelectric transistors hold great potential in low consumption devices. Due to the high film quality and clean system, two dimensional organic semiconductors are widely employed to fabricate high performance organic electronic devices and explore the modulation mechanism of the molecular packing on device performance. Here, we combine the ferroelectric hafnium oxide HfZrO x and two-dimensional molecular crystal 2,9-didecyldinaphtho[2,3-b:2′,3′-f]thieno[3,2b]thiophene (C10-DNTT) with controllable layers to study the molecular layer modulation of ferroelectric organic thin-film transistors (OTFTs). The contact resistance, driving current and transconductance are directly affected by the additional access resistance across the upper molecular layers at the source/drain contact region. Simultaneously, the capacitance of Schottky junction related to the molecular layer thickness could effectively adjust the gate potential acting on the organic channel, further controlling the devices’ subthreshold swing and transconductance efficiency. This work would promote the development of low voltage and high performance OTFTs.

Self-Doping Naphthalene Diimide Conjugated Polymers for Flexible Unipolar n-Type OTFTs.

The development of high-performance organic thin-film transistor (OTFT) materials is vital for flexible electronics. Numerous OTFTs are so far reported but obtaining high-performance and reliable OTFTs simultaneously for flexible electronics is still challenging. Herein, it is reported that self-doping in conjugated polymer enables high unipolar n-type charge mobility in flexible OTFTs, as well as good operational/ambient stability and bending resistance. New naphthalene diimide (NDI)-conjugated polymers PNDI2T-NM17 and PNDI2T-NM50 with different contents of self-doping groups on their side chains are designed and synthesized. The effects of self-doping on the electronic properties of resulting flexible OTFTs are investigated. The results reveal that the flexible OTFTs based on self-doped PNDI2T-NM17 exhibit unipolar n-type charge-carrier properties and good operational/ambient stability thanks to the appropriate doping level and intermolecular interactions. The charge mobility and on/off ratio are fourfold and four orders of magnitude higher than those of undoped model polymer, respectively. Overall, the proposed self-doping strategy is useful for rationally designing OTFT materials with high semiconducting performance and reliability.

Diketopyrrolopyrrole-based Conjugated Polymers as Representative Semiconductors for High-Performance Organic Thin-Film Transistors and Circuits

Diketopyrrolopyrrole-based Conjugated Polymers as Representative Semiconductors for High-Performance Organic Thin-Film Transistors and Circuits

Poly(ionic liquid) Gating Materials for High-Performance Organic Thin-Film Transistors: The Role of Block Copolymer Self-Assembly at the Semiconductor Interface.

The widespread realization of wearable electronics requires printable active materials capable of operating at low voltages. Polymerized ionic liquid (PIL) block copolymers exhibit a thickness-independent double-layer capacitance that makes them a promising gating medium for the development of organic thin-film transistors (OTFTs) with low operating voltages and high switching speed. PIL block copolymer structure and self-assembly can influence ion conductivity and the resulting OTFT performance. In an OTFT, self-assembly of the PIL gate on the semiconducting polymer may differ from bulk self-assembly, which would directly influence electrical double-layer formation. To this end, we used poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (P(NDI2OD-T2)) as a model semiconductor for our OTFTs, on which our PILs exhibited self-assembly. In this study, we explore this critical interface by grazing-incidence small-angle X-ray scattering (GISAXS) and atomic force microscopy (AFM) of P(NDI2OD-T2) and a series of poly(styrene)-b-poly(1-(4-vinylbenzyl)-3-butylimidazolium-random-poly(ethylene glycol) methyl ether methacrylate) (poly(S)-b-poly(VBBI+[X]-r-PEGMA)) block copolymers with varying PEGMA/VBBI+ ratios and three different mobile anions (where X = TFSI-, PF6-, or BF4-). We investigate the thin-film self-assembly of block copolymers as a function of device performance. Overall, a mixed orientation at the interface leads to improved device performance, while predominantly hexagonal packing leads to nonfunctional devices, regardless of the anion present. These PIL gated OTFTs were characterized with a threshold voltage below 1 V, making understanding of their structure-property relationships crucial to enabling the further development of high-performance gating materials.

Solution-processable and photocurable aromatic polyurea gate dielectrics for high-performance organic thin-film transistors

A solution-processable and photocurable polyurea (PU) with excellent electrical and dielectric properties is synthesized, characterized, and applied as a gate dielectric in high-performance organic thin-film transistors (OTFTs). The PU is synthesized from toluene-2,4-diisocyanate and 3,5-diaminobenzyl cinnamate via a condensation reaction. The PU containing cinnamoyl groups is photo-crosslinked without a photoinitiator and easily micropatterned by selective ultra-violet (UV) exposure. The photo-crosslinked PU (c-PU) film improves dielectric properties and thermal and chemical resistance compared with pristine PU films. We fabricated dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT)-based OTFTs with a c-PU gate dielectric layer to investigate the performance of the c-PU thin film as a gate dielectric. The field-effect mobility and on/off ratio of DNTT-based OTFTs with surface-treated c-PU gate dielectric are 0.75 ± 0.03 cm2 V−1s−1 and 1.60 × 106, respectively. In addition, solution-processed diketopyrrolopyrrole-thienothiophene copolymer-based OTFTs with c-PU gate dielectric show field-effect mobility of 0.07 ± 0.01 cm2 V−1s−1 and on/off ratio of 2.88 × 105. Furthermore, the complementary inverter was successfully demonstrated by using c-PU with excellent insulating property as the gate dielectric.

Low-voltage-operation of flexible organic C8-BTBT thin-film transistors with a reactively sputtered AlOx gate dielectric

High-k dielectrics are frequently used for organic thin-film transistors (OTFTs), which facilitate the reduction of the device's operating voltage and enhance the total electrical performance. Along these lines, in this work, the fabrication of high-k AlOx dielectrics with high capacitance and low leakage current is proposed. On top of that, low-voltage flexible OTFTs with a solution-processed 2,7-dioctyl benzothieno[3,2-b] benzothiophene channel layer were demonstrated. The AlOx dielectric film was deposited by employing the reactive magnetron sputtering technique from a metal Al target by using a gas mixture of Ar and O2 at room temperature. At the same time, the surface morphology of the semiconductor film was optimized by controlling the solid solubility of polystyrene and polymethyl methacrylate in the semiconductor solution, which is important for improving the device performance. In this way, the prepared flexible OTFTs showed a low operating voltage of 3 V, a high switch ratio of 4.2 × 107, a high mobility is 2.39 cm2/V s, and a steep subthreshold swing close to the theoretical limit of 68 mV/decade. It is, thus, expected that this method will be applicable to the development of high-performance OTFTs.

Improvement of the comprehensive properties of polymer templates via a novel dry blending method for fabricating OTFT electrodes

Polymer template is widely used in the fabrication of organic thin-film transistor (OTFT) electrodes. However, the mechanical/thermal properties of the polymer template are generally poor owing to their low Young’s modulus (E) and high coefficient of thermal expansion (CTE), resulting in the formation of massive wrinkles and cracks on the OTFT electrodes, as well as the electrodes incomplete transfer, which seriously affects the fabrication of the electrodes. Here, a novel dry blending method is described in which SiO2 nanoparticles are filled into a grooved silicon template, followed by permeation of polydimethylsiloxane (PDMS) into the SiO2 nanoparticle gaps. The SiO2 nanoparticles in the groove are extracted by curing and peeling off PDMS, affording a PDMS/SiO2 composite template with a nanoparticle content of 83.8 wt% and good transparency. The composite template has a Young’s modulus of 16.58 MPa and a CTE of 96 ppm/°C, which constitutes an increase of 91.56 % and a reduction of 69.23 %, respectively, compared with the original PDMS template. Finally, the high performance OTFT electrodes were fabricated using the composite template with good comprehensive properties.

Insulating Polymer Blend Organic Thin-Film Transistors Based on Bilayer-Type Alkylated Benzothieno[3,2-b]naphtho[2,3-b]thiophene.

Herein, we developed a practical method to produce high-performance organic thin-film transistors (OTFTs) based on highly layered crystalline organic semiconductors (OSCs) that form bilayer-type layered herringbone (b-LHB) packing and exhibit high intrinsic mobility. We applied the insulating polymer blend technique using a typical b-LHB OSC of 2-octyl-benzothieno[3,2-b]naphtho[2,3-b]thiophene (2-C8-BTNT) and fabricated polycrystalline thin-film transistors (TFTs) via short-duration spin coating and subsequent annealing. The use of blends and the choice of polymer additive strongly affected the performance of the polycrystalline TFTs, and poly(methyl methacrylate) (PMMA) blend TFTs exhibited a high mobility exceeding 4 cm2/(V s) and small device-to-device variations. Using extended techniques in atomic force microscopy (AFM), we investigated the thin-film morphologies by bimodal AFM and the carrier transport properties by Kelvin probe force microscopy (KPFM). We demonstrated that the PMMA blend system enables the formation of a well-ordered polycrystalline thin film induced by vertical phase separation between the OSC and PMMA over a large area, resulting in uniform TFT performance. These findings pave the way for obtaining high-performance TFTs using simple processes, representing a substantial advancement toward the realization of printed electronics.

Low-Temperature-Processed High-Performance Pentacene OTFTs with Optimal Nd-Ti Oxynitride Mixture as Gate Dielectric

When processed at a low temperature of 200 °C, organic thin-film transistors (OTFTs) with pentacene channel adopting high-k Neodymium-Titanium oxynitride mixtures (NdTiON) with various Ti contents as gate dielectrics are fabricated. The Ti content in the NdTiON is varied by co-sputtering a Ti target at 0 W, 10 W, 20 W and 30 W, respectively, while fixing the sputtering power of an Nd target at 45 W. High-performance OTFT is obtained for the 20 W-sputtered Ti, including a small threshold voltage of −0.71 V and high carrier mobility of 1.70 cm2/V·s. The mobility improvement for the optimal Ti content can be attributed to smoother dielectric surface and resultant larger overlying pentacene grains as reflected by Atomic Force Microscopy measurements. Moreover, this sample with the optimal Ti content shows much higher mobility than its counterpart processed at a higher temperature of 400 °C (0.8 cm2/V·s) because it has a thinner gate-dielectric/gate-electrode interlayer for stronger screening on the remote phonon scattering by the gate electrode. In addition, a high dielectric constant of around 10 is obtained for the NdTiON gate dielectric that contributes to a threshold voltage smaller than 1 V for the pentacene OTFT, implying the high potential of the Nd-Ti oxynitride in future high-performance organic devices.

Effects of Blended Poly(3-hexylthiophene) and 6,13-bis(triisopropylsilylethynyl) pentacene Organic Semiconductors on the Photoresponse Characteristics of Thin-Film Transistors

In this study, we demonstrate high-performance optical wavelength-selective organic thin-film transistors (TFTs) that incorporate heterogeneous organic semiconductor materials, poly(3-hexylthiophene) (P3HT) and 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene). The electrical characteristics of the fabricated transistors were analyzed in the dark to determine how the P3HT:TIPS-pentacene ratio of the semiconductor affected the performance of the transistor. Specifically, the P3HT:TIPS-pentacene weight ratio was varied (1:0, 1:0.25, 1:0.5, 1:0.75, 1:1, and 0:1) by blending 1 wt% P3HT dissolved in chloroform, and 1 wt% TIPS-pentacene dissolved in anisole. The UV-visible light absorbance characteristics of the films containing the P3HT, TIPS-pentacene, and P3HT:TIPS-pentacene blends were analyzed. Monochromatic light at wavelengths of 515 and 450 nm was used to clarify the influence of irradiation on the electrical characteristics of the TFTs. The results confirmed that the TFT containing P3HT:TIPS-pentacene at a blending ratio of 1:0.5 had the largest light-to-dark current ratio, i.e., approximately 33.8 and 23.5 when exposed to monochromatic light at wavelengths of 515 nm and 450 nm, respectively. The TFT with the P3HT:TIPS-pentacene blending ratio of 1:0.5 exhibited the highest photosensitivity values of 261.9 and 49.6 upon irradiation with light at wavelengths of 515 nm and 450 nm, respectively. The observed improvement in the performance of the heterogeneously blended organic transistors is discussed in relation to the morphological structure and charge transport path of the P3HT:TIPS-pentacene blended semiconductor films.

Mobility enhancement of DNTT and BTBT derivative organic thin-film transistors by triptycene molecule modification

Interface modification with a particular triptycene molecule that spontaneously forms a highly uniform molecule layer is a promising technique for realizing high-performance flexible organic thin-film transistors (OTFTs). Previous studies have shown that the triptycene-modified polymer surface enhances the crystallinity of the small-molecule organic semiconductor (OSC), DNTT (dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene), resulting in improved mobility. However, to date, the effectiveness of triptycene modification for the other OSCs has not been confirmed. Here, we report on the positive effect of triptycene layers on four different thienoacene-based OSCs (C10-DNTT, DPh-DNTT, C8-BTBT ([1]benzothieno[3,2-b][1]benzothiophene), and DPh-BTBT) in OTFTs. Regardless of the OSC type, triptycene-modified OTFTs are found to improve the field-effect mobility. A maximum effective mobility enhancement of 20-fold was obtained for C10-DNTT OTFTs. Furthermore, detailed observations of the surface morphology of the OSCs via scanning electron microscopy revealed that the crystal grain size of the OSC thin films can be increased when triptycene modification is conducted.

Editorial: High-Performance Organic Thin-Film Transistors

Organic electronics have attracted intensive research interest in the past decades Yao et al., 2020;Tang et al., 2021). The ever-increasing demand for organic electronics calls for the development of high-performance and high-stability organic thin-film transistors (OFETs). OFETs are considered key components to fabricate flexible and printable devices for promising applications, such as flexible displays, chemical sensors, wearable devices, and radio frequency identification (RFID) tags.The design and synthesis of novel structural materials (such as semiconductors, dielectrics, electrodes, and substrates), as well as the development of efficient interface engineering strategies, would further promote the development of flexible electronics and lay the foundation for their early commercialization (Ji et al., 2019;Ji et al., 2021). Overall, the articles published in this research topic cover the device materials for high-performance organic thin-film transistors. We hope that contributions published within this issue will contribute to a new insight into the field of organic electronics, exploring more possibilities of achieving high-performance optoelectronic devices.

Flexible, high mobility short-channel organic thin film transistors and logic circuits based on 4H\u201321DNTT

The low mobility and large contact resistance in organic thin-film transistors (OTFTs) are the two major limiting factors in the development of high-performance organic logic circuits. Here, solution-processed high-performance OTFTs and circuits are reported with a polymeric gate dielectric and 6,6 bis (trans-4-butylcyclohexyl)-dinaphtho[2,1-b:2,1-f]thieno[3,2-b]thiophene (4H–21DNTT) for the organic semiconducting layer. By optimizing and controlling the fabrication conditions, a high saturation mobility of 8.8 cm2 V−1 s−1 was demonstrated as well as large on/off ratios (> 106) for relatively short channel lengths of 15 μm and an average carrier mobility of 10.5 cm2 V−1 s−1 for long channel length OTFTs (> 50 μm). The pseudo-CMOS inverter circuit with a channel length of 15 μm exhibited sharp switching characteristics with a high signal gain of 31.5 at a supply voltage of 20 V. In addition to the inverter circuit, NAND logic circuits were further investigated, which also exhibited remarkable logic characteristics, with a high gain, an operating frequency of 5 kHz, and a short propagation delay of 22.1 μs. The uniform and reproducible performance of 4H–21DNTT OTFTs show potential for large-area, low-cost real-world applications on industry-compatible bottom-contact substrates.

Flexible high-performance organic thin film transistors and PMOS inverters: Trap controlled grain boundaries and contact resistance effect in different channel length devices

Printed high-performance organic thin film transistors (OTFTs) are fabricated on flexible polyethylene naphthalate (PEN) substrate for low-voltage operation. Gate, source and drain electrodes are inkjet-printed using conductive silver ink, while chemical vapor deposition grown parylene is used as dielectric layer. We used a blend of 2,7-dihexyl-dithieno[2,3-d;2′,3′-d′]benzo[1,2-b;4,5-b′]dithiophene (DTBDT-C6) and polystyrene (PS), for active p-type organic semiconducting material, and deposited by a dispenser system to achieve 0.66 cm2/Vs mobility at VGS = −6 V. For a set of 30 OTFT devices, the uniformity in device characteristics is illustrated by plotting the histograms for on current, mobility, and threshold voltage (VTh). Mobility decrease for small channel length due to greater contribution of contact resistance, while negative VTh shift occurs because of the generation of hole traps at dielectric-semiconductor interface attributed to moisture present. Electron traps on grain boundaries of the active semiconducting layer causes positive shift in VTh. By using these high mobility OTFT devices, two different low-voltage operation PMOS (P-channel metal-oxide-semiconductor) inverters are fabricated for which high signal gain value of 24.86 is achieved at VDD = −5 V.

Microstructure control of organic semiconductors via UV-ozone for high-sensitivity NO2 detection

Developing high sensitive organic semiconductors (OSCs) in organic thin-film transistors (OTFTs) is the key for OTFT based gas sensors. Herein, we report a simple processing route of highly sensitive OSCs for high performance OTFT based nitrogen dioxide (NO2) sensors, where the active OSC layer is based on ultraviolet-ozone (UVO) treated poly(3-hexylthiophene-2,5-diyl) (P3HT). Compared to conventional P3HT based OTFT sensors, the reported device exhibits a remarkable improvement of the gas response from 350% to 30000%. The studies in morphologies, chemical compositions and microstructures of the UVO-treated films reveal that a large number of carrier traps generated in the P3HT films is the decisive reason for the enhancement of sensing performance. Moreover, the optimized device shows great potential of practical applications on the stand points of sensitivity, selectivity, reusability and the ability of recovery, as well as limit of detection of ~7.3 ppb. This simple method provides an innovative understanding for the role of the carrier traps in sensing performance and demonstrates a bright future for developing high performance OTFT gas sensors.

High-Performance Pentacene Organic Thin-Film Transistor Based on Room-Temperature- Processed Hf0.13La0.87O as Gate Dielectric

High-performance pentace organic thin-film transistors (OTFTs) with room-temperature-processed Hf0.13 La0.87O as gate dielectric and Pd as gate electrode have been fabricated on both rigid Si and flexible polymide (PI) substrates. The OTFT on PI (Si) can achieve a high carrier mobility of 10.3 cm2V−1s−1 (12.5 cm2V−1s−1), and has a negligible hysteresis of −0.17 V (−0.08 V), small sub-threshold swing of 0.12 V/dec (0.11 V/dec) and low threshold voltage of −0.65 V (−0.86 V). After the devices have been exposed to the air for 30 days without encapsulation, their carrier mobilities degrade by less than 10%, indicating that the devices have good stability in the air.