Flexible Organic Thin-film Transistors Research Articles

Parylene-C-based flexible organic thin-film transistors and their reliability improvement using SU-8 passivation

Flexible and biocompatible organic thin-film transistors (OTFTs) can be well-suited for biological applications due to their compatibility with biomaterials. In this study, flexible OTFTs were fabricated with a Parylene-C substrate and gate dielectric, a material known for its flexibility and biocompatibility. We used poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] as organic channel material. To ensure the longevity and protection of the channel, SU-8, known for its biocompatibility and transparency, effectively safeguarded the OTFT and ensured its sustained operation. Flexible OTFTs were affixed to a curved fixture, referred to as a “curved condition.” The device parameters at −20 V of VD in the curved condition shows an Ion/off ratio of 3.5 × 104, threshold voltage (VTH) of −0.42 V, and mobility of 0.003 cm2/V s. The Parylene-C-based OTFT with SU-8 passivation demonstrated reliability by maintaining performance under curved conditions for 40 days. The results show that the proposed device is suitable for flexible electronics and sensor applications.

Analysis of electrical and hysteresis characteristics of flexible OTFT using solution-processable DPP-DTT polymer and Parylene-C

Organic thin-film transistors (OTFTs) fabricated on Parylene-C substrates have the advantages of a simple process, low cost, and flexible characteristics. This study introduces the manufacturing method and electrical characteristics of an OTFT using organic materials as the substrate, gate dielectric, and channel material. PDPP2T-TT-OD(DPP-DTT) is used as the channel material, which is a highly mobile p-type polymer with good air stability. The proposed OTFT device has flexible characteristics because it is fabricated on a Parylene-C substrate and can be used even in a curved state. Furthermore, the manufacturing process was largely achieved via a simple, low-cost solution process using spin-coating and photolithography with a photo-curable material. Under flat conditions, the threshold voltage (VTH) is approximately –3 V, the average ION/IOFF ratio is approximately 105, and the mobility is 0.84 cm2/Vs.

Low-voltage operating, high mobility top-gate structural flexible organic thin-film transistor with a one-step spin-coated binary polymer gate dielectric

Low-voltage operation is one of the prerequisites for the practical applications of the organic thin-film transistors (OTFTs). Up to date, the most reported low-voltage operatable OTFTs use a bottom-gate structure, and are fabricated by several different technologies in the whole process, in which the organic semiconductors and/or gate dielectrics are prepared in the expensive vacuum equipment. The simple fabricating technologies and fewer processes can better demonstrate the inherent advantages, and enhance the commercial competitiveness of OTFTs. Here, we propose a strategy to fabricate the binary polymers gate dielectric by one-step spin-coating in the top-gate structured flexible OTFTs, by which not only the device performances are prominently improved, but also the fabricating process of the OTFTs is minimized. As a result, the flexible OTFTs exhibit a high mobility over 0.5 cm2 Vs−1, low threshold voltage near to −0.5 V, and excellent mechanical bending durability with a very slightly performances degradation after the tensile and compressive bending at a small curvature radius of 3.0 mm over 1000 cycles.

Large‐Area Processable Ultrathin Organic Transistors with High Mobility and Mechanical Stabilities

AbstractFor applications involving the implantation of organic thin‐film transistors (OTFTs) in flexible environments, maintaining uniform device performance and ultrathin profiles with excellent mechanical stability is crucial for dynamic deformable conditions. This study presents high‐performance, mechanically flexible ultrathin OTFTs with a thickness under 5 µm, designed to be scalable for utilization in large areas. The OTFTs utilized an organosilicon polymer dielectric layer synthesized via initiated chemical vapor deposition combined with the crystalline organic semiconductor 2‐Decyl‐7‐phenyl[1]benzothieno[3,2‐b][1]benzothiophene (Ph‐BTBT‐10). Compared with conventional organic transistors, the OTFTs exhibited exceptional features, such as a charge carrier mobility of up to 7 cm2 V−1 s−1 and a near‐zero threshold voltage. Their performance is showcased across 900 devices on a 10 cm × 10 cm substrate to ensure uniformity and scalability. Additionally, the OTFTs demonstrate remarkable resilience to mechanical deformation, with a 97.4% mobility retention rate under a 20% tensile compression. After 1000 cycles of repeated deformation stress, they maintain a robust 94.6% performance level compared with the initial state. In summary, the ultrathin flexible OTFTs exhibit exceptional resilience to mechanical deformation, extended air exposure, uniformity in large arrays, and high mobility. This makes them promising for diverse applications, such as in vivo organ monitoring and skin displays.

All Electrohydrodynamic Printed Flexible Organic Thin Film Transistors

AbstractThe demand of cost‐effective fabrication of printed flexible transistors has dramatically increased in recent years due to the need for flexible interface devices for various application including e‐skins, wearables, and medical patches. In this study, electrohydrodynamic (EHD) printing processes are developed to fabricate all the components of polymer‐based organic thin film transistors (OTFTs), including source/drain and gate electrodes, semiconductor channel, and gate dielectrics, which streamline the fabrication procedure for flexible OTFTs. The flexible transistors with top‐gate‐bottom‐contact configuration are fabricated by integrating organic semiconductor (i.e., poly(3‐hexylthiophene‐2,5‐diyl) blended with small molecule 2,7‐dioctyl[1]benzothieno[3,2‐b][1]benzothiophene), conductive polymer (i.e., poly (3,4‐ethylenedioxythiophene) polystyrene sulfonate), and ion‐gel dielectric. These functional inks are carefully designed with orthogonal solvents to enable their compatible printing into multilayered flexible OTFTs. The EHD printing process of each functional component is experimentally characterized and optimized. The fully EHD‐printed OTFTs show good electrical performance with mobility of 2.86 × 10−1 cm2 V−1 s−1 and on/off ratio of 104, and great mechanical flexibility with small mobility change at bending radius of 6 mm and stable transistor response under hundreds of bending cycles. The demonstrated all printing‐based fabrication process provides a cost‐effective route toward flexible electronics with 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.

Optimization of Alkyl Side Chain Length in Polyimide for Gate Dielectrics to Achieve High Mobility and Outstanding Operational Stability in Organic Transistors

Alkyl chain modification strategies in both organic semiconductors and inorganic dielectrics play a crucial role in improving the performance of organic thin-film transistors (OTFTs). Polyimide (PI) and its derivatives have received extensive attention as dielectrics for application in OTFTs because of flexibility, high-temperature resistance, and low cost. However, low-temperature solution processing PI-based gate dielectric for flexible OTFTs with high mobility, low operating voltage, and high operational stability remains an enormous challenge. Furthermore, even though di-n-decyldinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (C10-DNTT) is known to have very high mobility as an air-stable and high-performance organic semiconductor, the C10-DNTT-based TFTs on the PI gate dielectrics still showed relatively low mobility. Here, inspired by alkyl side chain engineering, we design and synthesize a series of PI materials with different alkyl side chain lengths and systematically investigate the PI surface properties and the evolution of organic semiconductor morphology deposited on PI surfaces during the variation of alkyl side chain lengths. It is found that the alkyl side chain length has a critical influence on the PI surface properties, as well as the grain size and molecular orientation of semiconductors. Good field-effect characteristics are obtained with high mobilities (up to 1.05 and 5.22 cm2/Vs, which are some of the best values reported to date), relatively low operating voltage, hysteresis-free behavior, and high operational stability in OTFTs. These results suggest that the strategy of optimizing alkyl side-chain lengths opens up a new research avenue for tuning semiconductor growth to enable high mobility and outstanding operational stability of PI-based OTFTs.

High-gain, low-voltage unipolar logic circuits based on nanoscale flexible organic thin-film transistors with small signal delays.

One of the circuit topologies for the implementation of unipolar integrated circuits (circuits that use either p-channel or n-channel transistors, but not both) is the zero-VGS architecture. Zero-VGS circuits often provide excellent static performance (large small-signal gain and large noise margins), but they suffer from the large signal delay imposed by the load transistor. To address this limitation, we have used electron-beam lithography to fabricate zero-VGS circuits based on organic transistors with channel lengths as small as 120 nm on flexible polymeric substrates. For a supply voltage of 3 V, these circuits have characteristic signal-delay time constants of 14 ns for the low-to-high transition and 560 ns for the high-to-low transition of the circuit's output voltage. These signal delays represent the best dynamic performance reported to date for organic transistor-based zero-VGS circuits. The signal-delay time constant of 14 ns is also the smallest signal delay reported to date for flexible organic transistors.

A Sub-20nm Organic/Inorganic Hybrid Dielectric for Ultralow-Power Organic Thin-Film Transistor (OTFT) With Enhanced Operational Stability.

Organic/inorganic hybrid materials are utilized extensively as gate dielectric layers in organic thin-film transistors (OTFTs). However, inherently low dielectric constant of organic materials and lack of a reliable deposition process for organic layers hamper the broad application of hybrid dielectric materials. Here, a universal strategy to synthesize high-k hybrid dielectric materials by incorporating a high-k polymer layer on top of various inorganic layers generated by different fabrication methods, including AlOx and HfOx , is presented. Those hybrid dielectrics commonly exhibit high capacitance (>300 nF·cm-2 ) as well as excellent insulating properties. A vapor-phase deposition method is employed for precise control of the polymer film thickness. The ultralow-voltage (<3V) OTFTs are demonstrated based on the hybrid dielectric layer with 100% yield and uniform electrical characteristics. Moreover, the exceptionally high stability of OTFTs for long-term operation (current change less than 5% even under 30 h of voltage stress at 2.0 MV·cm-1 ) is achieved. The hybrid dielectric is fully compatible with various substrates, which allows for the demonstration of intrinsically flexible OTFTs on the plastic substrate. It is believed that this approach for fabricating hybrid dielectrics by introducing the high-k organic material can be a promising strategy for future low-power, flexible electronics.

Gold nanowire electrodes for flexible organic thin-film transistors

A flexible organic thin-film transistor (OTFT) with gold nanowire (AuNW) networks as source/drain/gate electrodes (AuNW-OTFT) was fabricated and characterized. Its fabrication was enabled by the successful formation of AuNW network electrodes on a flexible thin-film substrate using the thermal pressing transfer method. The fabricated AuNW-OTFT exhibited typical p-type characteristics (mobility: 6.0 × 10−2 cm2 V−1 s−1, on/off ratio: 105) and a lower contact resistance than that of an OTFT with vacuum-deposited Au electrodes in the same configuration. Furthermore, the AuNW-OTFT demonstrated high mechanical durability, as proven by the absence of significant characteristic changes before and after bending (bending radius:0.8 mm).

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.

Highly stable, low-voltage operable high-mobility flexible organic thin-film transistors based on a tri-layer gate dielectric

The application fields of organic thin-film transistors (OTFTs) depend on their field-effect mobility (μ). The low-voltage operation and good operating stability are the prerequisites for the practical applications of OTFTs. To date, it is still a huge challenge to obtain low-voltage operable OTFTs with high μ and good operating stability at the same time. In this work, we elaborately design and process a tri-layer gate dielectric, for pursuing the low-voltage operable, high-performance flexible OTFTs. The effects of the gate dielectrics on the device performances and the relevant mechanisms are discussed in detail. The tri-layer gate dielectric has a large gate capacitance per unit area, and simultaneously suppresses the polarization effect of the gate dielectric and the charge trapping at the interface and bulk of the gate dielectric, by which both the μ and the operating stability are significantly improved. As a result, the flexible OTFTs exhibit excellent figures of merit at low operating voltages below 4 V, with high μ up to 2.6 cm2 Vs−1, good bias stress stability with a negligible change of the channel current for 5000 s, and good mechanical flexibility with a negligible performance degradation after the tension bending for 1000 times at small curvature radii of both 5.0 mm and 3.0 mm in sequence. Our works provide a strategy for developing the high-performance flexible OTFTs with low-voltage operation, high μ and good operating stability simultaneously.

Subthreshold Swing of 59 mV decade−1 in Nanoscale Flexible Ultralow‐Voltage Organic Transistors

AbstractOrganic thin‐film transistors (TFTs) that provide subthreshold swings near the theoretical limit together with large on/off current ratios at very low operating voltages require high‐capacitance gate dielectrics with a vanishingly small defect density. A promising approach to the fabrication of such dielectrics at temperatures sufficiently low to allow TFT fabrication on polymeric substrates are hybrid dielectrics consisting of a thin metal oxide layer in combination with a molecular self‐assembled monolayer (SAM). Here, the electrical and surface properties of titanium oxide produced by the plasma‐assisted oxidation of the surface of vacuum‐deposited titanium gate electrodes and its use as the first component of a hybrid TiOx/SAM gate dielectric in flexible organic TFTs are investigated. These transistors have a gate‐dielectric capacitance of about 1 µF cm−2, a subthreshold swing of 59 mV decade−1 (within measurement error of the physical limit at room temperature) for a wide range of channel lengths as small as 0.7 µm, and an on/off current ratio of 107 for a gate‐source‐voltage range of 1 V.

Sequential manufacturing via intra-additive hybrid materials approach for fully roll-to-roll processed flexible organic thin film transistors

Conventional conductive films that are used as electrodes in organic thin film transistors (OTFTs) are susceptible to mechanical deformation, limiting the applicability of such films in flexible wearable electronic devices. To address this limitation, a hybrid conductive film (HCF) consisting of silver nanowires (NWs) and poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS) fabricated through roll-to-roll (R2R) slot-die coating is presented for use as a gate electrode in OTFTs. Mechanical stability compares by bending and taping test measurement quantitatively. Coating flow rate significantly influences sheet resistance and transmittance. A performance evaluation of the OTFTs with the Ag NW/PEDOT:PSS HCF as the gate electrode shows that the charge carrier mobility and on/off current ratio are 2.168 cm2 V−1 s−1 and 6 × 105, respectively. In addition, the HCF is highly flexible, and the HCF-based OTFTs exhibit 57% less mobility reduction than a conventional OTFT based on silver ink after 50,000 bending cycles with a 2 mm bending radius. These results highlight the potential of the fabricated HCF as an electrode material for flexible electronic devices.

Electrohydrodynamic-Jet-Printed Cinnamate-Fluorinated Cross-Linked Polymeric Dielectrics for Flexible and Electrically Stable Operating Organic Thin-Film Transistors and Integrated Devices.

Herein, printable polymer series containing different portions of cinnamate and perfluorinated phenyl functionalities, namely, polyperfluorostyrene-co-poly(vinylbenzyl cinnamates) (PFS-co-PVBCi (x:y)) copolymers, were synthesized and applied as gate dielectrics for organic thin-film transistors (OTFTs). The polymeric dielectrics were successfully printed via electrostatic force-assisted dispensing mode of electrohydrodynamic jet printing. The dielectric characteristics of the printed polymers, such as surface energy, dielectric constant, leakage current, atomic depth profiles, and deposited semiconducting layer characteristics, were clearly identified. In particular, the difference in driving stability of OTFTs according to the type of polymer was analyzed in detail and a possible mechanism was proposed. Results suggested that PFS-co-PVBCi (3:7) led to optimized consequences, yielding an almost negligible Vth shift under continuous bias stress. Through this, we successfully implemented flexible OTFT and logic devices using printed PFS-co-PVBCi (3:7) dielectrics with stable operation properties. Therefore, we believe that this study will facilitate the printing and synthesis of polymer dielectrics to produce printed and flexible OTFTs.

Flexible Organic Thin-Film Transistors With High Mechanical Stability on Polyimide Substrate by Chemically Plated Silver Electrodes

Conventionally, silver (Ag) electrodes for organic thin-film transistors (OTFTs) are prepared by evaporated method at high temperature. In this work, we report a new methodology to fabricate the Ag source/drain electrodes of flexible OTFTs arrays by chemically plated technique on polyimide substrate at room temperature. The indacenodithiophene-co-benzothiadiazole (IDT-BT) OTFTs with chemically plated electrodes obtain the saturation mobility of 0.25 cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{2}\text{V}^{-1}\text{s}^{-1}$ </tex-math></inline-formula> . Furthermore, compared with high-temperature thermal evaporation method, the subthreshold swing of the IDT-BT OTFTs decreases from 1.97 to 0.89 V/dec with chemically plated Ag source/drain electrodes. The smaller threshold voltage and contact resistance are also obtained, owing to the change of work function. The bending tests indicate that the electrical properties of the devices maintain unchanged during tensile bending at a radius up to 1.4 mm. Moreover, the electrical property of the devices remains stable at a tensile bending cycle of 1000 cycles at a radius of 2.5 mm. This proposed methodology has great potential for flexible and wearable electronic devices.

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.

Fröhlich polaron effect in flexible low-voltage organic thin-film transistors gated with high-k polymer dielectrics

The dielectric constant of the polymer insulator layer plays a key role in the charge transport and operation voltage of organic thin-film transistors (OTFT). However, the appropriate effects have not been well distinguished yet in donor-acceptor copolymer semiconductor-based OTFT. In this work, we demonstrate that the decreased mobilities in indacenodithiophene-co-benzothiadiazole OTFTs with the increase of gate dielectric constant is due to the Fröhlich polaron effect, especially for device gated with high-k relaxor ferroelectric polymer poly(vinylidene fluoride-trifluoroethylene-chlorofloroethylene). In order to eliminate this effect, a low-k polymer is introduced as a modification layer to construct a low-k/high-k bilayer dielectric to attain better mobility and improved bias stability. Finally, flexible top-gate OTFT is demonstrated on the polyethylene terephthalate substrate, showing good bending stability even after 1000 bending tests.

Flexible low-voltage organic thin-film transistors and PMOS inverters: the effect of channel width on noise margin

Low-operating-voltage (<2 V) organic thin-film transistors (OTFTs) and P-channel metal-oxide-semiconductor (PMOS) inverter circuits are fabricated on a 125 µm-thick flexible polyethylene naphthalate substrate using a blend of 2,7-dihexyl-dithieno[2,3-d;2′,3′-d′]benzo[1,2-b;4,5-b′]dithiophene and polystyrene as an active p-type organic semiconducting material. All three electrodes (gate, source, and drain) are inkjet-printed, while the active semiconducting material is deposited by a dispenser system to achieve a saturation mobility of 0.32 cm2 V−1 s−1 at V GS = −2 V. Two different PMOS inverters are fabricated, for which the signal gain peak values are resolved for an ultra-low supply voltage, V DD = −0.5 V. We achieve a signal gain of 2.73 at V DD = −0.5 V. The effect of channel width is demonstrated for both OTFTs and PMOS devices. The ‘on’ current increases with channel width, and the switching point of the PMOS inverters shifts toward the middle of the voltage transfer characteristics, and hence improves the noise margin.

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.