Organic Thin-film Transistor Array Research Articles

Ultrathin, Flexible, and Reusable Silicon Shadow Masks Manipulated via Transfer Printing

Stencil lithography is a facile patterning technique that offers a potential solution to the limitations of conventional photolithography. However, several critical factors such as enhanced pattern resolution, ability to pattern on nonplanar surfaces, and multilayer patterning should further be addressed to extend the applications of stencil lithography. This study presents a novel ultrathin silicon shadow mask which is flexible, reusable, and able to be precisely aligned and manipulated through transfer printing. The small thickness of the silicon shadow mask inherently provides it with the enhanced resolution patterning on both planar and nonplanar surfaces. To highlight these attractive benefits, substrates are patterned with metal deposition as well as by etching in a multilayer configuration. Moreover, it is found that the pattern resolution which is degraded as the shadow mask is repeatedly coated with metal and becomes warped can be restored after the mask cleaning procedure, demonstrating its effective reusability. Finally, as a device‐level application, top contact/bottom gate organic thin film transistor arrays are fabricated on both planar and curved surfaces using the shadow mask. These results show the promise of stencil lithography as a versatile and reliable high resolution patterning method for various applications by exploiting the presented ultrathin silicon shadow mask.

Fabrication of Large‐Area Organic Thin Film Transistor Array with Highly Uniform Water‐Borne Polyimide Gate Dielectric via Green Solvent‐Engineered Bar‐Coating Process

AbstractHere, the utilization of a highly uniform water‐borne polyimide (W‐PI) thin film as a gate dielectric layer for large‐scale organic thin film transistors (OTFTs) exceeding 100 cm2 is presented, employing the bar‐coating technique. The W‐PI thin films are obtained uniformly over a large area by systematically manipulating the surface tension of the water‐soluble poly(amic acid) salts (W‐PAAS) solution using alcohol as a co‐solvent in a “Green” process. The thickness of the W‐PI thin film is precisely controlled within the range of tens to hundreds of nanometers by adjusting key parameters, including the concentration of the W‐PAAS solution, wire diameter, and bar‐coating speed. As a result, 500 pentacene‐based OTFTs are successfully fabricated on a 10 × 10 cm2 substrate, utilizing a 280 nm thick W‐PI gate dielectric film. These devices exhibit excellent uniformity, with field‐effect mobility of 0.20 ± 0.02 cm2 V−1 s−1. Furthermore, the fabrication of electronic devices using an environmental‐friendly bar‐coating method on large‐area flexible substrates is demonstrated. This study highlights the considerable potential of eco‐friendly W‐PI thin films as dielectric materials, offering advantages such as cost‐effectiveness and high efficiency for reliable industrial applications.

Measurement of 64 organic thin-film transistors in an array test structure using a relay-switch board for efficient evaluation of long-term reliability

We propose an efficient array measurement test structure to measure many devices and to obtain statistical characteristics of organic thin-film-transistors (OTFT) for long-term reliability evaluation of new devices under development in laboratories and those that cannot achieve sufficient yield. We propose to adopt different devices from the device under test for the array control circuit. We implemented a separate control circuit as a dedicated relay-switch board without device consolidation. This relay-switch board can also apply voltage stress to any number of devices under test for negative bias temperature instability evaluation. The interface issue between the chip and board is resolved with a flexible flat cable and an anisotropic conductive film. The proposed measurement system successfully measured the OTFT arrays in (sweep time of source measure unit) × (number of OTFTs) period, and only requires 84 min for 64 nOTFTs (n-type OTFT) (552 points/device), which corresponds to 1.3 min/device in the experiment.

Reconfigurable Electronic Physically Unclonable Functions Based on Organic Thin‐Film Transistors with Multiscale Polycrystalline Entropy for Highly Secure Cryptography Primitives

AbstractIn this study, organic thin‐film transistors (OTFTs) are investigated as a promising platform for cost‐effective, reconfigurable, and strong electronic physically unclonable functions (PUFs) for highly secure cryptography primitives. Simple spin‐casting of solution‐processable small‐molecule organic semiconductors forms unique and unclonable fingerprint thin films with randomly distributed polycrystalline structures ranging from nanoscale molecular orientations to microcrystalline orientations, which provides a stochastic entropy source of device‐to‐device variations for OTFT arrays. Blending organic semiconductors with polymer materials is a promising strategy to improve the reliability of OTFT‐based PUFs. Studies on the relationship between the phase‐separated polycrystalline microstructure of organic semiconductor/polymer blend films and PUF characteristics reveal that the 2D mosaic microcrystalline structure of organic semiconductors in the vertically phase‐separated trilayered structure enables the implementation of OTFT‐based PUFs that simultaneously satisfy the requirements of being unclonable and unpredictable, with reliable cryptographic properties. The inherent multiscale randomness of the crystalline structure allows random distribution in OTFT‐based PUFs even with various channel dimensions. The secret bit stream generated from the OTFT‐based PUF developed in this study is reconfigurable by simply changing the gate bias, demonstrating the potential to counter evolving security attack threats.

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.

Direct fabrication of high-resolution and high-performance flexible electronics via surface-activation-localized electroless plating

Patterning high-resolution multi-metal layers without using subtractive lithography poses a substantial challenge but is indispensable for the development of the modern electronics industry, especially flexible electronics. Herein, the general and feasible additive manufacturing approach is reported to selectively deposit high-resolution multi-metal patterns via surface-activation-localized electroless plating (SALEP) and enable the fabrication of 3-dimensional (3D) interconnections and high-performance organic thin-film transistor (OTFT) arrays for flexible electronics. The SALEP approach comprises the direct layer-by-layer deposition of multi-metal patterns interconnected by via holes, which are defined by the selective adsorption of palladium catalysts and a metallization process. The localized adsorption of the catalyst is guided by the difference in surface wettability and adsorbability caused by vacuum ultraviolet (VUV)-induced photochemical modification. The low-temperature and 3D approach enables miniaturization of flexible electrical circuits down to 1 µm in width; it also enables integration of an OTFT array on a flexible substrate. The fabricated OTFT array with multi-metal Cu/Ni/Au as contact electrodes exhibits a high hole mobility exceeding 10 cm2 V−1 s−1, demonstrating the adaptability of the SALEP approach for the low-cost, high-efficiency, and scalable fabrication of high-performance flexible electronic devices.

Slot-die coating of sol–gel-based organic–inorganic nanohybrid dielectric layers for flexible and large-area organic thin film transistors

The slot-die coating method has enormous potential owing to the industrial applicability of its high throughput capability to the large-area fabrication of solution-processed films. Herein, we demonstrated a slot-die-coated sol–gel-based organic–inorganic (O–I) nanohybrid dielectric film for organic thin film transistors (OTFTs) with highly uniform properties, such as thickness, morphology, and electrical insulating characteristics. Through the modification of the solution properties and coating speed conditions, the slot-die coating process was used to successfully fabricate defect-free dielectric films with uniform morphology, ranging from below 50 to 800 nm. Additionally, from the OTFT behavior, we identified the clockwise hysteresis by the insulator bulk for the deposition of thick O–I nanohybrid dielectrics. Based on these analyses, we can produce flexible large-area OTFT arrays with optimum slot-die-coated dielectric layers, showing an average field-effect mobility of 0.675 cm2 V−1 s−1 with a low standard deviation of 0.052017 cm2 V−1 s−1. These results suggest that this printing technology has considerable potential in the manufacturing of flexible organic electronic devices that require industrial reliability and large-area processing.

Printed Flexible Organic Transistors with Tunable Aspect Ratios

AbstractA fabrication technique that allows aspect ratio modulation in a large array of organic thin film transistors (OTFTs) is demonstrated. In this design, discrete interdigitated source and drain electrodes can be individually selected to create a range of aspect ratios (W/L). It is shown that by increasing the aspect ratio, the drain current is increased respectively. Arrays of OTFTs are fabricated with a combination of inkjet printing and surface energy patterned blade coating. OTFTs with optimized aspect ratios are interconnected to form electronic circuits with the desired performance. A fabricated enhancement‐load inverter with selected aspect ratios of 2000 µm/50 µm for the drive OTFT and 250 µm/50 µm for the load transistor show good switching performance with a dc gain of approximately 6 at supply voltage of −20 V.

Chemical Sensing Platforms Based on Organic Thin-Film Transistors Functionalized with Artificial Receptors.

Organic thin-film transistors (OTFTs) have attracted intense attention as promising electronic devices owing to their various applications such as rollable active-matrix displays, flexible nonvolatile memories, and radiofrequency identification (RFID) tags. To further broaden the scope of the application of OTFTs, we focus on the host-guest chemistry combined with the electronic devices. Extended-gate types of OTFTs functionalized with artificial receptors were fabricated to achieve chemical sensing of targets in complete aqueous media. Organic and inorganic ions (cations and anions), neutral molecules, and proteins, which are regarded as target analytes in the field of host-guest chemistry, were electrically detected by artificial receptors. Molecular recognition phenomena on the extended-gate electrode were evaluated by several analytical methods such as photoemission yield spectroscopy in the air, contact angle goniometry, and X-ray photoelectron spectroscopy. Interestingly, the electrical responses of the OTFTs were highly sensitive to the chemical structures of the guests. Thus, the OTFTs will facilitate the selective sensing of target analytes and the understanding of chemical conversions in biological and environmental systems. Furthermore, such cross-reactive responses observed in our studies will provide some important insights into next-generation sensing systems such as OTFT arrays. We strongly believe that our approach will enable the development of new intriguing sensor platforms in the field of host-guest chemistry, analytical chemistry, and organic electronics.

Fabrication of Two-Dimensional Crystalline Organic Films by Tilted Spin Coating for High-Performance Organic Field-Effect Transistors.

We developed a facile method for fabricating large-area, two-dimensional (2D), organic, highly crystalline films and extended it to organic thin-film transistor arrays. Tilted spinning provided oriented flow at the three-phase contact line, and a 2D crystalline film that consisted of layer-by-layer stacked 2,7-diocty[1]benzothieno[3,2- b]benzothiophene (C8-BTBT) was obtained facilely for organic thin-film transistors (OTFTs). The extracted field-effect mobility is 4.6 cm2 V-1 s-1, but with nonideal features. By applying this method to microdroplet arrays, an oriented crystal was fabricated, and the channel region for OTFTs was covered by adjusting the spinning speed. By tuning the tilt angle (θ) of the revolving substrate, we fabricated high-performance OTFT arrays with average and maximum mobilities of 7.5 and 10.1 cm2 V-1 s-1, respectively, which exhibited high reliability factors of over 90% and were close to that of ideal transistors. These results suggest that high-quality crystalline films can be obtained via a facile tilted-spinning method.

High-Performance, Ultrathin, Ultraflexible Organic Thin-Film Transistor Array Via Solution Process.

Ultrathin organic thin-film transistors (OTFTs) have received extensive attention due to their outstanding advantages, such as extreme flexibility, good conformability, ultralight weight, and compatibility with low-cost and large-area solution-processed techniques. However, compared with the rigid substrates, it still remains a challenge to fabricate high-performance ultrathin OTFTs. In this study, a high-performance ultrathin 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) OTFT array is demonstrated via a simple spin-coating method, with mobility as high as 11 cm2 V-1 s-1 (average mobility: 7.22 cm2 V-1 s-1 ), on/off current ratio of over 106 , switching current of >1 mA, and a good yield ratio as high as 100%. The ultrathin thickness at ≈380 nm and the ultralight weight at ≈0.89 g m-2 enable the free-standing OTFTs to imperceptibly adhere onto human skin, and even a damselfly wing without affecting its flying. More importantly, the OTFTs show good electrical characteristics and mechanical stability when conformed onto the curved surfaces and even folded in a book after 100 folding cycles. These results illustrate the broad application potential of this simply fabricated ultrathin OTFT in next-generation electronics such as foldable displays and wearable devices.

42.1: Invited Paper: Fully transparent conformal organic thin‐film transistor array and its application as LED front driving

Fully transparent conformal thin‐film transistor (TFT) array, that enables adherence onto the flat and curved surfaces without affecting original visual appearance, is the fundamental unit for next‐generation “see‐through” electronics such as transparent displays for car windscreens, LED front driving, and conformal biological devices. Realization of these functions faces one key challenge that requires the development of all components consisting of TFT with excellent flexibility and visual transparency. Organic semiconductor is promising candidate as the active material of transparent conformal transistors because of its advantages in light weight, flexibility, and tunable optoelectronic properties. Until now, however, the colorless fully transparent conformal organic TFT array has not been reported.Here, fully transparent conformal organic TFT array is demonstrated based on the photolithography‐compatible metallic grid gate electrode and the solution‐processed C8‐BTBT film. The resulting TET array exhibits high optical transparency of &gt;80% over the visible spectrum, mobility of 2 cm2V−1s−1, on/off ratio of 105–106, switching current of &gt;0.1 mA, and excellent light stability. The transparent conformal transistor array is demonstrated to well adhere onto the flat and curved LEDs as front driving. These results present promising applications of the solution‐processed wide‐bandgap organic semiconductor thin films in transparent conformal active‐matrix displays.

Fully transparent conformal organic thin-film transistor array and its application as LED front driving.

A fully transparent conformal organic thin-film field-effect transistor array is demonstrated based on a photolithography-compatible ultrathin metallic grid gate electrode and a solution-processed C8-BTBT film. The resulting organic field-effect transistor array exhibits a high optical transparency of >80% over the visible spectrum, mobility up to 2 cm2 V-1 s-1, on/off ratio of 105-106, switching current of >0.1 mA, and excellent light stability. The transparent conformal transistor array is demonstrated to adhere well to flat and curved LEDs as front driving. These results present promising applications of the solution-processed wide-bandgap organic semiconductor thin films in future large-scale transparent conformal active-matrix displays.

Uniform Insulating Properties of Low-Temperature Curable Gate Dielectric for Organic Thin-Film Transistor Arrays on Plastic Substrate

Achieving a high-resolution display on a plastic substrate requires a low-temperature process to ensure dimensional stability during fabrication process, including the deposition of gate dielectrics. Evaluation platform to confirm the uniform insulating properties of organic dielectric material prior to actual application to organic thin-film transistor arrays were proposed. This test method enabled verification of the suitability of the low-temperature curable dielectric and chemical resistance during the fabrication process. A cross-linked poly(hydroxy imide) that can be cured at a low temperature of 130°C exhibited stable insulating properties in a large area that sudden breakdown was not observed in an electric field up to 4 MV/cm. Thiophene-thiazole-based copolymer semiconductor was used as an active layer and inkjet-printed. In all the processes, the temperature of the substrate was kept below 130 °C, and 4.8-inch electrophoretic display panels on a polyethylene naphthalate substrate with a resolution of 98 dpi was demonstrated.

Nanofluidic/nanoelectronic study on solvent-processed nanoscale organic transistors

Organic thin-film transistors (OTFTs) and other organic electronic devices have attracted more and more attention for next-generation wearable and flexible devices. Because of low conductivity of organic materials, working OTFTs need a channel structure with a very large aspect ratio (or ratio between channel width and length) to enable a sizable drive current. Therefore, to produce densely arranged OTFTs with microscale footprint areas, the OTFT channel length needs to be scaled down to sub-100 nm regimes. To enable cost-effective manufacturing of such nanoscale OTFT arrays, solvent-processing methods, such as spin-coating and roll-to-roll coating, are highly desirable, but such processes inevitably result in air voids in nanoscale OTFT channels, leading to poor and inconsistent gate modulation characteristics. In this work, the authors reveal the nanofluidic mechanisms responsible for the formation of air voids through characterizing the cross-sectional morphologies of as-fabricated nanoscale OTFT channels using electron microscopy and simulating the nanofluidic flows of organic materials into nanoscale transistor channel gaps using a computational fluidic dynamics tool. This work suggests that a post-fabrication thermal pressing process is needed for eliminating air voids and significantly improving gate modulation characteristics. Using this process, the authors demonstrate poly(3-hexylthiophene) OTFT arrays with channel length of 66 nm, which exhibits highly consistent on/off ratios up to ∼106. This work advances the technical and scientific knowledge for processing solution-based organic electronic materials into nanoscale devices. The presented thermal pressing process can be generically implemented for processing a broad range of solution-based organic materials.

Homogeneous dewetting on large-scale microdroplet arrays for solution-processed electronics

Unidirectional dewetting enables the production of large-area thin films with high efficiency at low cost. Herein, we report homogeneous unidirectional dewetting on large-area microdroplet arrays via gravity-induced deformation in droplets combined with alternating lyophilic/lyophobic patterns. This process allows the scaled-up deposition of thin films, including organic semiconductors and transition metal oxides, through the autogenous shrinkage of droplets, which further enables the fabrication of large-area organic thin-film transistor (OTFT) arrays. The resulting field-effect mobility and on/off ratio of the fully printed OTFTs exceeded 13 cm2 V−1 s−1 and 108, respectively. Therefore, the presented dewetting method is promising to realize the roll-to-roll manufacture of large-area flexible electronics.

Programmable and coherent crystallization of semiconductors.

The functional properties and technological utility of polycrystalline materials are largely determined by the structure, geometry, and spatial distribution of their multitude of crystals. However, crystallization is seeded through stochastic and incoherent nucleation events, limiting the ability to control or pattern the microstructure, texture, and functional properties of polycrystalline materials. We present a universal approach that can program the microstructure of materials through the coherent seeding of otherwise stochastic homogeneous nucleation events. The method relies on creating topographic variations to seed nucleation and growth at designated locations while delaying nucleation elsewhere. Each seed can thus produce a coherent growth front of crystallization with a geometry designated by the shape and arrangement of seeds. Periodic and aperiodic crystalline arrays of functional materials, such as semiconductors, can thus be created on demand and with unprecedented sophistication and ease by patterning the location and shape of the seeds. This approach is used to demonstrate printed arrays of organic thin-film transistors with remarkable performance and reproducibility owing to their demonstrated spatial control over the microstructure of organic and inorganic polycrystalline semiconductors.

Solution-Processed Organic Thin-Film Transistor Arrays with the Assistance of Laser Ablation

A key step toward commercialization of organic thin-film transistors (OTFTs) is to manufacture large-area OTFT arrays with desired uniform device performance. In this work, for the first time, solution-processed OTFT arrays were fabricated with the assistance of laser ablation. The source-drain electrodes and the whole devices were patterned by precise control of laser intensity and process path. Compared with traditional methods, this approach significantly simplifies the fabrication process of OTFT arrays with high quality and high yield. A careful selection of laser processing parameters is key to obtaining high quality and high performance OTFT arrays. The grazing incidence X-ray diffraction experiments and device performance tests ensured the selection of proper laser ablation intensity. Eventually, the OTFT arrays on silicon wafer and ITO glass exhibited uniform electrical characteristics with the mean mobility of 0.16 and 0.10 cm2 V-1 s-1, respectively. These results demonstrated that the laser ablation process provides a promising tool to simplify the fabrication of solution-processed OTFT arrays with low cost and high yield, which has great potential in upscaling of high performance OTFT arrays for display and circuits.

Fully inkjet-printed short-channel organic thin-film transistors and inverter arrays on flexible substrates

We report fully inkjet-printed organic thin-film transistors (OTFTs) and inverters with an average channel length of 9 μm and minimized overlap capacitance between gate and source/drain (S/D) electrodes on a flexible plastic substrate. Metal-organic precursor-type silver ink, poly(4-vinylphenol), and 6, 13-bis (triisopropylsilylethynyl)pentacene were used to form the gate and S/D electrodes, the gate dielectric, and the active semiconductor layer, respectively. Well-defined S/D electrodes with narrow width and spacing can be realized by introducing a separate printing process and interdigitated structure with a cartridge that can eject a 1 picoliter volume of ink droplets without any assistant surface treatments or structures. Fully inkjet-printed short channel OTFT arrays presenting an average mobility of 0.065 cm2 V−1 s−1 and on/off ratio of 4 × 103 and inverter arrays exhibiting a voltage gain of 5.5 at a VDD of −30 V and good frequency response were demonstrated.

Flexible All-organic, All-solution Processed Thin Film Transistor Array with Ultrashort Channel.

Shrinking the device dimension has long been the pursuit of the semiconductor industry to increase the device density and operation speed. In the application of thin film transistors (TFTs), all-organic TFT arrays made by all-solution process are desired for low cost and flexible electronics. One of the greatest challenges is how to achieve ultrashort channel through a cost-effective method. In our study, ultrashort-channel devices are demonstrated by direct inkjet printing conducting polymer as source/drain and gate electrodes without any complicated substrate’s pre-patterning process. By modifying the substrate’s wettability, the conducting polymer’s contact line is pinned during drying process which makes the channel length well-controlled. An organic TFT array of 200 devices with 2 μm channel length is fabricated on flexible substrate through all-solution process. The simple and scalable process to fabricate high resolution organic transistor array offers a low cost approach in the development of flexible and wearable electronics.