Fabrication of n- and p-type organic thin film transistors with minimized gate overlaps by self-aligned nanoimprinting

Rotating Cylinder-Assisted Nanoimprint Lithography for Enhanced Chemisorbable Filtration Complemented by Molecularly Imprinted Polymers

Rotating cylindrical stamp-based nanoimprint technique has many advantages, including the continuous fabrication of intriguing micro/nanostructures and rapid pattern transfer on a large scale. Despite these advantages, the previous nanoimprint lithography has rarely been used for producing sophisticated nanoscale patterns on a non-planar substrate that has many extended applications. Here, the simple integration of nanoimprinting process with a help of a transparent stamp wrapped on the cylindrical roll and UV optical source in the core to enable high-throughput pattern transfer, particularly on a fabric substrate is demonstrated. Moreover, as a functional resin material, this innovative strategy involves a synergistic approach on the synthesis of molecularly imprinted polymer, which are spatially organized free-standing perforated nanostructures such as nano/microscale lines, posts, and holes patterns on various woven or nonwoven blank substrates. The proposed materials can serve as a self-encoded filtration medium for selective separation of formaldehyde molecules. It is envisioned that the combinatorial fabrication process and attractive material paves the way for designing next-generation separation systems in use to capture industrial or household toxic substances.

Self-Assembled Monolayers as Patterning Tool for Organic Electronic Devices

The patterning of functional materials represents a crucial step for the implementation of organic semiconducting materials into functional devices. Classical patterning techniques such as photolithography or shadow masking exhibit certain limitations in terms of choice of materials, processing techniques and feasibility for large area fabrication. The use of self-assembled monolayers (SAMs) as a patterning tool offers a wide variety of opportunities, from the region-selective deposition of active components to guiding the crystallization direction. Here, we discuss general techniques and mechanisms for SAM-based patterning and show that all necessary components for organic electronic devices, i.e., conducting materials, dielectrics, organic semiconductors, and further functional layers can be patterned with the use of self-assembled monolayers. The advantages and limitations, and potential further applications of patterning approaches based on self-assembled monolayers are critically discussed.

Switching from weakly to strongly limited injection in self-aligned, nano-patterned organic transistors

Organic thin-film transistors for high frequency applications require large transconductances in combination with minimal parasitic capacitances. Techniques aiming at eliminating parasitic capacitances are prone to produce a mismatch between electrodes, in particular gaps between the gate and the interlayer electrodes. While such mismatches are typically undesirable, we demonstrate that, in fact, device structures with a small single-sided interlayer electrode gap directly probe the detrimental contact resistance arising from the presence of an injection barrier. By employing a self-alignment nanoimprint lithography technique, asymmetric coplanar organic transistors with an intentional gap of varying size (< 0.2 μm) between gate and one interlayer electrode are fabricated. An electrode overlap exceeding 1 μm with the other interlayer has been kept. Gaps, be them source or drain-sided, do not preclude transistor operation. The operation of the device with a source-gate gap reveals a current reduction up to two orders of magnitude compared to a source-sided overlap. Drift-diffusion based simulations reveal that this marked reduction is a consequence of a weakened gate-induced field at the contact which strongly inhibits injection.

Multilength Scale Patterning of Functional Layers by Roll-to-Roll Ultraviolet-Light-Assisted Nanoimprint Lithography

Top-down fabrication of nanostructures with high throughput is still a challenge. We demonstrate the fast (>10 m/min) and continuous fabrication of multilength scale structures by roll-to-roll UV-nanoimprint lithography on a 250 mm wide web. The large-area nanopatterning is enabled by a multicomponent UV-curable resist system (JRcure) with viscous, mechanical, and surface properties that are tunable over a wide range to either allow for usage as polymer stamp material or as imprint resist. The adjustable elasticity and surface chemistry of the resist system enable multistep self-replication of structured resist layers. Decisive for defect-free UV-nanoimprinting in roll-to-roll is the minimization of the surface energies of stamp and resist, and the stepwise reduction of the stiffness from one layer to the next is essential for optimizing the reproduction fidelity especially for nanoscale features. Accordingly, we demonstrate the continuous replication of 3D nanostructures and the high-throughput fabrication of multilength scale resist structures resulting in flexible polyethylenetherephtalate film rolls with superhydrophobic properties. Moreover, a water-soluble UV-imprint resist (JRlift) is introduced that enables residue-free nanoimprinting in roll-to-roll. Thereby we could demonstrate high-throughput fabrication of metallic patterns with only 200 nm line width.

Ultra-high gain diffusion-driven organic transistor

Emerging large-area technologies based on organic transistors are enabling the fabrication of low-cost flexible circuits, smart sensors and biomedical devices. High-gain transistors are essential for the development of large-scale circuit integration, high-sensitivity sensors and signal amplification in sensing systems. Unfortunately, organic field-effect transistors show limited gain, usually of the order of tens, because of the large contact resistance and channel-length modulation. Here we show a new organic field-effect transistor architecture with a gain larger than 700. This is the highest gain ever reported for organic field-effect transistors. In the proposed organic field-effect transistor, the charge injection and extraction at the metal–semiconductor contacts are driven by the charge diffusion. The ideal conditions of ohmic contacts with negligible contact resistance and flat current saturation are demonstrated. The approach is general and can be extended to any thin-film technology opening unprecedented opportunities for the development of high-performance flexible electronics.

Organic field-effect transistors offer limited gain due to the large contact resistance and the channel length modulation. Here, Torricelli et al. show a new transistor architecture where the charge injection and extraction are driven by the charge diffusion and a gain larger than 700 is achieved.

Diatom-inspired templates for 3D replication: natural diatoms versus laser written artificial diatoms

The diatoms are ubiquitous, exist in large numbers and show a great diversity of features on their porous silica structures. Therefore, they inspire the fabrication of nanostructured templates for nanoimprint processes (NIL), where large structured areas with nanometer precision are required. In this study, two approaches regarding the respective challenges and potential exploitations are followed and discussed: the first one takes advantage of a template that is directly made of natural occurring diatoms. Here, two replication steps via soft lithography are needed to obtain a template which is subsequently used for NIL. The second approach exploits the technical capabilities of the precise 3D laser lithography (3DLL) based on two-photon polymerization of organic materials. This method enables the fabrication of arbitrary artificial diatom-inspired micro- and nanostructures and the design of an inverse structure. Therefore, only one replication step is needed to obtain a template for NIL. In both approaches, a replication technique for true 3D structures is shown.

Toward printed integrated circuits based on unipolar or ambipolar polymer semiconductors

For at least the past ten years printed electronics has promised to revolutionize our daily life by making cost-effective electronic circuits and sensors available through mass production techniques, for their ubiquitous applications in wearable components, rollable and conformable devices, and point-of-care applications. While passive components, such as conductors, resistors and capacitors, had already been fabricated by printing techniques at industrial scale, printing processes have been struggling to meet the requirements for mass-produced electronics and optoelectronics applications despite their great potential. In the case of logic integrated circuits (ICs), which constitute the focus of this Progress Report, the main limitations have been represented by the need of suitable functional inks, mainly high-mobility printable semiconductors and low sintering temperature conducting inks, and evoluted printing tools capable of higher resolution, registration and uniformity than needed in the conventional graphic arts printing sector. Solution-processable polymeric semiconductors are the best candidates to fulfill the requirements for printed logic ICs on flexible substrates, due to their superior processability, ease of tuning of their rheology parameters, and mechanical properties. One of the strongest limitations has been mainly represented by the low charge carrier mobility (μ) achievable with polymeric, organic field-effect transistors (OFETs). However, recently unprecedented values of μ ∼ 10 cm(2) /Vs have been achieved with solution-processed polymer based OFETs, a value competing with mobilities reported in organic single-crystals and exceeding the performances enabled by amorphous silicon (a-Si). Interestingly these values were achieved thanks to the design and synthesis of donor-acceptor copolymers, showing limited degree of order when processed in thin films and therefore fostering further studies on the reason leading to such improved charge transport properties. Among this class of materials, various polymers can show well balanced electrons and holes mobility, therefore being indicated as ambipolar semiconductors, good environmental stability, and a small band-gap, which simplifies the tuning of charge injection. This opened up the possibility of taking advantage of the superior performances offered by complementary "CMOS-like" logic for the design of digital ICs, easing the scaling down of critical geometrical features, and achieving higher complexity from robust single gates (e.g., inverters) and test circuits (e.g., ring oscillators) to more complete circuits. Here, we review the recent progress in the development of printed ICs based on polymeric semiconductors suitable for large-volume micro- and nano-electronics applications. Particular attention is paid to the strategies proposed in the literature to design and synthesize high mobility polymers and to develop suitable printing tools and techniques to allow for improved patterning capability required for the down-scaling of devices in order to achieve the operation frequencies needed for applications, such as flexible radio-frequency identification (RFID) tags, near-field communication (NFC) devices, ambient electronics, and portable flexible displays.

Flexible and low-voltage integrated circuits constructed from high-performance nanocrystal transistors

Colloidal semiconductor nanocrystals are emerging as a new class of solution-processable materials for low-cost, flexible, thin-film electronics. Although these colloidal inks have been shown to form single, thin-film field-effect transistors with impressive characteristics, the use of multiple high-performance nanocrystal field-effect transistors in large-area integrated circuits has not been shown. This is needed to understand and demonstrate the applicability of these discrete nanocrystal field-effect transistors for advanced electronic technologies. Here we report solution-deposited nanocrystal integrated circuits, showing nanocrystal integrated circuit inverters, amplifiers and ring oscillators, constructed from high-performance, low-voltage, low-hysteresis CdSe nanocrystal field-effect transistors with electron mobilities of up to 22 cm(2) V(-1) s(-1), current modulation >10(6) and subthreshold swing of 0.28 V dec(-1). We fabricated the nanocrystal field-effect transistors and nanocrystal integrated circuits from colloidal inks on flexible plastic substrates and scaled the devices to operate at low voltages. We demonstrate that colloidal nanocrystal field-effect transistors can be used as building blocks to construct complex integrated circuits, promising a viable material for low-cost, flexible, large-area electronics.

Fabrication of transistors on flexible substrates: from mass-printing to high-resolution alternative lithography strategies

In this report, the development of conventional, mass-printing strategies into high-resolution, alternative patterning techniques is reviewed with the focus on large-area patterning of flexible thin-film transistors (TFTs) for display applications. In the first part, conventional and digital printing techniques are introduced and categorized as far as their development is relevant for this application area. The limitations of conventional printing guides the reader to the second part of the progress report: alternative-lithographic patterning on low-cost flexible foils for the fabrication of flexible TFTs. Soft and nanoimprint lithography-based patterning techniques and their limitations are surveyed with respect to patterning on low-cost flexible foils. These show a shift from fabricating simple microlense structures to more complicated, high-resolution electronic devices. The development of alternative, low-temperature processable materials and the introduction of high-resolution patterning strategies will lead to the low-cost, self-aligned fabrication of flexible displays and solar cells from cheaper but better performing organic materials.

Contact resistance and megahertz operation of aggressively scaled organic transistors

Bottom-gate, top-contact organic thin-film transistors (TFTs) with excellent static characteristics (on/off ratio: 10(7) ; intrinsic mobility: 3 cm(2) (V s)(-1) ) and fast unipolar ring oscillators (signal delay as short as 230 ns per stage) are fabricated. The significant contribution of the transfer length to the relation between channel length, contact length, contact resistance, effective mobility, and cutoff frequency of the TFTs is theoretically and experimentally analyzed.

Adhesive lithography for fabricating organic electronic and optoelectronics devices

Improvements in organic electronic materials have led to novel device applications, ranging from large-area flexible displays to lightweight plastic electronics. Progress on these applications would benefit from development of low-cost fabrication techniques for organic semiconductors. In this review, several fabrication processes based on adhesion force (i.e. van der Waals forces, thiol-metal reactions, and cold welding) are introduced. These patterning techniques are dry patterning techniques, i.e., the electronic materials are patterned from the raised regions of molds onto a substrate directly by additive or subtractive patterning methods. Patterning of organic small molecule, polymer thin films and metal electrodes by adhesive lithography is demonstrated. The operating properties of patterned organic light-emitting diodes (OLEDs) and organic thin film transistors (OTFTs) are comparable with the performance of devices fabricated by conventional evaporation deposition methods.