Macroscopic and high-throughput printing of aligned nanostructured polymer semiconductors for MHz large-area electronics

Charge transport in high-mobility conjugated polymers and molecular semiconductors

Conjugated polymers and molecular semiconductors are emerging as a viable semiconductor technology in industries such as displays, electronics, renewable energy, sensing and healthcare. A key enabling factor has been significant scientific progress in improving their charge transport properties and carrier mobilities, which has been made possible by a better understanding of the molecular structure-property relationships and the underpinning charge transport physics. Here we aim to present a coherent review of how we understand charge transport in these high-mobility van der Waals bonded semiconductors. Specific questions of interest include estimates for intrinsic limits to the carrier mobilities that might ultimately be achievable; a discussion of the coupling between charge and structural dynamics; the importance of molecular conformations and mesoscale structural features; how the transport physics of conjugated polymers and small molecule semiconductors are related; and how the incorporation of counterions in doped films-as used, for example, in bioelectronics and thermoelectric devices-affects the electronic structure and charge transport properties.

A New Frontier of Printed Electronics: Flexible Hybrid Electronics

The performance and integration density of silicon integrated circuits (ICs) have progressed at an unprecedented pace in the past 60 years. While silicon ICs thrive at low-power high-performance computing, creating flexible and large-area electronics using silicon remains a challenge. On the other hand, flexible and printed electronics use intrinsically flexible materials and printing techniques to manufacture compliant and large-area electronics. Nonetheless, flexible electronics are not as efficient as silicon ICs for computation and signal communication. Flexible hybrid electronics (FHE) leverages the strengths of these two dissimilar technologies. It uses flexible and printed electronics where flexibility and scalability are required, i.e., for sensing and actuating, and silicon ICs for computation and communication purposes. Combining flexible electronics and silicon ICs yields a very powerful and versatile technology with a vast range of applications. Here, the fundamental building blocks of an FHE system, printed sensors and circuits, thinned silicon ICs, printed antennas, printed energy harvesting and storage modules, and printed displays, are discussed. Emerging application areas of FHE in wearable health, structural health, industrial, environmental, and agricultural sensing are reviewed. Overall, the recent progress, fabrication, application, and challenges, and an outlook, related to FHE are presented.

Investigation and Control of Charge Transport Anisotropy in Highly Oriented Friction-Transferred Polythiophene Thin Films

Highly oriented thin films of poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) were prepared by friction-transfer technique followed by their characterization using polarized absorption spectroscopy, angle-dependent polarized Raman spectroscopy, and X-ray diffraction (XRD) techniques. Orientation in high-molecular-weight (M_W) polymers is hampered by chain folding or entanglements, which limit their macromolecular orientation. Interestingly, utilizing high-molecular-weight PBTTT (M_W > 50 kDa) and friction-transfer technique, successful fabrication of highly oriented thin films with very high dichroic ratio (∼30) was demonstrated. The role of the substrate's surface energy and its impact on the field-effect mobility (μ) of the oriented thin films were comprehensively investigated. The influence of annealing the thin films as prepared on the bare and self-assembled monolayer (SAM)-treated SiO₂ surfaces exhibiting differential trends of μ was systematically investigated. This was explained by partial and complete conformational transformation of macromolecules on bare and SAM-treated SiO₂ surfaces, respectively, after annealing them beyond liquid crystalline phase transition temperature, as revealed by in-plane and out-of-plane XRD results. On bare SiO₂, optimum μ up to 0.03 cm² V^-1 s^-1 along the backbone orientation was obtained for the thin films annealed to 120 °C; whereas, it reached up to 0.36 cm² V^-1 s^-1 on SAM-treated SiO₂ after annealing at 200 °C. Finally, a charge transport mechanism was proposed taking evidence from the anisotropic optical and electrical characteristics of the friction-transferred PBTTT films into consideration.

Deposition of nanoparticles from a volatile carrier liquid

HYPOTHESIS

Traversing length scales in a volatile suspension alters the various contributions to particle deposition from conjoining and disjoining surface forces and from convective and liquid evaporative effects, which is apparent in the deposit morphology.

EXPERIMENT

We investigate the particulate structures to result from the self-assembly of nanoparticles following the evaporation of a volatile carrier liquid from the level of the single particle and up to a level which is apparent to the naked eye, while quantifying the contributions of the main mechanisms that are involved in the deposition process.

FINDINGS

We show that from the level of the nanoparticles in our experiment and up to a length scale of approximately 10 μm, the morphology of the deposit is particularly sensitive to particle adhesion to the substrate and to liquid evaporation. At greater length scales, the morphology of the deposit is well correlated with the finite volume of particles and with particle convection effects. The particulate structures are in the form of detached particles and particle islands, stripes, and continuous coating, which may vary at different length scales of the same deposit.

Air and temperature sensitivity of n-type polymer materials to meet and exceed the standard of N2200

N-type organic semiconductors are notoriously unstable in air, requiring the design of new materials that focuses on lowering their LUMO energy levels and enhancing their air stability in organic electronic devices such as organic thin-film transistors (OTFTs). Since the discovery of the notably air stable and high electron mobility polymer poly{[N,N'-bis (2-octyldodecyl)- naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,29-bisthiophene)} (N2200), it has become a popular n-type semiconductor, with numerous materials being designed to mimic its structure. Although N2200 itself is well-studied, many of these comparable materials have not been sufficiently characterized to compare their air stability to N2200. To further the development of air stable and high mobility n-type organic semiconductors, N2200 was studied in organic thin film transistors alongside three N2200-based analogues as well as a recently developed polymer based on a (3E,7E)-3,7-bis(2-oxoindolin-3-ylidene)benzo[1,2-b:4,5-b']difuran-2,6(3 H,7 H)-dione (IBDF) core. This IBDF polymer has demonstrated promising field-effect mobility and air stability in drop-cast OTFTs. While N2200 outperformed its analogues, the IBDF-based polymer displayed superior air and temperature stability compared to N2200. Overall, polymers with more heteroatoms displayed greater air stability. These findings will support the development of new air-stable materials, and further demonstrate the persistent need for the development of novel n-type semiconductors.

Long-Range Order Self-Assembly of Conjugated Block Copolymers at Inclined Air-Liquid Interfaces

Here, we report that long-range order, direction-controlled, ultrathin conjugated polymer films can be formed by the self-assembly of conjugated block copolymers (i.e., poly(3-hexylthiophene)-block-poly(ethylene glycol)) at inclined air-water interfaces. Structure analyses revealed well-aligned nanowire arrays of poly(3-hexylthiophene) with a dramatically increased ordered domain size compared to the polymer films formed on a flat water surface. The improved degree of order was attributed to the flow field created by the enhanced solvent evaporation at the top of the water contact line. Note that it is challenging to prepare such well-ordered and molecularly thin films of conjugated polymers by conventional fabrication methods. The long-range order polymer film showed hole mobility an order of magnitude higher than polymer films formed on a flat interface when implemented as an active layer of field-effect transistor devices. This study demonstrates that a simple interface modification can significantly impact the self-assembly process, structure, and function of polymer films formed at the air-liquid interface.

Role of Multivalent Interactions in Dynamic-Template-Directed Assembly of Conjugated Polymers

Dynamic-template-directed assembly is a promising method to enhance molecular ordering and electronic properties of solution-coated polymer semiconductor thin films over a large area. In this work, we establish that multicomponent dynamic templates of complementary chemistries can promote polymer crystallization through cooperative multivalent interactions. We investigate this phenomenon using a combination of templating substrates including a fluoropolymer, a hydrogen-bonded liquid, and an ionic liquid (IL). Template-dependent multiscale morphology is studied by a comprehensive set of characterization techniques to understand how introducing diverse chemical moieties modulates polymer assembly. Our results clearly confirm synergistic effects between components of complementary chemistries constituting the dynamic template. The relative degree of crystallinity is improved by 50-150% for films deposited on multicomponent dynamic templates compared to their neat constituents. In addition, macroscopic alignment is increased significantly (2-5 times) compared to single-component templates. As a result, highly anisotropic charge transport is observed with apparent hole mobilities up to 3.6 cm² V^-1 s^-1. In contrast, such a synergistic effect is not observed when using a multicomponent dynamic template of comparable chemistries (i.e., IL and polymerized IL). We elucidate the origin of this synergistic effect by using attenuated total reflectance Fourier transform infrared spectroscopy and isothermal titration calorimetry. When the dynamic template comprises two or more components interacting with complementary binding sites on the conjugated polymer (CP) (esp. backbone vs side chain), the template-polymer interactions is significantly enhanced compared to the sum of single component contributions. These results provide valuable insights into surface-directed CP crystallization during large-area solution coating. Template dynamics is rarely studied and represents a new opportunity for guiding assembly of soft functional matter.

Motion-Programmed Bar-Coating Method with Controlled Gap for High-Speed Scalable Preparation of Highly Crystalline Organic Semiconductor Thin Films

Solution-processed organic semiconductor thin films with high charge carrier mobility are necessary for development of next-generation electronic applications, but the rapid processing speed demanded for the industrial-scale production of these thin films poses a challenge to control of their thin-film properties, such as crystallinity, morphology, and film-to-film uniformity. Here, we show a new solution coating method that is compatible with a roll-to-roll printing process at a rate of 2 mm s^-1 by using a gap-controllable wire bar, motion-programming strategy, and blended active inks. We demonstrate that the coating bar, the horizontal motion of which is repeatedly brought to an intermittent standstill, results in an improved vertically self-stratified structure and a high crystallinity for organic active inks comprising a semiconducting small molecule and a semiconducting polymer. Furthermore, organic transistors prepared by the developed method show superior hole mobility with high operational stability (hysteresis and kink-free transfer characteristics), high uniformity over a large area of a 4 in. wafer, good reproducibility, and superior electromechanical stabilities on a flexible plastic substrate. The bar-coating approach demonstrated here will be a step toward developing industrial-scale practical organic electronics applications.

Intrinsically distinct hole and electron transport in conjugated polymers controlled by intra and intermolecular interactions

It is still a matter of controversy whether the relative difference in hole and electron transport in solution-processed organic semiconductors is either due to intrinsic properties linked to chemical and solid-state structure or to extrinsic factors, as device architecture. We here isolate the intrinsic factors affecting either electron or hole transport within the same film microstructure of a model copolymer semiconductor. Relatively, holes predominantly bleach inter-chain interactions with H-type electronic coupling character, while electrons' relaxation more strongly involves intra-chain interactions with J-type character. Holes and electrons mobility correlates with the presence of a charge transfer state, while their ratio is a function of the relative content of intra- and inter-molecular interactions. Such fundamental observation, revealing the specific role of the ground-state intra- and inter-molecular coupling in selectively assisting charge transport, allows predicting a more favorable hole or electron transport already from screening the polymer film ground state optical properties.

All-printed large-scale integrated circuits based on organic electrochemical transistors

The communication outposts of the emerging Internet of Things are embodied by ordinary items, which desirably include all-printed flexible sensors, actuators, displays and akin organic electronic interface devices in combination with silicon-based digital signal processing and communication technologies. However, hybrid integration of smart electronic labels is partly hampered due to a lack of technology that (de)multiplex signals between silicon chips and printed electronic devices. Here, we report all-printed 4-to-7 decoders and seven-bit shift registers, including over 100 organic electrochemical transistors each, thus minimizing the number of terminals required to drive monolithically integrated all-printed electrochromic displays. These relatively advanced circuits are enabled by a reduction of the transistor footprint, an effort which includes several further developments of materials and screen printing processes. Our findings demonstrate that digital circuits based on organic electrochemical transistors (OECTs) provide a unique bridge between all-printed organic electronics (OEs) and low-cost silicon chip technology for Internet of Things applications.

Though designing digital circuits using organic electrochemical transistors (OECTs) is promising due to their high performance, inherent large footprint limits adoption. Here, the authors report staggered top-gate OECTs for all-printed integrated circuits with fast switching and small footprint.

The interplay of interfaces, supramolecular assembly, and electronics in organic semiconductors

Organic semiconductors, which include a diverse range of carbon-based small molecules and polymers with interesting optoelectronic properties, offer many advantages over conventional inorganic semiconductors such as silicon and are growing in importance in electronic applications. Although these materials are now the basis of a lucrative industry in electronic displays, many promising applications such as photovoltaics remain largely untapped. One major impediment to more rapid development and widespread adoption of organic semiconductor technologies is that device performance is not easily predicted from the chemical structure of the constituent molecules. Fundamentally, this is because organic semiconductor molecules, unlike inorganic materials, interact by weak non-covalent forces, resulting in significant structural disorder that can strongly impact electronic properties. Nevertheless, directional forces between generally anisotropic organic-semiconductor molecules, combined with translational symmetry breaking at interfaces, can be exploited to control supramolecular order and consequent electronic properties in these materials. This review surveys recent advances in understanding of supramolecular assembly at organic-semiconductor interfaces and its impact on device properties in a number of applications, including transistors, light-emitting diodes, and photovoltaics. Recent progress and challenges in computer simulations of supramolecular assembly and orientational anisotropy at these interfaces is also addressed.

Microstructural control suppresses thermal activation of electron transport at room temperature in polymer transistors

Recent demonstrations of inverted thermal activation of charge mobility in polymer field-effect transistors have excited the interest in transport regimes not limited by thermal barriers. However, rationalization of the limiting factors to access such regimes is still lacking. An improved understanding in this area is critical for development of new materials, establishing processing guidelines, and broadening of the range of applications. Here we show that precise processing of a diketopyrrolopyrrole-tetrafluorobenzene-based electron transporting copolymer results in single crystal-like and voltage-independent mobility with vanishing activation energy above 280 K. Key factors are uniaxial chain alignment and thermal annealing at temperatures within the melting endotherm of films. Experimental and computational evidences converge toward a picture of electrons being delocalized within crystalline domains of increased size. Residual energy barriers introduced by disordered regions are bypassed in the direction of molecular alignment by a more efficient interconnection of the ordered domains following the annealing process.

Though solution-processed conjugated polymers with inverted temperature activated transport have been reported, the origin of this behaviour is unclear. Here, the authors realize temperature-independent electron transport above 280 K in a donor-acceptor copolymer through microstructural engineering.

Mechanically Flexible Conductors for Stretchable and Wearable E-Skin and E-Textile Devices

Considerable progress in materials development and device integration for mechanically bendable and stretchable optoelectronics will broaden the application of "Internet-of-Things" concepts to a myriad of new applications. When addressing the needs associated with the human body, such as the detection of mechanical functions, monitoring of health parameters, and integration with human tissues, optoelectronic devices, interconnects/circuits enabling their functions, and the core passive components from which the whole system is built must sustain different degrees of mechanical stresses. Herein, the basic characteristics and performance of several of these devices are reported, particularly focusing on the conducting element constituting them. Among these devices, strain sensors of different types, energy storage elements, and power/energy storage and generators are included. Specifically, the advances during the past 3 years are reported, wherein mechanically flexible conducting elements are fabricated from (0D, 1D, and 2D) conducting nanomaterials from metals (e.g., Au nanoparticles, Ag flakes, Cu nanowires), carbon nanotubes/nanofibers, 2D conductors (e.g., graphene, MoS₂ ), metal oxides (e.g., Zn nanorods), and conducting polymers (e.g., poly(3,4-ethylenedioxythiophene):poly(4-styrene sulfonate), polyaniline) in combination with passive fibrotic and elastomeric materials enabling, after integration, the so-called electronic skins and electronic textiles.

Ion Gel Dynamic Templates for Large Modulation of Morphology and Charge Transport Properties of Solution-Coated Conjugated Polymer Thin Films

Dynamic surfaces play a critical role in templating highly ordered complex structures in living systems but are rarely employed for directing assembly of synthetic functional materials. We design ion gel templates with widely tunable dynamics ( T_g) to template solution-coated conjugated polymers. We hypothesize that the ion gel expedites polymer nucleation by reconfiguring its surface to facilitate cooperative multivalent interactions with the conjugated polymer, validated using both experimental and computational approaches. Varying ion gel dynamics enables large modulation of alignment, molecular orientation, and crystallinity in templated polymer thin films. At the optimal conditions, ion-gel-templated films exhibit 55 times higher dichroic ratio (grazing incidence X-ray diffraction) and 49% increase in the relative degree of crystallinity compared to those templated by the neat polymer matrix. As a result, the maximum hole mobilities increase by factors of 4 and 11 along the π-π stacking and the backbone directions. Intriguingly, we observe a synergistic effect between the gel matrix and the ionic liquid that produces markedly enhanced templating effect than either component alone. Molecular dynamics simulations suggest that complementary multivalent interactions facilitated by template reconfigurability underlie the observed synergy. We further demonstrate field-effect transistors both templated and gated by ion gels with average mobility exceeding 7 cm² V^-1 s^-1.

Vertical, electrolyte-gated organic transistors show continuous operation in the MA cm-2 regime and artificial synaptic behaviour

Until now, organic semiconductors have failed to achieve high performance in highly integrated, sub-100 nm transistors. Consequently, single-crystalline materials such as single-walled carbon nanotubes, MoS₂ or inorganic semiconductors are the materials of choice at the nanoscale. Here we show, using a vertical field-effect transistor design with a channel length of only 40 nm and a footprint of 2 × 80 × 80 nm², that high electrical performance with organic polymers can be realized when using electrolyte gating. Our organic transistors combine high on-state current densities of above 3 MA cm^-2, on/off current modulation ratios of up to 10⁸ and large transconductances of up to 5,000 S m^-1. Given the high on-state currents at such large on/off ratios, our novel structures also show promise for use in artificial neural networks, where they could operate as memristive devices with sub-100 fJ energy usage.

Polymer Electrolyte Blend Gate Dielectrics for High-Performance Ultrathin Organic Transistors: Toward Favorable Polymer Blend Miscibility and Reliability

We report on systematic mobility enhancements in electrolyte-gated organic field-effect transistors (OFETs) by thinning down the active layer and exploiting polymer solid-state electrolyte gate insulators (SEGIs). The SEGI is composed of homogeneous poly(vinylidene fluoride- co-hexafluoropropylene) [P(VDF-HFP)] polymer solution-ion gel blends of high areal capacitance of >10 μF cm^-2 at 1 Hz. By scaling up the poly(3-hexylthiophene) (P3HT) semiconducting layer by 1 order of magnitude (5-50 nm), an ultraviolet photoelectron spectroscopy examination reveals a downward vacuum-level shift generating a substantial hole injection barrier that originates from different interfacial dipole layer formations. The ultrathin (5.1 nm) P3HT FETs outperformed the other devices, exhibiting stable device characteristics with a highest field-effect mobility of >2 cm² V^-1 s^-1 (effective mobility of 0.83 ± 0.05 cm² V^-1 s^-1), on/off ratio of ∼10⁶, low threshold voltage of <-0.6 V, and low gate-leakage current levels of ∼10⁵ below the on-current levels in 10 μm channel length devices. We observed a positive threshold voltage shift in the P3HT/SEGI FETs with decreasing semiconductor thickness. The aforementioned mobility is at least 10 times greater than that of neat P(VDF-HFP) devices. The significant FET performance is attributed to a better insulator/semiconductor interface, efficient hole injection from the Au electrode resulting in a low contact resistance of <500 Ω  cm, and boosted charge-carrier densities in the transistor channel. This work demonstrates an excellent approach for carrier mobility enhancement and reliability assessment in low-voltage-operated electrolyte-gated OFETs.

Ion Electron-Coupled Functionality in Materials and Devices Based on Conjugated Polymers

The coupling between charge accumulation in a conjugated polymer and the ionic charge compensation, provided from an electrolyte, defines the mode of operation in a vast array of different organic electrochemical devices. The most explored mixed organic ion-electron conductor, serving as the active electrode in these devices, is poly(3,4-ethyelenedioxythiophene) doped with polystyrelensulfonate (PEDOT:PSS). In this progress report, scientists of the Laboratory of Organic Electronics at Linköping University review some of the achievements derived over the last two decades in the field of organic electrochemical devices, in particular including PEDOT:PSS as the active material. The recently established understanding of the volumetric capacitance and the mixed ion-electron charge transport properties of PEDOT are described along with examples of various devices and phenomena utilizing this ion-electron coupling, such as the organic electrochemical transistor, ionic-electronic thermodiffusion, electrochromic devices, surface switches, and more. One of the pioneers in this exciting research field is Prof. Olle Inganäs and the authors of this progress report wish to celebrate and acknowledge all the fantastic achievements and inspiration accomplished by Prof. Inganäs all since 1981.

Accessing MHz Operation at 2 V with Field-Effect Transistors Based on Printed Polymers on Plastic

Organic printed electronics are suitable for the development of wearable, lightweight, distributed applications in combination with cost-effective production processes. Nonetheless, some necessary features for several envisioned disruptive mass-produced products are still lacking: among these radio-frequency (RF) communication capability, which requires high operational speed combined with low supply voltage in electronic devices processed on cheap plastic foils. Here, it is demonstrated that high-frequency, low-voltage, polymer field-effect transistors can be fabricated on plastic with the sole use of a combination of scalable printing and digital laser-based techniques. These devices reach an operational frequency in excess of 1 MHz at the challengingly low bias voltage of 2 V, and exceed 14 MHz operation at 7 V. In addition, when integrated into a rectifying circuit, they can provide a DC voltage at an input frequency of 13.56 MHz, opening the way for the implementation of RF devices and tags with cost-effective production processes.

Wafer-Scale Fabrication of High-Performance n-Type Polymer Monolayer Transistors Using a Multi-Level Self-Assembly Strategy

Wafer-scale fabrication of high-performance uniform organic electronic materials is of great challenge and has rarely been realized before. Previous large-scale fabrication methods always lead to different layer thickness and thereby poor film and device uniformity. Herein, the first demonstration of 4 in. wafer-scale, uniform, and high-performance n-type polymer monolayer films is reported, enabled by controlling the multi-level self-assembly process of conjugated polymers in solution. Since the self-assembly process happened in solution, the uniform 2D polymer monolayers can be facilely deposited on various substrates, and theoretically without size limitations. Polymer monolayer transistors exhibit high electron mobilities of up to 1.88 cm² V^-1 s^-1 , which is among the highest in n-type monolayer organic transistors. This method allows to easily fabricate n-type conjugated polymers with wafer-scale, high uniformity, low contact resistance, and excellent transistor performance (better than the traditional spin-coating method). This work provides an effective strategy to prepare large-scale and uniform 2D polymer monolayers, which could enable the application of conjugated polymers for wafer-scale sophisticated electronics.

In Situ Observation of Alignment Templating by Seed Crystals in Highly Anisotropic Polymer Transistors

Due to the highly directional nature of transport in polymer-based organic field-effect transistors (OFETs), alignment of the polymer backbone can significantly affect device performance. While many methods of alignment have been detailed, the mechanism of alignment is rarely revealed-especially in cases of flow-induced alignment. Polymer aggregates are often observed in highly aligned systems, but their role is similarly unclear. Here, we present a comprehensive characterization of blade-coated P(NDI2OD-T2) (N2200) for OFET applications, including a rigorous, multimodal characterization of its in-plane alignment. Film thickness follows the expected power-law dependence on coating speed, while bulk polymer backbone orientation transitions from perpendicular to parallel to the coating direction as speed is increased. Charge carrier mobility >2 cm2/(V s) is achieved parallel to the coating direction for aligned N2200 coated at 5 mm/s and is found to be strongly correlated with the in-plane alignment of the fibrillar morphology at the film's surface, characterized with atomic force microscopy and near-edge X-ray absorption. We develop a model of N2200 crystal anisotropy through rotational scans of grazing incidence wide-angle X-ray scattering (GIWAXS) and use it to analyze simultaneous in situ GIWAXS and UV-vis reflectance data from polymer solutions coated at 5 mm/s. A small population of crystals align early in the drying process, but bulk alignment occurs very late in the drying process, likely mediated by a lyotropic liquid crystal phase transition templated by the aligned crystals. Our characterization also suggests that the majority of material in N2200 thin films is noncrystalline at these conditions.

Fast Deposition of Aligning Edge-On Polymers for High-Mobility Ambipolar Transistors

Fast deposition of aligning ambipolar polymers for high-performance organic field-effect transistors (OFETs) and inverter circuits are highly desired for both scientific studies and industry applications. Here, large-area and ordered polymer films are prepared by a bar-coating method at a rate of 120 mm s^-1 in air. Atomic force microscopy and grazing-incidence wide-angle X-ray scattering analysis indicate uniform edge-on poly(fluoroisoindigo-difluorobithiophene-fluoroisoindigo-bithiophene) (PFIBI-BT) in 11.7 ± 1 nm film (≈5 layers). The elongated, uniformly oriented grains can reduce the adverse effects of the grain boundaries and facilitate charge transport in polymers. Furthermore, OFETs based on parallel film show high hole/electron mobilities up to 5.5/4.5 cm² V^-1 s^-1 , which are approximately nine times of the devices prepared by spin-coating. The gain of the inverter is as high as 174, which is one of the highest values in polymer inventers currently. These results demonstrate that the excellent bipolar performance of few-layer PFIBI-BT can be ensured while achieving the compatibility of the experimental process with industrial preparation.

Difluorobenzothiadiazole and Selenophene-Based Conjugated Polymer Demonstrating an Effective Hole Mobility Exceeding 5 cm2 V-1 s-1 with Solid-State Electrolyte Dielectric

We report synthesis of a new poly(4-(4,4-bis(2-ethylhexyl)-4 H-silolo[3,2- b:4,5- b']dithiophene-2-yl)-7-(4,4-bis(2-ethylhexyl)-6-(selenophene-2-yl)-4 H-silolo[3,2- b:4,5- b']dithiophene-2-yl)-5,6-difluorobenzo[ c][1,2,5]thiadiazole (PDFDSe) polymer based on planar 4,7-bis(4,4-bis(2-ethylhexyl)-4 H-silolo[3,2- b:4,5- b']dithiophen-2-yl)-5,6-difluorobenzo[ c][1,2,5]thiadiazole (DFD) moieties and selenophene linkages. The planar backboned PDFDSe polymer exhibits highest occupied molecular orbital and lowest unoccupied molecular orbital levels of -5.13 and -3.56 eV, respectively, and generates well-packed highly crystalline states in films with exclusive edge-on orientations. PDFDSe thin film was incorporated as a channel material in top-gate bottom-contact organic thin-film transistor with a solid-state electrolyte gate insulator (SEGI) composed of poly(vinylidene difluoride-trifluoroethylene)/poly(vinylidene fluoride- co-hexafluroropropylene)/1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, which exhibited a remarkably high hole mobility up to μ = 20.3 cm² V^-1 s^-1 corresponding to effective hole mobility exceeding 5 cm² V^-1 s^-1 and a very low threshold voltage of -1 V. These device characteristics are associated with the high carrier density in the semiconducting channel region, induced by the high capacitance of the SEGI layer. The excellent carrier mobility from the PDFDSe/SEGI device demonstrates a great potential of semiconducting polymer thin-film transistors as electronic components in future electronic applications.

Drastic Improvement of Air Stability in an n-Type Doped Naphthalene-Diimide Polymer by Thionation

Organic thermoelectrics are attractive for the fabrication of flexible and cost-effective thermoelectric generators (TEGs) for waste heat recovery, in particular by exploiting large-area printing of polymer conductors. Efficient TEGs require both p- and n-type conductors: so far, the air instability of polymer n-type conductors, which typically lose orders of magnitude in electrical conductivity (σ) even for short exposure time to air, has impeded processing under ambient conditions. Here we tackle this problem in a relevant class of electron transporting, naphthalene-diimide copolymers, by substituting the imide oxygen with sulfur. n-type doping of the thionated copolymer gives rise to a higher σ with respect to the non-thionated one, and most importantly, owing to a reduced energy level of the lowest-unoccupied molecular orbital, σ is substantially stable over 16 h of air exposure. This result highlights the effectiveness of chemical tuning to improve air stability of n-type solution-processable polymer conductors and shows a path toward ambient large-area manufacturing of efficient polymer TEGs.

The evolution of artificial light actuators in living systems: from planar to nanostructured interfaces

Artificially enhancing light sensitivity in living cells allows control of neuronal paths or vital functions avoiding the wiring associated with the use of stimulation electrodes. Many possible strategies can be adopted for reaching this goal, including the direct photoexcitation of biological matter, the genetic modification of cells or the use of opto-bio interfaces. In this review we describe different light actuators based on both inorganic and organic semiconductors, from planar abiotic/biotic interfaces to nanoparticles, that allow transduction of a light signal into a signal which in turn affects the biological activity of the hosting system. In particular, we will focus on the application of thiophene-based materials which, thanks to their unique chemical-physical properties, geometrical adaptability, great biocompatibility and stability, have allowed the development of a new generation of fully organic light actuators for in vivo applications.

Bar-Coated Ultrathin Semiconductors from Polymer Blend for One-Step Organic Field-Effect Transistors

One-step deposition of bi-functional semiconductor-dielectric layers for organic field-effect transistors (OFETs) is an effective way to simplify the device fabrication. However, the proposed method has rarely been reported in large-area flexible organic electronics. Herein, we demonstrate wafer-scale OFETs by bar coating the semiconducting and insulating polymer blend solution in one-step. The semiconducting polymer poly(3-hexylthiophene) (P3HT) segregates on top of the blend film, whereas dielectric polymethyl methacrylate (PMMA) acts as the bottom layer, which is achieved by a vertical phase separation structure. The morphology of blend film can be controlled by varying the concentration of P3HT and PMMA solutions. The wafer-scale one-step OFETs, with a continuous ultrathin P3HT film of 2.7 nm, exhibit high electrical reproducibility and uniformity. The one-step OFETs extend to substrate-free arrays that can be attached everywhere on varying substrates. In addition, because of the well-ordered molecular arrangement, the moderate charge transport pathway is formed, which resulted in stable OFETs under various organic solvent vapors and lights of different wavelengths. The results demonstrate that the one-step OFETs have promising potential in the field of large-area organic wearable electronics.

A novel oligo-pyrazole-based thin film: synthesis, characterization, optical and morphological properties

Pyrazole-3,4-dicarboxylic acid 2 was synthesized via the hydrolysis of pyrazole-3-carboxylic acid 1 and subsequently heated with thionyl chloride to give the novel pyrazole-3,4-dicarbonyl dichloride 3, which was easily converted into oligo-pyrazole 4 upon its reaction with p-phenylene-diamine. These newly synthesized compounds were characterized by 1H-NMR, 13C-NMR, and FT-IR spectroscopy, and gel permission chromatography (GPC). Three novel oligo-pyrazole thin films were prepared using oligo-pyrazole 4 with these respective values of thickness: 20, 21, and 24 μm. The optical properties of the films, including the absorbance, transmittance, and optical band gap, were determined using UV-vis spectroscopy. The Eg values of the films were found to be 1.426, 1.537, and 1.648 eV for the 20, 21, and 24 μm thick organic films, respectively. Atomic force microscopy (AFM) was used to examine the surface morphology and properties of the organic films. In the AFM images, a few black regions were observed and several yellow regions appeared over a large area, and the surface of the oligo-pyrazole films had an extremely low roughness value. The as-synthesized oligo-pyrazole has great potential in optoelectronic applications according to the optical properties of the as-prepared films. Graphical abstract.

Aligning Ag Nanowires by a Facile Bioinspired Directional Liquid Transfer: Toward Anisotropic Flexible Conductive Electrodes

Recent years have witnessed the booming development of transparent flexible electrodes (TFEs) for their applications in electronics and optoelectronic devices. Various strategies have thus been developed for preparing TFEs with higher flexibility and conductivity. However, little work has focused on TFEs with anisotropic conductivity. Here, a facile strategy of directional liquid transfer is proposed, guided by a conical fibers array (CFA), based on which silver nanowires (AgNWs) are aligned on a soft poly(ethylene terephthalate) substrate in large scale. After further coating a second thin layer of the conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), a TFE with notable anisotropic conductivity and excellent optical transmittance of 95.2% is prepared. It is proposed that the CFA enables fine control over the receding of the three-phase contact line during the dewetting process, where AgNWs are guided and aligned by the as-generated directional stress. Moreover, anisotropic electrochemical deposition is enabled where the Cu nanoparticles deposit only on the oriented AgNWs, leading to a surface with anisotropic wetting behavior. Importantly, the approach enables alignment of AgNWs via multiple directions at one step. It is envisioned that the as-developed approach will provide an optional approach for simple and low-cost preparation of TFE with various functions.

Performance Comparisons of Polymer Semiconductors Synthesized by Direct (Hetero)Arylation Polymerization (DHAP) and Conventional Methods for Organic Thin Film Transistors and Organic Photovoltaics

C-C bond forming reactions are central to the construction of π-conjugated polymers. Classical C-C bond forming reactions such as the Stille and Suzuki coupling reactions have been widely used in the past for this purpose. More recently, direct (hetero)arylation polymerization (DHAP) has earned a place in the spotlight with an increasing number of π-conjugated polymers being produced using this atom-economic and more sustainable chemistry. As semiconductors in organic electronics, the device performances of the polymers made by DHAP are of great interest and importance. This review compares the device performances of some representative π-conjugated polymers made using the DHAP method with those made using the conventional C-C bond forming reactions when they are used as semiconductors in organic thin film transistors (OTFTs) and organic photovoltaics (OPVs).

Uniaxial Alignment of Conjugated Polymer Films for High-Performance Organic Field-Effect Transistors

Polymer semiconductors have been experiencing a remarkable improvement in electronic and optoelectronic properties, which are largely related to the recent development of a vast library of high-performance, donor-acceptor copolymers showing alternation of chemical moieties with different electronic affinities along their backbones. Such steady improvement is making conjugated polymers even more appealing for large-area and flexible electronic applications, from distributed and portable electronics to healthcare devices, where cost-effective manufacturing, light weight, and ease of integration represent key benefits. Recently, a strong boost to charge carrier mobility in polymer-based field-effect transistors, consistently achieving the range from 1.0 to 10 cm² V^-1 s^-1 for both holes and electrons, has been given by uniaxial backbone alignment of polymers in thin films, inducing strong transport anisotropy and favoring enhanced transport properties along the alignment direction. Herein, an overview on this topic is provided with a focus on the processing-structure-property relationships that enable the controlled and uniform alignment of polymer films over large areas with scalable processes. The key aspects are specific molecular structures, such as planarized backbones with a reduced degree of conformational disorder, solution formulation with controlled aggregation, and deposition techniques inducing suitable directional flow.

Design of New Isoindigo-Based Copolymer for Ambipolar Organic Field-Effect Transistors

We report the synthesis of a new conjugated polymer composed of isoindigo (IID) and 2,3-bis[thiophenyl-2-yl]thiophene acrylonitrile (CNTVT) subunits for high-performance n-type organic field-effect transistors (OFETs). To realize high electron mobility for the IID-based conjugated polymer, an electron-withdrawing nitrile group is incorporated into the vinylene unit, thereby shifting the energy of the lowest unoccupied molecular orbital for efficient electron injection from Au electrodes without disrupting the backbone planarity. Uniaxially aligned IID₂₄-CNTVT-conjugated polymer films for efficient intramolecular charge transport are achieved by off-center spin-coating from preaggregated solutions. To obtain its stable preaggregation in solution, a binary solvent system (a mixture of good and bad solvents) chosen with the assistance of Hansen solubility parameter simulation is used. Through this process, highly aligned IID₂₄-CNTVT films are obtained by off-center spin coating from a solvent mixture of 9:1 dichlorobenzene/2-methoxyethanol as the good and bad solvents, respectively. The properties of the aligned IID₂₄-CNTVT films are characterized with various analytical techniques, including UV-visible absorption spectroscopy, angle-resolved near-edge X-ray absorption fine structure spectroscopy, and grazing-incidence wide-angle X-ray scattering. Top-gate/bottom-contact OFETs with IID₂₄-CNTVT films aligned in the direction of charge transport exhibit a high-electron field-effect mobility of 0.83 ± 0.13 cm²/V·s.

Pronounced Side Chain Effects in Triple Bond-Conjugated Polymers Containing Naphthalene Diimides for n-Channel Organic Field-Effect Transistors

Three triple bond-conjugated naphthalene diimide (NDI) copolymers, poly{[ N, N'-bis(2-R₁)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]- alt-[(2,5-bis(2-R₂)-1,4-phenylene)bis(ethyn-2,1-diyl)]} (PNDIR₁-R₂), were synthesized via Sonogashira coupling polymerization with varying alkyl side chains at the nitrogen atoms of the imide ring and 2,5-positions of the 1,4-diethynylbenzene moiety. Considering their identical polymer backbone structures, the side chains were found to have a strong influence on the surface morphology/nanostructure, thus playing a critical role in charge-transporting properties of the three NDI-based copolymers. Among the polymers, the one with an octyldodecyl (OD) chain at the nitrogen atoms of imide ring and a hexadecyloxy (HO) chain at the 2,5-positions of 1,4-diethynylbenzene, P(NDIOD-HO), exhibited the highest electron mobility of 0.016 cm² V^-1 s^-1, as compared to NDI-based copolymers with an ethylhexyl chain at the 2,5-positions of 1,4-diethynylbenzene. The enhanced charge mobility in the P(NDIOD-HO) layers is attributed to the well-aligned nano-fiber-like surface morphology and highly ordered packing structure with a dominant edge-on orientation, thus enabling efficient in-plane charge transport. Our results on the molecular structure-charge transport property relationship in these materials may provide an insight into novel design of n-type conjugated polymers for applications in the organic electronics of the future.

Tattoo-Paper Transfer as a Versatile Platform for All-Printed Organic Edible Electronics

The use of natural or bioinspired materials to develop edible electronic devices is a potentially disruptive technology that can boost point-of-care testing. The technology exploits devices that can be safely ingested, along with pills or even food, and operated from within the gastrointestinal tract. Ingestible electronics can potentially target a significant number of biomedical applications, both as therapeutic and diagnostic tool, and this technology may also impact the food industry, by providing ingestible or food-compatible electronic tags that can "smart" track goods and monitor their quality along the distribution chain. Temporary tattoo-paper is hereby proposed as a simple and versatile platform for the integration of electronics onto food and pharmaceutical capsules. In particular, the fabrication of all-printed organic field-effect transistors on untreated commercial tattoo-paper, and their subsequent transfer and operation on edible substrates with a complex nonplanar geometry is demonstrated.

Carrier Charge Polarity in Mixed-Stack Charge-Transfer Crystals Containing Dithienobenzodithiophene

Dithieno[2,3- d;2'3'- d']benzo[1,2- b;4,5- b']dithiophene forms mixed-stack charge-transfer complexes with fluorinated tetracyanoquinodimethanes (F _nTCNQs, n = 0, 2, and 4) and dimethyldicyanoquinonediimine (DMDCNQI). The single-crystal transistors of the F _nTCNQ complexes exhibit electron transport, whereas the DMDCNQI complex shows hole transport as well. The dominance of electron transport is explained by the superexchange mechanism, where transfers corresponding to the acceptor-to-acceptor hopping ( t_e^eff) are more than 10 times larger than the donor-to-donor hopping ( t_h^eff). This is because the donor orbital next to the highest occupied molecular orbital makes a large contribution to the electron transport owing to the symmetry matching. Like this, inherently asymmetrical electron and hole transport in alternating stacks is understood by analyzing bridge orbitals other than the transport orbitals.

Tailoring Structure and Field-Effect Characteristics of Ultrathin Conjugated Polymer Films via Phase Separation

A phase-separation method has been developed to control the semiconductor thickness and molecular arrangement via the semiconducting/insulating polymer blend system. The thickness of the poly(3-hexylthiophene) film has been regulated from 10.5 ± 1.4 nm down to 1.9 ± 0.8 nm with a favorable self-assembly degree and the mobility ranging from 0.21 to 0.03 cm² V^-1 s^-1. The ultrathin films show high bias stability and weak decay after 24 days with a bottom-gate configuration. Benefited from a good molecular order, the films have low activation energy and a 2D charge transport profile in semiconductor layers. Moreover, this blending process can be used as a general strategy of thickness control in flexible low-voltage devices and donor-acceptor-conjugated polymers.

Measuring Dipole Inversion in Self-Assembled Nano-Dielectric Molecular Layers

A self-assembled nanodielectric (SAND) is an ultrathin film, typically with periodic layer pairs of high-k oxide and phosphonic-acid-based π-electron (PAE) molecular layers. IPAE, having a molecular structure similar to that of PAE but with an inverted dipole direction, has recently been developed for use in thin-film transistors. Here we report that replacing PAE with IPAE in SAND-based thin-film transistors induces sizable threshold and turn-on voltage shifts, indicating the flipping of the built-in SAND polarity. The bromide counteranion (Br^-) associated with the cationic stilbazolium portion of PAE or IPAE is of great importance, because its relative position strongly affects the electric dipole moment of the organic layer. Hence, a set of X-ray synchrotron measurements were designed and performed to directly measure and compare the Br^- distributions within the PAE and IPAE SANDs. Two trilayer SANDs, consisting of a PAE or IPAE layer sandwiched between an HfO_x and a ZrO_x layer, were deposited on the SiO_x surface of Si substrates or periodic Si/Mo multilayer substrates for X-ray reflectivity and X-ray standing wave measurements, respectively. Along with complementary DFT simulations, the spacings, elemental (Hf, Br, and Zr) distributions, molecular orientations, and Mulliken charge distributions of the PAE and IPAE molecules within each of the SAND trilayers were determined and correlated with the dipole inversion.

The meniscus-guided deposition of semiconducting polymers

The electronic devices that play a vital role in our daily life are primarily based on silicon and are thus rigid, opaque, and relatively heavy. However, new electronics relying on polymer semiconductors are opening up new application spaces like stretchable and self-healing sensors and devices, and these can facilitate the integration of such devices into our homes, our clothing, and even our bodies. While there has been tremendous interest in such technologies, the widespread adoption of these organic electronics requires low-cost manufacturing techniques. Fortunately, the realization of organic electronics can take inspiration from a technology developed since the beginning of the Common Era: printing. This review addresses the critical issues and considerations in the printing methods for organic electronics, outlines the fundamental fluid mechanics, polymer physics, and deposition parameters involved in the fabrication process, and provides future research directions for the next generation of printed polymer electronics.