Symposium EQ11—Materials, Processes and Device Structures Enabling Next-Generation High-Frequency Flexible Electronics

The evolution of organic electronics has now reached the commercial phase, with the recent market introduction of the first prototypes based on organic transistors and organic solar cell modules fabricated from solution. Understanding the impact of both the organic semiconductor design and processing conditions, on both molecular conformation and thin film microstructure has been demonstrated to be essential in achieving the required optical and electrical properties to enable these devices. Polymeric semiconductors offer an attractive combination in terms of appropriate solution rheology for printing processes, mechanical flexibility for rollable processing and applications, but their optical and electrical performance requires further improvement in order to fulfil their potential. Synthesis of conjugated aromatic polymers typically involves carbon coupling polymerisations utilising transition metal catalysts and metal containing monomers. This polymerisation chemistry creates polymers where the aromatic repeat units are linked by single carbon-carbon bonds along the backbone. In order to reduce potential conformational, and subsequently energetic, disorder due to rotation around these single bonds, an aldol condensation reaction was explored, in which a bisisatin monomer reacts with a bisoxindole monomer to create an isoindigo repeat unit that is fully fused along the polymer backbone. This aldol polymerization requires neither metal containing monomers or transition-metal catalysts, opening up new synthetic possibilities for conjugated aromatic polymer design, particularly where both monomers are electron deficient. Polymers with very large electron affinities can be synthesised by this method, resulting in air stable electron transport, demonstrated in solution processed organic thin film transistors. We present an electrical, optical and morphology characterisation of polymer thin films, illustrating structure-property relationships for this new class of polymers. Additionally, we will discuss the role of non-covalent short contacts on polymer backbone planarity, the subsequent minimisation of energetic disorder, and its effect on charge carrier mobility.
Organic electrochemical transistors (OECTs) have been shown to be promising devices for amplification of electrical signals and selective sensing of ions and biologically important molecules in an aqueous environment, and thus have potential to be utilised in bioelectronic applications. The sensitivity, selectivity and intensity of the response of this device is determined by the organic semiconducting polymer employed as the active layer. This work presents the design of new organic semiconducting materials which demonstrate significant improvements in OECT performance, through operation in accumulation mode, with high transconductance and low operating voltage.

The relentless downscaling of the silicon transistor has fuelled the continuous innovations witnessed in the semiconductor industry over the past fifty years. Unfortunately, adopting a similar approach to emerging forms of electronics is not straightforward both in terms of technology and economics. In this talk I will first discuss the main challenges that emerging technologies - such as printed large-area electronics (LAEs) - face in combing scalable manufacturing with the required performance. I will then present recent work from our laboratory on new materials and processing paradigms that enable the development of LAEs with numerous functionalities and performance characteristics superior to the current state-of-the-art. Emphasis will be placed on showcasing the key role of innovating additive manufacturing as an enabling agent for next generation LAEs.

The polarity control in Semiconducting Single-Walled Carbon Nanotubes field effect transistors (s-SWNT FETs) is important to promote their application in logic devices. The methods to turn the intrinsically ambipolar s-SWNT FETs into unipolar devices that have been proposed until now, require extra fabrication steps that make their preparation longer and more complex. Here, we demonstrate that by starting from a highly purified ink of semiconducting single-walled carbon nanotubes sorted by a conjugated polymer, and mixing them with additives we are able to achieve unipolar charge transport. The three additives used are benzyl viologen (BV), 4-(2,3-Dihydro-1,3-dimethyl-1H-benzimidazol-2-yl)-N,N-dimethylbenzenamine (N-DMBI), which give rise to n-type field effect transistors and 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄-TCNQ), which gives rise to p-type transistors. BV and N-DMBI transform the s-SWNTs transistors from ambipolar with mobility of the order of 0.7 cm²/Vs to n-type with mobility up to 5 cm²/Vs. F₄-TCNQ transform the ambipolar transistors in p-type with mobility up 16 cm²/Vs.

Conducting polymers are opening new possibilities that are impacting several technologies such as organic thermoelectrics but also opto- and bioelectronics applications.^[1] Among these polymers, PEDOT:PSS is the most successful one that can transport holes. With an electrical conductivity reaching thousands of S cm^-1 and an exceptional ambient stability, this ionic/electronic conductor has been integrated in multiple applications spanning from conducting layer in organic solar cells or light-emitting diodes to active material in sensors and actuators, supercapacitors or thermoelectrics.^[2] The versatility of the synthesis and processing of PEDOT:PSS is another reason of its success, being compatible with large scale deposition methods such as ink-jet printing or spray-coating, through an ink formulation that is water-based.
However, while PEDOT:PSS only transports holes (p-type), several opto- and bioelectronic applications include devices that require the integration of materials for transporting both charges; an n-type material able to transport electrons, combined with a p-type material that transport holes.
To fill this gap, several n-type polymers were developed, without entirely reaching the performances of their p-type counterpart PEDOT:PSS. Most of these n-type conducting polymers, when properly doped, demonstrates electron conductivity of tens of S cm^-1. However, their use at the industrial scale is extremely limited due to their processing involving halogenated solvents that are harmful for the environment. Moreover, they usually lack of ambient and thermal stability, and can be hardly overprocessed due to a poor solvent stability, resulting in mediocre performances. Various approaches are explored in order to enhance the electrical performances of this class of n-type materials, from design rationalizations and adjustments of the chemical structure to the careful choice of dopants. For instance, polymeric backbone planarization and increased rigidity is a strategy towards superior properties for high-performing n-type conducting polymers.^[3,4]

Here we report on the use of the rigid ladder-type electron-conducting polymer poly(benzimidazobenzophenanthroline) (BBL) composing an n-type conductive ink for printed electronics.^[5] The ink is alcohol-based and composed of nanoparticles of BBL produced from a solvent exchange process. This system is doped by the amine-based polymer poly(ethyleneimine) (PEI) and is processable by spray-coating in air. A thermal activation allows the BBL:PEI thin-film to demonstrate an electrical conductivity up to 8 S cm^-1, with an excellent thermal stability. Interestingly, this film is stable even when washed with common organic solvents typically used for microfabrication. Finally, we employed this material as active layer in thermoelectric generators demonstrating a power output of 56 nW for a △T of 50 K per p-n pair, and as ion-electron conductor in an organic electrochemical transistor (OECT) working in n-type depletion-mode regime. This device, coupled with a PEDOT:PSS-based OECT, was used for ternary logic application.

References

[1] X. Guo, A. Facchetti, Nature Materials 2020, 19, 922.
[2] H. Shi, C. Liu, Q. Jiang, J. Xu, Advanced Electronic Materials 2015, 1, 1500017.
[3] S. A. Jenekhe, P. O. Johnson, A. K. Agrawal, Macromolecules 1989, 22, 3216.
[4] Y. Lu, Z.-D. Yu, R.-Z. Zhang, Z.-F. Yao, H.-Y. You, L. Jiang, H.-I. Un, B.-W. Dong, M. Xiong, J.-Y. Wang, J. Pei, Angewandte Chemie International Edition 2019, 58, 11390.
[5] C.-Y. Yang, M.-A. Stoeckel, T.-P. Ruoko, H.-Y. Wu, X. Liu, N. B. Kolhe, Z. Wu, Y. Puttisong, C. Musumeci, M. Massetti, H. Sun, K. Xu, D. Tu, W. M. Chen, H. Y. Woo, M. Fahlman, S. A. Jenekhe, M. Berggren, S. Fabiano, Nat Commun 2021, 12, 2354.

Polymer semiconductors have recently gathered considerable interests for their potential to produce stretchable electronics for wearable and implantable device applications. The key target yet major challenge is to maintain high electrical performance under mechanical deformation. However, polymer semiconductors are typically π-conjugated systems with semi-crystalline thin film morphology, most of which experience device failure under low strain. In order to address this discrepancy, we iteratively developed three molecular design approaches.

We started with an in-depth investigation of structure-property relationships for a family of diketopyrrolopyrrole (DPP)-based conjugated polymers bearing hydrogen-bonding conjugation breakers (CBs) with different hydrogen-bonding strength and linker flexibilities. We concluded that CBs with a hydrogen-bonding self-association constant >0.7 and/or a denser packing tendency are able to induce higher polymer chain aggregation and crystallinity in thin films, resulting in a higher crack-on-set strain and a lower degradation in mobility during stretching. Detailed morphological characterizations suggested that crystallites alignment and hydrogen-bonding breakage are the main strain energy dissipation mechanisms. Such understanding well correlated molecular-level dynamic interactions with thin-film morphology and mechanical properties of polymer semiconductors, although this approach compromised the electrical performance.

To obtain highly stretchable polymer semiconductor without sacrifice in mobility, we investigated the potential of a high-performance conjugated polymer idacenodithiophene-co-benzothiadiazole (IDTBT) with originally low crystallinity. The polymer chains in amorphous region get involved in dissipating strain energy through conformation change. With this mechanism, IDTBT was found to be highly ductile and maintain sufficiently stable mobility of ~0.6 cm² V^-1 s^-1 at 100% strain without any crack formation. Despite its low crystallinity, the rigid and coplanar backbone configuration of IDTBT results in its efficient intrachain charge transport and thus high electrical performance. However, the plastic deformation of IDTBT leads to significant performance degradation after multiple stretching-releasing cycles due to wrinkle formation.

For realistic consumer electronics, robust elasticity is highly demanded. Therefore, we demonstrated a molecular design approach, covalently-embedded in-situ rubber matrix (iRUM) formation, which results in elastic semiconductors with high cyclic stability without comprising electrical performance. Additionally, iRUM approach can concurrently achieve solvent-resistance and photo-patternability, which are critical to realistic manufacturing. The rationally-designed good mixing of iRUM precursors with conjugated polymers significantly improves the crosslinking density of semiconductor films, serving as the key to achieve high elasticity. Besides, the charge transport is well maintained by leveraging reactivity difference between azide/C-H insertion and azide/C=C cycloaddition, leading to finely controlled competition between forming a rubber matrix and linking with conjugated polymer side chain. When applied in fully stretchable transistors, its mobility can be retained at 100% strain, and exhibits 0.93 cm² V^-1 s^-1 record-high average mobility retention after 1000 stretching-releasing cycles at 50% strain. Unprecedented 5000 stable stretching-releasing cycles are achieved and such a cycle life is five times longer than any reported work. A fully patterned, elastic transistor array was fabricated via photo-patterning of iRUM-semiconductor, demonstrating its compatibility with solution-processed multilayer electronic device manufacturing. Our work represents a transition from soft/stretchable electronics to developing practical, long-cycling, and multifunctional electronics.

The employment of organic semiconductors (OSCs) promises to widen the realm of electronics to countless new applications, thanks to the advantageous properties of soft matter. However, despite the plethora of fascinating opportunities, organic electronics has been progressing slower than expected due to difficulties in delivering high performance and reliability. In this framework, doping plays a fundamental role to overcome typical limitations. Indeed, even in the most successful organic-based technology -organic light-emitting diodes (OLEDs)- it was only when doped charge transport layers were introduced in standard device architectures, that the performance and the technological maturity required for industrialization of these devices were reached.
Interestingly, in recent years there has been a progressive transition from the simple employment of dopants in highly conducting layers, to their use into other fundamental parts of the devices. Active layers in organic thin-film transistors (OTFTs), for instance, have been almost entirely relying, until very recently, on pristine (i.e., non-doped) organic semiconductors. Extensive efforts in optimizing structure-property relationships in these layers led to a level of device optimization that pushed this approach to its limits. In this context, the addition of dopants enabled the demonstration of OTFTs with unprecedented performance owing to an improved charge transport and reduced contact resistance.^1–3 Similarly, molecular dopants have been successfully employed in bulk heterojunctions to fabricate solar cells⁴ and photodetectors⁵ displaying enhanced properties.
In this work we cover different materials systems and show how doping can lead to increased charge carrier mobilities owing to synergistic effects both in terms of electronic and structural properties. Indeed, the addition of low concentrations of molecular dopants to the channel of OTFTs can be accompanied by changes in their charge transport mechanism, with the onset of a band-like transport, besides the achievement of extremely high mobilities. Furthermore, we show how the microstructure of the OSCs can be controlled upon blending them with dopants aiding the transport. The improvements of our blend systems are not limited to transistors, but are also valid also for photodetectors, solar cells and diodes. The latter are in particular very promising, since high frequency rectifiers are a central component that determine the frequency of operation and the power conversion efficiency in wireless energy harvesting (WEH) systems. Here we present a novel strategy in which an adhesion lithography fabrication route, yielding to electrodes separated by nanogaps < 20 nm, is combined with doping to achieve co-planar nanogap Schottky diodes operating above 10 GHz.
Our results provide evidence that doping and innovative fabrication routes can pave the way towards future applications of GHz organic electronics.

References

1. Panidi, J. et al. Remarkable Enhancement of the Hole Mobility in Several Organic Small-Molecules, Polymers, and Small-Molecule:Polymer Blend Transistors by Simple Admixing of the Lewis Acid p-Dopant B(C6F5)3. Advanced Science 5, 1700290 (2018).
2. Scaccabarozzi, A. D. et al. Understanding Charge Transport in High-Mobility p-Doped Multicomponent Blend Organic Transistors. Advanced Electronic Materials 6, 2000539 (2020).
3. Waldrip, M., Jurchescu, O. D., Gundlach, D. J. & Bittle, E. G. Contact Resistance in Organic Field-Effect Transistors: Conquering the Barrier. Advanced Functional Materials 30, 1–31 (2020).
4. Lin, Y. et al. A Simple n-Dopant Derived from Diquat Boosts the Efficiency of Organic Solar Cells to 18.3%. ACS Energy Letters 3663–3671 (2020).
5. Wang, B. et al. Molecular Doping of Near-Infrared Organic Photodetectors for Photoplethysmogram Sensors. J. Mater. Chem. C (2021).

The achievement of high charge carrier mobility in solution processable organic semiconductors is a fundamental requirement for the development of organic field effect transistors operating at high frequency, therefore enabling middle-range wireless communication for the next generation of flexible electronic devices.
For this reason, in the last decades extensive efforts have been devoted to the chemical design of conjugated polymers and molecular semiconductors, reaching effective field effect mobilities >10 cm²V^-1s^-1, besides the employment of carbon nanostructures such as graphene and carbon nanotubes. Organic semiconductors consist in extended conjugated systems originating from overlapped π orbitals of adjacent sp2-hybridised carbon atoms. Herein, we report for the first time on the potential of sp-Carbon Atomic Wires as a new class of solution processable semiconductors for organic electronics. Carbon Atomic Wires are linear chains of sp-hybridized carbon atoms and represent the ultimate one-dimensional allotropic form of carbon. ^[1] They exhibit two possible isomeric configurations: cumulenes (sequence of double bonds) and polyynes (sequence of single and triple alternated bonds). Theoretical studies have predicted outstanding mechanical, optical and electronical properties for these structures. However, only a few experimental studies have investigated their electronic properties in solid-state devices and, so far, they have focused mainly on molecular junctions or monolayers. ^[2]
In this work we demonstrate the first example of organic field-effect-transistor (OFET) based on short cumulenic molecules, namely tetraphenylbutatriene [3]Ph. ^[3] Furthermore, we discuss on our latest achievements in understanding and optimizing charge-transport properties in solution processed thin films of [3]Ph. In particular, we show how the control of large-area thin-films deposited via wire-bar coating, a scalable printing technique, allows achievement of field-effect-mobilities above 0.1 cm²V^-1s^-1. Moreover, our [3]Ph-based transistors provide excellent operational stability in dark conditions, therefore addressing one of the major concerns for the application of Carbon Atomic Wires, and in general of organic semiconductors, in ambient conditions.
An in-depth characterisation of films obtained with different processing parameters allows to correlate microstructural and morphological features with the OFETs electrical performance. Our analysis highlights how these devices, based on the shortest of the cumulenes series, symbolize a promising starting point towards the exploration of sp-Carbon Atomic Wires with superior charge carrier mobility.
These results pave the way for the use of cumulenes as active material for organic electronics and open new possibilities for the chemical design of performant organic semiconductors based on sp-hybridised molecules.

[1] C. S. Casari, M. Tommasini, R. R. Tykwinski and A. Milani, Nanoscale 2016.
[2] M. R. Bryce, J. Mater. Chem. C 2021
[3] A. D. Scaccabarozzi, A. Milani, S. Peggiani, S. Pecorario, B. Sun, R. R. Tykwinski, M. Caironi, and C. S. Casari, J. Phys. Chem. Lett. 2020.

High-frequency flexible electronics demands high mobility channel materials suitable for low-temperature deposition. We present high-performance 2D In₂O₃ transistors printed from liquid Indium metal at temperatures compatible with low-cost polymer substrates. These ultrathin (2-4 nm) In₂O₃ channel materials demonstrate performance unseen in low-temperature sol-gel or nanoparticle systems, with low-field mobility greater than 40 cm²/Vs at 175 °C and 65 cm²/Vs at 250 °C processing compatible with PEN and Polyimide, respectively. We show through comparative XPS and XRD studies that a principal advantage of liquid-metal (LM) printed oxides relative to sol-gels is the ability to achieve crystalline and highly conducting films as deposited, avoiding hydroxide-dominated intermediate phases and eliminating the thermodynamic barrier posed by precursor decomposition.

Low-temperature Hall measurements reveal that, as deposited, these quantum-confined In₂O₃ channels exhibit Hall mobility of 12-20 cm²/Vs and electron concentrations of ~ 1*10¹⁹ cm^-3. Post-annealing in air further enhances electronic transport, allowing enhancement mode operation and tight control of the electronic density of states (eDOS) for monolayer and bilayer films, revealing the extremely steep band tail slope and lower deep state density of bilayer channels. Additionally, these devices exhibit ultra-low hysteresis and excellent current saturation, indicating the low defect density and stable back channel. This exceptional switching performance and the scalable simplicity of the LM printing method make these 2D oxide transistors promising devices for driving next-generation flexible electronic sensor and displays.

Organic thin-film transistors (TFTs) can often be fabricated at temperatures around or below 100 degrees Celsius and thus on a wide range of unconventional substrates, including a variety of mechanically flexible and optically transparent polymers, such as polyethylene naphthalate (PEN). This makes organic TFTs a potential alternative to TFTs based on inorganic semiconductors, most notably low-temperature polycrystalline silicon (LTPS) and indium gallium zinc oxide (IGZO), which typically require somewhat higher process temperatures that limit the choice of mechanically flexible substrate materials to ultrathin glass (which is prone to shattering) and polyimide (which has lower optical transmission compared with glass and PEN). For circuit and display applications, an important TFT parameter is the transit frequency, which is the highest frequency at which transistors are able to switch or amplify electrical signals. A field-effect transistor’s transit frequency depends critically on the channel length and the parasitic gate-to-source and gate-to-drain overlaps. Most of the highest transit frequencies reported for organic TFTs to date have been achieved with channel lengths and gate-to-contact overlaps of around 1 µm. To explore the static and dynamic performance of flexible organic TFTs with nanoscale dimensions, we have used electron-beam lithography and fabricated low-voltage organic TFTs with channel lengths and gate-to-contact overlaps as small as 100 nm on flexible PEN substrates. These TFTs display useful static and dynamic characteristics, including on/off current ratios of nine orders of magnitude, subthreshold swings below 100 mV/decade, turn-on voltages of 0 V, negligibly small threshold-voltage roll-off, contact resistances below 1 kOhm-cm, and switching delays below 50 ns.

Polymers have been used as flexible and easily processible dielectric materials for many years. Dielectric constant, dielectric loss and breakdown strength are critical for the successful application of a polymer dielectric. Numerous studies have been carried out to enhance the breakdown strength of polymers that have a reasonable dielectric constant. Taking advantage of their tailorability, this has been mainly achieved through the processing route where polymers are stretched to increase the orientation and superstructural organization, and hence the breakdown properties. This tends to lead to structural hierarchy forming tortuous path through thickness direction by the overlap of oriented crystalline and amorphous domains. However, it is challenging to understand the polymer responses under deformation. In this study we have investigated the effect of orientation on structural hierarchy (crystallinity, crystalline and amorphous chain orientation, superstructural organization) on the dielectric properties of Nylon 12. In addition to suppress the charge injection through the surfaces, SiO₂ coating was applied by using plasma enhanced chemical deposition.

Conjugated polymers are being asked to undergo more "polymer-like" behavior than ever before-- being strained, scratched, punctured, repeatedly elongated, and more. Computational modeling at the molecular dynamics level could speed up design of conjugated polymers considerably. However, these models must be able to consider "polymer-like" behavior as well. Accurate reflection of polymer nanostructure in molecular dynamics is key to determination of the polymer's mechanical properties. We find that the current state of modeling often neglects polymer rigidity. Forces meant to represent planarizing forces are often reduced or even eliminated by steric hindrance. We have thus developed a model that seperates out steric forces from forces associated with electron delocalization and internal conjugation. We additionally find that there are two key types of torsion present even in a neat polymer film-- improper and dihedral. Both of these forces must be dealt with simultaneously to achieve accuracy. Finally, we compare homopolymers to donor-acceptor polymers and find the difference in rigidity is less than might be expected.

Ongoing research into the miniaturization and optimization of electronic device manufacture has driven component study to the nanoscale, sparking interest in device design on nontraditional substrates for highly specific, targeted applications. The emerging field of smart ‘e-textiles’ examines the translation of traditional device manufacturing strategies onto flexible textile substrates, driven by commercial and fundamental interest in textile-nanomaterial integration and conductive network wiring within woven architectures to imbue useful functionality to worn garments. In the interest of adapting thermally driven electronic interconnection strategies to temperature-sensitive materials, we provide an investigation into the modification of textile surfaces with nanosolder-inks, establishing conductive channels for signal communication and joining resistive sensing elements through soldered interconnection. Low melting temperature tin/indium nanosolder particles were synthesized via aqueous chemical reduction and formulated into conductive ethylene-glycol based inks at various loading concentrations. These nanoinks were applied to individual threads of various fabric blends by dip-dry coating, with degree of coverage evaluated in relation to number of coats and associated thermal/electrical processing to improve adhesion. Printed threads were exposed to infrared heating, allowing for targeted melting of solder particles without harming the woven fiber structure, confirmed visually and via monitoring change in electrical resistance. After melting, threads modified by nanoinks with incorporated graphene oxide platelets demonstrated sensing capability for temperature, humidity, liquid phase salinity, and chemical vapors. Signal generated by these sensing fibers was transmitted via thread-to-thread soldered interconnection, establishing the conductive wiring groundwork for a textile device platform. This realization of non-destructive robust solder joining on thermally delicate textile substrates represents a significant step towards full translation of electronic device manufacturing strategies onto a woven garment.

Flexible electronics are poised to significantly change device technologies, with applications across biological/medicinal, environmental, consumer, and industrial markets. Devices that are thin, conformable, and compliant open myriad design opportunities that are currently infeasible with today’s rigid electronics. Some of the most promising material systems with these characteristics are semiconducting-polymer/elastomer (SPE) blends. The semiconducting polymer phase segregates into nanofibrils upon spin or blade coating and under certain conditions these fibrils can be aligned for directional charge transport. SPE blends have been shown to maintain excellent electronic properties under large strain, and have the advantage of being solution processable, enabling economical manufacturing scale-up.

In addition to the economic advantage solution-based processing confers, it also provides a large parameter space to optimize device fabrication. Solvent identity, polymer concentration, coating temperature, and coating speed are a few notable parameters available to tune. This allows for flexibility in the formulation process, but also creates a parameter space that can be difficult to optimize, and the optimum is often different with different coating methods. Of the many coating methods, meniscus-guided techniques are becoming more ubiquitous because of their ability to create aligned materials and are more directly translatable from lab to industrial scale manufacturing.

Here, we report on a long-wavelength instability in the blade coating process that leads to significant variation in the local semiconducting polymer fibril alignment for a DPP-TT:SEBS blend. Near-edge x-ray absorption fine structure (NEXAFS) is used to quantify the molecular alignment of DPP-TT in the fibrils and the surface composition of the films. The optical dichroic ratio from wide-field polarization imaging is directly correlated to fibril alignment as quantified by AFM, which provides a spatially resolved measurement of DPP-TT nanofibril alignment. These measurements are correlated to thin-film transistor data to characterize the impact on charge transport. Quantifying and potentially controlling coating instabilities are important for device optimization, especially in the scale-up process.

The ability to precisely control the equilibrium carrier concentration in organic semiconducting devices is of great interest. Solution processed doped layers are of extreme importance for high throughput production of organic electronic devices via roll-to-roll or ink-jet printing. In this talk, I will discuss tuning the conductivity of conjugated polymers containing Lewis basic sites by Lewis acids. Addition of the Lewis acid effectively p-dopes the hole transport in the parent polymer, leading to increases in the free hole density and conductivity. This methodology is advantageous since the polymer and Lewis acid have excellent solubility in organic solvents, negating the need for co-solvents that uses in molecular dopant such as F4TCNQ. We use a combination of techniques including electrical measurements, optical absorption, XPS, UPS, IPES, and electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR) and nuclear magnetic resonance (NMR) techniques in conjunction with density functional theory, to gain insight into the doping mechanism of 4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b′]dithiophene (CPDT) and 2,1,3-benzothiadiazole (BT)-based (PCPDTBT) by Lewis acids, BCF. We suggeste protonation of the CPDT moieties by the strong Brønsted acid BCF(H2O), followed by electron transfer from a neutral polymer chain segment to a protonated chain forming a neutral, protonated radical species and a positively charged radical species (a positive polaron or a hole). Neher et al Neher and colleagues investigated doping of poly(3-hexylthiophene) (P3HT) by BCF and suggested that the overall reaction resulted in formation of H2, rather than a radical species, as the final hydrogen-containing side product, with one possible route for H2 formation being reaction of two radical species. DFT calculations predict that the protonation reaction and H2 formation are highly endergonic. However, if we consider the formation of a larger anion, [BCF(OH)(OH₂)BCF]^–, in which [BCF(OH)]^- is hydrogen bonded to another BCF(OH₂) complex, the protonation reaction and H2 formation are exergonic.

Besides charge carrier mobility and contact resistance, the parasitic overlap capacitance is a key factor limiting the dynamic behavior of organic thin-film transistors. The most effective away to reduce parasitic overlap capacitances between source/drain and gate electrode is to employ self-aligned transistor architectures. However, so far self-alignment in organic transistors has only been achieved using non-scalable or complex fabrication processes. Here, we demonstrate quasi-self-aligned OTFTs in a coplanar top-gate architecture fabricated with reliable and scalable state-of-the-art fabrication techniques such as wet-chemical etching. Self-alignment is achieved by carrying out a wet-chemical
etching process on top of the channel interface. We prove that for the right choice
of etchant, the charge carrier transport properties of the channel interface are not deteriorated. The benefit of the self-aligned design lies in the reduction of the specific overlap capacitances which are two orders of magnitude (0.13 nF cm^-2) lower than for non-self-aligned devices (15 nF cm^-2). Furthermore, we highlight the potential of this approach for high-frequency operation of organic transistors and quantify the gain in cut-off frequency compared to non-self-aligned devices.

The field of magnetic metamaterials has spurred increasing interest since the first experimental demonstrations of a negative refractive index metamaterial¹. The appeal of these photonic devices lies in the possibility to engineer media with tailored electromagnetic response, imparting them with unconventional wave propagation properties that cannot be achieved by ‘natural’ materials². Examples of metamaterials-enabled applications include artificial magnetism, arbitrary wave-front shaping, and cloaking. To overcome the practical limitations of static metamaterials, recent efforts have focused on the integration of passive microwave metamaterials with a number of active tuning mechanisms. These dynamic metadevices grant unprecedented control over the propagation of microwaves, enabling for example real-time modulation of light intensity and phase, as well as tunable spectral selectivity³.
A plethora of tuning strategies has been so far proposed for the realization of active metadevices. Among electrical strategies, established approaches such as the addition of lumped elements (e.g. varactor and pin diodes) to microwave resonators, or the use of materials such as graphene and III-V compound semiconductors, come with high fabrication complexity and cost compared to printed organic electronics⁴. Furthermore, these approaches cannot be easily translated into platforms where large-area formats and mechanical conformability are of essence.
A new device configuration has recently been proposed, where organic electrochemical transistors – based on the conductive polymer PEDOT:PSS – actively tune a set of sub-5GHz microwave metasurfaces and metamaterial-inspired structures⁵. These devices consist in individual (or multiple) microwave Split-Ring Resonators in which the split region is bridged by a transistor, gated through common electrolytes. This device configuration enables a fully reversible modulation mechanism, which requires very low voltages (<1 V) thanks to the mixed ion-electron transporting ability of PEDOT:PSS⁶. This tuning strategy can be readily implemented on different resonant configurations, including magnetic and electric resonators, frequency-tuneable SRRs, dual-band devices, and in a reconfigurable metasurface.
The promising results achieved with PEDOT:PSS encourage a wider exploration of the continuously growing library of organic semiconductors. In this work, we demonstrate a set of tunable microwave devices using different p-type and n-type organic electrochemical transistors. By chemically engineering the organic semiconductors, we are able to operate our devices in selected voltage ranges, with fine control over the voltage dependence of the metadevices’ scattering properties. As an example of a possible application, we exploit this strategy within a wireless biosensing platform operating in the 2-2.5 GHz range.
In perspective, a metadevice tuning strategy based on organic electronic materials presents exciting opportunities for microwave technologies, possibly paving the way to a new generation of wireless interfaces for distributed sensing, fabricated on large-area flexible substrates via high-throughput techniques. Moreover, this class of metadevices could potentially complement other emerging neuromorphic technologies that are also based on organic electrochemical transistors, expanding the horizon of metamaterials research.
References:
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3. Zhao, X., Duan, G., Li, A., Chen, C. & Zhang, X. Microsyst. Nanoeng. 5, 1–17 (2019).
4. Caironi, M. & Noh, Y.-Y. Large area and flexible electronics. (Wiley-VCH, 2015).
5. Bonacchini, G. E. & Omenetto, F. G. Nat. Electron. (2021) - In press.
6. Rivnay, J. et al. Nat. Rev. Mater. 3, 17086 (2018).

Thin-film transistors (TFTs) based on amorphous metal-oxide semiconductors have received enormous attention. While this technology has set new standards as backplane for flat-panel displays, there is a wide range of applications that only becomes accessible, if metal-oxide are capable of high-frequency operation. To this end, the transit frequency (f_T) and maximum oscillation frequency (f_max) become important figures of merit, which have not received adequate attention in metal-oxide based electronics, yet. These numbers are critically limited by parasitic elements in the device (such as capacitances). Compared to devices based on single crystalline silicon (c-Si) technology, the f_T of most metal-oxide TFTs in the literature is typically low. In fact, the best f_T of metal-oxide TFTs based on large area compatible and cost-effective standard lithography did not reach the GHz regime, as of yet.
Here, we demonstrate the first a-IGZO based TFTs with a transit frequency f_T > 1 GHz. This is achieved by a rigorous reduction of parasitic elements (L_ov = 300 nm, C_GS + C_GD = 800 fF/mm) and a channel length L as low as 0.8 µm, resulting from a substantially optimized self-alignment concept without sacrificing the typical advantages of optical lithography. At the same time, our TFTs set a new record in f_max of 3.1 GHz, which states a new high-frequency limit for power amplification in metal-oxide electronics.
We expect that our results will inspire metal-oxide based high-frequency circuit applications, such as communications systems, on large-area and even flexible substrates. As such, metal-oxide TFTs ultimately may become a meaningful platform for the realization of concepts discussed in the framework of the IoT.

Organic transistor architectures that are able to interact with ions such as electrolyte-gated field effect transistors or electrochemical transistors are highly attractive devices for acquisition, transmission and processing of physiological signals. These devices are able to convert ions to electrons effectively and provide mechanical flexibility owing to their inherently soft channel materials. However, the temporal response of these devices, which directly translates into their operation speed, is defined by ion drift and mobility. As a result, they are limited to a few KHz speed regime. In addition, the lack of physically independent gate limits their ability to form integrated-circuits (ICs). Here we use internal ion-gated organic electrochemical transistors (IGTs) that are able to operate beyond the speeds dictated by ion mobility by creating local ion reservoirs inside the channel bulk. We demonstrate IGT-based amplifiers with uniform gain across their MHz range operation frequency. We characterize the electrical performance of these transistors with respect to various transistor sizes, contact area, and channel material composition, and determined the relationship between device geometry and gain, off current, leakage, and inter-transistor cross talk. Using these data and a high resolution vertical fabrication approach (capable of creating <1 µm transistors), we then created high-density IGT-based conformable circuits that performed time-division multiplexing at speeds comparable to silicon based ICs for physiological signal acquisition and multiplexing. The unique features of IGT-based devices facilitate potential applicability to a broad range of flexible circuits that require safe, chronic implantation in humans, including brain-machine interfaces, wearable electronics, and therapeutic responsive stimulation devices.

Radiation therapy is one of the most effective medical procedures for cancer treatment: high-energy ionizing radiation is very effective in destroying cancer cells, but the risks of malignancies induced by peripheral radiation in healthy tissues surrounding the target volumes represents a serious concern for all patients and doctors alike. Positioning of the patient, inhomogeneities in the target (muscles/bones/adipose tissue), and even minor movements (e.g. breathing), can alter the received dose and targeted volume, and thus affect the outcome of the procedure. Therefore, being able to measure radiation doses with high accuracy is a critical aspect of diagnostics and treatment. In this presentation I will introduce a new type of radiation dosimeter, the RAD-OFET (RAdiation Detector based on Organic Field-Effect Transistor), which can validate in real time the dose being delivered and ensure that for nearby regions an acceptable level of low dose is being received.¹ The RAD-OFETs exploit trap generation/annihilation in organic semiconductors,² are sensitive to doses relevant to many radiation treatment procedures and are robust when incorporated into conformal large-area electronic applications. Placement of the sensor directly onto the human body, coupled with the similarity in the Z-number between the electronically active layer and the human tissue, allows for direct measurement of the radiation dose, eliminating the need for extensive data processing faced by current technologies. The direct consequence is a greater precision and lower complexity in the medical equipment: their adoption in clinical settings will facilitate the application of therapeutic radiation with high precision, a process that will increase the effectiveness on treating cancerous tissue and minimize the impact on the surrounding healthy cells. These results uncover new opportunities for organic circuits that will improve the quality of healthcare through better, lower cost in vivo dose monitoring during radiation therapy.

[1] A. M. Zeidell, T. Ren, D. S. Filston, H. F. Iqbal, E. Holland, J. D. Bourland, J. E. Anthony, O. D. Jurchescu, Adv. Sci. 7 (18), 2001522 (2020).
[2] H. F. Haneef, A. M. Zeidell , O. D Jurchescu, J. Mater. Chem C 8, 759 (2020).

Energy autonomy and conformability are essential elements in the next generation of wearable and flexible electronics for healthcare, robotics and cyber-physical systems. This study presents ferroelectric polymer transducers and organic diodes for imperceptible sensing and energy harvesting systems, which are integrated on ultrathin (1-µm) substrates, thus imparting them with excellent flexibility [1]. It was found that these ultraflexible ferroelectric polymer transducers (UFPTs) develop improved ferroelectric properties through thermal annealing resulting in an increased crystallinity. After poling the remnant polarization reaches values up to 70 mC/m2 in close correlation with the crystallinity and the decrease of the dielectric constant. Simulations show that the sensitivity of ultraflexible ferroelectric polymer transducers can be strongly enhanced by the use of the ultrathin substrate. The ultrathin substrate furthermore allows the mounting of the transducers on 3D-shaped objects and the stacking in multiple layers. Indeed, UFPTs have improved sensitivity to strain and pressure as compared to devices on rigid thick substrates with values up to 15 nC/N; they have a fast response (<< 20 ms/N) and an excellent mechanical stability with a bending radius down to 40µm. Accordingly, they can be used as imperceptible wireless e-health patches for precise pulse and blood pressure monitoring.
For harvesting biomechanical energy, the transducers are combined with full wave organic rectifier circuits made of ultraflexible organic diodes. These diodes are based on organic thin film transistors with short-circuited drain and gate and are fabricated on the ultrathin substrate as well. The diodes have an excellent on/off ratio up 10⁷ and a transition voltage around 0V. Moreover they are exceptional in terms of operation frequency in the MHz regime.
Transducers, diodes and ultrathin capacitors (needed as storage elements) were further combined to form an imperceptible, 2.5 µm thin, energy harvesting device with an excellent peak power density of 3 mW×cm⁻³. When placed on body joints like elbow or knee we estimate that over a day 100 -200 mJ can be generated by biomechanical motions like knee bending with such a system. This should be enough for vital parameter reading several times per day.

[1] A. Petritz, E. Karner-Petritz, T. Uemura, P. Schäffner, T. Araki, B. Stadlober, T. Sekitani, Nat. Comm. (2021), 12:2399, https://doi.org/10.1038/s41467-021-22663-6

Conjugated polymeric electrochemical memristors (cPECMs) are a promising alternative towards next-gen bioinspired circuitry, such as memristive neuromorphic computers. Low gate switching voltages and low power consumption are easily achieved due to the wide range of redox potentials available. In the case of the p-type poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), the conjugated PEDOT allows mobile holes to hop from one chain to another where, in the neutral form, holes are compensated by PSS anions. Major limitations of PEDOT:PSS devices are due to the inherent nature of the material to oxidize, which can impede the state retention and cycle stability of a device. In this effort, a water soluble, partially reduced self-doped PEDOT (S-PEDOT) that is synthesized and fabricated into an all-solid-state memristive device. This three-terminal device with a pre- and post-synaptic terminal is able to tune the device’s conductance via electrochemical doping by means of voltage pulsing to the gate with a low power consumption for switching events (ca. 42 pJ/mm² for a 12 x 20 mm active area). S-PEDOT cPECMs are implemented to perform arithmetic and simple mathematics, execute information processing such as Boolean logic gates, display electrochromic behaviors, and can emulate essential artificial synaptic plasticities for learning and memory behaviors.

Implantable bioelectronics are poised to unravel causal relationships between neural activity and brain function due to their ability to directly interact with neurons over extended periods of time. To facilitate advancements in both these basic science and translational research avenues, responsive implantable electronics are required to perform high performance data acquisition, stimulation and low-latency digital signal processing to enable real-time manipulation of neural networks and affect behavior. We introduce a multiplex-then-amplify (MTA) scheme that in contrast to current approaches (which necessitate an equal number of amplifiers as number of channels), only requires one amplifier per multiplexer, significantly reducing the number of components and size of electronics in multi-channel acquisition systems. It also enables simultaneous stimulation of arbitrary waveforms on multiple independent channels. We validated the function of MTA by developing a fully implantable, responsive embedded system that merges ability to acquire individual neural action potentials using conformable conducting polymer-based electrodes with real-time onboard processing, low-latency arbitrary waveform stimulation, and local data storage within a miniaturized physical footprint. We demonstrate all of these features in chronically implanted freely behaving rats, verifying high quality neurophysiological signal acquisition from the hippocampal-prefrontal cortex network over weeks. We implement an automated kindling procedure to generate epileptic neural networks as well as previously established responsive neurostimulation protocols known to affect memory.
The MTA design enables effective, self-contained, chronic neural network manipulation with translational relevance to treatment of neuropsychiatric disease.

The diffusion of portable and wearable technologies results in a growing interest in electronic devices having features such as flexibility, lightness-in-weight and wireless operation. Organic electronics was proposed decades ago as a potential candidate to fulfill such need, in particular targeting pervasive Radio-Frequency (RF) applications. Still, limitations in terms of device performances at RF, particularly severe when large-area and scalable fabrication techniques are employed. In this work we demonstrated that rectification of an electromagnetic wave at 13.56 MHz thanks to a fully inkjet printed polymer diode is possible. The rectifier, a key enabling component of future pervasive wireless systems, is fabricated through scalable large-area methods on plastic. Thanks to fine tuning of the barriers at the semiconductor-metal interfaces, the diode has a rectification ratio higher than 10⁶: at the best of our knowledge this is the first time that a fully inkjet printed diode wit
h these performances has been obtained. When integrated in an organic rectifier, the excellent dynamic properties of the diode allow to rectify sinusoidal signals above 20 MHz, well covering the near-field communication range. As demonstrated in this work, our fully printed polymer diode opens the possibility to couple power to a plastic foil, or in future other non-rigid substrates such as paper, by rectifying a 13.56 MHz AC electromagnetic wave, as requested in near-field RFID applications.

We describe the key structure and material approaches towards recently demonstrated giga-Hertz large-area electronics (LAE) wireless systems using zinc-oxide (ZnO) thin-film transistors (TFTs) ^[1-4], and the resulting performance parameters. Today’s LAE, such as flat-panel displays, operate at frequencies up to ~100 MHz. The next target is wireless applications [Internet of Things (IoT), 5/6G], where bringing LAE to the giga-Hertz regime opens new capabilities, enabling meter-scale radiative apertures for enhanced spatial resolution and power efficiency.
The two central performance goals that guide TFT design, fabrication, and in-circuit use, are: (1) for active device operation, achieving gain at high frequency, especially recognizing the potential for power gain (bound by f_MAX) rather than current gain (bound by f_T), and exploiting f_MAX-limited circuit operation to generate power at giga-Hertz frequencies; (2) for passive switch operation, achieving high off-to-on impedance ratio to reconfigure antenna elements. In terms of physical electronics, these goals are met in the following ways:
- f_T, the frequency up to which the TFT provides current gain, is determined by the electron transit time t between source and drain, and electron shunting by parasitic capacitances. t depends on electron mobility, channel length, and source/drain (S/D) contact resistance; while the shunting capacitances depend on the gate-source/drain (G-S/D) overlap regions. These are the knobs to turn for high f_T: the electron mobility is primarily set by the choice of semiconductor; minimal S/D contact resistance is achieved by selective etching; and channel length and G-S/D overlaps are minimized by self-aligning S/D to gate electrode. The f_T thus achieved is 0.4 GHz, for a typical ZnO TFT with channel length of 1um, biased at V_GS=V_DS=6 V with V_T=2 V.
- f_MAX, the frequency up to which the TFT provides power gain, depends on f_T, but can be made larger than f_T by reducing the sources of loss within the TFT. In practice, a factor that limits f_MAX is the power loss due to the gate resistance. This loss can be reduced by using a thick and highly conductive gate metal stack and multi-finger layout (putting fingers in parallel to reduce resistance along gate width). The f_MAX thus achieved is 2.0 GHz for the typical ZnO TFT and biasing.
- |Z_OFF/Z_ON|, the off-to-on impedance ratio of a TFT switch, depends on the conductance around an off channel, through G-S/D parasitic capacitances (which sets |Z_OFF|), as well as the t through the ZnO channel (which sets |Z_ON|). Conduction through the parasitic capacitances can be minimized via: (1) small G-S/D overlaps; and (2) the use of resonant inductors, which can be formed with high quality factor thanks to the sq. mm dimensions and low-loss substrates available in LAE. One important approach to reducing |Z_ON| is to operate at high V_GS but low V_DS. This ensures high channel conductance while reducing TFT power dissipation to avoid thermal-induced breakdown ^[3]. The |Z_OFF/Z_ON| thus achieved is ~48 at 2.4 GHz, with the half-bandwidth of ~350 MHz.
Based on these advanced ZnO TFTs, we demonstrated a cross-coupled LC oscillator operating at 1.25 GHz ^[1] and an LAE-based phased array system operating at 1 GHz with beamforming capability ^[2]. In a reconfigurable antenna with tunable directionality and operating frequency, ZnO TFT switches operate in the 2.4 GHz frequency band ^[3]. These giga-Hertz system performance results suggest that LAE is a promising candidate for wireless applications in IoT and 5/6G.

[1] Y. Mehlman et al., IEEE 2019 DRC pp. 63-64.
[2] C. Wu et al., Nat. Electron., in press.
[3] C. Wu et al., IEEE 2020 IEDM, paper 33-6.
[4] C. Wu et al., IEEE 2021 28th AM-FPD, pp. 51-54.

This work was supported by grants from Semiconductor Research Corporation (SRC), DARPA, and Princeton Program in Plasma Science and Technology (PPST). This work also utilized the Princeton PRISM cleanroom facilities.

As advanced manufacturing techniques evolve, the mass production of lightweight, flexible electronics requires one to evaluate the levels of production variability that can be expected both in geometric dimensions and in the overall print fidelity of a direct-write (DW) printed design. These variations, introduced by inconsistent material effects in both the printing and curing stages of manufacture, generate behavior different from traditional rigid copper-milled designs. Even the smallest of these divergences in a critical dimension can significantly alter the high frequency response of the printed RF design.
As such, this project evaluates the feature fidelity of micro-dispensed RF antennas and waveguides at 5G frequencies (X-band) on flexible 0.127mm-thick ITera MT40 dielectric substrate with silver nano-inks to determine repeatability, critical dimensions, and acceptable geometric tolerances in 8 GHz patch antennas and microstrip waveguides. All printed designs were characterized for surface roughness, print thickness, and feature fidelity. Two measures of geometric accuracy were also used across multiple critical features: variation in average feature size from design dimensions, and standard deviation of a feature’s size from its average within a single print. The critical feature dimensions were determined to be waveguide width and impedance matching “inlets.” S-Parameters were used as a measurement of RF performance, with the experimental range of each of the five measured features plotted separately against the respective antennas’ responses to illuminate any outstanding trends between single feature variation and performance. These results were compared to simulation sets in order to determine whether trends were due to the feature alone, or to combinations of interdependent geometric variations. Based on these assessments, acceptable “as-printed” ranges of feature dimensions were developed to produce consistent, acceptable RF performance.
It was found that surface roughness and print thickness have no significant effect on performance, and that the level of variation in the microstrip width does not degrade port-to-patch transmission. However, variation in the dimensions of the impedance matching junction between the transmission line and patch plays a significant roll in determining overall RF performance. In general, establishing the average standard deviation of printed traces in an RF antenna design determines the target transmission line width due to the non-linear effects of junction variability on impedance matching. By altering design parameters away from the traditional “ideal” dimensions and instead designing for a highly variable process, performance of up to -36dB was achieved in this study, with 70% of the dataset reaching -20dB or below. This is a significant improvement over traditional design dimensions which were predicted to range from -36dB to just -15dB. All printed results were compared to simulations and dual-clad copper laminate PCBs to verify process, design, and simulation accuracy.
Given the trends analyzed in this work, it is reasonable to expect that developing similar sets of feature dimensions can be applied to any DW printing process. Each DW process will introduce its own level of variability, but by designing the RF structures using the methods described here it becomes possible to take full advantage of DW’s compatibility with flexible substrates. The end result will be higher acceptable through-put while moving towards automated design and fabrication of flexible, on-demand, high-frequency systems.

Samuel Fedorka¹, Bradley Pothier¹, Katherine Berry¹, Gary F. Walsh², Corey Shemelya¹
1. University of Massachusetts Lowell, Printed Electronics Research Collaborative, 1 University Ave., Lowell, Massachusetts 01854
2. U.S. Army Combat Capabilities Development Command Soldier Center, 15 General Greene Ave., Natick, Massachusetts 01760
The past decade has seen a surge in additive manufacturing materials, devices, and applications bringing new form-factors to both new and existing technologies. Additionally, this unprecedented growth has led to a massive increase in the potential application space of printed electronics ranging from sensing to Radio Frequency (RF) antennas, systems, and components. As printed technologies advance, one must also examine the related RF designs, material systems, and fabrication techniques in order to adapt the technology to a variety of substrates and application environments. For example: a variety of new ink technologies and printable systems have been specifically designed to advance the field of AM, but there has been little research into designing fully integrated 5G RF systems with respect to these novel printing techniques. As such, these fully integrated solutions, i.e. materials, methods, design rules, and design constraints, required for the success printed RF systems/subsystems remains relatively unexplored in additive manufacturing (AM) research.
This work explores direct-write (DW) AM using silver nano-ink and RF-transparent spray coated THV Fluoropolymer on substrates ranging from “thick” laminated ultrahigh molecular weight polyethylene to standard “thin” flexible low-loss dielectric RF material. Using these techniques, an example RF circuit operating at 5G frequencies was designed and fabricated as a case study for DW RF sensing. This circuit consists of a 5G antenna, a Low Noise Amplifier, and an RF detector IC. All RF designs were simulated in ANSYS HFSS to ensure both printed and spray coated features produced the optimal RF response. Initial conducting elements were printed using a Nordson Pro4 direct-write, micro-pen dispensing system with final designs applicable to transition to an Optomec 5x as the processes/models are finalized for conformal geometries. The spray-coated fluoropolymer was shown to provide an RF transparent coating to improve component adhesion (i.e., capacitors, resistors, IC chips) and protect printed traces from external effects such as oxidation, without adversely affecting the RF performance. Also examined were additional 5G antenna designs on novel/alternative substrates (UHMWPE) where the spray coated polymeric layer was shown to be useful as a base layer in order to provide a smooth, “printable” surface (i.e., wettable surface) for printed RF antenna and waveguide systems. Additionally, the spray coated fluoropolymer was enabled full structural encapsulation through the deposition of subsequent layers.
This research is aimed to provide insight into how RF circuit design can be modified with new design constraints, substrates, and materials for a printed/flexible form factors while still maintaining high levels of performance. As such, we aim to present work that enables printed RF design/test to move from single component analysis to complete RF systems with integrated RF ICs, RF structures, DC components, DC ICs, novel substrates, and novel coatings in a variety of form factors.

Flexible energy storage devices such as supercapacitors (SCs) and lithium-ion batteries (LIBs) have attracted great attention for promising performances, such as high longevity and cycle life, fast charging-discharging rate, and high-power density. As a result, all-solid-state supercapacitors (ASSSCs) are a great candidate for many applications, including wearable electronic devices, portable devices, and electric vehicles (EVs).
In this study, a high-performance flexible supercapacitor with gel polymer electrolyte was developed with optimal design, fabrication, mechanical, safety and electrochemical performance. The optimum ratio of materials was determined for durable, long cycle life, high performance, flexible, and environmentally friendly supercapacitors (SCs). A flexible thin film of electrolyte was fabricated with a thickness of 475 µm. Since the electrodes were composed of nanocarbon with a high specific area, the fabricated supercapacitor was lightweight, flexible, and low cost with high electrochemical performances.
Moreover, comprehensive mechanical testing of the electrolyte, such as stretchability, flexibility, yield stress, and ultimate tensile strength (UTS), was conducted by the MARK-10 apparatus. As a result, the elongation of the gel polymer electrolyte (GPE) was 355%. Besides flexibility, different design strategies were adopted to provide additional stretchability in the supercapacitors.
Furthermore, electrochemical tests, including cyclic voltammetry (CV) and galvanostatic charge and discharge (GCD), has been conducted to evaluate the performance of the fabricated supercapacitors. In addition, SEM, FTIR, and AFM were used to characterize the change in the performance while in different bending angles.

Large volumes of highly purified dispersions of semiconducting single-walled carbon nanotubes have become readily available through polymer-wrapping and shear-force mixing. Such nanotube inks can be further stabilized by small molecule additives and thus enable the reproducible deposition (e.g., by aerosol jet printing, spincoating or zone-casting) of semiconducting nanotube layers of variable thickness from sparse to dense. These networks are employed in thin film transistors with high hole and electron mobilities (5-50 cm²V^-1s^-1). Through intentional doping it is possible to create purely n-type or p-type transistors with on/off current ratios exceeding 10⁷, which are suitable for complementary and high-frequency circuits. Here, we will discuss the basic charge transport properties and limitations of nanotube networks, novel n-dopants as well as routes toward low-voltage circuits on flexible substrates.

Doping of semiconducting polymers has seen a surge in research interest driven by emerging applications in sensing, bioelectronics and thermoelectrics. A recent breakthrough was a doping technique based on ion-exchange, which separates the redox and charge compensation steps of the doping process. The improved microstructural control this process allows enables us for the first time to systematically address a longstanding but still poorly understood question: what limits the electrical conductivity at high doping levels? Is it the formation of charge carrier traps in the Coulomb potentials of the counterions, or is it the structural disorder in the polymer lattice? Here, we apply ion-exchange doping to several classes of high mobility conjugated polymers and identify experimental conditions that achieve near 100% doping efficiency under degenerate conditions with nearly 1 charge per monomer. We demonstrate very high conductivities up to 1200 S/cm in semicrystalline polymer systems, and show that in this regime conductivity is poorly correlated with ionic size, but strongly correlated with paracrystalline disorder. This observation, backed by a detailed electronic structure model that incorporates ion-hole and hole-hole interactions and a carefully parameterized model of disorder, indicates that trapping by dopant ions is negligible, and that maximizing crystalline order is critical to improving conductivity.

Achieving high-performance and reliable transistors based on metal halide perovskites has been pursued for decades yet remains challenging because of the absence of highly qualified perovskite channels. Herein, we report inorganic perovskite thin-film transistors with exceptional performance using high-crystallinity and uniform cesium-tin-triiodide-based semiconducting layers with moderate hole concentrations and superior Hall mobilities, which are enabled by the judicious engineering of film composition and crystallization. The optimized devices exhibit high field-effect hole mobilities of over 50 square centimeters per voltage-second and large current modulation greater than 10⁸ as well as high operational stability and reproducibility. Our results represent a key step towards employing printable perovskite semiconductors in cost effective transistors and circuits.

To realize the projected speed, efficiency, and scalability of carbon nanotube (CNT) based electronics, the key fundamental challenge is the growth of horizontally-aligned arrays of defect-free CNT with a semiconducting purity >99.99%. Until recently, selective CNT synthesis methods utilized either the native free energy or native stability difference between the growth of semiconducting CNTs (s-CNTs) and metallic CNTs (m-CNTs). However, such native energy separation is too small to sufficiently suppress the random thermal fluctuation at the 800-900°C growth temperature and fails to delivery semiconducting carbon nanotubes with required >99.99% s-CNT purity. Here we report an approach to open a larger gap in the free energy between m-CNT and s-CNT during both epitaxial growth and chirality change by contacting growing CNTs with low work-function electrodes such as Hafnium Carbide or Titanium Carbide. First, the remarkable difference of quantum capacitance between m-CNT and s-CNT separates the CNT free energy plot of CNTs into two branches. Second, when in contact with low work-function electrodes CNTs and tipped catalysts will be negatively charged (n-doped) due to the electrode work-function, which causes an increase in free energy gap between m-CNTs and s-CNT and a reduction of the twisting barrier from m-CNT to s-CNT. Third, the large aspect ratio of CNTs enables a strong local electric field generated by the charge stored at the CNT tip to contribute a prominent electrostatic effect that greatly affects the total free energy change. As a result, stable twisting of horizontally aligned CNTs from initially mixed m-CNT and s-CNT chiralities to s-CNT chiralities is achieved by applying a weak perturbation (10V/mm-alternating electric field). These results validate that a lower work-function contact does result in a higher selectivity towards s-CNTs at a reduced applied electric field. Our observations also open a possibility to select the catalytic pathway through the contact engineering and will pave the way for practical applications of CNT electronics.

Organic and molecular electronics is a promising field for many applications, particularly flexible electronics [1] [2], since it allows the processing of electronic circuits at low temperatures on a variety of substrates, including flexible sheets [3]. Despite continuous improvements in the charge carrier mobility of both n- and p-type thin-film transistors, the performance of these devices is not yet attractive enough for commercial applications.
A substantial hurdle to the realization of effective organic-based analog and digital circuits is the lack of scalable high-charge-carrier-mobility organic semiconductors with low contact resistances in transistors [4]. Devices based on organic single-crystalline rubrene with superior mobility of over 10 cm²V^-1s^-1 have been previously shown [5]. However, such single crystals are difficult to grow and handle and are less suited for integrated electronics, which limits the commercial attractiveness of this material.
Here, we demonstrate vacuum-processed and, subsequently, thermally annealed highly crystalline rubrene thin films which enable the realization of large-area high-frequency circuits. We fabricate organic thin-film transistors (OTFTs) based on highly ordered rubrene films with different underlayers and reach charge carrier mobilities over 5 cm²V^-1s^-1 (for a channel length of L = 100 μm and width of W = 1000 μm). Moreover, we significantly reduce the contact resistance of the transistors down to 1 kOhm cm by utilizing contact doping [6]. We analyze the uniformity of the crystals and show that such rubrene thin-film transistors are well suited for large-area electronics.

[1] H. Sirringhaus, Adv. Mater. 2014, 26, 1319.
[2] B. Lussem, C-M. Keum, D. Kasemann, B. Naab, Z. Bao, K. Leo, Chem. Rev. 2016, 116, 22, 13714.
[3] B. Lim, H. Sun, J. Lee, Y-Y Noh, Sci Rep. 2017, 7, 164.
[4] J-W. Borchert, B. Peng, F. Letzkus, J-N. Burghartz, P. Chan, K. Zojer, S. Ludwigs, H. Klauk, Nat Commun. 2019, 10, 1119.
[5] J. Takeya, M. Yamagishi, Y. Tominari, R. Hirahara, Y. Nakazawa, Appl. Phys. Lett. 2007, 90, 102120.
[6] S-J. Wang, M. Sawatzki, H. Kleemann, I. Lashkov, D. Wolf, A. Lubk, F. Talnack, S. Mannsfeld, Y. Krupskaya, B. Büchner, K. Leo, Mat. Today. Phys. 2021, 17, 100352.

Images such as large mammals have impacted psychology of human species since the beginning of their history. Indeed, demand for technologies of meter-size large-area digital advertisement has been essentially very large, though the current devices are not well-prevailed because of high cost, large weight, and high power consumption. We developed a method to produce LED pixels on printed backplanes of active matrices based on high-mobility organic semiconductors. Due to the light-weight film-based materials of low-cost printed organic active matrices, which is also very effective in reducing power consumption, the present technology meets the requirement for the large-area digital signage. The present devices are formed on a sheet of PEN film with the thickness of 0.1 mm, so that the large-area movies can be shown hung on the ceilings of buildings or under mobile drones.
High-performance solution-processed transistors are employed with recently developed technologies of ultrathin organic semiconductor single crystalline films suitable for large-area production with low energy consumption. The films are easily formed to large area from solution at relatively low temperature at 80 degrees centigrade [1]. Extremely thin crystal films are controllably grown to a-few molecular layers with the thickness of only 10 nm, so that material cost can be only 0.01 USD per cm². Such prospect bears increasing reality because of recent research innovations in the field of material chemistry, charge transport physics, and solution processes of printable organic semiconductors. Particularly important are new processing technologies for continuous growth of the organic single-crystalline semiconductor “wafers” from solution and for lithographical patterning of semiconductors and metal electrodes. The active-matrix transistors are successfully formed with 60-Hz frame rate and more than 1-mA on current to illuminate the LEDs, thanks to the high carrier mobility as high as 10 cm²/Vs in the single-crustal organic semiconductors.

[1] A. Yamamura, J. Takeya et. al., Adv. Electron. Mater. 3, 1600456 (2017); S. Watanabe, J. Takeya et al., Communications Physics 1, 37 (2018); A. Yamamura, S. Watanabe, J. Takeya et al., Sci. Adv. 4, eaao5758 (2018); T. Makita, S. Kumagai, A. Kumamoto, M. Mitani, J. Tsurumi, R. Hakamatani, M. Sasaki, T. Okamoto, Y. Ikuhara, S. Watanabe, and J. Takeya, PNAS (2019), DOI.org/10.1073/pnas.1909932116; T. Sawada, A. Yamamura, M. Sasaki, K. Takahira, T. Okamoto, S. Watanabe and J. Takeya, Nat. Commun. volume 11, Article number: 4839 (2020)
DOI.org/10.1038/s41467-020-18616-0

Organic thin-film transistor (OTFT) devices have been fabricated using a maximum processing temperature of 80 degrees Celsius, rendering them compatible with a range of low-cost, transparent and flexible substrates. Charge mobility at a transistor channel length (L) of 5 µm is 3cm²/Vs. Such perfrormance is compatible with a wide range of voltage or current driven display device applications fabricated on plastic or glass. A small variation of mobility with channel length suggests a very low contact resistance (~100 Ohms), enabling good use of channel length scaling to increase operating frequencies in digital circuits. Current driven applications, such as local dimming mini-LED backlights, AMOLED or voltage driven displays and sensor arrays have been demonstrated using SmartKem’s OTFT materials. These are fabricated using process techniques common to flat panel display backplanes (sputtering, photolithography, dry/wet etching), providing a low cost of manufacture per unit area substrate.
The design of digital circuitry using transistor devices is aided by electronic design automation (EDA) tools which enable the accurate layout of a circuit using a process technology helping to deliver a consistent device performance. This presentation describes work to establish a process/technology design kit (PDK/TDK) for devices using the SmartKem OSC material stack. The PDK contains the device dimensions, overlaps, etc relating to the OTFT device and these are coded in software into parameterizable cells (p-cells). Device modelling and circuit simulations are used to predict electrical performance of the designs prior to tape out of the mask set for fabrication. PDK roadmaps are presented relating to improvements in device operating voltage and fabrication techniques, showing the potential for achieving circuits with higher operating frequency at a lower voltage in subsequent batch runs.

Wearable devices seamlessly integrated into everyday objects and able to conform to the movement of soft artificial and natural surfaces are considered one of the next major steps in the development of electronics for consumer products and industrial applications. The main application scenarios of such imperceptible electronics include the collection of data from the environment or the interaction with humans, to enable e.g. smart personalized healthcare devices or efficient Industry 4.0 processes. This requires the wireless transmission of data and the conditioning of a variety of analogue sensor signals using flexible, cheap, and environmentally-friendly components.
Thin-film transistors (TFTs), based on oxide semiconductors, most prominently amorphous Indium-Gallium-Zinc-Oxide (IGZO), and fabricated on large-area polymer substrates, are a suitable approach to fabricate active electronics if high bendability and electrical performance are required simultaneously. Such devices are investigated since more than a decade, hence their DC performance is well understood and reliable fabrication processes for systems, such as display backplanes, are established. However, the realization of highly scaled sub-micron devices on thermally and mechanically instable plastic substrates, together with the optimization of their analogue and high-speed performance, is less investigated. Here, various fabrication techniques and device structures, as well as theoretical models to predict the AC performance, resulting in flexible IGZO TFTs with channel length as short as 160 nm, are presented. Their AC performance is analysed, and it is discussed how analogue integrated circuits, such as amplifiers, buffers, or multiplexers made from these TFTs, can be used to realize front-end conditioning circuits for wearable sensors.
The IGZO TFTs presented here are manufactured on free standing polyimide foils. These TFTs are fabricated using high-k atomic layer deposition (ALD) grown Al₂O₃ insulating and passivation layers, metallic contacts, and RF sputtered IGZO with a stoichiometric composition of In:Ga:Zn:O = 1:1:1:4. The maximum process temperature used is 150°C. They exhibit state of the art DC performance including field effect mobilities up to ≈15 cm²/Vs, on-off current ratios >10⁸, subthreshold swings of ≈125 mV/dec., and threshold voltages allowing the operation of devices and circuits at voltages <5 V. The TFTs also stay fully functional when exposed to tensile or compressive strain induced by bending radii as small as 50 µm.
Here, the AC performance of these TFTs will be shown. All devices are designed with ground-signal-ground contact pads to enable their reliable AC characterization using a two-port network analyser. To minimize the channel length and hence to maximize the operation frequency of flexible IGZO TFTs, different fabrication methods are evaluated. These include conventional bottom gate TFTs, transistors structured by self-alignment, focused ion beam milling, or direct laser writing, as well as vertical TFTs and double gate TFT geometries.
The characterisation of all these devices showed that conventional bottom gate TFTs, with a channel length of 1 µm, and gate overlaps of 5 µm can exhibit a transit frequency up to 47 MHz. This value is in agreement with corresponding transconductance and gate capacitance measurements. At the same time, the AC performance is limited by the lateral dimensions of the TFTs. It was found that although traditionally smaller feature structures are beneficial, in the case of short and flexible IGZO TFTs, a careful optimisation of the capacitances and contact resistances is required. Consequently, the shortest transistors do not exhibit the highest frequencies, but a transit frequency of 135 MHz is demonstrated for 500 nm long self-aligned transistors even while exposed to 0.5% tensile strain. These in turn, enable the fabrication of fully flexible circuits able to operate in the megahertz regime.

The organic thin-film transistor (TFT) is a key device for flexible electronics. The field-effect mobility for p-channel TFTs has reached up to 10 cm² V^-1 s^-1, which is close to those of InGaZnO TFTs used in flat panel displays. We have been trying to realize short channel, high mobility organic TFTs for high frequency operation, and control the threshold voltage for actual applications.
A key issue for realization of short channel, high mobility organic TFTs is reducing contact resistance between organic channel materials and contact electrodes. We have used contact electrodes modified with benzenethiol derivative to realize low contact resistance. The modification demonstrated wide work function tuning for contact electrodes. The measured work function of modified Au electrodes was in a relatively wide range between 4.24–6.02 eV. We have applied modified electrodes to bottom-contact organic TFTs. The organic TFT having C10 DNTT as a channel material exhibited very high channel conductance. This is due to the low contact resistance, which is about 20 Ω cm under a gate voltage of –10 V.
We have used modified electrodes for high frequency operation in organic TFTs. The current-gain cutoff frequency was measured for evaluation of operational frequency in organic TFTs. The cutoff frequency was estimated by direct measurement of the gate and drain modulation currents. The measured cutoff frequencies for both C60 and pentacene TFTs increase consistently with reducing channel length. Cutoff frequencies of about 28 and 11 MHz were obtained from C60 and pentacene TFTs with a channel length of 2 μm, respectively.
The threshold voltage control is an important issue for actual application. We have reported the threshold voltage control in organic TFTs realized by oxygen plasma treatment. Exposing a SiO2 gate dielectric surface in organic TFTs to oxygen plasma shifted the threshold voltage to positive gate voltage. The threshold voltage shift depended on plasma treatment time, AC power for plasma generation, and gate dielectric thickness. The threshold voltage change was explained by negative charges induced on and/or near the surface of the gate dielectric. The threshold voltage change on the order of 1 V was roughly proportional to plasma treatment time. The predictable change enables the control of threshold voltage in this range. In actual, we used organic TFTs with controlled threshold voltages for application to logic circuits. Operation of logic inverters consisting of organic TFTs with different threshold voltages was demonstrated as an application of TFTs with controlled threshold voltage. Furthermore, a ring oscillators consisting of organic inverters operated at supply voltages in the range of 15–25 V. Combination of techniques about short channel, high mobility, low contact resistance, and threshold voltage control will realize high performance circuit consisting of organic TFTs.

The necessity of higher operation speed in electronic circuits is inevitable as there is a big amount of data being produced and transferred in our daily lives through different ways of digital communication. This argument is valid for silicon based integrated circuits as well as for thin-film-transistor based integrated circuits. Improvements in device processes have equal importance as circuit design techniques in order to achieve high frequency operation within the field of thin film transistors (TFT). In this invited presentation, we will provide an overview of different circuit design approaches specifically focusing on fast operating thin-film circuits combined with two main technology aspects: scaling and higher mobility materials.
Transistor downscaling has been proven to be key for Si CMOS electronics to obtain better figures of merit and consequently improve the operating speed of the integrated circuits. A analogy can be made for TFT-based circuits, whereby the minimum achievable channel length of TFTs on flexible substrates is mainly depending on photolithography accuracy, limiting it in many cases to μm-ranges. A self-aligned amorphous Indium-Gallium-Zinc-Oxide (a-IGZO) TFT technology allows n-type only TFTs to have a channel length down to 1.5 μm. Thanks to the self-aligned structure, overlap the capacitance between source/drain and gate of a TFT are mostly eliminated. These have been instrumental in demonstrating an ISO14443-A compatible flexible NFC barcode tag operating at 13.56 MHz [1].
Besides scaling of the transistor footprint, upscaling of the number of routing metals also helps increasing the operation speed and reducing footprint of thin-film ICs. Combining the design strategies already available for unipolar technologies with these process developments, performance and robustness of digital thin-film ICs have been increased continuously. One example is a Two-Stage Resistor-Load Logic standard cell library for digital applications [2]. We have demonstrated that this logic topology shows better speed performance than the conventional one-stage resistor-load topology. It is also more robust against the impact of global variability. To demonstrate this, a 4-bit digital multiplier using the new topology with 0.8 μm channel length TFTs is fabricated. It shows a speed performance up to 1 MHz.
The second important technology factor in the performance of thin-film ICs is the semiconductor material. For instance, Low-temperature polysilicon (LTPS) is attractive because of its higher mobility and the availability of complementary transistors. Because of its polycrystalline structure, minimum channel length of LTPS TFTs is larger than a-IGZO devices. In the presentation, we will show a performance comparison between logic gates realized with a-IGZO and LTPS technologies. Having complementary transistors enables circuit designers to use similar techniques as used in silicon CMOS designs. To demonstrate the capabilities of LTPS, we will also compare the measurement results of two different flip-flop architectures in terms of speed performance and power consumption.
References:
[1] Myny, Kris, et al. "15.2 A flexible ISO14443-A compliant 7.5 mW 128b metal-oxide NFC barcode tag with direct clock division circuit from 13.56 MHz carrier." 2017 IEEE International Solid-State Circuits Conference (ISSCC). IEEE, 2017.
[2] Celiker, Hikmet, et al. "Two-Stage Resistor-Load Logic for Digital Applications on Flexible Substrates" 2021 IEEE International Conference on Flexible and Printable Sensors and Systems (FLEPS). IEEE, 2021. (to be published)
Acknowledgement - This work has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme under grant agreement No 716426 (FLICs project).

Recently Structural Health Monitoring (SHM) is being widely assessed by the aerospace industry to enhance the safety and consistency of aircraft structures. The use of built-in sensor networks coupled with continuous structural integrity monitoring on an aircraft can provide vital real-time information concerning the condition, damage state, and/or service environment of the structure, which could significantly reduce the cost compared to scheduled manual inspection. Sustainable and durable piezoelectric nanogenerators (PENGs) with good flexibility, high performance, and superior reliability under inconsistent environments are a promising candidate for modern Wireless Sensor Network (WSN)-based SHM application. Numerous materials have been used to fabricate PENGs such as lead zirconate titanate (PZT), barium titanate (BaTiO3), ZnO, Na/KNbO3 and ZnSnO3 nanoparticles have great piezoelectric coefficients and superior energy conversion efficiencies. However, research progress is often impeded due to fragility, heavyweight, low durability, complex synthesis and fabrication process, and high toxicity of many of these materials. This research demonstrates the prospect of comprehending a self-powered wireless sensing system based on a piezoelectric nanogenerator (PENG) which is prominently anticipated to address the aforementioned issues. Here we present a flexible self-powered PENG sensor for the quantitative measurement of abnormal vibration based on WSN with the distinctive mesoporous structure via a drop-cast technique, and successfully demonstrated proof-of-the-concept applications. The well-dispersed and controllable size of ZnO nanoparticles (NPs) endows the composite film with the advantages of ZnO NPs and the virtues of the poly vinylidene fluoride (PVDF) matrix. A high voltage electrical poling of the porous PVDF film was performed with an electric field of 70-120 Vµm-1 for 5-6 hours with a DC voltage of 0-6 kV. No short circuit or noticeable voltage fluctuation was detected up to the maximum voltage of 6 kV. The poled porous PVDF composite films were sandwiched between electrodes. For the characterization purpose, the electrical connections were made from both of the top and bottom electrodes by very thin and flexible copper conductors. The self-powered wireless structural health monitoring system is a combination of an energy generation part, a part of sensing, and an integrated circuit (IC) part. A custom-made IC is placed on top of the PENG device, which is driven by the PENG device, monitors the surrounding environment automatically and transfers the signal to a receiver wirelessly. The performance of PENG based on the porous PVDF is systematically studied. The open-circuit voltage of the PENG device of 50wt.% ZnO was measured within a frequency range from 10Hz to 50Hz (varied frequency by Vibration Research's VR9500 Revolution unit) to examine the feasibility which is close to oscillation frequencies under a practical environment. The peak-to-peak output open-circuit voltage of the PENG was measured to be 84.5V at 30 Hz frequency. The peak output power (P=I²R) 78µW and the corresponding peak power density of 12µW/cm² were achieved at a load resistance of 7MΩ, which is sufficient for driving the entire system. Such a PENG device produces electrical energy from the shaker vibration, and charge an input capacitor (1 µF) through a rectifier circuit. When the charging voltage of the input capacitor reaches ~ 5V, which was regulated by a zener diode, it discharges energy through a buck converter module to an output storage capacitor (220 µF). The charging and discharging cycle of the input capacitor continues until the output capacitor is charged up to ~ 3.1V, and by then, it empowers the whole system and sends the signal to a remote receiver. These results demonstrate the great potential of porous PVDF based film for the simple and cost-effective fabrication of high-performance piezoelectric energy harvesting devices.

Emerging flexible electronics technologies including wearable biodevices, soft robotics, and stretchable sensors demand efficient thermal management to dissipate and distribute heat. One significant challenge in these systems is that the low thermal conductivity of flexible substrates limits device functionality by accumulating heat which leads to deteriorated charge transport and unstable electro-mechanical performance. To overcome these challenges, liquid metal (LM) dispersed elastomers have shown unique combinations of soft mechanical response with exceptional electrical and thermal functionalities. However, approaches to control LM microstructure in composites are needed to further enhance properties to enable heat dissipation and improved interfacial compatibility in flexible electronic systems. Here, we present an innovative approach to control LM microstructure in soft composites to enhance heat distribution while maintaining the soft, mechanical characteristics needed for flexible electronics. Through a mechanical reconfiguration and thermal relaxation process, we orient and elongate LM droplets in elastomers at high concentration (up to 70% by volume). Through this microstructural control in soft composites, we demonstrate a material which simultaneously achieves a high thermal conductivity of 13 W/mK (> 70 × increase over polymer matrix) and low modulus (< 1.0 MPa) in stress-free conditions. As a lightweight, conductive, and transferrable medium for flexible substrates, we also extend the LM morphology programming concept for micron sized thin films. The thickness tunable (30-70 μm) thin films are composed of disk-shaped LM droplets which can be electrically activated for circuit integration. This enables high stretchability (strain limit > 700%) and metallic electrical conductivity (sheet resistance < 0.1 Ω/sq) in soft films. We demonstrate these materials for rapid heat dissipation of high-power devices on flexible substrate, wearable robotics, and electronics integrated flexible circuits. We envision that the material, process, and device development strategy using the LM programmed composites will enable next-generation high-frequency flexible transistors, stretchable electronics, and soft robotics.

Triboelectric generator (TEG) devices have attracted considerable interest in the field of mechanical energy harvesting due to their low weight and mechanical softness. The charge transfer in TEG devices has been attributed to electron transfer, ion transfer, and material transfer [1] with the discussions still ongoing. There are many innovative TEG devices from low-cost materials presented in the literature for mechanical energy harvesting from human motion, vibrations, wind, water flow, and waves.[2-4] However, all TEG devices as a rule contain movable parts to magnify the dipole moment between triboelectric charges obtained from contact-separation. This specific requirement complicates the integration of TEG devices into energy harvesting systems. Alternative soft materials that exhibit bulk electromechanical response are piezoelectric polymers and their nanocomposites. For example, fluoropolymers such as polyvinylidene fluoride (PVDF) and PVDF co-polymers exhibit the piezoelectric d₃₃ coefficient as high as 20–28 pC N⁻¹ due to large dipole polarization between hydrogen and fluorine atoms.[5] However, to obtain high electromechanical response, the poling procedure at high electric fields (10–30 kV cm⁻¹ ) for prolonged periods (24 h or more) has to be applied. The necessity to apply high electric field limits moreover, the alignment of dipoles in the polymer is not stable and depolarization of PVDF is often observed over the course of days even at room temperature.
Herein we are presenting the triboelectric laminates where the polymer bilayers with different triboelectric properties are stacked to form laminates, together with the separator mesh between each bilayer and for an electrodes we use flexible polyisoprene conductive carbon black composite . The proposed design ensures the formation of aligned macroscopic dipoles across the volume of triboelectric laminate and provides the desired electromechanical response. When the force is applied, the triboelectric charges on opposite surfaces of bilayers are formed due to triboelectrification. The mesh provides the macroscopic dipole formation between the distinct triboelectric layers. Different triboelectric surface properties of polymer bilayers result in dipole alignment. Triboelectric laminate generates the open-circuit voltage (V_OC) of 29 V and short-circuit current (I_SC) of 15 nA. Volumetric energy density of 21.13 µJ cm⁻³ and a peak average power density of 46.19 µW cm⁻³ are observed. Triboelectric laminate shows an effective piezoelectric constant d₃₃ of 9.83 pC N⁻¹ , higher than reported for piezoelectric polymer like nylon-11 and there is no need to apply an external electric field to polarize the material and to observe the piezoelectric response. In addition, the electromechanical response of the triboelectric laminate does not disappear over time. The triboelectric laminate is produced from cheap, mechanically flexible materials that can be produced in large quantities.

This work has been supported by the European Regional Development Fund within the Activity 1.1.1.2 “Post-doctoral Research Aid” of the Specific Aid Objective 1.1.1 “To increase the research and innovative capacity of scientific institutions of Latvia and the ability to attract external financing, investing in human resources and infrastructure” of the Operational Programme “Growth and Employment” (No.1.1.1.2/VIAA/3/19/404)

[1] S. Lin, L. Xu, C. Xu, X. Chen, A. C. Wang, B. Zhang, P. Lin, Y. Yang, H. Zhao, Z. L. Wang, Adv. Mater. 2019, 31, 1808197.
[2] Z. Tian, J. He, X. Chen, T. Wen, C. Zhai, Z. Zhang, J. Cho, X. Choua, C. Xue, RSC Adv. 2018, 8, 2950.
[3] L. Zhao, L. Liu, X. Yang, H. Hong, Q. Yang, J. Wang, Q. Tang, J. Mater. Chem. A 2020, 8, 7880.
[4] C. Rodrigues, D. Nunes, D. Clemente, N. Mathias, J. M. Correia, P. Rosa-Santos, F. Taveira-Pinto, T. Morais, A. Pereira, J. Ventura, Energy Environ. Sci. 2020, 13, 2657.
[5] K. S. Ramadan, D. Sameoto, S. Evoy, Smart Mater. Struct. 2014, 23, 033001.

Fabricating high quality semiconductor thin films on plastic substrates is becoming increasingly important which is crucial in realizing high-speed electronic devices on plastic substrates. Germanium (Ge) thin film is one of the promising candidates among semiconductors which can be fabricated on flexible substrates at low temperature. Ge has many excellent properties such as higher electron and hole mobilities and smaller energy bandgap compared to silicon (Si), which is advantageous in applications such as thin-film transistor, solar cell, and next generation MOSFET. Metal-induced crystallization (MIC) technique by gold (Au) catalyst is one of the promising candidates to crystallize Ge films because of its low crystallization temperature. Nevertheless, further knowledge about the crystallization process is needed to improve the quality of the film. In this work, the Au layer thickness dependence of the crystallization process has been investigated in detail. It is found that the grain size, crystal orientation and crystallization temperature are sensitive to the Au layer thickness. Ge films crystallized with thinner Au films show lower crystallization temperature and higher (111) crystal orientation. A hole mobility as high as 50 cm²/Vs is achieved by annealing a Ge(30nm)/Au(9nm) bilayer at 220 °C, which is low enough to fabricate electronic devices on plastic substrates such as polyimide.

The Au and Ge films were fabricated by RF magnetron sputtering process on Si wafer with 100 nm thermally grown oxide (SiO₂) layer. Au films ranging from 9 to 34 nm were deposited on the wafer and subsequently Ge films (27 and 55 nm) were deposited on the Au layer. Au/Ge bilayers were crystallized by using MIC technique at 220 °C under N₂ ambient. Structural properties were examined by X-ray diffraction (XRD), scanning electron microscope (SEM) and electron backscattered diffraction (EBSD). Ge films crystallized with thinner Au film showed higher Ge (111) orientation, which was confirmed by XRD and EBSD observation. From the Hall effect measurement, p-type behavior and hole mobility as high as 50 cm²/Vs was achieved for the Ge film crystallized by annealing a Ge(30nm)/Au(9nm) bilayer film. Transistor action was also demonstrated successfully by using these films as channel layers. These findings will open up the possibility to apply MIC technique to crystallize Ge thin film at temperatures as low as 220 °C, which is low enough to fabricate thin-film semiconductor on various inexpensive plastic substrates.

The current generation of organic electronic devices such as the established OLED technology are based on amorphous thin-films with low carrier mobility due to their disordered molecular orientation. For high frequency device operation, higher carrier mobility is generally required. Here, we present the growth of highly crystalline organic thin-film crystals using vacuum deposition technique where the thickness of the films can be precisely tuned using epitaxy [1, 2]. In addition, organic dopants can be incorporated into the thin-film crystals during the deposition process that allow us to tune the properties of the individual layers. Finally, we integrate these doped highly crystalline thin films into devices to enable novel applications.

[1] S. Wang, M. Sawatzki, H. Kleemann, I. Lashkov, D. Wolf, A. Lubk, F. Talnack, S.11
Mannsfeld, Y. Krupskaya, B. Büchner, K. Leo, Mater. Today Phys. 2021, 17, 100352.
[2] M. Sawatzki, H. Kleemann, B. Boroujeni, S. Wang, J. Vahland, F. Ellinger, K. Leo, Adv.
Sci. 2021, 8, 2003519.

In this presentation, an analytical theoretical description of the ac transit frequency of organic field-effect transistors is proposed. The model is built upon an advanced physical description of the contact resistance as a key mathematical component. Such a treatment self-consistently and predictively correlates the transit frequency to a number of materials, geometrical, and operational parameters. By navigating a broad parametric space, it is found that the ambitious gigahertz operation is observable only in highly downscaled devices, and the intrinsic carrier mobilities and charge-injection barriers required to reach that regime are specified.

The shortwave infrared spectral region is particularly powerful for a variety of applications including environmental monitoring and medical diagnosis, enabling greater penetration depth and improved resolution in comparison to visible light. Upconversion imagers that combine photosensing and display in a compact structure are attractive since they avoid the costly and complex process of pixilation. Here, we have implemented polymeric upconversion imagers that combined photo-sensing and display in a compact structure, to extend the capability of human and machine vision to 1400 nm. The photoresponse exhibited a high external quantum efficiency of 35% at a low bias of ≤3 V with –3 dB bandwidth of 10 kHz. The large active area of 2 cm² enabled demonstrations such as object inspection, imaging through smog, and concurrent recording of blood vessel location and blood flow pulses.
In addition to visualisation of infrared radiation, this class of narrow bandgap polymers showed highly delocalized redox states and operated as stable n-type anodes in energy storage devices. The redox polymer was used in supercapacitors and achieved a high areal power density of 227 mW/cm², allowing rapid charging and relevant to ac line filtering applications. The capacitance retention was 84% after 11,000 full redox cycles, offering the critical benefit of long cycle life. This work demonstrated the application of a new class of stable redox-active materials suitable to meet the energy storage needs for short-range wireless electronics.

Despite the impressive advances that organic electronics has faced in the past 30 years, which led to the introduction of a variety of devices for a great number of different applications (such as, for example, the mobile phone industry and the biomedical field), organic field effect transistors still suffer from undeniable limitations, and present clearly subpar performances, especially with respect to standard silicon-based devices. In particular, despite the many advantages that organic transistor have in terms of low fabrication costs, easy processing techniques (often from solution), flexibility, and large area deposition, their actual employment for high-performance applications is still slowed down by several issues, such as the low mobility of organic semiconductors, the not-so-easy miniaturization, and poor long term stability of organic molecules, which generally lead to not always trivial encapsulation steps in order to avoid early aging due to the extreme sensitivity of such molecules to oxygen and moisture. These limitations have repercussions in the use of OFET-based devices in applications where high operation frequencies are needed, such as IoT, where smart devices are thought to intercommunicate and exchange continuous information in real time, and front-end circuits developing. An obvious approach to somehow dodge this limitation, which is in fact due to the intrinsic nature of the employed materials, and thus obtaining transistors with a suitable switching rate, is related to finding new ways to shorten the transistor channel length. However, a further challenge is doing so without excessively complicating the fabrication process. In fact, a lot of effort has been put in the past 20 years towards the reduction of devices dimensions, which is generally obtained using rather complicated approaches.

In order to meet the aforementioned goals, an accurate design is mandatory in order to minimize all the side effects that could limit the operational frequency of these devices. In this work we will report the approach that we have developed along the years in order to improve OFET performances. First of all, cut off frequency is strongly limited by all the device parasitic capacitances, therefore, thanks to the transparency of the employed substrates we have developed a self-alignment process that allowed us to achieve operational frequencies, very close to the MHz even with relatively low mobility semiconductors. More recently, in order to further close the gap with standard semiconductor-based devices, we have developed and optimized an easy and upscalable procedure for routinely fabricating submicrometer size OFETs with limited parasitic capacitances. These two factors turned out to be crucial to achieve cut off frequencies well above the MHz even with the employment of low mobility semiconductors, and without the need of employing expensive high resolution techniques. The proposed approach, thanks to its versatility and impressive performance can open up interesting new paths in the field of organic electronics for logic and front-end circuits developing and IoT applications.

Relatively long channel lengths of lateral-channel dual-gate organic thin-film transistors typically leads to slow inverter operation. Vertical channel dualgate organic thin-film transistors are a promising alternative due to their short channel lengths, however, the lack of appropriate p- and n-type devices has limited the development of complementary inverter circuits. Here we show that organic vertical n-channel permeable single- and dualbase transistors and vertical p-channel permeable base transistors can be used to create integrated complementary inverters and ring oscillators. With operating voltages < 2.0 V, the threshold voltages of the n-type OPDBTs are
changed across a wide range from 0.12 to 0.82 V by varying the voltage of the additional base. Thus, the vertical dual-base transistors enable a widerange of switching voltage controllability of complementary inverters. In addition, the inverters exhibit small switching time constants at 10 MHz, and the 7-stage complementary organic ring oscillators exhibit short signal propagation delays of 11 ns per stage at a supply voltage of 4 V. This underlines the potential of vertical organic transistors for high-frequency logic circuit applications