Mechanical and Electronic Properties of Thin-Film Transistors on Plastic, and Their Integration in Flexible Electronic Applications

Flexible Graphene Field-Effect Transistors and Their Application in Flexible Biomedical Sensing

Flexible electronics are transforming our lives by making daily activities more convenient. Central to this innovation are field-effect transistors (FETs), valued for their efficient signal processing, nanoscale fabrication, low-power consumption, fast response times, and versatility. Graphene, known for its exceptional mechanical properties, high electron mobility, and biocompatibility, is an ideal material for FET channels and sensors. The combination of graphene and FETs has given rise to flexible graphene field-effect transistors (FGFETs), driving significant advances in flexible electronics and sparked a strong interest in flexible biomedical sensors. Here, we first provide a brief overview of the basic structure, operating mechanism, and evaluation parameters of FGFETs, and delve into their material selection and patterning techniques. The ability of FGFETs to sense strains and biomolecular charges opens up diverse application possibilities. We specifically analyze the latest strategies for integrating FGFETs into wearable and implantable flexible biomedical sensors, focusing on the key aspects of constructing high-quality flexible biomedical sensors. Finally, we discuss the current challenges and prospects of FGFETs and their applications in biomedical sensors. This review will provide valuable insights and inspiration for ongoing research to improve the quality of FGFETs and broaden their application prospects in flexible biomedical sensing.

Solution-processed high-k photopatternable polymers for low-voltage electronics

High dielectric constant (k) polymers have been widely explored for flexible, low-power-consumption electronic devices. In this work, solution-processable high-k polymers were designed and synthesized by ultraviolet (UV) triggered crosslinking at a low temperature (60 °C). The highly crosslinked network allows for high resistance to organic solvents and high breakdown strength over 2 MV cm-1. The UV-crosslinking capability of the polymers enables them to achieve a high-resolution pattern with a feature size down to 1 μm. Further investigation suggests that the polar cyano pendants in side chains are responsible for increasing the dielectric constant up to 10 in a large-area device array, thereby contributing to a low driving voltage of 5 V and high field-effect mobility exceeding 20 cm2 V-1 s-1 in indium gallium zinc oxide (IGZO) thin-film transistors (TFTs). In addition, the solution-processable high-k dielectric polymers were utilized to fabricate flexible low-voltage organic TFTs, which show highly reliable and reproducible mechanical stability at a bending radius of 5 mm after 1000 cycles. And also, the high radiation stability of the dielectric polymers was observed in a UV-sensitive TFT device, thereby achieving highly reproducible pattern recognition, which is promising for artificial optic nerve circuits.

Pressure effects on both fluorescent emission and charge transport properties of organic semiconductors: a computational study

External pressure can regulate the photophysical property and charge transport performance of organic semiconductors, however, the underlying mechanism at the microscopic level is still elusive. Using thermal vibrational correlation function coupled quantum mechanics/molecular mechanics and full quantum charge transfer rate theory, we systematically explore the influence of pressure on fluorescence emission and charge transport behaviours of representative cyclooctatetrathiophene (COTh). It is found that, upon pressurization, the intramolecular configurations of COTh became more twisted, leading to the blue-shifted emission. The fluorescence quantum efficiency (FQE) of COTh crystals decreases monotonically in a wide pressure range of 0-4.38 GPa, because the increase of intermolecular electronic energy transfer rate constant (keet) is larger than the decrease of internal conversion rate constant (kic), and the variation of keet is dominant. The decrease in kic is attributed to the decreasing reorganization energy, reflecting the suppression of the low-frequency flipping vibrations of four thiophene rings and the high-frequency stretching vibrations of central cyclooctatetraene, while the keet increase is due to the simultaneous increase in exciton coupling and spectra overlap. Moreover, we predicted that the hole mobility of COTh increases monotonically by nearly an order of magnitude from 0.39 to 3.00 cm2 V-1 s-1 upon compression, because of the increase in transfer integral and the decrease of charged reorganization energy. Furthermore, its hole mobility exhibits obvious anisotropy. Our work systematically builds the external pressure, molecular packing, luminescence and transport properties relationships of organic semiconductors and provides theoretical guidance for the rational design of pressure responsive organic semiconductors with excellent photoelectric performance.

Flexible and Stretchable Organic Electrochemical Transistors for Physiological Sensing Devices

Flexible and stretchable bioelectronics provides a biocompatible interface between electronics and biological systems and has received tremendous attention for in situ monitoring of various biological systems. Considerable progress in organic electronics has made organic semiconductors, as well as other organic electronic materials, ideal candidates for developing wearable, implantable, and biocompatible electronic circuits due to their potential mechanical compliance and biocompatibility. Organic electrochemical transistors (OECTs), as an emerging class of organic electronic building blocks, exhibit significant advantages in biological sensing due to the ionic nature at the basis of the switching behavior, low driving voltage (<1 V), and high transconductance (in millisiemens range). During the past few years, significant progress in constructing flexible/stretchable OECTs (FSOECTs) for both biochemical and bioelectrical sensors has been reported. In this regard, to summarize major research accomplishments in this emerging field, this review first discusses structure and critical features of FSOECTs, including working principles, materials, and architectural engineering. Next, a wide spectrum of relevant physiological sensing applications, where FSOECTs are the key components, are summarized. Last, major challenges and opportunities for further advancing FSOECT physiological sensors are discussed.

Impact of Planar and Vertical Organic Field-Effect Transistors on Flexible Electronics

The development of flexible and conformable devices, whose performance can be maintained while being continuously deformed, provides a significant step toward the realization of next-generation wearable and e-textile applications. Organic field-effect transistors (OFETs) are particularly interesting for flexible and lightweight products, because of their low-temperature solution processability, and the mechanical flexibility of organic materials that endows OFETs the natural compatibility with plastic and biodegradable substrates. Here, an in-depth review of two competing flexible OFET technologies, planar and vertical OFETs (POFETs and VOFETs, respectively) is provided. The electrical, mechanical, and physical properties of POFETs and VOFETs are critically discussed, with a focus on four pivotal applications (integrated logic circuits, light-emitting devices, memories, and sensors). It is pointed out that the flexible function of the relatively newer VOFET technology, along with its perspective on advancing the applicability of flexible POFETs, has not been reviewed so far, and the direct comparison regarding the performance of POFET- and VOFET-based flexible applications is most likely absent. With discussions spanning printed and wearable electronics, materials science, biotechnology, and environmental monitoring, this contribution is a clear stimulus to researchers working in these fields to engage toward the plentiful possibilities that POFETs and VOFETs offer to flexible electronics.

Halogen Bond-Driven Aggregation-Induced Emission Skeleton: N-(3-(Phenylamino)allylidene) Aniline Hydrochloride

Aggregation-induced emission (AIE) is a unique photophysical process, and its emergence brings a revolutionary change in luminescence. However, AIE-based research has been limited to a few classical molecular skeletons, which is unfavorable for in-depth studies of the photophysical characteristics of AIE and the full exploitation of their potential values. There is an urgent need to develop new skeletons to rise to the challenges of an insufficient number of AIE core structures and difficult modification. Here, we report a novel dumbbell AIE skeleton, in which two phenyls are connected through (E)-3-iminoprop-1-en-1-amine. This skeleton shows extremely strong solid-state emission with an absolute quantum yield up to 69.5%, a large Stokes shift, and typical AIE characteristics, which well resolves the challenge of difficult modification and low luminous efficiency of the traditional AIE skeletons. One-step reaction, high yield, and diversified modification endow the skeleton with great scalability from simple to complicated, or from symmetrical to asymmetrical structures, which establishes the applicability of the skeleton in various scenarios. These molecules self-assemble into highly ordered layer-, rod-, petal-, hollow pipe-, or helix-like nanostructures, which contribute to strong AIE emission. Crystallographic data reveal the highly ordered layer structures of the aggregates with different substituents, and a novel halogen bond-driven self-assembly mechanism that restricts intramolecular motion is clearly discovered. Taking advantage of these merits, a full-band emission system from green to red is successfully established, which displays great potential in the construction of light-emitting films and advanced light-emitting diodes. The discovery of this AIE skeleton may motivate a huge potential application value in luminescent materials and lead to hitherto impossible technological innovations.

Strong Cellulose-Based Light-Management Film with Ultraviolet Blocking and Near-Infrared Shielding Performance

Light-management materials play a great significant role in efficient-energy buildings because they reduce indoor energy consumption by adjusting to natural sunlight, enhancing heat insulation, and creating comfortable indoor lighting. Here, we fabricated an ecofriendly and sustainable lignocellulose-based light-management film via a facile homogenization and mechanical hot-pressing method without complicated treatment or toxic reagents. The resulting film exhibited a favorable ultraviolet (UV) blocking performance of 82.25% for UVA, high visible light transmittance, and high haze, with a desirable tensile strength of 197 MPa, which was significantly higher than those of most petroleum-based plastics. Accordingly, the film was further endowed with near-infrared absorption performance by spray-coating with lanthanum hexaboride (LaB₆) nanoparticles─there were almost no adverse effects on its light transmittance or mechanical strength. Meanwhile, the high haze of the film implied that it met the requirement for privacy protection in smart buildings. The MFC/lignin/LaB₆ composite film has potential applications in energy-saving buildings and optoelectronic devices.

Nano-Scale Ga2O3 Interface Engineering for High-Performance of ZnO-Based Thin-Film Transistors

Thin-film transistor (TFT) is a essential device for future electronics driving the next level of digital transformation. The development of metal-oxide-semiconductor (MOS) TFTs is considered one of the most advantageous devices for next-generation, large-area flexible electronics. This study demonstrates the systematic study of the amorphous gallium oxide (a-Ga₂O₃) and its application to nanocrystalline ZnO TFTs. The TFT with a-Ga₂O₃/c-ZnO-stack channel exhibits a field-effect mobility of ∼41 cm² V^-1 s^-1 and excellent stability under positive-bias-temperature stress. The a-Ga₂O₃/c-ZnO-stack TFT on polyimide (PI) substrate exhibits a negligible threshold voltage shift upon 100k bending cycles with a radius of 3 mm and is very stable under environmental test. The smooth morphology with tiny grains of ∼12 nm diameter with fewer grain boundary states improves the charge transport in Ga₂O₃/ZnO-stack TFT. The existence of amorphous a-Ga₂O₃ in between very thin ZnO layers helps to enhance the heterointerfaces and reduce the defect density in Ga₂O₃/ZnO interface. Therefore, integrating a-Ga₂O₃ in the ZnO channel in stacked TFT can increase mobility and enhance stability for next-generation flexible TFT electronics.

N-Phenyl-substituted poly(benzimidazole imide)s with high glass transition temperature and low coefficient of thermal expansion

To obtain high thermostable materials for flexible display substrates, a series of copoly(benzimidazole imide)s was prepared using 5-amine-2-(4-aminobenzene)-1-phenyl-benzimidazole (N-PhPABZ) and 6(5)-amino-2-(4-aminobenzene)-benzimidazole (PABZ). Incorporating N-phenyl groups effectively healed the brittleness of the poly(benzimidazole imide)s (PBIIs) derived from pyromellitic dianhydride (PMDA), and the resultant homo- and copoly(benzimidazole imide)s displayed an outstandingly high glass transition temperature (T g > 450 °C) and a low coefficient of thermal expansion (CTE < 10 ppm K-1). Furthermore, the influence of removing intermolecular hydrogen bonds on the properties of these poly(benzimidazole imide)s was systematically analyzed. These data provide a feasible method to prepare superheat-resistant poly(benzimidazole imide)s without H-bonding.

Self-Aligned Bilayers for Flexible Free-Standing Organic Field-Effect Transistors

Free-standing and flexible field-effect transistors based on 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene)/polystyrene bilayers are obtained by well-controlled phase separation of both components. The phase separation is induced by solvent vapor annealing of initially amorphous blend films, leading to crystallization of TIPS-pentacene as the top layer. The crystallinity and blend morphology strongly depend on the molecular weight of polystyrene, and under optimized conditions, distinct phase separation with a well-defined and trap-free interface between both fractions is achieved. Due to the distinct bilayer morphology, the resulting flexible field-effect transistors reveal similar charge carrier mobilities as rigid devices and additionally pronounced environmental and bias stress stabilities. The performance of the flexible transistors remains stable up to a strain of 1.8%, while above this deformation, a close relation between current and strain is observed that is required for applications in strain sensors.

Mechanical Stress Stability of Flexible Amorphous Zinc Tin Oxide Thin-Film Transistors

Due to their low-temperature processing capability and ionic bonding configuration, amorphous oxide semiconductors (AOS) are well suited for applications within future mechanically flexible electronics. Over the past couple of years, amorphous zinc tin oxide (ZTO) has been proposed as indium and gallium-free and thus more sustainable alternative to the widely deployed indium gallium zinc oxide (IGZO). The present study specifically focuses on the strain-dependence of elastic and electrical properties of amorphous zinc tin oxide thin-films sputtered at room temperature. Corresponding MESFETs have been compared regarding their operation stability under mechanical bending for radii ranging from 5 to 2 mm. Force-spectroscopic measurements yield a plastic deformation of ZTO as soon as the bending-induced strain exceeds 0.83 %. However, the electrical properties of ZTO determined by Hall effect measurements at room temperature are demonstrated to be unaffected by residual compressive and tensile strain up to 1.24 %. Even for the maximum investigated tensile strain of 1.26 %, the MESFETs exhibit a reasonably consistent performance in terms of current on/off ratios between six and seven orders of magnitude, a subthreshold swing around 350 mV/dec and a field-effect mobility as high as 7.5 cm2V−1s−1. Upon gradually subjecting the transistors to higher tensile strain, the channel conductivity steadily improves and consequently, the field-effect mobility increases by nearly 80 % while bending the devices around a radius of 2 mm. Further, a reversible threshold voltage shift of about −150 mV with increasing strain is observable. Overall, amorphous ZTO provides reasonably stable electrical properties and device performance for bending-induced tensile strain up to at least 1.26 % and thus represent a promising material of choice considering novel bendable and transparent electronics.

Facile one-step fabrication of all cellulose composites with unique optical performance from wood and bamboo pulp

Cellulosic substrates completely originating from biomass have gained increasing attention for utilization in photoelectric devices due to their biodegradability, sustainability, and renewability. Herein, a simple one-step strategy was used to fabricate transparent (84.2%-90% at 550 nm) all-cellulose composites (ACCs) with customized optical haze (14.7%-83.7% at 550 nm) from wood and bamboo pulp due to their variable solubility. Surface roughness, coagulation bath composition, and the size of the undissolved cellulose fibers contributed to optical haze regulation. Fabricated ACCs demonstrated water resistance, thermal stability, and good mechanical properties. Moreover, an enhancement in the power conversion efficiency of a perovskite solar cell was achieved by simple attachment. Compared with non-sustainable petroleum base materials, ACCs exhibit biodegradability and renewability, which makes the composites promising in large-scale production and various applications due to their tunable haze.

New Simulation Method for Dependency of Device Degradation on Bending Direction and Channel Length

The dependency of device degradation on bending direction and channel length is analyzed in terms of bandgap states in amorphous indium-gallium-zinc-oxide (a-IGZO) films. The strain distribution in an a-IGZO film under perpendicular and parallel bending of a device with various channel lengths is investigated by conducting a three-dimensional mechanical simulation. Based on the obtained strain distribution, new device simulation structures are suggested in which the active layer is defined as consisting of multiple regions. The different arrangements of a highly strained region and density of states is proportional to the strain account for the measurement tendency. The analysis performed using the proposed structures reveals the causes underlying the effects of different bending directions and channel lengths, which cannot be explained using the existing simulation methods in which the active layer is defined as a single region.

Photo-induced Reactive Oxygen Species Activation for Amorphous Indium-Gallium-Zinc Oxide Thin-Film Transistors Using Sodium Hypochlorite

We proposed a novel material named sodium hypochlorite (NaClO) solution as a source of activation for amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs). We reduced the activation temperature from 300 to 150 °C using NaClO solution (concentration: 50%) and obtained satisfactory electrical characteristics of a-IGZO TFTs. The field-effect mobility, threshold voltage, on/off ratio, subthreshold swing, and threshold voltage (V_th) shift under negative bias illumination stress (V_G = -20 V and V_D = 10.1 V for 10,000 s) of NaClO (50%)-activated a-IGZO TFTs were 10.41 cm²/V·s, 1.51 V, 2.78 × 10⁸, 0.37 V/dec, and -5.43 V, respectively. Also, the V_th shifts of the NaClO (50%)-activated a-IGZO TFTs (150 °C) under the positive bias stress test decreased from 5.01 to 1.87 V (V_G = 20 V and V_D = 10.1 V for 10,000 s) compared with that of only-annealed (300 °C) a-IGZO TFTs. Also, the mechanism of NaClO activation for a-IGZO TFTs is clarified through photo-assisted oxygen radical (POR) and heat-driven oxygen radical (HOR) effects. The POR and HOR effects generated the reactive oxygen species (ROS) from NaClO solution (50%), which activated a-IGZO TFTs at a low temperature (150 °C). When the NaClO solution (50%) was exposed to external energy, it generated ROS such as hydroxyl radicals (OH^•), hydroperoxyl radicals (HO₂^•), and oxygen radicals (O^•), which promoted the formation of strong metal-oxide bonds in a-IGZO TFTs. Furthermore, NaClO solution (50%) was applied to a-IGZO TFTs on a flexible polyimide substrate and electrohydrodynamic printing process for selective deposition.

Amorphous NdIZO Thin Film Transistors with Contact-Resistance-Adjustable Cu S/D Electrodes

High-performance amorphous oxide semiconductor thin film transistors (AOS-TFT) with copper (Cu) electrodes are of great significance for next-generation large-size, high-refresh rate and high-resolution panel display technology. In this work, using rare earth dopant, neodymium-doped indium-zinc-oxide (NdIZO) film was optimized as the active layer of TFT with Cu source and drain (S/D) electrodes. Under the guidance of the Taguchi orthogonal design method from Minitab software, the semiconductor characteristics were evaluated by microwave photoconductivity decay (μ-PCD) measurement. The results show that moderate oxygen concentration (~5%), low sputtering pressure (≤5 mTorr) and annealing temperature (≤300 °C) are conducive to reducing the shallow localized states of NdIZO film. The optimized annealing temperature of this device configuration is as low as 250 °C, and the contact resistance (R_C) is modulated by gate voltage (V_G) instead of a constant value when annealed at 300 °C. It is believed that the adjustable R_C with V_G is the key to keeping both high mobility and compensation of the threshold voltage (V_th). The optimal device performance was obtained at 250 °C with an I_on/I_off ratio of 2.89 × 10⁷, a saturation mobility (μ_sat) of 24.48 cm²/(V·s) and V_th of 2.32 V.

Impact of Backbone Amide Substitution at the Meta- and Para-Positions on the Gas Barrier Properties of Polyimide

This study designed and synthesised a meta-amide-substituted dianiline monomer (m-DABA) as a stereoisomer of DABA, a previously investigated para-amide-substituted dianiline monomer. This new monomer was polymerised with pyromellitic dianhydride (PMDA) to prepare a polyimide film (m-DABPI) in a process similar to that employed in a previous study. The relationship between the substitution positions on the monomer and the gas barrier properties of the polyimide film was investigated via molecular simulation, wide-angle X-ray diffraction (WXRD), and positron annihilation lifetime spectroscopy (PALS) to gain deeper insights into the gas barrier mechanism. The results showed that compared with the para-substituted DABPI, the m-DABPI exhibited better gas barrier properties, with a water vapour transmission rate (WVTR) and an oxygen transmission rate (OTR) as low as 2.8 g·m⁻²·d⁻¹ and 3.3 cm³·m⁻²·d⁻¹, respectively. This was because the meta-linked polyimide molecular chains were more tightly packed, leading to a smaller free volume and lower molecular chain mobility. These properties are not conducive to the permeation of small molecules into the film; thus, the gas barrier properties were improved. The findings have significant implications for the structural design of high-barrier materials and could promote the development of flexible display technology.

Electrochemical Generation of Hydrated Zinc Vanadium Oxide with Boosted Intercalation Pseudocapacitive Storage for a High-Rate Flexible Zinc-Ion Battery

With the surging development of flexible wearable and stretchable electronic devices, flexible energy-storage devices with excellent electrochemical properties are in great demand. Herein, a flexible Zn-ion battery comprised by hydrated zinc vanadium oxide/carbon cloth (ZnVOH/CC) as the cathode is developed, and it shows a high energy density, superior lifespan, and good safety. ZnVOH/CC is obtained by the in situ transformation of hydrated vanadium oxide/carbon cloth (VOH/CC) by an electrochemical method, and the intercalation pseudocapacitive reaction mechanism is discovered for ZnVOH/CC. The co-insertion/deinsertion of H⁺/Zn²⁺ is observed; the H⁺ insertion dominates in the initial discharge stage and the high-rate electrochemical process, while Zn²⁺ insertion dominates the following discharge stage and the low-rate electrochemical procedure. An ultrastable reversible capacity of 135 mAh g^-1 at 20 A g^-1 is obtained after 5000 cycles without capacity fading. Moreover, the as-assembled flexible zinc-ion battery can operate normally under rolled, folded, and punched conditions with superior safety. It is capable to deliver a high discharge capacity of 184 mAh g^-1 at 10 A g^-1 after 170 cycles. This work paves a new way for designing low-cost, safe, and quick-charging energy-storage devices for flexible electronics.

Highly Deformable, Ultrathin Large-Area Poly(methyl methacrylate) Films

Poly(methyl methacrylate) (PMMA) is a glassy engineering polymer that finds extensive use in a number of applications. Over the past decade, thin films of PMMA were combined with graphene or other two-dimensional materials for applications in the area of nanotechnology. However, the effect of size upon the mechanical behavior of this thermoplastic polymer has not been fully examined. In this work, we adopted a homemade nanomechanical device to assess the yielding and fracture characteristics of freestanding, ultrathin (180-280 nm) PMMA films of a loaded area as large as 0.3 mm2. The measured values of Young's modulus and yield strength were found to be broadly similar to those measured in the bulk, but in contrast, all specimens exhibited a quite surprisingly high strain at failure (>20%). Detailed optical examination of the specimens during tensile loading showed clear evidence of craze development which however did not lead to premature fracture. This work may pave the way for the development of glassy thermoplastic films with high ductility at ambient temperatures.

Performance Enhancement and Bending Restoration for Flexible Amorphous Indium Gallium Zinc Oxide Thin-Film Transistors by Low-Temperature Supercritical Dehydration Treatment

For high-performance and high-lifetime flexible and wearable electronic applications, a low-temperature posttreatment method is highly expected to enhance the device performance and repair the defects induced by the low-temperature fabrication process intrinsically. Particularly, if the method can repair the traces induced by the multiple cycles of bending or deforming, it would overcome current fatal obstacles and provide a vital solution to the rapid development of flexible electronics. In this work, we propose a method to apply low-temperature supercritical CO₂ fluid with a dehydration function to improve or even restore the performance of flexible amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs). After the treatment, the a-IGZO TFT exhibits 3 times improvement drivability up to 0.24 μA/μm, a smaller subthreshold swing of 0.18 V dec^-1, a smaller V_th of 0.25 V, and a larger I_on/I_off ratio of 3.8 × 10⁷. Additionally, the posttreated a-IGZO TFTs possess relatively good uniformity and reproducibility with an on-current standard deviation of 0.047 μA/μm, and the performance of the a-IGZO TFT after the treatment remains almost unchanged even after bending 2500 times at a bending radius of 5 mm. These characteristics are attributed to the improved quality of the channel and gate dielectric. It is worth noting that when this is applied to a flexible TFT-driven organic light-emitting diode lighting system, this treatment method can restore the performance of not only the TFT but also the lighting system, even after the system has been bent more than 600 times and has failed. To date, this is the first time that the bending-track erasing function of the supercritical fluid for flexible systems has been reported, which has the potential to prolong the lifetime of flexible electronics.

Material innovation and mechanics design for substrates and encapsulation of flexible electronics: a review

Advances in materials and mechanics designs have led to the development of flexible electronics, which have important applications to human healthcare due to their good biocompatibility and conformal integration with biological tissue. Material innovation and mechanics design have played a key role in designing the substrates and encapsulations of flexible electronics for various bio-integrated systems. This review first introduces the inorganic materials and novel organic materials used for the substrates and encapsulation of flexible electronics, and summarizes their mechanics properties, permeability and optical transmission properties. The structural designs of the substrates are then introduced to ensure the reliability of flexible electronics, including the patterned and pre-strained designs to improve the stretchability, and the strain-isolation and -limiting substrates to reduce the deformation. Some emerging encapsulations are presented to protect the flexible electronics from degradation, environmental erosion or contamination, though they may slightly reduce the stretchability of flexible electronics.

Measuring the multilayer silicon based microstructure using differential reflectance spectroscopy

The yield of a large-area ultra-thin display panel depends on the realization of designed thickness of multilayer films of all pixels. Measuring the thicknesses of multilayer films of a single pixel is crucial to the accurate manufacture. However, the thinnest layer is reaching the sub-20nm level, and different layers feature remarkable divergence in thickness with similar optical constants. This turns to a key obstruction to the thickness characterization by optical spectroscopy. Based on the tiny differences in absorptivity, a fast method for measuring the film thickness in a single pixel is proposed which combines the layer number reducing model and micro-area differential reflectance spectroscopy. The lower layers can be considered as semi-infinite in the corresponding spectral range whose thickness is infinite in the fitting algorithm. Hence, the thickness of the upper layer is fitted in a simplified layer structure. For demonstration, a multilayer silicon microstructure in a single pixel, p-Si/a-Si/n-Si (10nm/950nm/50nm) on complex substrate, is measured. The light spot diameter is about 60 microns with measuring-time in 2 seconds. The measurement deviation is 3% compared by a commercial ellipsometer. To conclude, the proposed method realizes the layer number reduction for fitting multilayer thickness with large thickness difference and similar optical constants, which provides a powerful approach for multilayer microstructure characterizations.

Atomic-Level and Modulated Interfaces of Photocatalyst Heterostructure Constructed by External Defect-Induced Strategy: A Critical Review

Despite the existence of numerous photocatalyst heterostructures, their separation efficiency and charge flow precision remain low due to the poor study on interfacial properties. The photocatalysts with confined defects can effectively control the photogenerated carrier migration, but the metastability of such defects considerably decreases the photocatalyst stability. Meanwhile, the introduction of defective region can increase the coordinative unsaturation and delocalize local electrons to promote their interactions with the molecules/ions in that region. The selective growth of modulated heterogeneous interface by defect-induced strategy may not only increase the stability of defective structures, but also enhance the migration of interfacial charges. Using this method, photocatalytic heterostructures with low contact resistances and intimate interfaces are constructed to achieve the optimal charge migration in terms of efficiency and accuracy. In this work, the point, linear, and planar heterogeneous interfaces and related defect engineering techniques are discussed. Particularly, it is focused on the external, defect-induced interfacial heterogeneities with various spatial and dimensional configurations, which exhibit modulated and controllable interfacial properties. Furthermore, the main aspects of fabricating photocatalyst heterostructures by the defect-induced strategy, including the i) controllable generation of defects, ii) advanced characterization methods, and iii) elaborate construction of the minimal interface, are described.

Strain Engineering in 2D Material-Based Flexible Optoelectronics

Flexible optoelectronics, as promising components hold shape-adaptive features and dynamic strain response under strain engineering for various intelligent applications. 2D materials with atomically thin layers are ideal for flexible optoelectronics because of their high flexibility and strain sensitivity. However, how the strain affects the performance of 2D materials-based flexible optoelectronics is confused due to their hypersensitive features to external strain changes. It is necessary to establish an evaluation system to comprehend the influence of the external strain on the intrinsic properties of 2D materials and the photoresponse performance of their flexible optoelectronics. Here, a focused review of strain engineering in 2D materials-based flexible optoelectronics is provided. The first attention is on the mechanical properties and the strain-engineered electronic properties of 2D semiconductors. An evaluation system with relatively comprehensive parameters in functionality and service capability is summarized to develop 2D materials-based flexible optoelectronics in practical application. Based on the parameters, some strategies to improve the functionality and service capability are proposed. Finally, combining with strain engineering in future intelligence devices, the challenges and future perspective developing 2D materials-based flexible optoelectronics are expounded.

Picene and PTCDI based solution processable ambipolar OFETs

Facile and efficient solution-processed bottom gate top contact organic field-effect transistor was fabricated by employing the active layer of picene (donor, D) and N,N'-di(dodecyl)-perylene-3,4,9,10-tetracarboxylic diimide (acceptor, A). Balanced hole (0.12 cm2/Vs) and electron (0.10 cm2/Vs) mobility with Ion/off of 104 ratio were obtained for 1:1 ratio of D/A blend. On increasing the ratio of either D or A, the charge carrier mobility and Ion/off ratio improved than that of the pristine molecules. Maximum hole (µmax,h) and electron mobilities (µmax,e) were achieved up to 0.44 cm2/Vs for 3:1 and 0.25 cm2/Vs for 1:3, (D/A) respectively. This improvement is due to the donor phase function as the trap center for minority holes and decreased trap density of the dielectric layer, and vice versa. High ionization potential (- 5.71 eV) of 3:1 and lower electron affinity of (- 3.09 eV) of 1:3 supports the fine tuning of frontier molecular orbitals in the blend. The additional peak formed for the blends at high negative potential of - 1.3 V in cyclic voltammetry supports the molecular level electronic interactions of D and A. Thermal studies supported the high thermal stability of D/A blends and SEM analysis of thin films indicated their efficient molecular packing. Quasi-π-π stacking owing to the large π conjugated plane and the crystallinity of the films are well proved by GIXRD. DFT calculations also supported the electronic distribution of the molecules. The electron density of states (DOS) of pristine D and A molecules specifies the non-negligible interaction coupling among the molecules. This D/A pair has unlimited prospective for plentiful electronic applications in non-volatile memory devices, inverters and logic circuits.

Devising Materials Manufacturing Toward Lab-to-Fab Translation of Flexible Electronics

Flexible electronics have witnessed exciting progress in academia over the past decade, but most of the research outcomes have yet to be translated into products or gain much market share. For mass production and commercialization, industrial adoption of newly developed functional materials and fabrication techniques is a prerequisite. However, due to the disparate features of academic laboratories and industrial plants, translating materials and manufacturing technologies from labs to fabs is notoriously difficult. Therefore, herein, key challenges in the materials manufacturing of flexible electronics are identified and discussed for its lab-to-fab translation, along the four stages in product manufacturing: design, materials supply, processing, and integration. Perspectives on industry-oriented strategies to overcome some of these obstacles are also proposed. Priorities for action are outlined, including standardization, iteration between basic and applied research, and adoption of smart manufacturing. With concerted efforts from academia and industry, flexible electronics will bring a bigger impact to society as promised.

Self-polarized Poly(vinylidene fluoride) Ultrathin Film and Its Piezo/Ferroelectric Properties

Organic nonvolatile memory with ultralow power consumption is a critical research demand for next-generation memory applications. However, obtaining a large-area, highly oriented ferroelectric ultrathin film with low leakage current and stable ferroelectric switching remains a challenge for achieving low operation voltage in ferroelectric memory transistors. Here, an ideal ferroelectric neat PVDF ultrathin film with a high degree of orientation is fabricated by a melt-draw technique without post-thermal treatment and assisted stabilization process. The PVDF ultrathin film is self-polarized with predominantly vertical orientation of dipole moments, exhibiting a d₃₃ of 25 pm V^-1 and the ultralow coercive voltage of approximately 3 V characterized by piezoresponse force microscopy. A remnant polarization of 6.3 μC cm^-2 is identified based on a PVDF capacitor with an active layer formed by six layers of melt-drawn thin films. By employing a single-layer melt-drawn PVDF ultrathin film as an insulation layer, a bottom-gate-top-contact ferroelectric field-effect transistor is fabricated with a very low operation voltage of 5 V. It exhibits a memory window with an on/off current ratio of 10³ at zero gate bias and threshold voltage shift of around 2 V.

3D-metal-embroidered electrodes: dreaming for next generation flexible and personalizable energy storage devices

Flexible and Personalizable battery is a promising candidate for energy storage, but suffers from the weldablity and large-scale producibility of the electrode. To address the issues, we design a nickel foam catalyzed electroless deposition (NFED) derived 3D-metal-pattern embroidered electrodes. This is the first attempt to utilize this type of electrode in battery field. It is found that the current collector can be embroidered on any selected areas of any complex-shape electrodes, with high controllability and economical feasibility. As a result, the electronic conductivity of the flexible electrodes can be improved by nearly one order of magnitude, which can be easily and firmly weldded to the metal tab using the industry generic ultrasonic heating process. The embroidered electrodes could substantially promote the electrochemical performance under bending deformation, with both Li-S and Li-LiFePO₄ batteries as the models. This innovation is also suitable to embroider all the VIII group elements on any electrodes with personalized shapes, which is widely attractive for the development of next generation flexible and personalizable energy storage devices.

Mechanochemical and Thermal Treatment for Surface Functionalization to Reduce the Activation Temperature of In-Ga-Zn-O Thin-film Transistors

Amorphous indium-gallium-zinc oxide (a-IGZO) films, which are widely regarded as a promising material for the channel layer in thin-film transistors (TFTs), require a relatively high thermal annealing temperature to achieve switching characteristics through the formation of metal-oxygen (M-O) bonding (i.e., the activation process). The activation process is usually carried out at a temperature above 300 °C; however, achieving activation at lower temperatures is essential for realizing flexible display technologies. Here, a facile, low-cost, and novel technique using cellophane tape for the activation of a-IGZO films at a low annealing temperature is reported. In terms of mechanochemistry, mechanical pulling of the cellophane tape induces reactive radicals on the a-IGZO film surface, which can give rise to improvements in the properties of the a-IGZO films, leading to an increase in the number of M-O bonds and the carrier concentration via radical reactions, even at 200 °C. As a result, the a-IGZO TFTs, compared to conventionally annealed a-IGZO TFTs, exhibited improved electrical performances, such as mobility, on/off current ratio, and threshold voltage shift (under positive bias temperature and negative bias temperature stress for 10,000 s at 50 °C) from 8.25 to 12.81 cm²/(V·s), 2.85 × 10⁷ to 1.21 × 10⁸, 6.81 to 3.24 V, and -6.68 to -4.93 V, respectively.

Stress Release Effect of Micro-hole Arrays for Flexible Electrodes and Thin Film Transistors

The effects of micro-hole arrays in the thin metal films were studied as a method to release bending stress in flexible electrodes and flexible thin film transistors (TFTs). Interest in flexible electronics is increasing, and many approaches have been suggested to solve the issue of the electrical failure of electrodes or electrical components such as TFTs after repeated bending. Here, we demonstrate a micro-hole array structure as a common solution to release bending stress. Although micro-size cracks were generated and propagated from the hole edges, the cracks stopped within a certain range when enough stress was released. Moreover, since the crack sites were predictable and controllable, a fatal electrical breakdown in a conductive layer such as a metal electrode or the semiconducting junction of a TFT can be prevented by specifically arranging the hole arrays. Thin film layers fabricated without holes suffered an electrical breakdown due to random crack propagation during bending tests. Aluminum thin film electrodes prepared with arrays of 3 μm diameter holes and 25% hole area showed excellent durability after 300,000 bending cycles. The change in resistance was below 3%. The electrical characteristics of an a-IGZO TFT with the micro-hole structure were almost equivalent to a standard a-IGZO TFT. After 10,000 bending cycles, I_ON and the ratio of I_ON/I_OFF remained >10⁷ A and ∼10⁷, respectively. Since the effective hole diameter is micrometer in size, fabrication does not require additional process steps or expensive process equipment. Therefore, the approach can be an important way to enhance the reliability of various electrical devices in flexible and wearable applications.

Ink synthesis and inkjet printing of electrostatically stabilized multilayer graphene nanoshells

HYPOTHESIS

Most functional inkjet inks are sterically stabilized nanoparticle dispersions that require a post-printing-process to remove stabilizing materials and gain functionality. This post-process limits material selection and increases fabrication time and complexity for printed devices. By optimizing the electrostatic stability of a carbon nanomaterial dispersed in water or ethylene glycol via pH adjustment, a stable and printable ink should be attainable without a steric stabilizing material and hence the post-process may be avoided.

EXPERIMENTS

The electrostatic stability of multilayer graphene nanoshells (MGNS)-an inexpensive and net carbon-negative nanomaterial-dispersed in water and ethylene glycol was studied by measuring zeta potential as a function of pH and modeling energetic potentials between particles. Requirements for electrical percolation of printed MGNS were analyzed and corroborated with electrical measurements.

FINDINGS

Electrostatic stability improved with increased zeta potential caused by an increased pH. Ionic strength also increased with pH, causing strong destabilization. By increasing zeta potential while minimizing ionic strength, the maximum solid-loading of MGNS in DI water and ethylene glycol was increased up to 20%. For the MGNS solid-loading achieved here, electrical percolation occurs with 20-30 consecutively printed layers producing a resistivity of 30 Ω-cm. The inexpensive, environmentally-friendly MGNS are a promising material for printed, flexible electronics.

Roles of interfaces in the ideality of organic field-effect transistors

Organic field-effect transistors (OFETs) are fundamental building blocks for flexible and large-area electronics due to their superior solution-processability, flexibility and stretchability. OFETs with high ideality are essential to their practical applications. In reality, however, many OFETs still suffer from non-ideal behaviors, such as gate-dependent mobility, which thus hinders the extraction of their intrinsic performance. It is much desired to gain a comprehensive understanding of the origins of these non-idealities. OFETs are primarily interface-related devices, and hence their performance and ideality are highly dependent on the interface properties between each device component. This review will focus on the recent progress in investigating the non-ideal behaviors of OFETs. In particular, the roles of interfaces, including the organic semiconductor (OSC)/dielectric interface, OSC/electrode interface and OSC/atmosphere interface, in determining the ideality of OFETs are summarized. Viable approaches through interface optimization to improve the device ideality are also reviewed. Finally, an overview of the outstanding challenges as well as the future development directions for the construction of ideal OFETs is given.

Transparent Flexible Nanoline Field-Effect Transistor Array with High Integration in a Large Area

Transparent flexible transistor array requests large-area fabrication, high integration, high manufacturing throughput, inexpensive process, uniformity in transistor performance, and reproducibility. This study suggests a facile and reliable approach to meet the requirements. We use the Al-coated polymer nanofiber patterns obtained by electrohydrodynamic (EHD) printing as a photomask. We use the lithography and deposition to produce highly aligned nanolines (NLs) of metals, insulators, and semiconductors on large substrates. With these NLs, we demonstrate a highly integrated NL field-effect transistor (NL-FET) array (10⁵/(4 × 4 in²), 254 pixel-per-inch) made of pentacene and indium zinc oxide semiconductor NLs. In addition, we demonstrate a NL complementary inverter (NL-CI) circuit consisting of pentacene and fullerene NLs. The NL-FET array shows high transparency (∼90%), flexibility (stable at 2.5 mm bending radius), uniformity (∼90%), and high performances (mobility = 0.52 cm²/(V s), on-off ratio = 7.0 × 10⁶). The NL-CI circuit also shows high transparency, flexibility, and typical switching characteristic with a gain of 21. The reliable large-scale fabrication of the various NLs proposed in this study is expected to be applied for manufacturing transparent flexible nanoelectronic devices.

Relieving the Photosensitivity of Organic Field-Effect Transistors

It is generally believed that the photoresponse behavior of organic field-effect transistors (OFETs) reflects the intrinsic property of organic semiconductors. However, this photoresponse hinders the application of OFETs in transparent displays as driven circuits due to the current instability resulting from the threshold voltage shift under light illumination. It is necessary to relieve the photosensitivity of OFETs to keep the devices stable. 2,6-diphenyl anthracene thin-film and single-crystal OFETs are fabricated on different substrates, and it is found that the degree of molecular order in the conducting channels and the defects at the dielectric/semiconductor interface play important roles in determining the phototransistor performance. When highly ordered single-crystal OFETs are fabricated on polymeric substrates with low defects, the photosensitivity (P) decreases by more than 10⁵ times and the threshold voltage shift (ΔV_T ) is almost eliminated compared with the corresponding thin-film OFETs. This phenomenon is further verified by using another three organic semiconductors for similar characterizations. The decreased P and ΔV_T of OFETs ensure a good current stability for OFETs to drive organic light-emitting diodes efficiently, which is essential to the application of OFETs in flexible and transparent displays.

Wrinkled titanium nitride nanocomposite for robust bendable electrodes

Electrical contacts and interconnections are critical components for all electronic devices. Bendable electrodes with enhanced electro-mechanical properties are highly desirable to develop innovative wearable electronic devices. Herein we report on a fabrication method for robust bendable coatings based on titanium nitride (TiN) thin films and silver nanowires (Ag NWs). TiN and TiN-AgNWs nanocomposites were deposited on polyethylene terephthalate (PET) substrates using a plasma enhanced pulsed laser deposition (PLD) technique. The resulting TiN coatings exhibit excellent adhesion to PET and their sheet resistance can be tuned using a dual frequency PLD process and further decreased by incorporating Ag NWs into the TiN layers. Sample sheet resistance was decreased down to values as low as [Formula: see text] corresponding to the formation of TiN-AgNWs nanocomposites. The electro-mechanical robustness of TiN based coatings were evaluated by four-probe resistance measurements in situ under cyclic bending tests. We show that the TiNAgNWs nanocomposites surpass both ITO and Ag NWs coatings in terms of mechanical robustness and electrical conductivity respectively. These nanocomposites withstand high strain fatigue loading up to ϵ = 2.6%, keeping R _S below 5 Ω/□. The data demonstrates that the incorporation of Ag NWs in TiN coatings improve the mechanical robustness, limiting the crack growth and propagation, with low optical transmittance decrease (≈11%). These results indicate that Ag NWs based nanocomposites are attractive materials for flexible electronic devices.

Improving Performance of Tin-Doped-Zinc-Oxide Thin-Film Transistors by Optimizing Channel Structure

In this paper, we investigated the performance of thin-film transistors (TFTs) with different channel configurations including single-active-layer (SAL) Sn-Zn-O (TZO), dual-active-layers (DAL) In-Sn-O (ITO)/TZO, and triple-active-layers (TAL) TZO/ITO/TZO. The TAL TFTs were found to combine the advantages of SAL TFTs (a low off-state current) and DAL TFTs (a high mobility and a low threshold voltage). The proposed TAL TFTs exhibit superior electrical performance, e.g. a high on-off state current ratio of 2 × 10⁸, a low threshold voltage of 0.63 V, a high field effect mobility of 128.6 cm²/Vs, and a low off-state current of 3.3 pA. The surface morphology and characteristics of the ITO and TZO films were investigated and the TZO film was found to be C-axis-aligned crystalline (CAAC). A simplified resistance model was deduced to explain the channel resistance of the proposed TFTs. At last, TAL TFTs with different channel lengths were also discussed to show the stability and the uniformity of our fabrication process. Owing to its low-processing temperature, superior electrical performance, and low cost, TFTs with the proposed TAL channel configuration are highly promising for flexible displays where the polymeric substrates are heat-sensitive and a low processing temperature is desirable.

Synthesis of Conjugated Polymers Containing B←N Bonds with Strong Electron Affinity and Extended Absorption

The B←N is isoelectronic to the C-C, with the former having stronger dipole moment and higher electron affinity. Replacing the C-C bonds in conjugated polymers with B←N bonds is an effective pathway toward novel polymers with strong electron affinity and adjustable optoelectronic properties. In this work, we synthesize a conjugated copolymer, namely, BNIDT-DPP, based on a B←N embedded unit, BNIDT, and a typical electron-deficient unit, diketopyrrolopyrrole (DPP). For comparison, the C-C counterpart, i.e., IDT-DPP, is also synthesized. In contrast to IDT-DPP, the B←N embedded polymer BNIDT-DPP shows an extended absorption edge (836 versus 978 nm), narrowed optical bandgap (1.48 versus 1.27 eV), and higher electron affinity (3.54 versus 3.74 eV). The Gaussian simulations reveal that the B←N embedded polymer BNIDT-DPP is more electron-deficient in contrast to IDT-DPP, supporting the decreased bandgap and energy levels of BNIDT-DPP. Organic thin-film transistor (OTFT) tests indicate a well-defined p-type characteristic for both IDT-DPP and BNIDT-DPP. The hole mobilities of IDT-DPP and BNIDT-DPP tested by OTFTs are 0.059 and 0.035 cm²/V·s, respectively. The preliminary fabrication of all-polymer solar cells based on BNIDT-DPP and PBDB-T affords a PCE of 0.12%. This work develops a novel B←N embedded polymer with strong electron affinity and extended absorption, which is potentially useful for electronic device application.

Stepped Annealed Inkjet-Printed InGaZnO Thin-Film Transistors

The preparation of thin-film transistors (TFTs) using ink-jet printing technology can reduce the complexity and material wastage of traditional TFT fabrication technologies. We prepared channel inks suitable for printing with different molar ratios of their constituent elements. Through the spin-coated and etching method, two different types of TFTs designated as depletion and enhancement mode were obtained simply by controlling the molar ratios of the InGaZnO channel elements. To overcome the problem of patterned films being prone to fracture during high-temperature annealing, a stepped annealing method is proposed to remove organic molecules from the channel layer and to improve the properties of the patterned films. The different interfaces between the insulation layers, channel layers, and drain/source electrodes were processed by argon plasma. This was done to improve the printing accuracy of the patterned InGaZnO channel layers, drain, and source electrodes, as well as to optimize the printing thickness of channel layers, reduce the defect density, and, ultimately, enhance the electrical performance of printed TFT devices.

Polyol Reduction: A Low-Temperature Eco-Friendly Solution Process for p-Channel Copper Oxide-Based Transistors and Inverter Circuits

An optimized polyol reduction method was proposed for p-type Cu_xO deposition using a spin-coating method at low temperatures. The film characterizations and the electrical properties of integrated thin-film transistors (TFTs) were investigated as a function of annealing temperature and polyol type. The Cu_xO TFTs derived from propylene glycol showed the optimal performance with an average field-effect hole mobility of 0.15 cm² V^-1 s^-1, an on/off current ratio of ∼10⁴, and a threshold voltage of -7 V at a low temperature of 220 °C. Considering few investigations on the operational/air stability of solution-processed p-type oxide TFTs, we carried out systematical studies and revealed different roles of O₂ and H₂O on the device performance. The low activation energy (0.16 eV) for hole transport and high voltage gain (37) of the composed complementary inverter highlight great potential for the construction of all oxide-based transparent flexible electronics and circuits.

Highly Improved Quasi-Two-Dimensional Oxide Transistors via Non-centrosymmetric Nitrogen Dioxide Treatment, toward Extremely Low Process Temperature and Operant Self-Aligned Coplanar Structure

Rapid degradations are typically encountered in low-temperature processed oxide thin-film transistors (TFTs) with a high indium composition and quasi-two-dimensional (Q2D) thin channel, owing to the breaking of numerous surface bonds of the Q2D oxide and the ineffectiveness of oxidation treatment. Strategically, a novel approach is proposed for the effective use of non-centrosymmetric nitrous oxide (NO₂) as a reactive oxidizer gas for realizing the highly robust and rapid field-effect mobility properties of low-temperature-processed Q2D amorphous indium zinc oxide (a-IZO) TFTs. From the surface chemical analysis, it is found that NO₂ stably reconstructs surface chemical bonding with NO₃^- ions by capturing the charged electrons and oxygen and the regions with and without NO₂ treatment display extreme differences in their electrical conductivity. Thus, a new process design can be suggested for the fabrication of self-aligned coplanar Q2D transistors, with the aim of scaling down and replacing conventional hydrogen treatment or ultraviolet irradiation. This concept is tactically designed considering the problematic aging effect and impact of the NO₂ treatment. The self-aligned coplanar top-gate Q2D a-IZO TFTs exhibit outstanding device performance with a field-effect mobility of 30.1 cm² V^-1 s^-1 and a relatively low positive bias stress shift of 1.3 V at an extremely low process temperature of 80 °C.