Non-volatile organic memory with sub-millimetre bending radius

A floating-gate field-effect transistor memory device based on organic crystals with a built-in tunneling dielectric by a one-step growth strategy

A floating-gate organic field-effect transistor (FG-OFET) memory device is becoming a promising candidate for emerging non-volatile memory applications due to the advantages of its sophisticated data-storage mechanism and reliable long-term data retention capacity. However, a conventional FG-OFET memory device suffers from complex fabrication technologies and poor mechanical flexibility, which limits its practical applications. Here, we propose a facile one-step liquid-surface drag coating strategy to fabricate a layered stack of 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (Dif-TES-ADT) crystals and high-quality insulating polymer polystyrene (PS). The liquid surface enhances the spreading area of an organic solution and facilitates the unidirectional growth of organic crystals. In the bilayer-structured blend, the bottom PS polymer and the top Dif-TES-ADT semiconductor serve as a tunneling dielectric and an active memory layer of an FG-OFET memory device, respectively. Consequently, a flexible FG-OFET memory device with a large memory window of 41.4 V, a long retention time of 5000 s, and a high current ON/OFF ratio of 105 could be achieved, showing the best performance ever reported for organic thin film-based FG-OFET memory devices. In addition, multi-level data storage (3 bits per cell) can be achieved by tuning the gate voltage magnitude. Our work not only provides a general strategy for the growth of high-quality organic crystals, but also paves the way towards high-performance flexible memory devices.

Nonvolatile Memory Organic Light-Emitting Transistors

In the field of active-matrix organic light emitting display (AMOLED), large-size and ultra-high-definition AMOLED applications have escalated the demand for the integration density of driver chips. However, as Moore's Law approaches the limit, the traditional technology of improving integration density that relies on scaling down device dimension is facing a huge challenge. Thus, developing a multifunctional and highly integrated device is a promising route for improving the integration density of pixel circuits. Here, a novel nonvolatile memory ferroelectric organic light-emitting transistor (Fe-OLET) device which integrates the switching capability, light-emitting capability and nonvolatile memory function into a single device is reported. The nonvolatile memory function of Fe-OLET is achieved through the remnant polarization property of ferroelectric polymer, enabling the device to maintain light emission at zero gate bias. The reliable nonvolatile memory operations are also demonstrated. The proof-of-concept device optimized through interfacial modification approach exhibits 20 times improved field-effect mobility and five times increased luminance. The integration of nonvolatile memory, switching and light-emitting capabilities within Fe-OLET provides a promising internal-storage-driving paradigm, thus creating a new pathway for deploying storage capacitor-free circuitry to improve the pixel aperture ratio and the integration density of circuits toward the on-chip advanced display applications.

Oxygen-Induced Barrier Lowering for High-Performance Organic Field-Effect Transistors

Organic field-effect transistors (OFETs) have the advantages of low-cost, large-area processing and could be utilized in a variety of emerging applications. However, the generally large contact resistance (R_c) limits the integration and miniaturization of OFETs. The R_c is difficult to reduce due to an incompatibility between obtaining strong orbit coupling and the barrier height reduction. In this study, we developed an oxygen-induced barrier lowering strategy by introducing oxygen (O₂) into the nanointerface between the electrodes and organic semiconductors layer and achieved an ultralow channel width-normalized R_c (R_c·W) of 89.8 Ω·cm and a high mobility of 11.32 cm² V^-1 s^-1. This work demonstrates that O₂ adsorbed at the nanointerface of metal-semiconductor contact can significantly reduce the R_c from both experiments and theoretical simulations and provides guidance for the construction of high-performance OFETs, which is conducive to the integration and miniaturization of OFETs.

Fluoropolymer ferroelectrics: Multifunctional platform for polar-structured energy conversion

Ferroelectric materials are currently some of the most widely applied material systems and are constantly generating improved functions with higher efficiencies. Advancements in poly(vinylidene fluoride) (PVDF)-based polymer ferroelectrics provide flexural, coupling-efficient, and multifunctional material platforms for applications that demand portable, lightweight, wearable, and durable features. We highlight the recent advances in fluoropolymer ferroelectrics, their energetic cross-coupling effects, and emerging technologies, including wearable, highly efficient electromechanical actuators and sensors, electrocaloric refrigeration, and dielectric devices. These developments reveal that the molecular and nanostructure manipulations of the polarization-field interactions, through facile defect biasing, could introduce enhancements in the physical effects that would enable the realization of multisensory and multifunctional wearables for the emerging immersive virtual world and smart systems for a sustainable future.

Ferroelectric Wide-Bandgap Metal Halide Perovskite Field-Effect Transistors: Toward Transparent Electronics

Transparent field-effect transistors (FETs) are attacking intensive interest for constructing fancy "invisible" electronic products. Presently, the main technology for realizing transparent FETs is based on metal oxide semiconductors, which have wide-bandgap but generally demand sputtering technique or high-temperature (>350 °C) solution process for fabrication. Herein, a general device fabrication strategy for metal halide perovskite (MHP) FETs is shown, by which transparent perovskite FETs are successfully obtained using low-temperature (<150 °C) solution process. This strategy involves the employment of ferroelectric copolymer poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) as the dielectric, which conquers the challenging issue of gate-electric-field screening effect in MHP FETs. Additionally, an ultra-thin SnO₂ is inserted between the source/drain electrodes and MHPs to facilitate electron injection. Consequently, n-type semi-transparent MAPbBr₃ FETs and fully transparent MAPbCl₃ FETs which can operate well at room temperature with mobility over 10^-3  cm²  V^-1  s^-1 and on/off ratio >10³ are achieved for the first time. The low-temperature solution processability of these FETs makes them particularly attractive for applications in low-cost, large-area transparent electronics.

Surface Modification of Hetero-phase Nanoparticles for Low-Cost Solution-Processable High-k Dielectric Polymer Nanocomposites

The surface modification of nanoparticles (NPs) is crucial for fabricating polymer nanocomposites (NCs) with high dielectric permittivity. Here, we systematically studied the effect of surface functionalization of TiO₂ and BaTiO₃ NPs to enhance the dielectric permittivity of polyvinylidene fluoride (PVDF) NCs by 23 and 74%, respectively, measured at a frequency of 1 kHz. To further increase the dielectric permittivity of PVDF/NPs-based NCs, we developed a new hetero-phase filler-based approach that is cost-effective and easy to implement. At a 1:3 mixing ratio of TiO₂:BaTiO₃ NPs, the dielectric constant of the ensuing NC is found to be 50.2, which is comparable with the functionalized BaTiO₃-based NC. The highest dielectric constant value of 76.1 measured at 1 kHz was achieved using the (3-aminopropyl)triethoxysilane (APTES)-modified hetero-phase-based PVDF composite at a volume concentration of 5%. This work is an important step toward inexpensive and easy-to-process high-k nanocomposite dielectrics.

Fully Photoswitchable Phototransistor Memory Comprising Perovskite Quantum Dot-Based Hybrid Nanocomposites as a Photoresponsive Floating Gate

Tremendous research efforts have been dedicated into the field of photoresponsive nonvolatile memory devices owing to their advantages of fast transmitting speed, low latency, and power-saving property that are suitable for replacing current electrical-driven electronics. However, the reported memory devices still rely on the assistance of gate bias to program them, and a real fully photoswitchable transistor memory is still rare. Herein, we report a phototransistor memory device comprising polymer/perovskite quantum dot (QD) hybrid nanocomposites as a photoresponsive floating gate. The perovskite QDs offer an effective discreteness with an excellent photoresponse that are suitable for photogate application. In addition, a series of ultraviolet (UV)-sensitive insulating polymer hosts were designed to investigate the effect of UV light on the memory behavior. We found that a fully photoswitchable memory device was fulfilled by using the independent and sequential photoexcitation between a UV-sensitive polymer host and a visible light-sensitive QD photogates, which produced decent photoresponse, memory switchability, and highly stable memory retention with a memory ratio of 10⁴ over 10⁴ s. This study not only unraveled the mystery in the fully photoswitchable functionality of nonvolatile memory but also enlightened their potential in the next-generation electronics for light-fidelity application.

Bird-Inspired Self-Navigating Artificial Synaptic Compass

Extrasensory neuromorphic devices that can recognize, memorize, and learn stimuli imperceptible to human beings are of considerable interest in interactive intelligent electronics research. This study presents an artificially intelligent magnetoreceptive synapse inspired by the magnetocognitive ability used by birds for navigation and orientation. The proposed synaptic platform is based on arrays of ferroelectric field-effect transistors with air-suspended magneto-interactive top-gates. A suspended gate of an elastomeric composite with superparamagnetic particles laminated with an electrically conductive polymer is mechanically deformed under a magnetic field, facilitating control of the magnetic-field-dependent contact area of the suspended gate with an underlying ferroelectric layer. The remanent polarization of the ferroelectric layer is electrically programmed with the deformed suspended gate, resulting in analog conductance modulation as a function of the magnitude, number, and time interval of the input magnetic pulses. The proposed extrasensory magnetoreceptive synapse may be used as an artificially intelligent synaptic compass that facilitates barrier-adaptable navigation and mapping of a moving object.

Soft Ferroelectrics Enabling High-Performance Intelligent Photo Electronics

Soft ferroelectrics based on organic and organic-inorganic hybrid materials have gained much interest among researchers owing to their electrically programmable and remnant polarization. This allows for the development of numerous flexible, foldable, and stretchable nonvolatile memories, when combined with various crystal engineering approaches to optimize their performance. Soft ferroelectrics have been recently considered to have an important role in the emerging human-connected electronics that involve diverse photoelectronic elements, particularly those requiring precise programmable electric fields, such as tactile sensors, synaptic devices, displays, photodetectors, and solar cells for facile human-machine interaction, human safety, and sustainability. This paper provides a comprehensive review of the recent developments in soft ferroelectric materials with an emphasis on their ferroelectric switching principles and their potential application in human-connected intelligent electronics. Based on the origins of ferroelectric atomic and/or molecular switching, the soft ferroelectrics are categorized into seven subgroups. In this review, the efficiency of soft ferroelectrics with their distinct ferroelectric characteristics utilized in various human-connected electronic devices with programmable electric field is demonstrated. This review inspires further research to utilize the remarkable functionality of soft 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.

Optical-field driven charge-transfer modulations near composite nanostructures

Optical activation of material properties illustrates the potentials held by tuning light-matter interactions with impacts ranging from basic science to technological applications. Here, we demonstrate for the first time that composite nanostructures providing nonlocal environments can be engineered to optically trigger photoinduced charge-transfer-dynamic modulations in the solid state. The nanostructures explored herein lead to out-of-phase behavior between charge separation and recombination dynamics, along with linear charge-transfer-dynamic variations with the optical-field intensity. Using transient absorption spectroscopy, up to 270% increase in charge separation rate is obtained in organic semiconductor thin films. We provide evidence that composite nanostructures allow for surface photovoltages to be created, which kinetics vary with the composite architecture and last beyond optical pulse temporal characteristics. Furthermore, by generalizing Marcus theory framework, we explain why charge-transfer-dynamic modulations can only be unveiled when optic-field effects are enhanced by nonlocal image-dipole interactions. Our demonstration, that composite nanostructures can be designed to take advantage of optical fields for tuneable charge-transfer-dynamic remote actuators, opens the path for their use in practical applications ranging from photochemistry to optoelectronics.

Controlling and modulating charge transfer dynamics in composite nanostructures, though promising for optoelectronic applications, remains a challenge. Here, the authors report optical control of charge separation and recombination processes in organic semiconductor-based composite nanostructures.

Synthetic Bilayers on Mica from Self-Assembly of Hydrogen-Bonded Triazines

This study describes organic thin films prepared under a range of conditions from a model series of bis-N-alkyl chloro-triazines functionalized with short alkyl chains from ethyl to hexyl. The pure films were characterized using atomic force microscopy (AFM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). When cast on mica, these compounds assemble as crystalline sheets made up of a synthetic bilayer along the crystallographic ab-plane with an internal hydrogen-bonded domain between external alkyl chains. These micron-scale surfaces stack along the c-axis, and increasing the alkyl chain length results in changes to the crystal morphology from needles to nanoscale plates. Thicker films produce nanoscale, pyramidal stacks of bilayers. Compared to atomically flat mica, a rougher, unetched silicon substrate produced irregular domains in the secondary bilayer. Films of mixtures comprising the ethyl derivative with butyl, pentyl, or hexyl derivative were imaged using time-of-flight secondary-ion mass spectrometry (ToF-SIMS) that indicated a trend toward a constant stoichiometry with increasing alkyl chain length. AFM of mixed films on mica showed single bilayers of height <2 nm, with an acceptable correlation to the XRD measurements, supporting a constant stoichiometry. These materials permit easy modification of mica to a micron-scale, atomically flat hydrophobic surface, and the use of mixtures with different alkyl chain lengths suggests a method to improve the quality of functional organic thin films.

Recent innovations in artificial skin

The skin is a "smart", multifunctional organ that is protective, self-healing and capable of sensing and many forms of artificial skins have been developed with properties and functionalities approximating those of natural skin. Starting from specific commercial products for the treatment of burns, progress in two fields of research has since allowed these remarkable materials to be viable skin replacements for a wide range of dermatological conditions. This review maps out the development of bioengineered skin replacements and synthetic skin substitutes, including electronic skins. The specific behaviors of these skins are highlighted, and the performances of both types of artificial skins are evaluated against this. Moving beyond mere replication, highly advanced artificial skin materials are also identified as potential augmented skins that can be used as flexible electronics for health-care monitoring and other applications.

Tailored Poly(vinylidene fluoride-co-trifluoroethylene) Crystal Orientation for a Triboelectric Nanogenerator through Epitaxial Growth on a Chitin Nanofiber Film

Tailoring the crystal orientation of poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) has attracted widespread interest because of its effects on the ferroelectric properties required for various electronic devices. In this study, we investigated the epitaxial growth of PVDF-TrFE on a chitin film for developing triboelectric nanogenerators (TENGs). The crystallographic match between the chitin and PVDF-TrFE enables the development of the intended crystal orientation, with the PVDF-TrFE polarization axis aligned perpendicular to the substrate. In addition, the epitaxially grown PVDF-TrFE on chitin not only enhances the performance of the TENG but also increases the stability of the hygroscopic chitin film against water. The corresponding TENG exhibits a significantly higher output current compared to that of a nonepitaxial PVDF-TrFE/chitin film. Furthermore, the triboelectric sensors based on epitaxial PVDF-TrFE/chitin films allow the monitoring of subtle pressures, suggesting that tailoring the crystal orientation of PVDF-TrFE is a promising approach for developing high-performance TENGs.

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.

One-dimensional organic artificial multi-synapses enabling electronic textile neural network for wearable neuromorphic applications

One-dimensional (1D) devices are becoming the most desirable format for wearable electronic technology because they can be easily woven into electronic (e-) textile(s) with versatile functional units while maintaining their inherent features under mechanical stress. In this study, we designed 1D fiber-shaped multi-synapses comprising ferroelectric organic transistors fabricated on a 100-μm Ag wire and used them as multisynaptic channels in an e-textile neural network for wearable neuromorphic applications. The device mimics diverse synaptic functions with excellent reliability even under 6000 repeated input stimuli and mechanical bending stress. Various NOR-type textile arrays are formed simply by cross-pointing 1D synapses with Ag wires, where each output from individual synapse can be integrated and propagated without undesired leakage. Notably, the 1D multi-synapses achieved up to ~90 and ~70% recognition accuracy for MNIST and electrocardiogram patterns, respectively, even in a single-layer neural network, and almost maintained regardless of the bending conditions.

Tuning Charge Transport in PVDF-Based Organic Ferroelectric Transistors: Status and Outlook

The use of polymer ferroelectric dielectrics in organic field-effect transistors (FETs) for nonvolatile memory application was demonstrated more than 15 years ago. The ferroelectric dielectric polyvinylidene fluoride (PVDF) and its copolymers are most widely used for such applications. In addition to memory applications, polymer ferroelectrics as a dielectric layer in organic FETs yield insights into interfacial transport properties. Advantages of polymer ferroelectric dielectrics are their high dielectric constant compared to other polymer dielectrics and their tunable dielectric constant with temperature. Further, the polarization strength may also be tuned by externally poling the ferroelectric dielectric layer. Thus, PVDF and its copolymers provide a unique testbed not just for investigating polarization induced transport in organic FETs, but also enhancing device performance. This article discusses recent developments of PVDF-based ferroelectric organic FETs and capacitors with a focus on tuning transport properties. It is shown that FET carrier mobilities exhibit a weak temperature dependence as long as the dielectric is in the ferroelectric phase, which is attributed to a polarization fluctuation driven process. The low carrier mobilities in PVDF-based FETs can be enhanced by tuning the poling condition of the dielectric. In particular, by using solution-processed small molecule semiconductors and other donor-acceptor copolymers, it is shown that selective poling of the PVDF-based dielectric layer dramatically improves FET properties. Finally, the prospects of further improvement in organic ferroelectric FETs and their challenges are provided.

Well-ordered nanoparticle arrays for floating gate memory applications

A non-volatile floating gate memory device containing well-ordered Au nanoparticles (NPs) is fabricated as a metal-oxide-semiconductor capacitor structure. With superior control on the size, shape and position of nanoparticles, the presented nano-floating gate memory (NFGM) device possesses almost perfect precision of device geometry. The well-ordered Au NPs embedded within the memory device exhibit large memory window at low operation voltages (8.8V @ ± 15V), fast operation time (<10^-4 s) and good retention (up to 10⁷ s). In this work, the structural properties of the NFGM device are correlated with the examined electrical properties. The current results are compared with the other studies in the literature to emphasis the advantages of the precise ordering and geometry of the NPs.

A kirigami-inspired island-chain design for wearable moistureproof perovskite solar cells with high stretchability and performance stability

A range of power generating approaches, such as integration with clothing, fashion accessories, or textiles, allow electronic devices to be charged in environmentally friendly ways. Stretchable, efficient, stable, and even washable solar cells are considered the next necessary component to supply continuous wearable energy. However, ultra-thin photo-active materials are often fragile, which inevitably raises challenges for electron conduction during stretching and washing processes, thus resulting in unsatisfactory output performance. Herein, we have removed the stumbling block by designing a kirigami-inspired unique island-chain structure with serpentine interconnects, which prevented the photo-active layer of subcells from being subjected to excessive strain. Notably, this is the first time perovskite solar cell arrays met the above wearable requirements simultaneously. The obtained devices exhibited a high yet stable power output (efficiency of 17.68%) accompanied by a robust cycling performance (87% of the initial PCE) even after 300 times of continuous stretching with a large ratio of 80%. The efficiency of the optimized PSCs maintains promising stability after being exposed in a harsh environment (80% humidity) for 10 days. As textile-compatible power sources, the successfully designed stretchable and moisture-resistant photovoltaics add power-generation functionality to clothing, opening a new avenue for applications as long-term power sources for wearable electronics.

Photo-/Thermal-Responsive Field-Effect Transistor upon Blending Polymeric Semiconductor with Hexaarylbiimidazole toward Photonically Programmable and Thermally Erasable Memory Device

It is shown that the semiconducting performance of field-effect transistors (FETs) with PDPP4T (poly(diketopyrrolopyrrole-quaterthiophene)) can be reversibly tuned by UV light irradiation and thermal heating after blending with the photochromic hexaarylbiimidazole compound (p-NO₂ -HABI). A photo-/thermal-responsive FET with a blend thin film of PDPP4T and p-NO₂ -HABI is successfully fabricated. The transfer characteristics are altered significantly with current enhanced up to 10⁶ -fold at V_G = 0 V after UV light irradiation. However, further heating results in the recovery of the transfer curve. This approach can be extended to other semiconducting polymers such as P3HT (poly(3-hexyl thiophene)), PBTTT (poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b] thiophene)) and PDPPDTT (poly(diketopyrrolopyrrole-dithienothiophene)). It is hypothesized that TPIRs (2,4,5-triphenylimidazolyl radicals) formed from p-NO₂ -HABI after UV light irradiation can interact with charge defects at the gate dielectric-semiconductor interface and those in the semiconducting layer to induce more hole carriers in the semiconducting channel. The application of the blend thin film of PDPP4T and p-NO₂ -HABI is further demonstrated to fabricate the photonically programmable and thermally erasable FET-based nonvolatile memory devices that are advantageous in terms of i) high ON/OFF current ratio, ii) nondestructive reading at low electrical bias, and iii) reasonably highly stable ON-state and OFF-state.

Sensing and memorising liquids with polarity-interactive ferroelectric sound

The direct sensing and storing of the information of liquids with different polarities are of significant interest, in particular, through means related to human senses for emerging biomedical applications. Here, we present an interactive platform capable of sensing and storing the information of liquids. Our platform utilises sound arising from liquid-interactive ferroelectric actuation, which is dependent upon the polarity of the liquid. Liquid-interactive sound is developed when a liquid is placed on a ferroelectric polymer layer across two in-plane electrodes under an alternating current field. As the sound is correlated with non-volatile remnant polarisation of the ferroelectric layer, the information is stored and retrieved after the liquid is removed, resulting in a sensing memory of the liquid. Our pad-type allows for identifying the position of a liquid. Flexible tube-type devices offer a route for in situ analysis of flowing liquids including a human serum liquid in terms of sound.

Oxide Heteroepitaxy-Based Flexible Ferroelectric Transistor

With the rise of Internet of Things, the presence of flexible devices has attracted significant attention owing to design flexibility. A ferroelectric field-effect transistor (FeFET), showing the advantages of high speed, nondestructive readout, and low-power consumption, plays a key role in next-generation technology. However, the performance of these devices is restricted since conventional flexible substrates show poor thermal stability to integrate traditional ferroelectric materials, limiting the compatibility of wearable devices. In this study, we adopt flexible muscovite mica as a substrate due to its good thermal properties and epitaxial integration ability. A flexible FeFET composed of oxide heteroepitaxy on muscovite is realized by combining an aluminum-doped zinc oxide film as the semiconductor channel layer and a Pb(Zr_0.7Ti_0.3)O₃ film as the ferroelectric gate dielectric. The excellent characteristics of the transistor together with superior thermal stability and mechanical flexibility are demonstrated through various mechanical bending and temperature measurements. The on/off current ratio of the FeFET is higher than 10³, which based on the field effect in the transfer curve. The smallest bending radius that can be achieved is 5 mm with a cyclability of 300 times and a retention of 100 h. This study opens an avenue to use oxide heteroepitaxy to construct a FeFET for next-generation flexible electronic systems.

A Universal Strategy for Stretchable Polymer Nonvolatile Memory via Tailoring Nanostructured Surfaces

Building stretchable memory is an effective strategy for developing next-generation memory technologies toward stretchable and wearable electronics. Here we demonstrate a universal strategy for the fabrication of high performance stretchable polymer memory via tailoring surface morphology, in which common conjugated polymers and sharp reduced graphene oxide (r-rGO) films are used as active memristive layers and conductive electrodes, respectively. The fabricated devices feature write-once-read-many-times (WORM) memory, with a low switching voltage of 1.1 V, high ON/OFF current ratio of 10⁴, and an ideal long retention time over 12000 s. Sharp surface-induced resistive switching behavior has been proposed to explore the electrical transition. Moreover, the polymer memory show reliable electrical bistable properties with a stretchability up to 30%, demonstrating their great potential candidates as high performance stretchable memory in soft electronics.

High-Performance Nanofloating Gate Memory Based on Lead Halide Perovskite Nanocrystals

Lead halide perovskites have been extensively investigated in a host of optoelectronic devices, such as solar cells, light-emitting diodes, and photodetectors. The halogen vacancy defects arising from the halogen-poor growth environment are normally regarded as an unfavorable factor to restrict the device performance. Here, for the first time, we demonstrate the utilization of the vacancy defects in lead halide perovskite nanostructures for achieving high-performance nanofloating gate memories (NFGMs). CH₃NH₃PbBr₃ nanocrystals (NCs) were uniformly decorated on the CdS nanoribbon (NR) surface via a facile dip-coating process, forming a CdS NR/CH₃NH₃PbBr₃ NC core-shell structure. Significantly, owing to the existence of sufficient carrier trapping states in CH₃NH₃PbBr₃ NCs, the hybrid device possessed an ultralarge memory window up to 77.4 V, a long retention time of 12 000 s, a high current ON/OFF ratio of 7 × 10⁷, and a long-term air stability for 50 days. The memory window of the device is among the highest for the low-dimensional nanostructure-based NFGMs. Also, this strategy shows good universality and can be extended to other perovskite nanostructures for the construction of high-performance NFGMs. This work paves the way toward the fabrication of new-generation, high-capacity nonvolatile memories using lead halide perovskite nanostructures.

High-efficiency blue thermally activated delayed fluorescence from donor-acceptor-donor systems via the through-space conjugation effect

The photophysical optimization of donor (D)-acceptor (A) molecules is a real challenge because of the intrinsic limitation of their charger transfer (CT) excited states. Herein, two D-A-D molecules featuring blue thermally activated delayed fluorescence (TADF) are developed, in which a homoconjugated acceptor 5,10-diphenyl-5,10-dihydrophosphanthrene oxide (DPDPO2A) is incorporated to bridge four carbazolyl or 3,6-di-t-butyl-carbazolyl groups for D-A interaction optimization without immoderate conjugation extension. It is shown that the through-space conjugation effect of DPDPO2A can efficiently enhance intramolecular CT (ICT) and simultaneously facilitate the uniform dispersion of the frontier molecular orbitals (FMO), which remarkably reduces the singlet-triplet splitting energy (ΔE ST) and increases FMO overlaps for radiation facilitation, resulting in the 4-6 fold increased rate constants of reverse intersystem crossing (RISC) and singlet radiation. The maximum external quantum efficiency beyond 20% and the state-of-the-art efficiency stability from sky-blue TADF OLEDs demonstrate the effectiveness of the "conjugation modulation" strategy for developing high-performance optoelectronic D-A systems.

pJ-Level Energy-Consuming, Low-Voltage Ferroelectric Organic Field-Effect Transistor Memories

Ferroelectric organic field-effect transistors (Fe-OFETs) have attracted considerable attention because of their promising potential for memory applications, while a critical issue is the large energy consumption mainly caused by a high operating voltage and slow data switching. Here, we employ ultrathin ferroelectric polymer and semiconducting molecular crystals to create low-voltage Fe-OFET memories. Devices require only pJ-level energy consumption. The writing and erasing processes require ∼1.2 and 1.6 pJ/bit, respectively, and the reading energy is ∼1.9 pJ/bit (on state) and ∼0.2 fJ/bit (off state). Thus, our memories consume only <0.1% of the energy required for devices using bulk functional layers. Besides, our devices also exhibit low contact resistance and steep subthreshold swing. Therefore, we provide a strategy that opens up a path for Fe-OFETs toward emerging applications, such as wearable electronics.

Facile and Microcontrolled Blade Coating of Organic Semiconductor Blends for Uniaxial Crystal Alignment and Reliable Flexible Organic Field-Effect Transistors

The ability to fabricate uniform and high-quality patterns of organic semiconductors using a simple method is necessary to realize high-performance and reliable organic field-effect transistors (OFETs) for practical applications. Here, we report the facile fabrication of chemically patterned substrates in order to provide solvent wetting/dewetting regions and grow patterned crystals during blade coating of a small-molecule semiconductor/insulating polymer blend solution. Polyurethane acrylate is selected as the solvent dewetting material, not only because of its hydrophobicity but also because its patterns are easily produced by selective UV irradiation onto precursor films. 6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS-PEN) crystal patterns are grown on the line-shaped wetting regions of the patterned film, and the crystallinity of TIPS-PEN and alignment of molecules are found using various crystal analysis tools depending on the pattern widths. The smallest width of 5 μm yielded an OFET showing the highest field-effect mobility value of 1.63 cm²/(V·s), which is much higher than the value of the OFET based on the unpatterned TIPS-PEN crystal. Notably, we demonstrate flexible and low-voltage-operating OFETs for practical use of the patterned crystals, and the OFETs show highly stable operation under sustained gate bias stress thanks to the patterned crystals.

Organic crystalline materials in flexible electronics

Flexible electronics have attracted considerable attention recently given their potential to revolutionize human lives. High-performance organic crystalline materials (OCMs) are considered strong candidates for next-generation flexible electronics such as displays, image sensors, and artificial skin. They not only have great advantages in terms of flexibility, molecular diversity, low-cost, solution processability, and inherent compatibility with flexible substrates, but also show less grain boundaries with minimal defects, ensuring excellent and uniform electronic characteristics. Meanwhile, OCMs also serve as a powerful tool to probe the intrinsic electronic and mechanical properties of organics and reveal the flexible device physics for further guidance for flexible materials and device design. While the past decades have witnessed huge advances in OCM-based flexible electronics, this review is intended to provide a timely overview of this fascinating field. First, the crystal packing, charge transport, and assembly protocols of OCMs are introduced. State-of-the-art construction strategies for aligned/patterned OCM on/into flexible substrates are then discussed in detail. Following this, advanced OCM-based flexible devices and their potential applications are highlighted. Finally, future directions and opportunities for this field are proposed, in the hope of providing guidance for future research.

Using Artificial Skin Devices as Skin Replacements: Insights into Superficial Treatment

Artificial skin devices are able to mimic the flexibility and sensory perception abilities of the skin. They have thus garnered attention in the biomedical field as potential skin replacements. This Review delves into issues pertaining to these skin-deep devices. It first elaborates on the roles that these devices have to fulfill as skin replacements, and identify strategies that are used to achieve such functionality. Following which, a comparison is done between the current state of these skin-deep devices and that of natural skin. Finally, an outlook on artificial skin devices is presented, which discusses how complementary technologies can create skin enhancements, and what challenges face such devices.

Ultrathin Conformable Organic Artificial Synapse for Wearable Intelligent Device Applications

Ultrathin conformable artificial synapse platforms that can be used as on-body or wearable chips suggest a path to build next-generation, wearable, intelligent electronic systems that can mimic the synaptic operations of the human brain. So far, an artificial synapse architecture with ultimate mechanical flexibility in a freestanding form while maintaining its functionalities with high stability and accuracy on any conformable substrate has not been demonstrated yet. Here, we demonstrate the first ultrathin artificial synapse (∼500 nm total thickness) that features freestanding ferroelectric organic neuromorphic transistors (FONTs), which can stand alone without a substrate or an encapsulation layer. Our simple dry peel-off process allows integration of the freestanding FONTs with an extremely thin film that is transferable to various conformable substrates. The FONTs exhibit excellent and reliable synaptic functions, which can be modulated by diverse electrical stimuli and relative timing (or temporal order) between the pre- and postsynaptic spikes. Furthermore, the FONTs show sustainable synaptic plasticity even under folded condition ( R = 50 μm, ε = 0.48%) for more than 6000 input spikes. Our study suggests that the ultrathin conformable organic artificial synapse platforms are considered as one of key technologies for realization of wearable intelligent electronics in the future.

Highly efficient walking perovskite solar cells based on thermomechanical polymer films

Inspired by heliotropism in nature, a passive walking perovskite solar cell is constructed as a prototype actuator with an advanced structure.

Nonvolatile Memory Based on Molecular Ferroelectric/Graphene Field Effect Transistor

Ferroelectric thin films are extensively attractive as next-generation nonvolatile memories. Recently, molecular ferroelectrics (MFe), as an emerging new class, have been a new research focus because of their desirable characteristics such as good solution processability, tunable chemical properties, and bio-friendly compositions. However, traditional uniaxial MFe only possess one polar axis which greatly limits their application, as it requires restricted orientational control in single crystal. To achieve macroscopic ferroelectricity and thus fully realize technological advantages of MFe, development of multiaxes is imperative to maximize effective polarization in specific crystallographic orientations. Herein, we present an early exploration on polycrystalline multiaxial MFe thin films of [Hdabco][ReO₄] with a two-dimensional graphene hybrid nonvolatile memory device. The polarization switching of MFe is experimentally realized by the nonvolatile modulation of two current states in graphene. Such a hybrid device can exhibit large memory window ∼35 V implying its great potential in memory applications. Hence, by taking the advantages of multiple polarization axes of MFe, the low cost and large area MFe/graphene hybrid memory manifests new possibilities for the integration of these materials as flexible next generation memory devices.

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

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

Multifunctional Dithiadiazolyl Radicals: Fluorescence, Electroluminescence, and Photoconducting Behavior in Pyren-1'-yl-dithiadiazolyl

The pyren-1'-yl-functionalized dithiadiazolyl (DTDA) radical, C₁₆H₉CNSSN (1), is monomeric in solution and exhibits fluorescence in the deep-blue region of the visible spectrum (440 nm) upon excitation at 241 nm. The salt [1][GaCl₄] exhibits similar emission, reflecting the largely spectator nature of the radical in the fluorescence process, although the presence of the radical leads to a modest quenching of emission (Φ_F = 98% for 1⁺ and 50% for 1) through enhancement of non-radiative decay processes. Time-dependent density functional theory studies on 1 coupled with the similar emission profiles of both 1⁺ and 1 are consistent with the initial excitation being of predominantly pyrene π-π* character. Spectroscopic studies indicate stabilization of the excited state in polar media, with the fluorescence lifetime for 1 (τ = 5 ns) indicative of a short-lived excited state. Comparative studies between the energies of the frontier orbitals of pyren-1'-yl nitronyl nitroxide (2, which is not fluorescent) and 1 reveal that the energy mismatch and poor spatial overlap between the DTDA radical SOMO and the pyrene π manifold in 1 efficiently inhibit the non-radiative electron-electron exchange relaxation pathway previously described for 2. Solid-state films of both 1 and [1][GaCl₄] exhibit broad emission bands at 509 and 545 nm, respectively. Incorporation of 1 within a host matrix for OLED fabrication revealed electroluminescence, with CIE coordinates of (0.205, 0.280) corresponding to a sky-blue emission. The brightness of the device reached 1934 cd/m² at an applied voltage of 16 V. The crystal structure of 1 reveals a distorted π-stacked motif with almost regular distances between the pyrene rings but alternating long-short contacts between DTDA radicals. Solid state measurements on a thin film of 1 reveal emission occurs at shorter wavelengths (375 nm) whereas conductivity measurements on a single crystal of 1 show a photoconducting response at longer wavelength excitation (455 nm).

A Fabrication and Measurement Method for a Flexible Ferroelectric Element Based on Van Der Waals Heteroepitaxy

Flexible non-volatile memories have received much attention as they are applicable for portable smart electronic device in the future, relying on high-density data storage and low-power consumption capabilities. However, the high-quality oxide based nonvolatile memory on flexible substrates is often constrained by the material characteristics and the inevitable high-temperature fabrication process. In this paper, a protocol is proposed to directly grow an epitaxial yet flexible lead zirconium titanate memory element on muscovite mica. The versatile deposition technique and measurement method enable the fabrication of flexible yet single-crystalline non-volatile memory elements necessary for the next generation of smart devices.

Fully Printed Wearable Vital Sensor for Human Pulse Rate Monitoring using Ferroelectric Polymer

The ability to monitor subtle changes in vital and arterial signals using flexible devices attached to the human skin can be valuable for the detection of various health conditions such as cardiovascular disease. Conventional Si device technologies are being utilised in traditional clinical systems; however, its fabrication is not easy owing to the difficulties in adapting to conventional processes. Here, we present the development of a fully printed, wearable, ferroelectric-polymer vital sensor for monitoring the human pulse wave/rate on the skin. This vital sensor is compact, thin, sufficiently flexible, and conforms to the skin while providing high pressure sensitivity, fast response time, superior operational stability, and excellent mechanical fatigue properties. Moreover, the vital sensor is connected to a communication amplifier circuit for monitoring the pulse waves with a wireless sensing system. This sensor system can realise the development of new healthcare devices for wearable sensor applications.

A High-Performance Optical Memory Array Based on Inhomogeneity of Organic Semiconductors

Organic optical memory devices keep attracting intensive interests for diverse optoelectronic applications including optical sensors and memories. Here, flexible nonvolatile optical memory devices are developed based on the bis[1]benzothieno[2,3-d;2',3'-d']naphtho[2,3-b;6,7-b']dithiophene (BBTNDT) organic field-effect transistors with charge trapping centers induced by the inhomogeneity (nanosprouts) of the organic thin film. The devices exhibit average mobility as high as 7.7 cm² V^-1 s^-1 , photoresponsivity of 433 A W^-1 , and long retention time for more than 6 h with a current ratio larger than 10⁶ . Compared with the standard floating gate memory transistors, the BBTNDT devices can reduce the fabrication complexity, cost, and time. Based on the reasonable performance of the single device on a rigid substrate, the optical memory transistor is further scaled up to a 16 × 16 active matrix array on a flexible substrate with operating voltage less than 3 V, and it is used to map out 2D optical images. The findings reveal the potentials of utilizing [1]benzothieno[3,2-b][1]benzothiophene (BTBT) derivatives as organic semiconductors for high-performance optical memory transistors with a facile structure. A detailed study on the charge trapping mechanism in the derivatives of BTBT materials is also provided, which is closely related to the nanosprouts formed inside the organic active layer.

Recent Advances of Flexible Data Storage Devices Based on Organic Nanoscaled Materials

Following the trend of miniaturization as per Moore's law, and facing the strong demand of next-generation electronic devices that should be highly portable, wearable, transplantable, and lightweight, growing endeavors have been made to develop novel flexible data storage devices possessing nonvolatile ability, high-density storage, high-switching speed, and reliable endurance properties. Nonvolatile organic data storage devices including memory devices on the basis of floating-gate, charge-trapping, and ferroelectric architectures, as well as organic resistive memory are believed to be favorable candidates for future data storage applications. In this Review, typical information on device structure, memory characteristics, device operation mechanisms, mechanical properties, challenges, and recent progress of the above categories of flexible data storage devices based on organic nanoscaled materials is summarized.

S-Layer Protein for Resistive Switching and Flexible Nonvolatile Memory Device

In this work, a flexible resistive switching memory device consisting of S-layer protein (Slp) is demonstrated for the first time. This novel device (Al/Slp/indium tin oxide/polyethylene terephthalte) based on a simple and easy fabrication method is capable of bistable switching to low resistive state (LRS) and high resistive state (HRS). This device exhibits bistable memory behavior with stability and a long retention time (>4 × 10³ s), being stable up to a 500 cycle endurance test and with significant HRS/LRS ratio. The device possesses consistent switching performance for more than 100 times bending, corresponding to desired applicability for biocompatible wearable electronics. The memory mechanism is attributed to a trapping/de-trapping process in S-layer protein. These promising results of the flexible memory device could find a way in the wearable storage applications like smart bands and sports equipments' sensors.

Solution-Processed Flexible Organic Ferroelectric Phototransistor

In this article, we demonstrate ferroelectric insulator, P(VDF-TrFE), can be integrated with red light sensitive polymeric semiconductor, P(DPP-TzBT), toward ferroelectric organic phototransistors (OPTs). This ferroelectricity-modulated phototransistor possesses different nonvolatile and tunable dark current states due to P(VDF-TrFE)'s remnant polarization. As a result, the OPT is endowed with a tunable dark current level ranging from 1 nA to 100 nA. Once the OPT is programmed or electrically polarized, its photo-to-dark (signal-to-noise) ratio can be "flexible" during photodetection process, without gate bias application. This kind of organic ferroelectric phototransistor has great potential in detecting wide ranges of light signals with good linearity. Moreover, its tuning mechanism discussed in this work can be helpful to understand the operation mechanism of organic phototransistor (OPT). It can be promising for novel photodetection application in plastic electronic devices.

Direct Writing and Aligning of Small-Molecule Organic Semiconductor Crystals via "Dragging Mode" Electrohydrodynamic Jet Printing for Flexible Organic Field-Effect Transistor Arrays

Patterning and aligning of organic small-molecule semiconductor crystals over large areas is an important issue for their commercialization and practical device applications. This Letter reports "dragging mode" electrohydrodynamic jet printing that can simultaneously achieve direct writing and aligning of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PEN) crystals. Dragging mode provides favorable conditions for crystal growth with efficient controls over supply voltages and nozzle-to-substrate distances. Optimal printing speed produces millimeter-long TIPS-PEN crystals with unidirectional alignment along the printing direction. These crystals are highly crystalline with a uniform packing structure that favors lateral charge transport. Organic field-effect transistors (OFETs) based on the optimally printed TIPS-PEN crystals exhibit high field-effect mobilities up to 1.65 cm²/(V·s). We also demonstrate the feasibility of controlling pattern shapes of the crystals as well as the fabrication of printed flexible OFET arrays.

Organic flash memory on various flexible substrates for foldable and disposable electronics

With the emergence of wearable or disposable electronics, there grows a demand for a flash memory realizable on various flexible substrates. Nevertheless, it has been challenging to develop a flash memory that simultaneously exhibits a significant level of flexibility and performance. This is mainly due to the scarcity of flexible dielectric materials with insulating properties sufficient for a flash memory, which involves dual dielectric layers, respectively, responsible for tunneling and blocking of charges. Here we report ultra-flexible organic flash memories based on polymer dielectrics prepared by initiated chemical vapor deposition. Using their near-ideal dielectric characteristics, we demonstrate flash memories bendable down to a radius of 300 μm that exhibits a relatively long-projected retention with a programming voltage on par with the present industrial standards. The proposed memory technology is then applied to non-conventional substrates, such as papers, to demonstrate its feasibility in a wide range of applications.

Flexible flash memory is crucial to modern electronics, but its fabrication is challenging in the absence of suitable dielectric materials. Here, Lee et al. realize organic memory with retention over 10 years using tunneling and blocking dielectric layers prepared by initiated chemical vapor deposition.

Excellent low-voltage operating flexible ferroelectric organic transistor nonvolatile memory with a sandwiching ultrathin ferroelectric film

The high operating voltage is a primary issue preventing the commercial application of the ferroelectric organic field-effect transistor (Fe-OFET) nonvolatile memory (NVM). In this work, we propose a novel route to resolve this issue by employing two ultrathin AlO_X interfacial layers sandwiching an ultrathin ferroelectric polymer film with a low coercive field, in the fabricated flexible Fe-OFET NVM. The operation voltage of Fe-OFET NVMs decreases with the downscaling thickness of the ferroelectric film. By inserting two ultrathin AlO_X interfacial layers at both sides of the ultrathin ferroelectric film, not only the gate leakage is prominently depressed but also the mobility is greatly improved. Excellent memory performances, with large mobility of 1.7 ~ 3.3 cm² V^-1 s^-1, high reliable memory switching endurance over 2700 cycles, high stable data storage retention capability over 8 × 10⁴ s with memory on-off ratio larger than 10², are achieved at the low operating voltage of 4 V, which is the lowest value reported to data for all Fe-OFET NVMs. Simultaneously, outstanding mechanical fatigue property with the memory performances maintaining well over 7500 bending cycles at a bending radius of 5.5 mm is also achieved in our flexible FE-OFET NVM.

Stretchable Polymer Dielectrics for Low-Voltage-Driven Field-Effect Transistors

A stretchable and mechanical robust field-effect transistor is essential for soft wearable electronics. To realize stretchable transistors, elastic dielectrics with small current hysteresis, high elasticity, and high dielectric constants are the critical factor for low-voltage-driven devices. Here, we demonstrate the polar elastomer consisting of poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP):poly(4-vinylphenol) (PVP). Owing to the high dielectric constant of PVDF-HFP, the device can be operated under less than 5 V and shows a linear-regime hole mobility as high as 0.199 cm² V^-1 s^-1 without significant current hysteresis. Specifically, the PVDF-HFP:PVP blends induce the vertical phase separation and significantly reduce current leakage and reduce the crystallization of PVDF segments, which can contribute current hysteresis in the OFET characteristics. All-stretchable OFETs based on these PVDF-HFP:PVP dielectrics were fabricated. The device can still keep the hole mobility of approximately 0.1 cm²/(V s) under a low operation voltage of 3 V even as stretched with 80% strain. Finally, we successfully fabricate a low-voltage-driven stretchable transistor. The low voltage operating under strains is the desirable characteristics for soft and comfortable wearable electronics.

Flexible, Semitransparent, and Inorganic Resistive Memory based on BaTi0.95 Co0.05 O3 Film

Perovskite ceramics and single crystals are commonly hard and brittle due to their small maximum elastic strain. Here, large-scale BaTi_0.95 Co_0.05 O₃ (BTCO) film with a SrRuO₃ (SRO) buffered layer on a 10 µm thick mica substrate is flexible with a small bending radius of 1.4 mm and semitransparent for visible light at wavelengths of 500-800 nm. Mica/SRO/BTCO/Au cells show bipolar resistive switching and the high/low resistance ratio is up to 50. The resistive-switching properties show no obvious changes after the 2.2 mm radius memory being written/erased for 360 000 cycles nor after the memory being bent to 3 mm radius for 10 000 times. Most importantly, the memory works properly at 25-180 °C or after being annealed at 500 °C. The flexible and transparent oxide resistive memory has good prospects for application in smart wearable devices and flexible display screens.