A photochromic diode with a continuum of intermediate states: towards high density multilevel storage

Arylazopyrazole-modulated stable dual-mode phototransistors

High-performance organic devices with dynamic and stable modulation are essential for building devices adaptable to the environment. However, the existing reported devices incorporating light-activated units exhibit either limited device stability or subpar optoelectronic properties. Here, we synthesize a new optically tunable polymer dielectric functionalized with photochromic arylazopyrazole units with a cis-isomer half-life of as long as 90 days. On this basis, stable dual-mode organic transistors that can be reversibly modulated are successfully fabricated. The trans-state devices exhibit high carrier mobility reaching 7.4 square centimeters per volt per second and excellent optical figures of merit, whereas the cis-state devices demonstrate stable but starkly different optoelectronic performance. Furthermore, optical image sensors are prepared with regulatable nonvolatile memories from 36 hours (cis state) to 108 hours (trans state). The achievement of dynamic light modulation shows remarkable prospects for the intelligent application of organic optoelectronic devices.

Tunable Photochromism of Spirooxazine in the Solid State: A New Design Strategy Based on the Hypochromic Effect

As an important organic photofunctional material, spirooxazine (SO) usually does not exhibit photochromism in the solid state since the intermolecular π-π stacking impedes photoisomerization. Developing photochromic SO in the solid state is crucial for practical applications but is still full of challenges. Here, a series of spirooxazine derivatives (SO1-SO4) with bulky aromatic substituents at the 4- and 7-positions of the skeleton, which provide them with a large volume with which to undergo solid-state photochromism under mild conditions, is designed and synthesized. All the compounds SO1-SO4 exhibit tunable solid photochromism without ground colors, excellent fatigue resistance, and high thermal stability. Notably, it takes only 15 s for SO4 to reach the saturation of absorption intensity, thought to represent the fastest solid-state photoresponse of spirooxazines. X-ray crystal structures of the intermediate compound SO0 and the products SO1-SO2 as well as computational studies suggest that the bulky aromatic groups can lead to a hypochromic effect, allowing for the photochromism of SO in the solid state. The ideal photochromic properties of these spirooxazines open a new avenue for their applications in UV printing, quick response code, and related fields.

Molecular-Switch-Embedded Solution-Processed Semiconductors

Recent improvements in the performance of solution-processed semiconductor materials and optoelectronic devices have shifted research interest to the diversification/advancement of their functionality. Embedding a molecular switch capable of transition between two or more metastable isomers by light stimuli is one of the most straightforward and widely accepted methods to potentially realize the multifunctionality of optoelectronic devices. A molecular switch embedded in a semiconductor can effectively control various parameters such as trap-level, dielectric constant, electrical resistance, charge mobility, and charge polarity, which can be utilized in photoprogrammable devices including transistors, memory, and diodes. This review classifies the mechanism of each optoelectronic transition driven by molecular switches regardless of the type of semiconductor material or molecular switch or device. In addition, the basic characteristics of molecular switches and the persisting technical/scientific issues corresponding to each mechanism are discussed to help researchers. Finally, interesting yet infrequently reported applications of molecular switches and their mechanisms are also described.

Non-volatile ternary memristors based on a polymer containing a carbazole donor with CuO NPs embedded

A D–A-type polymer PCz–BMBI was synthesized and non-volatile ternary memory devices of ITO/PCz–BMBI:CuO/Al were fabricated with an ON2/ON1/OFF ratio of 105.3 : 102.3 : 1.

Development of oxidized hydroxyethyl cellulose-based hydrogel enabling unique mechanical, transparent and photochromic properties for contact lenses

With the development of smart devices, higher requirements are put forward for the stimuli-responsive materials. Stimuli-hydrogels as one kind of stimuli-responsive materials with hydrophilicity, demonstrate huge potential in developing intelligent devices for biomedical application. On this basis, we herein report that a sample method was devised to develop a novel composite hydrogel mainly based on oxidized hydroxyethyl cellulose and allyl co-polymer. Subsequently, a series of tests toward this oxidized hydroxyethyl cellulose-based hydrogel due to its structure and performance was applied. Here, the oxidized hydroxyethyl cellulose molecular chains were used as biomacromolecule templates to form Schiff base, borate and hydrogen bonds to obtain unique mechanical properties (fast recovery with almost no-hysteresis and remarkable compressive capacity), while a double bond functionalized spirooxazine (allyl spirooxazine derivative) was applied to endow photo- and pH sensitivity to the oxidized hydroxyethyl cellulose-based transparent hydrogel (T% = 93%) substrate. Furthermore, the oxidized hydroxyethyl cellulose-based hydrogel did exhibit good pH environment adaptability and noncytotoxicity in vitro test. Based on the advanced characteristics, the designed oxidized hydroxyethyl cellulose-based hydrogel has potential applications prospect in the development of safe, fashionable and pH- detectable contact lenses, thereby providing a new strategy for the development of smart, stylish contact lenses.

Protonation-Induced Enhanced Optical-Light Photochromic Properties of an Inorganic-Organic Phosphomolybdic Acid/Polyaniline Hybrid Thin Film

A phosphomolybdic acid/polyaniline (PMoA/PANI) optical-light photochromic inorganic/organic hybrid thin film was successfully synthesized by protonation between the the multiprotonic acid phosphomolybdic acid (H₃PO₄·12MoO₃) and the conductive polymer polyaniline. The stable Keggin-type structure of PMoA was maintained throughout the process. Protonation and proton transfer successfully transformed the quinone structure of eigenstate PANI into the benzene structure of single-polarized PANI in the PMoA/PANI hybridized thin film, and proton transfer transformed the benzene structure of single-polarized PANI back to the quinone structure of eigenstate PANI in the PMoA/PANI hybrid thin film, as verified by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The average distribution of PMoA/PANI was observed by atom force microscopy (AFM). Interestingly, protonation of PMoA caused PANI to trigger transformation of the quinone structure into the single-polarized benzene structure, which enhanced the electron delocalization ability and vastly enhanced the maximum light absorption of the PMoA/PANI hybrid thin film as confirmed by density functional theory (DFT), electrochemistry, and ultraviolet-visible spectroscopy (UV-Vis) studies. Under optical-light illumination, the pale-yellow PMoA/PANI hybrid thin film gradually turned deep blue, thus demonstrating a photochromic response, and reversible photochromism was also observed in the presence of hydrogen peroxide (H₂O₂) or oxygen (O₂). After 40 min of optical-light illumination, 36% of the Mo⁵⁺ species in PMoA was photoreduced via a protonation-induced proton transfer mechanism, and this proton transfer resulted in a structural change of PANI, as observed by XPS, generating a dominant structure with high maximum light absorption of 3.46, when compared with the literature reports.

Redox gated polymer memristive processing memory unit

Memristors with enormous storage capacity and superior processing efficiency are of critical importance to overcome the Moore’s Law limitation and von Neumann bottleneck problems in the big data and artificial intelligence era. In particular, the integration of multifunctionalities into a single memristor promises an essential strategy of obtaining a high-performance electronic device that satisfies the nowadays increasing demands of data storage and processing. In this contribution, we report a proof-of-concept polymer memristive processing-memory unit that demonstrates programmable information storage and processing capabilities. By introducing redox active moieties of triphenylamine and ferrocene onto the pendants of fluorene skeletons, the conjugated polymer exhibits triple oxidation behavior and interesting memristive switching characteristics. Associated with the unique electrochemical and electrical behavior, the polymer device is capable of executing multilevel memory, decimal arithmetic operations of addition, subtraction, multiplication and division, as well as simple Boolean logic operations.

Though designing conductive polymers for memory devices is attractive for future low-cost flexible electronics, a proof-of-concept device has yet to be realized. Here, the authors report a redox-gated polymer memristive processing unit with programmable multilevel storage and logic functionalities.

Mussel-Inspired Polydopamine Coating for Flexible Ternary Resistive Memory

In recent years, numerous organic molecules and polymers carrying various functional groups were synthesized and used in fabrication of wearable electronic devices. Compared to previous materials that suffer from poisonousness, stiffness and complex film fabrication, we circumvent above matters by taking advantage of mussel-inspired polydopamine as our active material to realize resistive random access memories (RRAMs). Polydopamine thin films were grown on indium tin oxide glass catalyzed by Cu₂ SO₄ /H₂ O₂ and characterized by Fourier infrared spectroscopy (FT-IR), UV/Vis spectroscopy and scanning electron microscopy. The Al/Polydopamine film/ITO devices possess ternary memory behavior with good ternary device yield with two threshold voltages around 1.50 V and 3.50 V, long data retention over 10⁴  s of continuous reading or 10⁴ pulse reading. The two resistance switchings are attributed to defects functioning as charge traps and the formation of conductive filaments. A flexible device based on Al/polydopamine film/ITO/polyethylene terephthalate retains its ternary memory behavior after being bent with a bending radius of 1.54 cm and bending cycles up to 5000, demonstrating good compatibility and flexibility of polydopamine.

Self-Assembled Photochromic Molecular Dipoles for High-Performance Polymer Thin-Film Transistors

The development of high-performance multifunctional polymer-based electronic circuits is a major step toward future flexible electronics. Here, we demonstrate a tunable approach to fabricate such devices based on rationally designed dielectric super-lattice structures with photochromic azobenzene molecules. These nanodielectrics possessing ionic, molecular, and atomic polarization are utilized in polymer thin-film transistors (TFTs) to realize high-performance electronics with a p-type field-effect mobility (μ_FET) exceeding 2 cm² V^-1 s^-1. A crossover in the transport mechanism from electrostatic dipolar disorder to ionic-induced disorder is observed in the transistor characteristics over a range of temperatures. The facile supramolecular design allows the possibility to optically control the extent of molecular and ionic polarization in the ultrathin nanodielectric. Thus, we demonstrate a 3-fold increase in the capacitance from 0.1 to 0.34 μF/cm², which results in a 200% increase in TFT channel current.

Thiadizoloquinoxaline-Based N-Heteroacenes as Active Elements for High-Density Data-Storage Device

A novel thiadiazoloquinoxaline (TQ)-based donor-acceptor (D-A)-type N-heteroacene (Py-1-TQ) has been demonstrated for promising applications in organic multilevel resistive memory devices. Compared with its counterparts (Py-0-TQ and Py-2-TQ), which show flash-type binary memory behaviors, Py-1-TQ exhibits excellent nonvolatile write-once-read-many-times-type ternary memory effects with high ON2/ON1/OFF current ratios (10^5.8:10^3.4:1), which can be attributed to the different electron-withdrawing abilities between the pyrazine unit and TQ species that can induce stepwise D-A charge-transfer processes. These results suggest that TQ-based N-heteroacenes can be potentially useful in ultrahigh-density data-storage devices through the rational D-A tuning.

Pseudohalide-Induced 2D (CH3 NH3 )2 PbI2 (SCN)2 Perovskite for Ternary Resistive Memory with High Performance

Recently, organic-inorganic hybrid perovskites (OIHP) are studied in memory devices, but ternary resistive memory with three states based on OIHP is not achieved yet. In this work, ternary resistive memory based on hybrid perovskite is achieved with a high device yield (75%), much higher than most organic ternary resistive memories. The pseudohalide-induced 2D (CH₃ NH₃ )₂ PbI₂ (SCN)₂ perovskite thin film is prepared by using a one-step solution method and fabricated into Al/perovskite film/indium-tin oxide (glass substrate as well as flexible polyethylene terephthalate substrate) random resistive access memory (RRAM) devices. The three states have a conductivity ratio of 1:10³ :10⁷ , long retention over 10 000 s, and good endurance properties. The electrode area variation, impedance test, and current-voltage plotting show that the two resistance switches are attributable to the charge trap filling due to the effect of unscreened defect in 2D nanosheets and the formation of conductive filaments, respectively. This work paves way for stable perovskite multilevel RRAMs in ambient atmosphere.

Fluorine-Induced Highly Reproducible Resistive Switching Performance: Facile Morphology Control through the Transition between J- and H-Aggregation

Improving the reproducibility and air-endurance of organic resistance switching (RS) devices, in particular multilevel-cell RS devices, is critical for the confirmation of its competency to realize big data storage capability. However, such enhancement still remains challenging. In this report, we demonstrated that fluorine (F)-embedding should be an effective way to enhance the overall performance of RS devices. Four new azo-cored analogues (IDAZO, FIDAZO, F₂IDAZO, and F₄IDAZO) have been designed and synthesized. These four compounds have similar structures with different numbers of F substituents. Interestingly, UV-vis measurements reveal that upon F-embedding, an exceptional transition from molecular J-aggregation to H-aggregation is achieved. As a result, the morphology of RS films becomes more and more uniform, as determined by AFM and XRD. Meanwhile, the hydrophobicity of RS film is promoted, which further improves the device atmospheric stability. The total RS reproducibility increases to 96% (the uppermost value), and the tristage RS reproducibility rises to 64%, accompanied by a more stable OFF state and lower logic SET voltages. Our study suggests F-embedding would be a promising strategy to achieve highly reproducible and air-endurable organic multilevel-cell RS devices.

Light-Controlled Reversible Modulation of Frontier Molecular Orbital Energy Levels in Trifluoromethylated Diarylethenes

Among bistable photochromic molecules, diarylethenes (DAEs) possess the distinct feature that upon photoisomerization they undergo a large modulation of their π-electronic system, accompanied by a marked shift of the HOMO/LUMO energies and hence oxidation/reduction potentials. The electronic modulation can be utilized to remote-control charge- as well as energy-transfer processes and it can be transduced to functional entities adjacent to the DAE core, thereby regulating their properties. In order to exploit such photoswitchable systems it is important to precisely adjust the absolute position of their HOMO and LUMO levels and to maximize the extent of the photoinduced shifts of these energy levels. Here, we present a comprehensive study detailing how variation of the substitution pattern of DAE compounds, in particular using strongly electron-accepting and chemically stable trifluoromethyl groups either in the periphery or at the reactive carbon atoms, allows for the precise tuning of frontier molecular orbital levels over a broad energy range and the generation of photoinduced shifts of more than 1 eV. Furthermore, the effect of different DAE architectures on the transduction of these shifts to an adjacent functional group is discussed. Whereas substitution in the periphery of the DAE motif has only minor implications on the photochemistry, trifluoromethylation at the reactive carbon atoms strongly disturbs the isomerization efficiency. However, this can be overcome by using a nonsymmetrical substitution pattern or by combination with donor groups, rendering the resulting photoswitches attractive candidates for the construction of remote-controlled functional systems.

Photochromic benzo[b]phosphole oxide with excellent thermal irreversibility and fatigue resistance in the thin film solid state via direct attachment of dithienyl units to the weakly aromatic heterocycle

A novel photochromic benzo[b]phosphole oxide has been demonstrated to display photochromic properties with excellent fatigue resistance and thermal irreversibility in polymethylmethacrylate (PMMA) thin film under ambient conditions. The remarkable photochromic behaviour can be achieved by rational molecular design, in which the weakly aromatic phosphole oxide is directly incorporated into the photo-responsive dithienylethene units. Photopatterning via photoinduced colouration and decolouration has been performed to demonstrate the repeatable and distinct transformation between the bistable states, making it a promising candidate with photoswitching properties for optoelectronic applications.

Synthesis and improved photochromic properties of pyrazolones in the solid state by incorporation of halogen

Four novel photochromic pyrazolones have been prepared by introducing halogen atoms as substituents on the benzene ring. All as-synthesized compounds exhibited excellent reversible photochromic performances in the solid state. Upon UV light irradiation, the as-synthesized compounds can change their structures from E-form to K-form with yellow coloration. Further processed by heating, they rapidly reverted to their initial states at 120°С. Their photo-response and thermal bleaching kinetics were detailed investigated by UV absorption spectra. The results showed that the time constants were higher than that of our previously reported compounds at least one order of magnitude and the rate constants of the as-synthesized compounds were significantly influenced by the size and electronegativity of different halogen atoms. The fluorescence emission were modulated in a high degree via photoisomerization of pyrazolones, which might be due to the efficient energy transfer from E-form to K-form isomers for their partly overlaps between their E-form absorption spectra and K-form fluorescence spectra.

Flexible non-volatile optical memory thin-film transistor device with over 256 distinct levels based on an organic bicomponent blend

Organic nanomaterials are attracting a great deal of interest for use in flexible electronic applications such as logic circuits, displays and solar cells. These technologies have already demonstrated good performances, but flexible organic memories are yet to deliver on all their promise in terms of volatility, operational voltage, write/erase speed, as well as the number of distinct attainable levels. Here, we report a multilevel non-volatile flexible optical memory thin-film transistor based on a blend of a reference polymer semiconductor, namely poly(3-hexylthiophene), and a photochromic diarylethene, switched with ultraviolet and green light irradiation. A three-terminal device featuring over 256 (8 bit storage) distinct current levels was fabricated, the memory states of which could be switched with 3 ns laser pulses. We also report robustness over 70 write-erase cycles and non-volatility exceeding 500 days. The device was implemented on a flexible polyethylene terephthalate substrate, validating the concept for integration into wearable electronics and smart nanodevices.

Design of a Photoactive Hybrid Bilayer Dielectric for Flexible Nonvolatile Organic Memory Transistors

Organic field-effect transistors (OFETs) featuring a photoactive hybrid bilayer dielectric (PHBD) that comprises a self-assembled monolayer (SAM) of photochromic diarylethenes (DAEs) and an ultrathin solution-processed hafnium oxide layer are described here. We photoengineer the energy levels of DAE SAMs to facilitate the charging and discharging of the interface of the two dielectrics, thus yielding an OFET that functions as a nonvolatile memory device. The transistors use light signals for programming and electrical signals for erasing (≤3 V) to produce a large, reversible threshold-voltage shift with long retention times and good nondestructive signal processing ability. The memory effect can be exercised by more than 10(4) memory cycles. Furthermore, these memory cells have demonstrated the capacity to be arrayed into a photosensor matrix on flexible plastic substrates to detect the spatial distribution of a confined light and then store the analog sensor input as a two-dimensional image with high precision over a long period of time.

Enhanced visible-active photochromism of a polyoxometalates/TiO2 composite film by combining Bi2O3 nanoparticles

Photochromic hybrid film sensitive to visible light was fabricated by introducing Bi₂O₃ into PMoA/TiO₂ composite as a visible light sensitizer.

Photoswitchable organic field-effect transistors and memory elements comprising an interfacial photochromic layer

Memory devices with superior electrical characteristics were designed using an interfacial spirooxazine layer introduced between dielectric and semiconductor layers in OFETs.

25th anniversary article: organic electronics marries photochromism: generation of multifunctional interfaces, materials, and devices

Organic semiconductors have garnered significant interest as key components for flexible, low-cost, and large-area electronics. Hitherto, both materials and processing thereof seems to head towards a mature technology which shall ultimately meet expectations and efforts built up over the past years. However, by its own organic electronics cannot compete or complement the silicon-based electronics in integrating multiple functions in a small area unless novel solutions are brought into play. Photochromic molecules are small organic molecules able to undergo reversible photochemical isomerization between (at least) two (meta)stable states which exhibit markedly different properties. They can be embedded as additional component in organic-based materials ready to be exploited in devices such as OLEDs, OFETs, and OLETs. The structurally controlled incorporation of photochromic molecules can be done at various interfaces of a device, including the electrode/semiconductor or dielectric/semiconductor interface, or even as a binary mixture in the active layer, in order to impart a light responsive nature to the device. This can be accomplished by modulating via a light stimulus fundamental physico-chemical properties such as charge injection and transport in the device.

Inorganic–organic hybrid polymer with multiple redox for high-density data storage

A multi-redox polyoxometalate-based hybrid polymer has been demonstrated to show multilevel resistive switching memory behaviors.