Optically switchable organic light-emitting transistors

Customizable Organic Charge-Transfer Cocrystals for the Dual-Mode Optoelectronics in the NIR (II) Window

Organic molecules have been regarded as ideal candidates for near-infrared (NIR) optoelectronic active materials due to their customizability and ease of large-scale production. However, constrained by the intricate molecular design and severe energy gap law, the realization of optoelectronic devices in the second near-infrared (NIR (II)) region with required narrow band gaps presents more challenges. Herein, we have originally proposed a cocrystal strategy that utilizes intermolecular charge-transfer interaction to drive the redshift of absorption and emission spectra of a series BFXTQ (X = 0, 1, 2, 4) cocrystals, resulting in the spectra located at NIR (II) window and reducing the optical bandgap to ∼0.98 eV. Significantly, these BFXTQ-based optoelectronic devices can exhibit dual-mode optoelectronic characteristics. An investigation of a series of BFXTQ-based photodetectors exhibits detectivity (D*) surpassing 1013 Jones at 375 to 1064 nm with a maximum of 1.76 × 1014 Jones at 1064 nm. Moreover, the radiative transition of CT excitons within the cocrystals triggers NIR emission over 1000 nm with a photoluminescence quantum yield (PLQY) of ∼4.6% as well as optical waveguide behavior with a low optical-loss coefficient of 0.0097 dB/μm at 950 nm. These results promote the advancement of an emerging cocrystal approach in micro/nanoscale NIR multifunctional optoelectronics.

Robust, Ultrafast and Reversible Photoswitching in Bulk Polymers Enabled by Octupolar Molecule Design

Improving the photoswitching rate and robustness of photochromic molecules in bulk solids is paramount for practical applications but remains an on-going challenge. Here, we introduce an octupolar design paradigm to develop a new family of visible light organic photoswitches, namely multi-branched octupolar Stenhouse Adducts (MOPSAs) featuring a C3-symmetrical A3-(D-core) architecture with a dipolar donor-acceptor (D-A) photochrome in each branch. Our design couples multi-dimensional geometric and electronic effects of MOPSAs to enable robust ultrafast reversible photoswitching in bulk polymers. Specifically, the optimal MOPSA (4 wt %) in commercial polyurethane films accomplishes nearly 100 % discoloration in 6 s under visible light with ∼ 100 % thermal-recovery in 17.4 s at 60 °C, while the acquired kinetics constants are 3∼7 times that of dipolar DASA counterpart and 1∼2 orders of magnitude higher than those of reported DASAs in polymers. Importantly, the MOPSA-doped polymer films sustain 500 discoloration/recovery cycles with slow degradation, superior to the existing DASAs in polymers (≤30 cycles). We discover that multi-dipolar coupling in MOPSA enables enhanced polarization and electron delocalization, promoting the rate-determining thermal cyclization, while the branched and non-planar geometry of MOPSA induces large free volume to facilitate the isomerization. This design can be extended to develop spiropyran or azobenzene-based ultrafast photochromic films. The superior photoswitching performance of MOPSAs together with their high-yield and scalable synthesis and facile film processing inspires us to explore their versatile uses as smart inks or labels for time-temperature indicators, optical logic encryption and multi-levelled data encryption.

Dynamical Effects of Solvation on Norbornadiene/Quadricyclane Systems

Molecules that can undergo reversible chemical transformations following the absorption of light, the so-called molecular photoswitches, have attracted increasing attention in technologies, such as solar energy storage. Here, the optical and thermochemical properties of the photoswitch are central to its applicability, and these properties are influenced significantly by solvation. We investigate the effects of solvation on two norbornadiene/quadricyclane photoswitches. Emphasis is put on the energy difference between the two isomers and the optical absorption as these are central to the application of the systems in solar energy storage. Using a combined classical molecular dynamics and quantum mechanical/molecular mechanical computational scheme, we showcase that the dynamic effects of solvation are important. In particular, it is found that standard implicit solvation models generally underestimate the energy difference between the two isomers and overestimate the strength of the absorption, while the explicit solvation spectra are also less red-shifted than those obtained using implicit solvation models. We also find that the absorption spectra of the two systems are strongly correlated with specific dihedral angles. Altogether, this highlights the importance of including the dynamic effects of solvation.

Photophore-Anchored Molecular Switch for High-Performance Nonvolatile Organic Memory Transistor

Over the past decade, molecular-switch-embedded memory devices, particularly field-effect transistors (FETs), have gained significant interest. Molecular switches are integrated to regulate the resistance or current levels in FETs. Despite substantial efforts, realizing large memory window with a long retention time, a critical factor in memory device functionality, remains a challenge. This is due to the inability of an isomeric state of a molecular switch to serve as a stable deep trap state within the semiconductor layer. Herein, the study addresses this limitation by introducing chemical bonding between molecular switch and conjugated polymeric semiconductor, facilitating closed isomer of diarylethene (DAE) to operate as a morphologically stable deep trap state. Azide- and diazirine-anchored DAEs are synthesized, which form chemical bonds to the polymer through photocrosslinking, thereby implementing permanent and controllable trapping states nearby conjugated backbone of polymer semiconductor. Consequently, when diazirine-anchored DAE is blended with F8T2 and subjected to photocrosslinking, the resulting organic FETs exhibit remarkable memory performance, including a memory window of 22 V with a retention time over 106 s, a high photoprogrammable on/off ratio over 103, and a high operational stability over 100 photocycles. Further, photophore-anchored DAEs can achieve precise patterning, which enables meticulous control over the semiconductor layer structure.

Molecular Electronics: From Nanostructure Assembly to Device Integration

Integrated electronics and optoelectronics based on organic semiconductors have attracted considerable interest in displays, photovoltaics, and biosensing owing to their designable electronic properties, solution processability, and flexibility. Miniaturization and integration of devices are growing trends in molecular electronics and optoelectronics for practical applications, which requires large-scale and versatile assembly strategies for patterning organic micro/nano-structures with simultaneously long-range order, pure orientation, and high resolution. Although various integration methods have been developed in past decades, molecular electronics still needs a versatile platform to avoid defects and disorders due to weak intermolecular interactions in organic materials. In this perspective, a roadmap of organic integration technologies in recent three decades is provided to review the history of molecular electronics. First, we highlight the importance of long-range-ordered molecular packing for achieving exotic electronic and photophysical properties. Second, we classify the strategies for large-scale integration of molecular electronics through the control of nucleation and crystallographic orientation, and evaluate them based on factors of resolution, crystallinity, orientation, scalability, and versatility. Third, we discuss the multifunctional devices and integrated circuits based on organic field-effect transistors (OFETs) and photodetectors. Finally, we explore future research directions and outlines the need for further development of molecular electronics, including assembly of doped organic semiconductors and heterostructures, biological interfaces in molecular electronics and integrated organic logics based on complementary FETs.

High Mobility Emissive Excimer Organic Semiconductor Towards Color-Tunable Light-Emitting Transistors

Organic light-emitting transistors (OLETs) are highly integrated and minimized optoelectronic devices with significant potential superiority in smart displays and optical communications. To realize these various applications, it is urgently needed for color-tunable emission in OLETs, but remains a great challenge as a result of the difficulty for designing organic semiconductors simultaneously integrating high carrier mobility, strong solid-state emission, and the ability for potential tunable colors. Herein, a high mobility emissive excimer organic semiconductor, 2,7-di(2-anthryl)-9H-fluorene (2,7-DAF) was reasonably designed by introducing a rotatable carbon-carbon single bond connecting two anthracene groups at the 2,7-sites of fluorene, and the small torsion angles simultaneously guarantee effective conjugation and suppress fluorescence quenching. Indeed, the unique stable dimer arrangement and herringbone packing mode of 2,7-DAF single crystal enables its superior integrated optoelectronic properties with high carrier mobility of 2.16 cm2  ⋅ V-1  ⋅ s-1 , and strong excimer emission with absolute photoluminescence quantum yield (PLQY) of 47.4 %. Furthermore, the voltage-dependent electrically induced color-tunable emission from orange to blue was also demonstrated for an individual 2,7-DAF single crystal based OLETs for the first time. This work opens the door for a new class of high mobility emissive excimer organic semiconductors, and provides a good platform for the study of color-tunable OLETs.

Enhancing the Potential of Fused Heterocycle-Based Triarylhydrazone Photoswitches

Triarylhydrazones represent an attractive class of photochromic compounds offering many interesting features including high molar absorptivity, good addressability, and extraordinary thermal stability. In addition, unlike most other hydrazone-based photoswitches, they effectively absorb light above 365 nm. However, previously prepared triaryhydrazones suffer from low quantum yields of the Z→E photoisomerization. Here, we have designed a new subclass of naphthoyl-benzothiazole hydrazones that balance the most beneficial features of previously reported naphthoyl-quinoline and benzoyl-pyridine triarylhydrazones. These preserve the attractive absorption characteristics, exhibit higher thermal stability of the metastable form than the former and enhance the rate of the Z→E photoisomerization compared to the later, as a result of the weakening of the intramolecular hydrogen bonding between the hydrazone hydrogen and the benzothiazole moiety. Introducing the benzothiazole motif extends the tunability of the photochromic behaviour of hydrazone-based switches.

Contributions of Light to Novel Logic Concepts Using Optoelectronic Materials

Instead of the current method of transmitting voltage or current signals in electronic circuit operation, light offers an alternative to conventional logic, allowing for the implementation of new logic concepts through interaction with light. This manuscript examines the use of light in implementing new logic concepts as an alternative to traditional logic circuits and as a future technology. This article provides an overview of how to implement logic operations using light rather than voltage or current signals using optoelectronic materials such as 2D materials, metal-oxides, carbon structures, polymers, small molecules, and perovskites. This review covers the various technologies and applications of using light to dope devices, implement logic gates, control logic circuits, and generate light as an output signal. Recent research on logic and the use of light to implement new functions is summarized. This review also highlights the potential of optoelectronic logic for future technological advancements.

Phototunable and Photopatternable Polymer Semiconductors

ConspectusIn recent decades, there has been rapid development in the field of polymer semiconductors, particularly those based on conjugated donor-acceptor (D-A) polymers exhibiting high charge mobilities. Furthermore, the application of polymer semiconductors has been successfully extended to a wide range of functional devices, including sensors, photodetectors, radio frequency identification (RFID) tags, electronic paper, skin electronics, and artificial synapses. Over the past few years, there has been a growing focus on stimuli-responsive polymer semiconductors, which have the potential to impart additional functionalities to conventional field-effect transistors, garnering increased attention within the research community. In this context, phototunable polymer semiconductors have received significant attention due to their ability to utilize light as an external stimulus, enabling remote control of device performance with high spatiotemporal resolution. Meanwhile, integration of field-effect transistors with polymer semiconductors can enable the realization of complex functions. To achieve this, precise and controllable patterning of polymer semiconductors becomes essential. In this Account, we discuss our research findings in the context of phototunable and photopatternable polymer semiconductors. These developments encompass the following key aspects: (i) polymer semiconductors, such as poly(diketopyrrolopyrrole-quaterthiophene) (PDPP4T), exhibit phototunability when blended with the photochromic compound hexaarylbiimidazole (HABI). The photo/thermal-responsive field-effect transistors (FETs) can be fabricated using blending thin films. Remarkably, these photo/thermal-responsive transistors can function as photonically programmable and thermally erasable nonvolatile memory devices. (ii) By incorporating photoswitchable groups like azo and spiropyran into the side chains of conjugated D-A polymers, we can create phototunable polymer semiconductors. The reversible isomerization of azo and spiropyran groups significantly influences the charge transport properties of these polymer semiconductors. Consequently, the performance of the resulting FETs can be reversibly tuned through UV/visible or near-infrared light (NIR) irradiation. Notably, the incorporation of two distinct azo groups into the side chains leads to polymer semiconductors with tristable semiconducting states, offering the ability to logically control device performance using light irradiation at three different wavelengths. (iii) Photopatterning of p-type, n-type, and ambipolar semiconductors featuring alkyl side chains can be achieved using a diazirine-based, four-armed photo-cross-linker (4CNN) with a loading concentration of no more than 3% (w/w). Furthermore, the semiconducting performances of FETs with patterned thin films were found to be satisfactorily uniform. Importantly, the cross-linked thin films are robust and show good resistance to organic solvents, which is useful for fabricating all-solution processable multilayer electronic devices. (iv) The introduction of azide groups into the side chains of conjugated polymers results in a single-component semiconducting photoresist. The presence of azide groups renders the side chains with photo-cross-linking ability, enabling the successful formation of uniform patterns, even as small as 5 μm, under UV light irradiation. Benefiting from the single component feature, field-effect transistors with individual patterned thin films display satisfactorily uniform performances. Moreover, this semiconducting photoresist has proven effective for efficiently photopatterning other polymer semiconductors, demonstrating its versatility.

Photo-Modulated Ionic Polymer as an Adaptable Electron Transport Material for Optically Switchable Pixel-Free Displays

In addition to electrically driven organic light-emitting diode (OLED) displays that rely on complicated and costly circuits for switching individual pixel illumination, developing a facile approach that structures pixel-free light-emitting displays with exceptional precision and spatial resolution via external photo-modulation holds significant importance for advancing consumer electronics. Here, optically switchable organic light-emitting pixel-free displays (OSPFDs) are presented and fabricated by judiciously combining an adaptive photosensitive ionic polymer as electron transport materials (ETM) with external photo-modulation as the switching mode while ensuring superior illumination performance and seamless imaging capability. By irradiating the solution-processed OSPFDs with light at specific wavelengths, efficient and reversible tuning of both electron transport and electroluminescence is achieved simultaneously. This remarkable control is achieved by altering the energetic matching within OSPFDs, which also exhibits a high level of universality and adjustable flexibility in the three primary color-based light-emitting displays. Moreover, the ease of creating and erasing desired pixel-free emitting patterns through a non-invasive photopatterning process within a single OSPFD is demonstrated, thereby rendering this approach promising for commercial displaying devices and highly precise pixelated illuminants.

High-Efficiency Area-Emissive White Organic Light-Emitting Transistor for Full-Color Display

The construction of high-performance white organic light-emitting transistor (OLET) with uniform area emission is crucial for smart display technologies but remains greatly challenging. Herein, high-efficiency uniform area-emissive OLETs based on a unique lateral-integrated device configuration which incorporates efficient energy transfer of phosphorescent and fluorescent guests, enabling color-tunable and white emission, are demonstrated. Through precisely regulating the energy transfer between host and guests, high external quantum efficiency of 13.9% for white-emission OLETs is achieved due to the improved high exciton utilization and light outcoupling efficiency which is the highest value reported so far for OLETs and prevents exciton-charge annihilation and electrode photon losses. Moreover, good loop stability is also achieved, along with effective gate tunability and ultralow driving voltage of below 5 V. Finally, a 4 × 6 white-emission OLET array for full-color display is demonstrated for the first time, suggesting its great potential applications for advanced display technologies.

Dibenzothiophene Sulfone-Based Ambipolar-Transporting Blue-Emissive Organic Semiconductors Towards Simple-Structured Organic Light-Emitting Transistors

The development of blue-emissive ambipolar organic semiconductor is an arduous target due to the large energy gap, but is an indispensable part for electroluminescent device, especially for the transformative display technology of simple-structured organic light-emitting transistor (SS-OLET). Herein, we designed and synthesized two new dibenzothiophene sulfone-based high mobility blue-emissive organic semiconductors (DNaDBSOs), which demonstrate superior optical property with solid-state photoluminescent quantum yield of 46-67 % and typical ambipolar-transporting properties in SS-OLETs with symmetric gold electrodes. Comprehensive experimental and theoretical characterizations reveal the natural of ambipolar property for such blue-emissive DNaDBSOs-based materials is ascribed to a synergistic effect on lowering LUMO level and reduced electron injection barrier induced by the interfacial dipoles effect on gold electrodes due to the incorporation of appropriate DBSO unit. Finally, efficient electroluminescence properties with high-quality blue emission (CIE (0.179, 0.119)) and a narrow full-width at half-maximum of 48 nm are achieved for DNaDBSO-based SS-OLET, showing good spatial control of the recombination zone in conducting channel. This work provides a new avenue for designing ambipolar emissive organic semiconductors by incorporating the synergistic effect of energy level regulation and molecular-metal interaction, which would advance the development of superior optoelectronic materials and their high-density integrated optoelectronic devices and circuits.

Does the Traditional Band Picture Describe the Electronic Structure of Doped Conjugated Polymers? TD-DFT and Natural Transition Orbital Study of Doped P3HT

Polarons and bipolarons are created when one or two electrons are removed from the π-system of a p-type conjugated polymer, respectively. In the traditional band picture, the creation of a polaron causes two electronic energy levels to move into the band gap. The removal of a second electron to form a bipolaron causes the two intragap states to move further into the gap. Several groups, however, who looked at the energies of the Kohn-Sham orbitals from DFT calculations, have recently argued that the traditional band picture is incorrect for explaining the spectroscopy of doped conjugated polymers. Instead, the DFT calculations suggest that polaron creation causes only one unoccupied state to move into the band gap near the valence band edge while half-filled state in the valence band and the conduction band bend downward in energy. To understand the discrepancy, we performed TD-DFT calculations of polarons and bipolarons on poly(3-hexylthiophene) (P3HT). Not only do the TD-DFT-calculated absorption spectra match the experimental absorption spectra, but an analysis using natural transitional orbitals (NTOs), which provides an approximate one-electron picture from the many-electron TD-DFT results, supports the traditional band picture. Our TD-DFT/NTO analysis indicates that the traditional band picture also works for bipolarons, a system for which DFT calculations were unable to determine the electronic structure.

Searching the Chemical Space of Bicyclic Dienes for Molecular Solar Thermal Energy Storage Candidates

Photoswitches are molecular systems that are chemically transformed subsequent to interaction with light and they find potential application in many new technologies. The design and discovery of photoswitch candidates require intricate molecular engineering of a range of properties to optimize a candidate to a specific applications, a task which can be tackled efficiently using quantum chemical screening procedures. In this paper, we perform a large scale screening of approximately half a million bicyclic diene photoswitches in the context of molecular solar thermal energy storage using ab initio quantum chemical methods. We further device an efficient strategy for scoring the systems based on their predicted solar energy conversion efficiency and elucidate potential pitfalls of this approach. Our search through the chemical space of bicyclic dienes reveals systems with unprecedented solar energy conversion efficiencies and storage densities that show promising design guidelines for next generation molecular solar thermal energy storage systems.

Organic Polarized Light-Emitting Transistors

Electrically driven polarized light-emitting sources are central to various applications including quantum computers, optical communication, and 3D displays, but serious challenges remain due to the inevitable incorporation of complex optical elements in conventional devices. Here, organic polarized light-emitting transistors (OPLETs), a kind of novel device that integrates the functions of organic field-effect transistors, organic light-emitting diodes, and polarizers into one unique device, are demonstrated with a degree of polarization (DOP) as high as 0.97, which is comparable to completely linearly polarized light (DOP = 1). Under the modulation of gate voltage, robust and efficient polarization emission is proven, ascribed to the intrinsic in-plane anisotropy of the molecular transition dipole moment in organic semiconductors and the open-ended feature of OPLETs instead of other factors. As a result, high-contrast optical imaging and anti-counterfeiting security are successfully demonstrated based on OPLETs, establishing a new direction for photonic and electronic integration toward on-chip miniaturized optoelectronic applications.

Single-Component Color-Tunable Smart Organic Emitters with Simultaneous Multistage Stimuli-Responsiveness and Multimode Emissions

Achieving color-tunable emission in single-component organic emitters with multistage stimuli-responsiveness is of vital significance for intelligent optoelectronic applications, but remains enormously challenging. Herein, we present an unprecedented example of a color-tunable single-component smart organic emitter (DDOP) that simultaneously exhibits multistage stimuli-responsiveness and multimode emissions. DDOP based on a highly twisted amide-bridged donor-acceptor-donor structure has been found to facilitate intersystem crossing, form multimode emissions, and generate multiple emissive species with multistage stimuli-responsiveness. DDOP pristine crystalline powders exhibit abnormal excitation-dependent emissions from a monomer-dominated blue emission centered at 470 nm to a dimer-dominated yellow emission centered at 550 nm through decreasing the ultraviolet (UV) excitation wavelengths, whereas DDOP single crystals show a wide emission band with a main emission peak at 585 nm when excited at different wavelengths. The emission behaviors of pristine crystalline powders and single crystals are different, demonstrating emission features that are closely related to the aggregation states. The work has developed color-tunable single-component organic emitters with simultaneous multistage stimuli-responsiveness and multimode emissions, which is vital for expanding intelligent optoelectronic applications, including multilevel information encryption, multicolor emissive patterns, and visual monitoring of UV wavelengths.

Indefinite and bidirectional near-infrared nanocrystal photoswitching

Materials whose luminescence can be switched by optical stimulation drive technologies ranging from superresolution imaging^1-4, nanophotonics⁵, and optical data storage^6,7, to targeted pharmacology, optogenetics, and chemical reactivity⁸. These photoswitchable probes, including organic fluorophores and proteins, can be prone to photodegradation and often operate in the ultraviolet or visible spectral regions. Colloidal inorganic nanoparticles^6,9 can offer improved stability, but the ability to switch emission bidirectionally, particularly with near-infrared (NIR) light, has not, to our knowledge, been reported in such systems. Here, we present two-way, NIR photoswitching of avalanching nanoparticles (ANPs), showing full optical control of upconverted emission using phototriggers in the NIR-I and NIR-II spectral regions useful for subsurface imaging. Employing single-step photodarkening^10-13 and photobrightening^12,14-16, we demonstrate indefinite photoswitching of individual nanoparticles (more than 1,000 cycles over 7 h) in ambient or aqueous conditions without measurable photodegradation. Critical steps of the photoswitching mechanism are elucidated by modelling and by measuring the photon avalanche properties of single ANPs in both bright and dark states. Unlimited, reversible photoswitching of ANPs enables indefinitely rewritable two-dimensional and three-dimensional multilevel optical patterning of ANPs, as well as optical nanoscopy with sub-Å localization superresolution that allows us to distinguish individual ANPs within tightly packed clusters.

Thin-Film Morphology and Optical Properties of Photoisomerizable Donor-Acceptor Oligothiophenes

It was recently reported that the most popular electron-accepting units introduced to π-conjugated oligomers studied for organic photovoltaic applications are susceptible to unwanted and even destructive photochemical reactions. The consequences of Z/E photoisomerization of the popular 2-(1,1-dicyanomethylene)rhodanine (RCN) unit on the optical and morphological properties of a homologous series of RCN-functionalized oligothiophenes are studied here. Oligomers consisting of one, two, or three thiophene units were studied as pure Z isomers and with E isomer compositions of 25, 53, and 45%, respectively, for Z/E mixtures. Solutions of Z isomers and Z/E mixtures were characterized by UV-vis and photoluminescence spectroscopy, wherein changes to optical properties were evaluated on the basis of E isomer content. X-ray diffraction of thin-film Z/E mixtures reveals crystalline domains of both Z and E forms after thermal annealing for mono- and bithiophene oligomers, with greater interplanar spacing for E crystalline domains than the Z counterparts along the substrate normal direction. The surface morphology viewed by atomic force microscopy also shows fiberlike structures for the E form with a much larger aspect ratio than for the Z domains in the bithiophene oligomer. Optical characterization reveals drastic changes in the solid state upon introduction of the E form for the mono- and bithiophene derivatives, whereas subtle consequences are noted for the terthiophene analogue. Most notably, a 132 nm redshift in maximum absorption occurs for the bithiophene oligomer films containing 53% E isomer compared to the pure Z counterpart. Finally, although solid-state photoisomerization experiments find no evidence of Z → E isomerization in polycrystalline Z films, E → Z isomerization is observed and becomes more restrictive in films with higher crystallinity (i.e., after thermal annealing). This structure-property study, which elucidates the consequences of the RCN configuration on solid-state packing and optical properties, is expected to guide the development of more efficient and stable organic optoelectronic devices.

Molecular modification effects on the electrochromic and photochromic properties of diarylethene with intramolecular isomerization behavior

Diarylethene (DAE) is one of the most widely used functional units for electrochromic or photochromic materials. To better understand the molecular modification effects on the electrochromic and photochromic properties of DAE, two modification strategies, substitution with functional groups or heteroatoms, were investigated theoretically by density functional theory calculations. It is found that red-shifted absorption spectra caused by a decreased highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap and S₀ → S₁ transition energy during the ring-closing reaction become more significant by adding different functional substituents. In addition, for two isomers, the energy gap and S₀ → S₁ transition energy decreased by heteroatom substitution of S atoms with O or NH, while they increased by replacing two S atoms with CH₂. For intramolecular isomerization, one-electron excitation is the most effective way to trigger the closed-ring (O → C) reaction, while the open-ring (C → O) reaction occurs most readily in the presence of one-electron reduction. Moreover, it is confirmed that substitution with strong electron donating groups (-OCH₃/-NH₂) or with one O/two CH₂ heteroatoms leads to a more favorable closed-ring (O → C) reaction. Functionalized with strong electron-withdrawing groups (-NO₂ and -COOH) or one/two NH heteroatom substitutions, the open-ring (C → O) reaction is easier. Our results confirmed that the photochromic and electrochromic properties of DAE can be tuned effectively by molecular modifications, which provides theoretical guidance for the design of new DAE-based photochromic/electrochromic materials.

Light-Induced Bipolar Photoresponse with Amplified Photocurrents in an Electrolyte-Assisted Bipolar p-n Junction

The p-n junction with bipolar characteristics sets the fundamental unit to build electronics while its unique rectification behavior constrains the degree of carrier tunability for expanded functionalities. Herein, a bipolar-junction photoelectrode employed with a gallium nitride (GaN) p-n homojunction nanowire array that operates in electrolyte is reported, demonstrating bipolar photoresponse controlled by different wavelengths of light. Significantly, with rational decoration of a ruthenium oxides (RuO_x ) layer on nanowires guided by theoretical modeling, the resulting RuO_x /p-n GaN photoelectrode exhibits unambiguously boosted bipolar photoresponse by an enhancement of 775% and 3000% for positive and negative photocurrents, respectively, compared to the pristine nanowires. The loading of the RuO_x layer on nanowire surface optimizes surface band bending, which facilitates charge transfer across the GaN/electrolyte interface, meanwhile promoting the efficiency of redox reaction for both hydrogen evolution reaction and oxygen evolution reaction which corresponds to the negative and positive photocurrents, respectively. Finally, a dual-channel optical communication system incorporated with such photoelectrode is constructed with using only one photoelectrode to decode dual-band signals with encrypted property. The proposed bipolar device architecture presents a viable route to manipulate the carrier dynamics for the development of a plethora of multifunctional optoelectronic devices for future sensing, communication, and imaging systems.

Harvesting Triplet Excitons in High Mobility Emissive Organic Semiconductor for Efficiency Enhancement of Light-Emitting Transistors

Organic light-emitting transistors (OLETs), a kind of highly integrated and minimized optoelectronic device, demonstrate great potential applications in various fields. The construction of high-performance OLETs requires the integration of high charge carrier mobility, strong emission, and high triplet exciton utilization efficiency in the active layer. However, it remains a significant long-term challenge, especially for single component active layer OLETs. Herein, the successful harvesting of triplet excitons in a high mobility emissive molecule, 2,6-diphenylanthracene (DPA), through the triplet-triplet annihilation process is demonstrated. By incorporating a highly emissive guest into the DPA host system, an obvious increase in photoluminescence efficiency along with exciton utilization efficiency results in an obvious enhancement of external quantum efficiency of 7.2 times for OLETs compared to the non-doped devices. Moreover, well-tunable multi-color electroluminescence, especially white emission with Commission Internationale del'Eclairage  of (0.31, 0.35), from OLETs is also achieved by modulating the doping concentration with a controlled energy transfer process. This work opens a new avenue for integrating strong emission and efficient exciton utilization in high-mobility organic semiconductors for high-performance OLETs and advancing their related functional device applications.

Recent advances in n-type and ambipolar organic semiconductors and their multi-functional applications

Organic semiconductors have received broad attention and research interest due to their unique integration of semiconducting properties with structural tunability, intrinsic flexibiltiy and low cost. In order to meet the requirements of organic electronic devices and their integrated circuits, p-type, n-type and ambipolar organic semiconductors are all necessary. However, due to the limitation in both material synthesis and device fabrication, the development of n-type and ambipolar materials is quite behind that of p-type materials. Recent development in synthetic methods of organic semiconductors greatly enriches the range of n-type and ambipolar materials. Moreover, the newly developed materials with multiple functions also put forward multi-functional device applications, including some emerging research areas. In this review, we give a timely summary on these impressive advances in n-type and ambipolar organic semiconductors with a special focus on their synthesis methods and advanced materials with enhanced properties of charge carrier mobility, integration of high mobility and strong emission and thermoelectric properties. Finally, multi-functional device applications are further demonstrated as an example of these developed n-type and ambipolar materials.

Van der Waals Multilayer Heterojunction for Low-Voltage Organic RGB Area-Emitting Transistor Array

Organic light-emitting transistors (OLETs) have garnered considerable attention from academy and industry due to their potential applications in next-generation display technologies, multifunctional devices, and organic electrically pumped lasers. However, overcoming the trade-offs among power consumption, external quantum efficiency (EQE), and uniform area emission remains a long-standing issue for OLETs. Herein, a van der Waals multilayer heterojunction methodology is proposed to enhance the layer-to-layer interfacial interaction and contact, resulting in better dipole shielding, carrier transport, exciton recombination, and current density distribution. The prepared multilayer heterojunction OLET (MLH-OLET) array shows uniform and bright RGB area emission and low operating voltage (<30 V among the lowest applied voltage of reported lateral LETs). Additionally, a high brightness under area emission of 1060 cd m^-2 , a high EQE value of 0.85%, and a high loop stability (over 380 cycles, outperforming state-of-the-art OLETs) indicate that the proposed multilayer heterojunction is obviously superior to the reported lateral device configuration. The van der Waals multilayer heterojunction developed for the preparation of OLET arrays sufficiently meets the low-voltage, high-performance, and low-cost requirements of future display technologies.

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.

Data-driven discovery of molecular photoswitches with multioutput Gaussian processes

Photoswitchable molecules display two or more isomeric forms that may be accessed using light. Separating the electronic absorption bands of these isomers is key to selectively addressing a specific isomer and achieving high photostationary states whilst overall red-shifting the absorption bands serves to limit material damage due to UV-exposure and increases penetration depth in photopharmacological applications. Engineering these properties into a system through synthetic design however, remains a challenge. Here, we present a data-driven discovery pipeline for molecular photoswitches underpinned by dataset curation and multitask learning with Gaussian processes. In the prediction of electronic transition wavelengths, we demonstrate that a multioutput Gaussian process (MOGP) trained using labels from four photoswitch transition wavelengths yields the strongest predictive performance relative to single-task models as well as operationally outperforming time-dependent density functional theory (TD-DFT) in terms of the wall-clock time for prediction. We validate our proposed approach experimentally by screening a library of commercially available photoswitchable molecules. Through this screen, we identified several motifs that displayed separated electronic absorption bands of their isomers, exhibited red-shifted absorptions, and are suited for information transfer and photopharmacological applications. Our curated dataset, code, as well as all models are made available at https://github.com/Ryan-Rhys/The-Photoswitch-Dataset.

STM-induced ring closure of vinylheptafulvene molecular dipole switches on Au(111)

Dihydroazulene/vinylheptafulvene pairs are known as molecular dipole switches that undergo a ring-opening/-closure reaction by UV irradiation or thermal excitation. Herein, we show that the ring-closure reaction of a single vinylheptafulvene adsorbed on the Au(111) surface can be induced by voltage pulses from the tip of a scanning tunneling microscope. This cyclization is accompanied by the elimination of HCN, as confirmed by simulations. When inducing lateral movements by applying voltage pulses with the STM tip, we observe that the response of the single molecules changes with the ring closing reaction. This behaviour is discussed by comparing the dipole moment and the charge distribution of the open and closed forms on the surface.

Nanofloating gate modulated synaptic organic light-emitting transistors for reconfigurable displays

The use of postsynaptic current to drive long-lasting luminescence holds a disruptive potential for harnessing the next-generation of smart displays. Multiresponsive long afterglow emission can be achieved by integrating light-emitting polymers in electric spiked transistors trigged by distinct presynaptic signals inputs. Here, we report a highly effective electric spiked long afterglow organic light-emitting transistor (LAOLET), whose operation relies on a nanofloating gate architecture. Long afterglow emission with reconfigurable brightness and retention time is observed upon applying specific positive gate voltage spiked. Conversely, when negative gate voltage stimulus is applied, these LAOLETs function as click-on display. Interestingly, upon endowing the device with force sensing capabilities, it can operate as a long afterglow pressure sensor that emits long-lasting green light subsequently to a controlled extrusion action.

Ultraviolet Photodetectors Based on Polymer Microwire Arrays toward Wearable Medical Devices

Polymer micro/nanoarchitectures have attracted intense interest for wearable medical applications due to their excellent mechanical flexibility, solution processability, and tunable optoelectronic properties. Based on polymer micro/nanostructures, high-performance ultraviolet (UV) photodetectors can not only functionalize the accurate image sensing but also sustain the biocomfortable flexible devices for real-time health monitoring. The main challenges are focused on the integration of medical wearable devices, which requires large-scale assembly of polymer micro/nanostructures with controlled morphology and strict alignment. Herein, we utilized a confined assembly system through the cautious regulation for the growth of high-quality polymer 1D arrays. UV photodetectors based on these polymer microwire arrays perform a high on/off ratio of 137 and responsivity of 19.1 mA W^-1. Polymer microarray photodetectors facilitate the scale-up fabrication of 14 × 18 multiplexed image sensors for highly accurate capturing the signals of Arabic numerals "1," "2," and "3." Flexible UV photodetectors based on these arrays present excellent flexibility and bending durability, maintaining 97% of their original on/off ratio after 4000 cycles with a 10 mm bending radius. UV photodetection signals were also collected from the attached flexible devices on the back skin of the mouse, demonstrating the great potential in wearable medical photodetection.

Stepping Out of the Blue: From Visible to Near-IR Triggered Photoswitches

Light offers unique opportunities for controlling the activity of materials and biosystems with high spatiotemporal resolution. Molecular photoswitches are chromophores that undergo reversible isomerization between different states upon irradiation with light, allowing a convenient means to control their influence over the system of interest. However, a significant limitation of classical photoswitches is the requirement to initiate the switching in one or both directions using deleterious UV light with poor tissue penetration. Red-shifted photoswitches are hence in high demand and have attracted keen recent research interest. In this Review, we highlight recent progress towards the development of visible- and NIR-activated photoswitches characterized by distinct photochromic reaction mechanisms. We hope to inspire further endeavors in this field, allowing the full potential of these tools in biotechnology and materials chemistry applications to be realized.

Molecular doped, color-tunable, high-mobility, emissive, organic semiconductors for light-emitting transistors

Developing high-mobility emissive organic semiconductors with tunable colors is crucial for organic light-emitting transistors (OLETs), a pivotal component of integrated optoelectronic devices, but remains a great challenge. Here, we demonstrate a series of color-tunable, high-mobility, emissive, organic semiconductors via molecular doping with a high-mobility organic semiconductor, 2,6-diphenylanthracene, as the host. The well-matched molecular structures and sizes with efficient energy transfer between the host and guest enable the intrinsically high charge transport with tunable colors. High mobility with the highest value >2 cm² V^-1 s^-1 and strong emission with photoluminescence quantum yield >15.8% are obtained for these molecular-doped organic semiconductors. Last, a large color gamut for constructed OLETs is up to 59% National Television System Committee standard, meanwhile with an extremely high current density approaching 326.4 kA cm^-2, showing great potential for full-color smart display, organic electrically pumped lasers and other related logic circuitries.

Molecular Dynamics Study of the Thermodynamics of Integer Charge Transfer vs Charge-Transfer Complex Formation in Doped Conjugated Polymers

Molecular dopants such as 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) can interact with conjugated polymers such as poly(3-hexylthiophene-2,5-diyl) (P3HT) in two different ways: they can undergo integer charge transfer (ICT) or they can form a partial-charge-transfer complex (CTC). Both are seen experimentally, but the CTC has been challenging to characterize, making it difficult to answer questions such as the following. Which polymorph is more stable? Do they have similar barriers for formation? Is there a thermodynamic route to convert one to the other? Here, we study the structure and the thermodynamics of bulk F4TCNQ-doped P3HT with all-atom molecular dynamics simulations, using thermodynamic integration to calculate the relative free energies. We find that the ICT and CTC polymorphs have similar thermodynamic stabilities. The barrier to create the ICT polymorph, however, is lower than that to make the CTC polymorph, because the ICT polymorph has a small critical nucleus, but the critical nucleus for the CTC polymorph is larger than what we can simulate. Moreover, simulated thermal annealing shows that the activation barrier for converting the CTC polymorph to the ICT polymorph is relatively modest. Overall, the simulations explain both the observed structures and the thermodynamics of F4TCNQ-doped P3HT and offer guidelines for targeting the production of a desired polymorph for different applications.

Intrinsic Linear Dichroism of Organic Single Crystals toward High-Performance Polarization-Sensitive Photodetectors

The ability to detect light in photodetectors is central to practical optoelectronic applications, which has been demonstrated in inorganic semiconductor devices. However, so far, the study of polarization-sensitive organic photodetectors, which have unique applications in flexible and wearable electronics, has not received much attention. Herein, the construction of polarization-sensitive photodetectors based on the single crystals of a superior optoelectronic organic semiconductor, 2,6-diphenyl anthracene (DPA), is demonstrated. The systematic characterization of two-dimensionally grown DPA crystals with various techniques definitely show their strong anisotropy in molecular vibration, optical reflectance and optical absorption. In terms of polarization sensitivity, DPA-crystal based photodetectors exhibit a linear dichroic ratio up to ≈1.9. Theoretical calculations confirm that intrinsic linear dichroism, originated from the anisotropic in-plane crystal structure, is responsible for the polarization sensitivity of DPA crystals. This work opens up a new door for exploiting organic semiconductors for developing highly compact polarization photodetectors and providing new functionalities in novel flexible optical and optoelectronic applications.

A DNA‐Based Two‐Component Excitonic Switch Utilizing High‐Performance Diarylethenes

Nucleosidic diarylethenes (DAEs) are an emerging class of photochromes but have rarely been used in materials science. Here, we have developed doubly methylated DAEs derived from 2′‐deoxyuridine with high thermal stability and fatigue resistance. These new photoswitches not only outperform their predecessors but also rival classical non‐nucleosidic DAEs. To demonstrate the utility of these new DAEs, we have designed an all‐optical excitonic switch consisting of two oligonucleotides: one strand containing a fluorogenic double‐methylated 2′‐deoxyuridine as a fluorescence donor and the other a tricyclic cytidine (tC) as acceptor, which together form a highly efficient conditional Förster‐Resonance‐Energy‐Transfer (FRET) pair. The system was operated in liquid and solid phases and showed both strong distance‐ and orientation‐dependent photochromic FRET. The superior ON/OFF contrast was maintained over up to 100 switching cycles, with no detectable fatigue.

A robust vertical nanoscaffold for recyclable, paintable, and flexible light-emitting devices

Organic light-emitting devices are key components for emerging opto- and nanoelectronics applications including health monitoring and smart displays. Here, we report a foldable inverted polymer light-emitting diode (iPLED) based on a self-suspended asymmetrical vertical nanoscaffold replacing the conventional sandwich-like structured LEDs. Our empty vertical-yet-open nanoscaffold exhibits excellent mechanical robustness, proven by unaltered leakage current when applying 1000 cycles of 40-kilopascal pressure loading/unloading, sonication, and folding, with the corresponding iPLEDs displaying a brightness as high as 2300 candela per square meter. By using photolithography and brush painting, arbitrary emitting patterns can be generated via a noninvasive and mask-free process with individual pixel resolution of 10 μm. Our vertical nanoscaffold iPLED can be supported on flexible polyimide foils and be recycled multiple times by washing and refilling with a different conjugated polymer capable of emitting light of different color. This technology combines the traits required for the next generation of high-resolution flexible displays and multifunctional optoelectronics.

Redistributed Current Density in Lateral Organic Light-Emitting Transistors Enabling Uniform Area Emission with Good Stability and Arbitrary Tunability

Organic light-emitting transistors (OLETs), integrating the functions of an organic field-effect transistor (OFET) and organic light-emitting diode (OLED) in a single device, are promising for the next-generation display technology. However, the great challenge of achieving uniform area emission in OLETs with good stability and arbitrary tunability hinders their development in this field. Herein, an effective solution to obtain well-defined area emission in lateral OLETs by incorporating a charge-transport buffer (CTB) layer between the conducting channel and emitting layer is proposed. Comprehensive theoretical simulation and experimental results demonstrate redistributed potential beneath the drain electrode under the shielding effect of the CBT layer, resulting in a highly uniform current density. In this case, uniform recombination of balanced holes and electrons can be guaranteed, which is essential for the formation of area emission in the following OLETs. RGB OLETs with uniform area emission are constructed, which show good gate tunable ability (ON/OFF ratio 10⁶ ), high loop stability (over 200 cycles) and high aperture ratio (over 80%) due to the arbitrary tunability of the device geometry. This work provides a new avenue for constructing area-emission lateral OLETs, which have great potential for display technology because of their good compatibility with conventional fabrication techniques.

Directional, Low-Energy Driven Thermal Actuating Bilayer Enabled by Coordinated Submolecular Switching

The authors reveal a thermal actuating bilayer that undergoes reversible deformation in response to low-energy thermal stimuli, for example, a few degrees of temperature increase. It is made of an aligned carbon nanotube (CNT) sheet covalently connected to a polymer layer in which dibenzocycloocta-1,5-diene (DBCOD) actuating units are oriented parallel to CNTs. Upon exposure to low-energy thermal stimulation, coordinated submolecular-level conformational changes of DBCODs result in macroscopic thermal contraction. This unique thermal contraction offers distinct advantages. It's inherently fast, repeatable, low-energy driven, and medium independent. The covalent interface and reversible nature of the conformational change bestow this bilayer with excellent repeatability, up to at least 70 000 cycles. Unlike conventional CNT bilayer systems, this system can achieve high precision actuation readily and can be scaled down to nanoscale. A new platform made of poly(vinylidene fluoride) (PVDF) in tandem with the bilayer can harvest low-grade thermal energy and convert it into electricity. The platform produces 86 times greater energy than PVDF alone upon exposure to 6 °C thermal fluctuations above room temperature. This platform provides a pathway to low-grade thermal energy harvesting. It also enables low-energy driven thermal artificial robotics, ultrasensitive thermal sensors, and remote controlled near infrared (NIR) driven actuators.

Molecular Approach to Engineer Two-Dimensional Devices for CMOS and beyond-CMOS Applications

Two-dimensional materials (2DMs) have attracted tremendous research interest over the last two decades. Their unique optical, electronic, thermal, and mechanical properties make 2DMs key building blocks for the fabrication of novel complementary metal-oxide-semiconductor (CMOS) and beyond-CMOS devices. Major advances in device functionality and performance have been made by the covalent or noncovalent functionalization of 2DMs with molecules: while the molecular coating of metal electrodes and dielectrics allows for more efficient charge injection and transport through the 2DMs, the combination of dynamic molecular systems, capable to respond to external stimuli, with 2DMs makes it possible to generate hybrid systems possessing new properties by realizing stimuli-responsive functional devices and thereby enabling functional diversification in More-than-Moore technologies. In this review, we first introduce emerging 2DMs, various classes of (macro)molecules, and molecular switches and discuss their relevant properties. We then turn to 2DM/molecule hybrid systems and the various physical and chemical strategies used to synthesize them. Next, we discuss the use of molecules and assemblies thereof to boost the performance of 2D transistors for CMOS applications and to impart diverse functionalities in beyond-CMOS devices. Finally, we present the challenges, opportunities, and long-term perspectives in this technologically promising field.

Thermoresponsive multicolor-emissive materials based on solid lipid nanoparticles

Despite the recent advances in the field of thermofluorochromism, the fabrication of thermoresponsive multicolor-emissive materials in a simple, low-cost and versatile manner still remains a challenge. Herein we accomplish this goal by expanding the concept of matrix-induced thermofluorochromism, where a sudden two-state variation of dyes' emission is promoted by the solid-liquid transition of a surrounding phase change material (e.g., paraffins). We demonstrate that this behavior can be transferred to the nanoscale by the synthesis of dye-loaded solid lipid nanoparticles, different types of which can then be combined into a single platform to obtain multicolor thermofluorochromism using a single type of emitter. Because of the reduced dimensions of these particles, they can be utilized to prepare transparent nanocomposites and inkjet-printed patterns showing complex thermoresponsive luminescence signals and applications ranging from smart displays to thermal sensing and high-security anti-counterfeiting.

Application of Triplet-Triplet Annihilation Upconversion in Organic Optoelectronic Devices: Advances and Perspectives

Organic semiconductor materials have been widely used in various optoelectronic devices due to their rich optical and/or electrical properties, which are highly related to their excited states. Therefore, how to manage and utilize the excited states in organic semiconductors is essential for the realization of high-performance optoelectronic devices. Triplet-triplet annihilation (TTA) upconversion is a unique process of converting two non-emissive triplet excitons to one singlet exciton with higher energy. Efficient optical-to-electrical devices can be realized by harvesting sub-bandgap photons through TTA-based upconversion. In electrical-to-optical devices, triplets generated after the combination of electrons and holes also can be efficiently utilized via TTA, which resulted in a high internal conversion efficiency of 62.5%. Currently, many interesting explorations and significant advances have been demonstrated in these fields. In this review, a comprehensive summary of these intriguing advances on developing efficient TTA upconversion materials and their application in optoelectronic devices is systematically given along with some discussions. Finally, the key challenges and perspectives of TTA upconversion systems for further improvement for optoelectronic devices and other related research directions are provided. This review hopes to provide valuable guidelines for future related research and advancement in organic optoelectronics.

Synaptic Plasticity Powering Long-Afterglow Organic Light-Emitting Transistors

Long-lasting luminescence in optoelectronic devices is highly sought after for applications in optical data storage and display technology. While in light-emitting diodes this is achieved by exploiting long-afterglow organic materials as active components, such a strategy has never been pursued in light-emitting transistors, which are still rather unexplored and whose technological potential is yet to be demonstrated. Herein, the fabrication of long-afterglow organic light-emitting transistors (LAOLETs) is reported whose operation relies on an unprecedented strategy based on a photoinduced synaptic effect in an inorganic indium-gallium-zinc-oxide (IGZO) semiconducting channel layer, to power a persistent electroluminescence in organic light-emitting materials. Oxygen vacancies in the IGZO layer, produced by irradiation at λ = 312 nm, free electrons in excess yielding to a channel conductance increase. Due to the slow recombination kinetics of photogenerated electrons to oxygen vacancies in the channel layer, the organic material can be fueled by postsynaptic current and displays a long-lived light-emission (hundreds of seconds) after ceasing UV irradiation. As a proof-of-concept, the LAOLETs are integrated in active-matrix light-emitting arrays operating as visual UV sensors capable of long-lifetime green-light emission in the irradiated regions.

Unveiling electron transfer in a supramolecular aggregate for adaptive and autonomous photochromic response

Interactive and responsive materials are dynamic molecular systems that are capable of modulating their behavior and adapting to environment autonomously. Photochromic materials are among the fascinating class of dynamic responsive systems and widely used in molecular switches and optoelectronic devices. However, the phototriggered color changing largely relies on the conformation transformation of the photochromic motif, which significantly limited their usage in organic liquid solutions. Herein, we demonstrate a photochromic organic supramolecular system by using electron-rich N,N-dimethylacetamide (DMA) and electron-deficient naphthalenediimide (NDI) as a donor and acceptor, respectively. In the binary system, the photo-induced electron transfer through lone pair···π interactions pathway leads to dynamic photochromic response not only in the solution but also in the crystalline aggregated state. Furthermore, by incorporating the supramolecules in the polymer matrix, the transparent polymeric film also exhibits rapid photochromic response, which makes it a promisingly interactive and adaptive photochromic material.