Phototunable Full‐Color Emission of Cellulose‐Based Dynamic Fluorescent Materials

An iridescent chameleon‐like material that can change its colors under different circumstances is always desired in color‐on‐demand applications. Herein, a strategy based on trichromacy and the dynamically tunable fluorescence resonance energy transfer (FRET) process to design and prepare these chameleon‐like fluorescent materials is proposed. A set of trichromic (red, green, and blue), solid fluorescent materials are synthesized by covalently attaching spiropyran, fluorescein, and pyrene onto cellulose chains independently. After simply mixing them together, a full range of color is realized. The chameleon‐like nature of these materials is based on the dynamic tunable FRET process between donors (green and blue) and acceptors (red) in which the energy transfer efficiency can be finely tuned by irradiation. Ultimately, the reversible and nonlinear regulation of fluorescence properties, including color and intensity, is achieved on a timescale recognizable by the naked eye. Benefited by the excellent processability inherited from the cellulose derivatives, the as‐prepared materials are feasibly transformed into different forms. Particularly, a fluorescent ink with the complicated fluorescent input–output dependence suggests more than a proof‐of‐concept; indeed, it suggests a unique method of information encryption, security printing, and dynamic anticounterfeiting.     

Single‐Walled Carbon Nanotube/Phase Change Material Composites: Sunlight‐Driven,Reversible, Form‐Stable Phase Transitions for Solar Thermal Energy Storage

The development of solar energy conversion materials is critical to the growth of a sustainable energy infrastructure in the coming years. A novel hybrid material based on single‐walled carbon nanotubes (SWNTs) and form‐stable polymer phase change materials (PCMs) is reported. The obtained materials have UV‐vis sunlight harvesting, light‐thermal conversion, thermal energy storage, and form‐stable effects. Judicious application of this efficient photothermal conversion to SWNTs has opened up a rich field of energy materials based on novel SWNT/PCM composits with enhanced performance in energy conversion and storage.     

A General Electrochemical Strategy for Synthesizing Charge‐Transfer Complex Micro/Nanowires

Universal strategies for synthesizing one‐dimensional organic nanomaterials are of fundamental importance in the development of more flexible, cheaper and lighter electronics. Charge‐transfer (CT) complexes, the major kind of organic conductors, are in the long‐term attractive materials owing to their unique crystal structures and conductive properties. In this article, a general strategy for the synthesis of CT complex micro/nanowires based on the localized nanoelectrochemistry using tiny carbon nanotube (CNT) electrodes is presented. This strategy is successfully demonstrated over 12 typical CT complexes, and a general rule for the preparation of various kinds of CT complex micro/nanowires is summarized. The CT complex micro/nanowires thus synthesized have high aspect ratios and long lengths as compared with traditional macroscopic planar electrodes, originating from the one‐dimensional structural feature with fewer or no defects and the ultrasmall surface area of the CNT. This work provides a more versatile material basis for the fundamental and application studies of low‐dimensional organic conductor materials.     

Pyridine‐Containing Electron‐Transport Materials for Highly Efficient Blue Phosphorescent OLEDs with Ultralow Operating Voltage and Reduced Efficiency Roll‐Off

A series of pyridine‐containing electron‐transport materials are developed as an electron‐transport layer for the FIrpic‐based blue phosphorescent organic light‐emitting diodes. Their energy levels can be tuned by the introduction of pyridine rings in the framework and on the periphery of the molecules. Significantly reduced operating voltage is achieved without compromising external quantum efficiency by solely tuning the nitrogen atom orientations of those pyidine rings. Unprecedented low operating voltages of 2.61 and 3.03 V are realized at 1 and 100 cd m^?2, giving ever highest power efficiency values of 65.8 and 59.7 lm W^?1, respectively. In addition, the operating voltages at 100 cd m^?2 can be further reduced to 2.70 V by using a host material with a small singlet‐triplet exchange energy, and the threshold voltage for electroluminescence can even be 0.2–0.3 V lower than the theoretical minimum value of the photon energy divided by electron charge. Aside from the reduced operating voltage, a further reduced roll‐off in efficiency is also achieved by the combination of an appropriate host material.     

Copper Iodide Based Hybrid Phosphors for Energy‐Efficient General Lighting Technologies

Solid‐state‐lighting (SSL) is a new lighting technology that is rapidly replacing conventional lighting sources because it is much more energy efficient, longer lasting, and contributes significantly to environmental protection. A main branch of SSL technology is light‐emitting diodes (LEDs), and white‐light LEDs (WLEDs) are in the greatest demand for general lighting and illumination applications. Current WLED devices rely heavily on rare‐earth elements (REEs), which will likely suffer from cost and supply risks and environmental consequences in the near future. Crystalline inorganic–organic hybrid materials based on I–VII binary semiconductors represent a promising material class as REE‐free phosphor alternatives. This article provides a brief overview of recent advancement on this material family, with a focus on the rational design, energy‐efficient and low‐cost synthesis, systematic modification, and optimization of their electronic, optical, and thermal properties. A particular emphasis will be made on our own progress over the past several years in developing four classes of CuI(L) structures with substantially improved performance as energy‐saving lighting phosphors. General strategies for structural design, synthesis, and property optimization of these materials will also be discussed.     

Monothiatruxene‐Based,Solution‐Processed Green,Sky‐Blue,and Deep‐Blue Organic Light‐Emitting Diodes with Efficiencies Beyond 5% Limit

The development of blue materials with good efficiency, even at high brightness, with excellent color purity, simple processing, and high thermal stability assuring adequate device lifetime is an important remaining challenge for organic light‐emitting didoes (OLEDs) in displays and lightning applications. Furthermore, these various features are typically mutually exclusive in practice. Herein, four novel green and blue light‐emitting materials based on a monothiatruxene core are reported together with their photophysical and thermal properties, and performance in solution‐processed OLEDs. The materials show excellent thermal properties with high glass transition temperatures ranging from 171 to 336 °C and decomposition temperatures from 352 to 442 °C. High external quantum efficiencies of 3.7% for a deep‐blue emitter with CIE color co‐ordinates (0.16, 0.09) and 7% for green emitter with color co‐ordinates (0.22, 0.40) are achieved at 100 cd m^?2. The efficiencies observed are exceptionally high for fluorescent materials with photoluminescence quantum yields of 24% and 62%, respectively. The performance at higher brightness is very good with only 38% and 17% efficiency roll‐offs at 1000 cd m^?2. The results indicate that utilization of this unique molecular design is promising for efficient deep‐blue highly stable and soluble light‐emitting materials.     

Surface‐Functionalization‐Mediated Direct Transfer of Molybdenum Disulfide for Large‐Area Flexible Devices

The transfer of synthesized large‐area 2D materials to arbitrary substrates is expected to be a vital step for the development of flexible device fabrication processes. The currently used hazardous acid‐based wet chemical etching process for transferring large‐area MoS₂ films is deemed to be unsuitable because it significantly degrades the material and damages growth substrates. Surface energy‐assisted water‐based transfer processes do not require corrosive chemicals during the transfer process; however, the concept is not investigated at the wafer scale due to a lack of both optimization and in‐depth understanding. In this study, a wafer‐scale water‐assisted transfer process for metal–organic chemical vapor‐deposited MoS₂ films based on the hydrofluoric acid treatment of a SiO₂ surface before the growth is demonstrated. Such surface treatment enhances the strongly adhering silanol groups, which allows the direct transfer of large‐area, continuous, and defect‐free MoS₂ films; it also facilitates the reuse of growth substrate. The developed transfer method allows direct fabrication of flexible devices without the need for a polymeric supporting layer. It is believed that the proposed method can be an alternative defect‐ and residue‐free transfer method for the development of MoS₂‐based next‐generation flexible devices.     

Aggregate States and Energetic Disorder in Highly Ordered Nanostructures of para‐Sexiphenyl Grown by Hot Wall Epitaxy

We report on photoluminescence (PL) and thermally stimulated luminescence (TSL) in highly ordered nanostructures of para‐sexiphenyl (PSP) grown by hot wall epitaxy (HWE). A low‐energy broad band is observed in the PL spectra that can be attributed to the emission from molecular aggregates. While the intrinsic exciton emission in steady‐state PL dominates at low temperatures, the emission from aggregates increases with elevating temperature and its magnitude depends sensitively on film preparation conditions. Time‐resolved PL measurements showed that the aggregate emission decays with a life‐time of ≈ 4 ns, which is approximately an order of magnitude larger than the lifetime of singlet excitons. TSL data suggests the presence of an energetically disordered distribution of localized states for charge carriers in PSP films, which results from an intrinsic disorder in this material. A low‐temperature TSL peak with the maximum at around 30 K evidences for a weak energy disorder in PSP films, and has been interpreted in terms of a hopping model of TSL in disordered organic materials.     

High‐Performance,Phosphorescent, Top‐Emitting Organic Light‐Emitting Diodes with p–i–n Homojunctions

High‐performance, green, orange, and red top‐emitting organic light‐emitting diodes (TOLEDs) with p–i–n homojunction are demonstrated. An excellent ambipolar host, 2,5‐bis(2‐(9H‐carbazol‐9‐yl)phenyl)‐1,3,4‐oxadiazole (o‐CzOXD), which has good thermal and morphological stabilities, a high triplet energy level, and equally high electron and hole mobilities, is chosen as the organic host material for the homojunction devices. By electrical doping, the carrier injection and transporting characteristics are greatly improved. The optical structure is optimized in view of light emission of different colors to enhance the color purity and improve the view characte­ristics. As a result, high efficiency p–i–n homojunction TOLEDs with saturated intrinsic emission of the emitting materials and angular independence of the emission are realized. The performances of these p–i–n homojunction TOLEDs are even higher than the multi‐layer heterojunction bottom‐emitting devices using the same emitting layers.     

Boosting Efficiency of Near‐Infrared Organic Light‐Emitting Diodes with Os(II)‐Based Pyrazinyl Azolate Emitters

Tremendous effort has been devoted to developing novel near‐infrared (NIR) emitters and to improving the performance of NIR organic light‐emitting diodes (OLEDs). Os(II) complexes are known to be an important class of NIR electroluminescent materials. However, the highest external quantum efficiency achieved so far for Os(II)‐based NIR OLEDs with an emission peak wavelength exceeding 700 nm is still lower than 3%. A new series of Os(II) complexes ( 1 – 4 ) based on functional pyrazinyl azolate chelates and dimethyl(phenyl)phosphane ancillaries is presented. The reduced metal‐to‐ligand charge transfer (MLCT) transition energy gap of pyrazinyl units in the excited states results in efficient NIR emission for this class of metal complexes. Consequently, NIR OLEDs based on 1 – 4 show excellent device performance, among which complex 4 with a triazolate fragment gives superior performance with maximum external quantum efficiency of 11.5% at peak wavelength of 710 nm, which represent the best Os(II)‐based NIR‐emitting OLEDs with peak maxima exceeding 700 nm.     

Efficient Light‐Emitting Devices Based on Phosphorescent Polyhedral Oligomeric Silsesquioxane Materials

Synthesis, photophysical, and electrochemical characterizations of iridium‐complex anchored polyhedral oligomeric silsesquioxane (POSS) macromolecules are reported. Monochromatic organic light‐emitting devices based on these phosphorescent POSS materials show peak external quantum efficiencies in the range of 5–9%, which can be driven at a voltage less than 10 V for a luminance of 1000 cd m^?2. The white‐emitting devices with POSS emitters show an external quantum efficiency of 8%, a power efficiency of 8.1 lm W^?1, and Commission International de'lÉclairage coordinates of (0.36, 0.39) at 1000 cd m^?2. Encouraging efficiency is achieved in the devices based on hole‐transporting and Ir‐complex moieties dual‐functionalized POSS materials without using host materials, demonstrating that triplet‐dye and carrier‐transporting moieties functionalized POSS material is a viable approach for the development of solution‐processable electrophosphorescent devices.     

Versatile,Benzimidazole/Amine‐Based Ambipolar Compounds for Electroluminescent Applications: Single‐Layer,Blue, Fluorescent OLEDs,Hosts for Single‐Layer,Phosphorescent OLEDs

A series of compounds containing arylamine and 1,2‐diphenyl‐1H‐benz[d]imidazole moieties are developed as ambipolar, blue‐emitting materials with tunable blue‐emitting wavelengths, tunable ambipolar carrier‐transport properties and tunable triplet energy gaps. These compounds possess several novel properties: (1) they emit in the blue region with high quantum yields; (2) they have high morphological stability and thermal stability; (3) they are capable of ambipolar carrier transport; (4) they possess tunable triplet energy gaps, suitable as hosts for yellow‐orange to green phosphors. The electron and hole mobilities of these compounds lie in the range of 0.68–144 × 10^?6 and 0.34–147 × 10^?6 cm² V^?1 s^?1, respectively. High‐performance, single‐layer, blue‐emitting, fluorescent organic light‐emitting diodes (OLEDs) are achieved with these ambipolar materials. High‐performance, single‐layer, phosphorescent OLEDs with yellow‐orange to green emission are also been demonstrated using these ambipolar materials, which have different triplet energy gaps as the host for yellow‐orange‐emitting to green‐emitting iridium complexes. When these ambipolar, blue‐emitting materials are lightly doped with a yellow‐orange‐emitting iridium complex, white organic light‐emitting diodes (WOLEDs) can be achieved, as well by the use of the incomplete energy transfer between the host and the dopant.     

Electrolyte‐Regulated Solid‐Electrolyte Interphase Enables Long Cycle Life Performance in Organic Cathodes for Potassium‐Ion Batteries

Organic cathode materials as economical and environment‐friendly alternatives to inorganic cathode materials have attracted comprehensive attention in potassium‐ion batteries (KIBs). Nonetheless, active material dissolution and mismatched electrolytes result in insufficient cycle life that definitely hinders their practical applications. Here, a significantly improved cycle life of 1000 cycles (80% capacity retention) on a practically insoluble organic cathode material, anthraquinone‐1,5‐disulfonic acid sodium salt, is realized, in KIBs through a solid‐electrolyte interphase (SEI) regulation strategy by ether‐based electrolytes. Such an excellent performance is attributed to the robust SEI film and fast reaction kinetics. More importantly, the ether‐electrolyte‐derived SEI film has a protective inorganic‐rich inner layer arising from the prior decomposition of potassium salts to solvents, as revealed by X‐ray photoelectron spectroscopy analysis and computational studies on molecular orbital energy levels. The findings shed light on the critical roles of electrolytes and the corresponding SEI films in enhancing performance of organic cathodes in KIBs.     

Energy‐Level Alignment at the Organic/Electrode Interface in Organic Optoelectronic Devices

It is commonly believed that the work‐function reduction effect of the cathode interfacial material in organic electronic devices leads to better energy‐level alignment at the organic/electrode interface, which enhances the device performance. However, there is no agreement on the exact dipole direction in the literature. In this study, a peel‐off method to reveal the buried organic/metal interface to examine the energy‐level alignment is developed. By splitting the device at different interfaces, it is discovered that oppositely oriented dipoles are formed at different surfaces of the interfacial layer. Moreover, the function of the electrode interface differs in different device types. In organic light‐emitting diodes, the vacuum‐level alignment generally occurs at the organic/cathode interface, while in organic photovoltaic devices, the Fermi‐level pinning commonly happens. Both are determined by the integer charge‐transfer levels of the organic materials and the work‐function of the electrode. As a result, the performance enhancement by the cathode interfacial material in organic photovoltaic devices cannot be solely explained by the energy‐level alignment. The clarification of the energy‐level alignment not only helps understand the device operation but also sets up a guideline to design the devices with better performance.     

Non‐Volatile Polymer Electroluminescence Programmable with Ferroelectric Field‐Induced Charge Injection Gate

Electroluminescence (EL) of organic and polymeric fluorescent materials programmable in the luminance is extremely useful as a non‐volatile EL memory with the great potential in the variety of emerging information storage applications for imaging and motion sensors. In this work, a novel non‐volatile EL memory in which arbitrarily chosen EL states are programmed and erased repetitively with long EL retention is demonstrated. The memory is based on utilizing the built‐in electric field arising from the remnant polarization of a ferroelectric polymer which in turn controls the carrier injection of an EL device. A device with vertically stacked components of a transparent bottom electrode/a ferroelectric polymer/a hole injection layer/a light emitting layer/a top electrode successfully emits light upon alternating current (AC) operation. Interestingly, the device exhibits two distinctive non‐volatile EL intensities at constant reading AC voltage, depending upon the programmed direct current (DC) voltage on the ferroelectric layer. DC programmed and AC read EL memories are also realized with different EL colors of red, green and blue. Furthermore, more than four distinguishable EL states are precisely addressed upon the programmed voltage input each of which shows excellent EL retention and multiple cycle endurance of more than 10⁵ s and 10² cycles, respectively.     

Pyrite‐Based Bi‐Functional Layer for Long‐Term Stability and High‐Performance of Organo‐Lead Halide Perovskite Solar Cells

Organo‐lead halide perovskite solar cells (PSCs) have received great attention because of their optimized optical and electrical properties for solar cell applications. Recently, a dramatic increase in the photovoltaic performance of PSCs with organic hole transport materials (HTMs) has been reported. However, as of now, future commercialization can be hampered because the stability of PSCs with organic HTM has not been guaranteed for long periods under conventional working conditions, including moist conditions. Furthermore, conventional organic HTMs are normally expensive because material synthesis and purification are complicated. It is herein reported, for the first time, octadecylamine‐capped pyrite nanoparticles (ODA‐FeS₂ NPs) as a bi‐functional layer (charge extraction layer and moisture‐proof layer) for organo‐lead halide PSCs. FeS₂ is a promising candidate for the HTM of PSCs because of its high conductivity and suitable energy levels for hole extraction. A bi‐functional layer based on ODA‐FeS₂ NPs shows excellent hole transport ability and moisture‐proof performance. Through this approach, the best‐performing device with ODA‐FeS₂ NPs‐based bi‐functional layer shows a power conversion efficiency of 12.6% and maintains stable photovoltaic performance in 50% relative humidity for 1000 h. As a result, this study has the potential to break through the barriers for the commercialization of PSCs.     

Mechanistic Study on High Efficiency Deep Blue AIE‐Based Organic Light‐Emitting Diodes by Magneto‐Electroluminescence

Aggregation‐induced emission (AIE) materials are highly attractive because of their excellent properties of high efficiency emission in nondoped organic light‐emitting diodes (OLEDs). Therefore, a deep understanding of the working mechanisms, further improving the electroluminescence (EL) efficiency of the resulting AIE‐based OLEDs, is necessary. Herein, the conversion process from higher energy triplet state (T₂) to the lowest singlet state (SS₁) is found in OLEDs based on a blue AIE material, 4′‐(4‐(diphenylamino)phenyl)‐5′‐phenyl‐[1,1′:2′,1′′‐terphenyl]‐4‐carbonitrile (TPB‐AC), obviously relating to the device efficiency, by magneto‐EL (MEL) measurements. A special line shape with rise at low field and reduction at high field is observed. The phenomenon is further clarified by theoretical calculations, temperature‐dependent MELs, and transient photoluminescence emission properties. On the basis of the T₂‐S₁ conversion process, the EL performances of the blue OLEDs based on TPB‐AC are further enhanced by introducing a phosphorescence doping emitter in the emitting layer, which effectively regulates the excitons on TPB‐AC molecules. The maximum external quantum efficiency (EQE) reaches 7.93% and the EQE keeps 7.57% at the luminance of 1000 cd m^?2. This work establishes a physical insight for designing high‐performance AIE materials and devices in the future.     

Symmetric and Asymmetric Decoration of Graphene: Bimetal‐Graphene Sandwiches

Low‐dimensional carbon materials, i.e., graphene and its functionalization with a number of semiconductor or conductor materials, such as noble metal nanostructures, have primary importance for their potential exploitation as electro‐active materials, i.e., as new generation catalysts. Here, low‐cost, solution chemistry‐based, two‐step functionalization of an individual, free‐standing, chemical vapor‐deposited graphene monolayer is reported, with noble metal (Au, Pt, Pd) nanoparticles to build up two‐side decorated graphene‐based metal nanoclusters. Either the same metal (symmetric decoration) or different metals (asymmetric decoration) are used for the preparation of bimetal graphene sandwiches, which are adsorbed at the liquid/liquid (organic/water) interface. The successful fabrication of such dual‐decorated graphene‐based metal nanocomposites is confirmed using various microscopic techniques (scanning electron and atomic force microscopies) and several spectroscopic methods (x‐ray photoelectron, energy dispersive x‐ray, mapping mode Raman spectroscopy, and electron energy loss spectroscopy). Taken together, it is inferred from these techniques that the location of deposited metal nanoparticles is on opposite sides of the graphene.     

Inkjet‐Printed Organic Electrodes for Bottom‐Contact Organic Field‐Effect Transistors

A graphite thin film was investigated as the drain and source electrodes for bottom‐contact organic field‐effect transistors (BC OFETs). Highly conducting electrodes (10² S cm^?1) at room temperature were obtained from pyrolyzed poly(l,3,4‐oxadiazole) (PPOD) thin films that were prepatterned with a low‐cost inkjet printing method. Compared to the devices with traditional Au electrodes, the BC OFETs showed rather high performances when using these source/drain electrodes without any further modification. Being based on a graphite‐like material these electrodes possess excellent compatibility and proper energy matching with both p‐ and n‐type organic semiconductors, which results in an improved electrode/organic‐layer contact and homogeneous morphology of the organic semiconductors in the conducting channel, and finally a significant reduction of the contact resistance and enhancement of the charge‐carrier mobility of the devices is displayed. This work demonstrates that with the advantages of low‐cost, high‐performance, and printability, PPOD could serve as an excellent electrode material for BC OFETs.     

n‐Doping of a Low‐Electron‐Affinity Polymer Used as an Electron‐Transport Layer in Organic Light‐Emitting Diodes

n‐Doping electron‐transport layers (ETLs) increases their conductivity and improves electron injection into organic light‐emitting diodes (OLEDs). Because of the low electron affinity and large bandgaps of ETLs used in green and blue OLEDs, n‐doping has been notoriously more difficult for these materials. In this work, n‐doping of the polymer poly[(9,9‐dioctylfluorene‐2,7‐diyl)‐alt‐(benzo[2,1,3]thiadiazol‐4,7‐diyl)] (F8BT) is demonstrated via solution processing, using the air‐stable n‐dopant (pentamethylcyclopentadienyl)(1,3,5‐trimethylbenzene)ruthenium dimer [RuCp*Mes]₂. Undoped and doped F8BT films are characterized using ultraviolet and inverse photoelectron spectroscopy. The ionization energy and electron affinity of the undoped F8BT are found to be 5.8 and 2.8 eV, respectively. Upon doping F8BT with [RuCp*Mes]₂, the Fermi level shifts to within 0.25 eV of the F8BT lowest unoccupied molecular orbital, which is indicative of n‐doping. Conductivity measurements reveal a four orders of magnitude increase in the conductivity upon doping and irradiation with ultraviolet light. The [RuCp*Mes]₂‐doped F8BT films are incorporated as an ETL into phosphorescent green OLEDs, and the luminance is improved by three orders of magnitude when compared to identical devices with an undoped F8BT ETL.