Deepening Insights into Aggregation Effect of Intermolecular Charge-Transfer Aggregates for Highly Efficient Near-Infrared Non-Doped Organic Light-Emitting Diodes over 780 nm

Though urgently needed, high-efficiency near-infrared (NIR) organic light-emitting diode (OLED) is still rare due to the energy-gap law. Formation of intermolecular charge-transfer aggregates (CTA) with nonadiabatic coupling suppression can decelerate non-radiative decay rates for high-efficiency NIR-OLEDs. However, the aggregation effect of CTA is still not fully understood, which limits the rational design of CTA. Herein, two CTA molecules with a same π-framework but different terminal substituents are developed to unveil the aggregation effect. In highly ordered crystalline states, the terminal substituents substantially affect the molecular packing motifs and intermolecular charge-transfer states, thus leading to distinct photophysical properties. In comparison, in amorphous states, these two CTA demonstrate similar photophysical behaviors and properties due to their similar molecular packing and intermolecular interactions as evidenced by molecular dynamics simulations. Importantly, the formations of amorphous CTA trigger multifunction improvements such as aggregation-induced NIR emission, aggregation-induced thermally activated delayed fluorescence, self-doping and self-host features. The non-doped OLEDs demonstrate NIR emissions centered at 788 and 803 nm, and high maximum external quantum efficiencies of 2.6% and 1.5% with small efficiency roll-off, respectively. This study provides deeper insight into the aggregation effect of CTA and lays a foundation for the development of high-efficiency NIR non-doped OLEDs.     

Sb3+-Doping in Cesium Zinc Halides Single Crystals Enabling High-Efficiency Near-Infrared Emission

Luminescent metal halide materials with flexible crystallography/electronic structures and tunable emission have demonstrated broad application prospects in the visible light region. However, designing near-infrared (NIR) light-emitting metal halides remains a challenge. Here, an enlightening prototype is proposed to explore the high-efficiency broadband NIR emission in metal halide systems by incorporating Sb³⁺ into the Cs₂ZnCl₄ matrix. Combined experimental analysis and density functional theory calculations reveal a modified self-trapped excitons model to elaborate the NIR emission. The high photoluminescence quantum yield of 69.9% peaking at 745 nm and large full width at half maximum of 175 nm, along with excellent air/thermal stability, show the unique advantages of lead-free metal halide Cs₂ZnCl₄:Sb³⁺ as the NIR light source. The substitution of Cl⁻ by Br⁻ further enables the red-shift of emission peak from 745 to 823 nm. The NIR light-emitting diode device based on Cs₂ZnCl₄:Sb³⁺ demonstrates potential as a non-visible light source in night vision. This study puts forward an effective strategy to design the novel eco-friendly and high-efficiency NIR emissive materials and provides guidance for expanding the application scope of luminescent metal halides.     

Perovskite-Organic Coupling WLED: Progress and Perspective

Perovskite shows great potential in lighting and display owing to its advantages of low cost, high efficiency, and whole visible light tunability. However, how to realize high-efficiency white perovskite light-emitting diodes (WPeLEDs) still faces challenges such as the stability of devices and the energy regulation between different emission centers. In recent years, some organic molecules are introduced into the perovskite system because of their role in stabilizing perovskite crystals and enhancing photoelectric properties. In this review, the strategy of perovskite-organic combination and coupling emission are emphasized, hoping to promote the development of high-efficiency WPeLEDs. First, the research status of perovskite-organic coupling WLEDs (POC-WLEDs) is summarized in detail. Then, the development direction and possibility of POC-WLEDs are proposed by combining them with some recent reports on POC methods. Finally, an outlook on POC-WLEDs is proposed. It is a considerable strategy to introduce organic luminescent molecules into perovskite systems as ligands or A-site organic cations for coupling emission with perovskite. In addition, the technologies of the organic polymer matrix, interfacial exciplex, and organic crosslinking are noteworthy exploration directions. They will promote the development of POC-WLEDs in improving the stability of perovskite electroluminescence and regulating the energy transfer efficiency between perovskite/organic-molecule emitting centers.     

Molecularly Engineered Near‐Infrared Light‐Emitting Electrochemical Cells

The development of near‐infrared (NIR) luminescent materials has emerged as a promising research field with important applications in solid‐state lighting (SSL), night‐vision‐readable displays, and the telecommunication industry. Over the past two decades, remarkable advances in the development of light‐emitting electrochemical cells (LECs) have stunned the SSL community, which has in turn driven the quest for new classes of stable, more efficient NIR emissive molecules. In this review, an overview of the state of the art in the field of near‐infrared light‐emitting electrochemical cells (NIR‐LEC) is provided based on three families of emissive compounds developed over the past 25 years: i) transition metal complexes, ii) ionic polymers, and iii) host–guest materials. In this context, ionic and conductive emitters are particularly attractive since their emission can be tuned via molecular design, which involves varying the chemical nature and substitution pattern of their ancillary ligands. Herein, the challenges and current limitations of the latter approach are highlighted, particularly with respect to developing NIR‐LECs with high external quantum efficiencies. Finally, useful guidelines for the discovery of new, efficient emitters for tailored NIR‐LEC applications are presented, together with an outlook towards the design of new NIR‐SSL materials.     

Anthracene derivatives as efficient emitting hosts for blue organic light-emitting diodes utilizing triplet–triplet annihilation

The molecular design strategies for the host materials suitable for highly efficient, blue fluorescent organic light-emitting diodes (OLEDs) are demonstrated. The device characteristics of blue fluorescent OLEDs are compared with different host materials. Some devices exhibit a highly efficient blue electroluminescence with a high external quantum efficiency of more than 7%. The correlation between OLED efficiency and triplet–triplet annihilation is characterized by measuring the up-conversion of triplet excited states into singlet ones. The host materials require an anthracene unit and a bulky molecular structure to prevent the overlap of anthracene units between adjacent molecules in the film.     

High Solid‐State Near Infrared Emissive Organic Fluorophores from Thiadiazole[3,4‐c]Pyridine Derivatives for Efficient Simple Solution‐Processed Nondoped Near Infrared OLEDs

Many efforts have been dedicated to developing near infrared (NIR) fluorescent emitters with strong emission especially in the range of 700–1000 nm due to their potential applications in biomedical and optoelectronic fields. However, high solid state NIR emission fluorophores are still rare for applications. Herein, two efficient donor‐π‐acceptor type NIR emitters, C3HTP and C4HTP , are designed and synthesized by end‐capping two isomeric bis(n‐hexylthienyl)thiadiazole[3,4‐c]pyridines as π‐acceptor with structural bulky, electron rich tercarbazole moiety. They exhibit excellent solid state NIR emission with an emission peak at 725 nm, especially C3HTP , reaching a record high photoluminescence quantum yield (Φ_PL) of 34% for NIR organic fluorescent materials. By taking advantage of their Φ_PL values in the film state (Φ_PL = 10–34%), suitable energy levels (highest occupied molecular orbital (HOMO) level ≈ ?5.3 eV), high hole mobility (5.49 × 10^?8 cm² V^?1 s^?1) as well as good amorphous film forming ability by solution casting, they are used to fabricate a nondoped emissive layer (EML) in simple double‐layer solution processed NIR electroluminescent (EL) devices. The device containing C3HTP as the EML shows a NIR emission peaking at 726 nm and excellent EL performance with a high external quantum efficiency of 1.51%, which is the best solution processed nondoped NIR organic light‐emitting diodes reported to date. Importantly, this represents an advance in near infrared organic fluorescent materials and EL devices that meet the requirements of many applications.     

Temperature and modulation characteristics of resonant-cavitylight-emitting diodes

Resonant-cavity light-emitting diodes (RCLED) are novel, high-efficiency light-emitting diodes which employ optical microcavities. These diodes have higher intensities and higher spectral purity as compared to conventional LEDs. Analytical formulas are derived for the enhancement of the spontaneous emission along the optical axis of the cavity. The design rules for high-efficiency operation of RCLEDs are established. The temperature dependence of the emission intensity is analyzed in the range 20-80° and it is described by an exponential dependence with a characteristic temperature of 112 K. The modulation characteristics of RCLEDs exhibit 3 dB frequencies of 580 MHz. Eye diagrams at transmission rates of 622 Mb/s are wide open indicating the suitability of RCLEDs for high-speed data transmission     

In-plane light emission of organic light-emitting transistors with bilayer structure using ambipolar semiconducting polymers

The concept of using an ambipolar bilayer semiconducting heterostructure in organic light-emitting transistors (OLETs) is introduced to provide a new approach to achieve surface emission. The properties of top-gate-type bilayer OLETs with ambipolar materials based on two types of fluorene-type polymers used as an emissive layer and an electron blocking layer are investigated. Line-shaped yellow–green emission occurs near a hole-injection electrode. When hole transport is dominant in the upper layer which acts as an electron blocking layer, and electrons are injected into the lower layer, an in-plane light-emitting pattern is observed. The measured in-plane emission zone confirms that both hole and electron transport are determined to occur mainly along the different organic layers between the source and drain electrodes, and an in-plane recombination zone of electrons and holes exists near the bilayer organic interface. This work is anticipated to be useful for the development of in-plane light-emitting transistors.     

Controllable molecular doping in organic single crystals toward high-efficiency light-emitting devices

Organic single-crystalline semiconductors have drawn significant attention in the area of organic electronic and optoelectronic devices due to their superiorities of highly ordered structure, high carrier mobility and low impurity content. Molecular doping technique has made great progress in improving device performance via optimizing the optical and electrical properties of organic semiconductors. In particular, this technique has been attempted by taking fluorescent dye-molecules as the emissive dopants to tune emission color and improve device performance of organic single crystals. Up to now, there are few reports about the use of molecular doping in organic single crystals to optimize their intrinsic electrical properties. Here, we have introduced the controllable molecular doping as a feasible approach toward manipulating charge carrier transport properties of organic single crystals. Upon optimization of doping concentration, balanced carrier transport can be realized in 5,5′-bis(4-trifluoromethyl phenyl) [2,2’] bithiophene (P2TCF₃)-doped 1,4-bis(4-methylstyryl) benzene (BSB–Me) crystals. Organic light-emitting devices (OLEDs) based on these doped crystals achieve a maximum luminance of 423 cd/m² and current efficiency of 0.48 cd/A. It demonstrates that high-efficiency crystal-based OLEDs are of great significance for the development of organic electronics, especially for display and lighting applications.     

Frontiers in Circularly Polarized Phosphorescent Materials

Circularly polarized luminescence (CPL) materials have received increasing attention in recent years. Amongst various CPL materials, circularly polarized phosphorescence (CPP) materials featuring long life-time represent a novel research frontier and exhibit promising applications in various fields. Herein, the state-of-the-art advances of CPP materials are systematically summarized, as classified into transition metal complexes, organic small molecules, polymers, and organic/inorganic hybrid materials. Besides, the recent applications of CPP materials in organic light-emitting diodes and encryption display are also summarized. Furthermore, the current challenges and future perspectives are put forward. It is expected that this review will offer more inspirations for the future rational design of advanced CPP materials, thus further promoting their future practical applications.     

Rational Design of Perylenediimide‐Substituted Triphenylethylene to Electron Transporting Aggregation‐Induced Emission Luminogens (AIEgens) with High Mobility and Near‐Infrared Emission

Organic materials with both high electron mobility and strong solid‐state emission are rare although for their importance to advanced organic optoelectronics. In this paper, triphenylethylenes with varying number of perylenediimide (PDI) unit (TriPE‐nPDIs, n = 1?3) are synthesized and their optical and charge‐transporting properties are systematically investigated. All the molecules exhibit strong solid‐stated near infrared (NIR) emission and some of them exhibit aggregation‐enhanced emission characteristics. Organic field‐effect transistors (OFETs) using TriPE‐nPDIs are fabricated. TriPE‐3PDI shows the best performance with maximum quantum yield of ≈30% and optimized electron mobility of over 0.01 cm² V^?1 s^?1, which are the highest values among aggregation‐induced emission luminogens with NIR emissions reported so far. Photophysical property investigation and theoretical calculation indicate that the molecular conformation plays an important role on the optical properties of TriPE‐nPDI, while the result from film microstructure study reveals that the film crystallinity influences greatly their OFET device performance.     

En Route to Wide Area Emitting Organic Light-Emitting Transistors for Intrinsic Drive-Integrated Display Applications: A Comprehensive Review

Organic light-emitting transistors (OLET) evolved from the fusion of the switching functionality of field-effect transistors (FET) with the light-emitting characteristics of organic light-emitting diode (OLED) that can simplify the active-matrix pixel device architecture and hence offer a promising pathway for future flat panel and flexible display technology. This review systematically analyzes the key device/molecular engineering tactics that assist in improving the electrode edge narrow emission to wide-area emission for display applications via three different topics, that is, narrow to wide-area emission, vertical architecture, and impact of high-κ dielectric on the device performance. Source–drain electrode engineering such as symmetric/asymmetric, planar/non-planar arrangement, semitransparent nature, multilayer approach comprising charge transport, and work function modification layers enable widening the emission zone. Vertical OLET architecture offers short channel lengths with a high aperture ratio, pixel type area emission, and stable light-emitting area. Transistors utilizing high-κ dielectric materials have assisted in lowering the operating voltage, enhancing luminance and air stability. The promising development in achieving wide-area emission provides a solid basis for constructing OLET research toward display applications; however, it relies on developing highly luminescent and fast charge transporting materials, suitable semitransparent source/drain electrodes, high-κ -dielectrics, and device architectural engineering.     

Horizontal orientation of linear-shaped organic molecules having bulky substituents in neat and doped vacuum-deposited amorphous films

Organic amorphous films fabricated by vacuum deposition have been widely used in organic light-emitting devices, making use of their high-performance optical and electrical characteristics and taking advantage of the easy fabrication of pinhole-free thin smooth layers of a desired thickness. However, random orientation in amorphous films often makes it difficult to utilize their best optical and electrical potential. Here the authors demonstrate that the linear-shaped molecules of fluorescent styrylbenzene derivatives are horizontally oriented in organic amorphous films fabricated by conventional vacuum deposition even when the molecules are doped in an isotropic host matrix film. The longer the molecular length is, the larger the anisotropy of the molecular orientation becomes. The weak interaction between adjacent molecules and the linear-shaped molecular structure probably cause the horizontal orientation. The fact that the horizontal molecular orientation occurs on any underlying layers shows the high versatility of the horizontal orientation for various applications. Their findings will provide a new guideline for molecular designs that can be used to improve optical and electrical characteristics of organic optoelectronic devices, such as organic light-emitting diodes and organic laser devices.     

Tuning the optical absorption of potential blue emitters

The quest for more efficient blue emitters to be applied in organic light-emitting diodes is one of the challenging tasks of contemporary nanotechnologies. An approach to enhance substantially the intrinsic efficiency of luminescent organic molecules is the so-called thermally activated delayed fluorescence. A prerequisite for its occurrence is a vanishing energy separation between the first singlet and triplet excited states. A series of donor–acceptor molecules is investigated theoretically within this study in order to validate a molecular model for design of efficient organic blue emitters with closely spaced singlet and triplet excited states. The model is based on meta-linkage of the donor and acceptor residues to a spacer ensuring frontier molecular orbitals partitioning. The optimal geometries of the molecules are obtained with density functional theory (B3LYP/6-31G^*) and the singlet and triplet absorption spectra are simulated within the time-dependent density functional framework. The excited singlet-triplet energy gap is estimated and correlated to structural and energetic characteristics of the donors and acceptors. Several requirements for achieving high-energy triplet states at the molecular level in such donor–acceptor systems are outlined, the main being disjoint character of the molecular orbitals on the spacer and sufficient energy separation of the two topmost occupied orbitals. It is shown that by variation of the acceptor moiety the optical absorption transitions of the compounds can be fine-tuned in a systematic fashion. Molecules with degenerate singlet and triplet first excited states are proposed, combining bisdimethylaminotriphenylamine or phenoxazine as donors with diphenyloxadiazole or diphenyl-2,2′-bipyridine as acceptors. Bipolar molecules derived from this model could be used as prospective building blocks for efficient emissive materials in blue organic light-emitting diodes.     

Near‐Infrared (NIR) Organic Light‐Emitting Diodes (OLEDs): Challenges and Opportunities

The rapid development of the science and technology of organic semiconductors has already led to mass application of organic light‐emitting diodes (OLEDs) in television monitors of outstanding quality as well as in a large variety of smaller displays found in smartphones, tablets, and other gadgets, while introduction of the technology to the illumination sector is imminent. Notably, the requirements of all such applications for emission in the visible range of the electromagnetic spectrum are well tuned to the optical and electronic properties of typical organic semiconductors, thereby representing relatively “low‐hanging fruits,” in terms of material development and exploitation. However, the question arises as to whether developing materials suited for efficient near‐infrared (NIR, 700–1000 nm) emission is possible, and, crucially, desirable to enable new classes of applications spanning from through‐space, short‐range communications to biomedical sensors, night vision, and more generally security applications to name but a few. Here, the major fundamental hurdles to be overcome to achieve efficient NIR emission from organic π‐conjugated systems are discussed, recent progress is reviewed, and an outlook for further development of both materials and applications is provided.     

Intrinsically Flexible and Aging Resistant Fluorene-Based Rod-Coil Copolymer for Bendable Deep-Blue PLEDs

In the field of flexible light-emitting display, goal-oriented intelligent molecular design is used to control various behaviors of molecules, which provides potential for the development of flexible light-emitting conjugated polymers (LCPs). The introduction of non-conjugated units into polymer molecules is a key prerequisite for realizing the intrinsic flexibility, but its easy interchain slip will also lead to the formation of interchain excited states, which is detrimental to the efficiency of light-emitting diodes. Herein, two kinds of fluorene-based rod-coil copolymer with stable deep blue emission characteristics is presented and with Commission Internationale de L'Eclairage (CIE) coordinates of (0.18, 0.14) and (0.15, 0.09), respectively. Surprisingly, the copolymer films show efficient blue emission even at 100% tension. Meanwhile, the rod-coil copolymer possesses better aging resistance compared to rigid π-conjugated counterparts. Finally, both rigid and flexible light-emitting diodes based on rod-coil copolymer exhibit stable deep blue emission, and the G2-based PLED with CIE coordinates of (0.16, 0.08), which approach National Television System Committee standard blue specification. These results confirm the validity of rod-coil copolymer design strategy in constructing inherently flexible polymers with deep blue emission, which have great application potential in flexible PLEDs.     

Preface to the Special Issue on Flexible Energy Devices

Flexible energy devices are the building blocks for next-generation wearable electronics.Flexible energy devices are expected to have multiple functions,such as energy conver-sion from light to electricity and vice versa,energy genera-tion from triboelectric,energy storage and so on.These func-tions can be efficiently realized by solar cells,light-emitting di-odes (LEDs),triboelectric nanogenerators (TENG),batteries and supercapacitors,etc.The flexible energy devices can be in-tegrated into flexible,wearable,and/or portable platforms to enable wide application prospects in the fields of informa-tion,energy,medical care,national defense,etc.However,flex-ible energy devices face more challenges when compared to their rigid counterparts,which requires more breakthroughs and research efforts on fabrication techniques,materials innov-ation,novel structure designs,and deep physical understand-ings.     

Tenfold Enhancement in Light Emission from Low-Temperature Operational Doped OLEDs by Detrapping Super-Long-Lived Trapped Charges

Although doping organic fluorophores into host matrix is a common way to obtain high-efficiency organic light-emitting diodes (OLEDs), trapped charges are often observed on dopant molecules that are considered to be detrimental to the further enhancement of their photoelectric performances. Surprisingly, trapped charges with a super-long lifetime of >2 h are detected in doped OLEDs operated at 20 K, which has never been discovered previously in the literature. However, the observations demonstrate that the longer lifetimes of these trapped charges are primarily governed by the larger spacing distances between trapped electrons and holes, rather than being solely determined by the well-accepted energy-level depth of trap states. More amazingly, compared with the device driven only by a conventional constant voltage source, a tenfold enhancement in light emission is achieved in low-temperature operational doped OLEDs powered by an alternating positive and negative pulse voltage, because the applied negative pulse voltage can assist these super-long-lived trapped electrons and holes to detrap and then recombine with each other for producing enhanced light-emission. Therefore, this study clarifies the dynamic behaviors of trapped charges and paves the pathway for obtaining high-performance OLEDs working in low-temperature circumstances such as outer-space or aerospace fields.     

Fluorescent Photochromic α-Cyanodiarylethene Molecular Switches: An Emerging and Promising Class of Functional Diarylethene

Fluorescent photochromic molecules that exhibit distinct light-triggered changes in their emission colors are highly desirable for the fabrication of smart soft materials and advanced photonic devices. α-Cyanodiarylethenes, that is, α-cyano-functionalized diarylethenes, as alternative “non-azo” Z/E photochromic molecular switches, are popular choices due to their unique characteristics such as their aggregation-induced emission or aggregation-induced-enhanced emission behavior in their self-assembled states, and visible changes in fluorescence colors during Z/E photoisomerization. In recent years, the development of fluorescent photochromic α-cyanodiarylethene-based compounds including α-cyanostilbenes, dicyanodistyrylbenzenes, and diaryldicyanoethenes, has mainly focused on molecular design, photochemical and photophysical behavior in solution, and smart soft matter technologies. In this review, recent significant achievements in light-responsive systems based on the Z/E photoisomerization of fluorescent photochromic α-cyanodiarylethene switches that span the range from liquid crystals to gels and finally to self-assembled nanostructures, are highlighted. The smart soft materials constructed from α-cyanodiarylethene molecular switches find use in a plethora of areas, including display, sensing, encrypting, actuating, and biomedical imaging applications, among others. The review concludes with a brief perspective on some major challenges and opportunities for the future development of light-responsive smart soft photonic materials.     

Unravelling the Molecular Origin of Organic Semiconductors with High-Performance Thermoelectric Response

A decisive prerequisite toward systematic development of high-efficiency organic thermoelectric materials is not only thoroughly understanding the microscopic physical processes controlling the performance, but also precisely correlating such processes and the macroscopic properties to the basic chemical structures. Here, by using multiscale first-principles calculations, the interplay among thermoelectric properties, microscopic transport parameters, and molecular structures for the whole family of small-molecule organic thermoelectric materials is rationalized, and general molecular design principles are concurrently formulated. It is unveiled that thermoelectric power factor of a wide variety of molecular semiconductors is directly proportional to a unified quality factor, and high-performance thermoelectric response demands to boost the intermolecular electronic coupling, and to suppress the interaction of electron with lattice vibrations. Furthermore, it is uncovered that extending the π-conjugated backbones along the long axis, and maximizing the networks of intermolecular S···S or C H···π contacts meet the proposed material design rule.