Toward Highly Efficient Solid‐State White Light‐Emitting Electrochemical Cells: Blue‐Green to Red Emitting Cationic Iridium Complexes with Imidazole‐Type Ancillary Ligands

Using imidazole‐type ancillary ligands, a new class of cationic iridium complexes ( 1 – 6 ) is prepared, and photophysical and electrochemical studies and theoretical calculations are performed. Compared with the widely used bpy (2,2′‐bipyridine)‐type ancillary ligands, imidazole‐type ancillary ligands can be prepared and modified with ease, and are capable of blueshifting the emission spectra of cationic iridium complexes. By tuning the conjugation length of the ancillary ligands, blue‐green to red emitting cationic iridium complexes are obtained. Single‐layer light‐emitting electrochemical cells (LECs) based on cationic iridium complexes show blue‐green to red electroluminescence. High efficiencies of 8.4, 18.6, and 13.2 cd A^?1 are achieved for the blue‐green‐emitting, yellow‐emitting, and orange‐emitting devices, respectively. By doping the red‐emitting complex into the blue‐green LEC, white LECs are realized, which give warm‐white light with Commission Internationale de L'Eclairage (CIE) coordinates of (0.42, 0.44) and color‐rendering indexes (CRI) of up to 81. The peak external quantum efficiency, current efficiency, and power efficiency of the white LECs reach 5.2%, 11.2 cd A^?1, and 10 lm W^?1, respectively, which are the highest for white LECs reported so far, and indicate the great potential for the use of these cationic iridium complexes in white LECs.     

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.     

Emission Color Trajectory and White Electroluminescence Through Supramolecular Control of Energy Transfer and Exciplex Formation in Binary Blends of Conjugated Polyrotaxanes

White electroluminescence and fine‐tuning of the emission color from binary blends of a blue‐emitting polymer and a green/yellow‐emitting threaded molecular wire consisting of a conjugated polymer supramolecularly encapsulated by functionalized cyclodextrins are demonstrated. Encapsulation controls the minimum intermolecular distance on the nanoscale, resulting in suppressed energy‐transfer between the blend constituents and reduced formation of interchain charge‐transfer complexes. The use of a green‐emitting polyrotaxane significantly improves the electrical properties with respect to blends of a blue electroluminescent polyrotaxane and leads to a significant reduction in the turn‐on voltage required for achieving white electroluminescence (V_ON = 3 V), with only 20% by weight of the encapsulated material.     

Blue‐Emitting Iridium(III) Complexes for Light‐Emitting Electrochemical Cells: Advances,Challenges, and Future Prospects

Light‐emitting electrochemical cells (LECs) are one of the most promising technologies for solid‐sate lighting. Among them, LECs based on phosphorescent iridium(III) complexes have attracted significant research interest in the past 15 years, because of their high efficiency and tunable emission color across the entire visible spectrum. To fabricate white LECs for lighting, high‐performance blue LECs are the first prerequisite. Huge efforts have been devoted to improving the performances of blue LECs based on iridium(III) complexes either by developing new blue‐emitting complexes or by engineering the devices. Nevertheless, blue LECs have still shown much lower performances (brightness, efficiency, stability, etc.) compared to the red, orange‐red, yellow, and green counterpart devices. In particular, a single blue LEC with satisfactory blue‐color purity, high efficiency, and high stability is still missing. Here, the advances in blue‐emitting iridium(III) complexes for LECs and the device engineering on LECs using the complexes are reported. The challenges ahead are discussed, and future prospects are outlined.     

Tuning the Emitting Color of Organic Light‐Emitting Diodes Through Photochemically Induced Transformations: Towards Single‐Layer,Patterned, Full‐Color Displays and White‐Lighting Applications

Photochemically induced emission tuning for the definition of pixels emitting the three primary colors, red, green, blue (RGB), in a single conducting polymeric layer is investigated. The approach proposed is based on an acid‐induced emission shift of the (1‐[4‐(dimethylamino)phenyl]‐6‐phenylhexatriene) (DMA‐DPH) green emitter and acid‐induced quenching of the red fluorescent emitter (4‐dimethylamino‐4′‐nitrostilbene) (DANS). The two emitters are dispersed in the wide bandgap conducting polymer poly(9‐vinylcarbazole) (PVK), along with a photoacid generator (PAG). In the unexposed film areas, red emission is observed because of efficient energy transfer from PVK and DMA‐DPH to DANS. Exposure of selected areas of the film at different doses results in quenching of the red emitter's fluorescence and the formation of green, blue, or even other color‐emitting pixels, depending on the exposure dose and the relative concentrations of the different compounds in the film. Organic light‐emitting diodes having the PVK polymer containing the appropriate amounts of DMA‐DPH, DANS, and PAG as the emitting layer are fabricated and electroluminescence spectra are recorded. The time stability of induced emission spectrum changes and the color stability during device operation are also examined, and the first encouraging results are obtained.     

Device Physics of White Polymer Light‐Emitting Diodes

The charge transport and recombination in white‐emitting polymer light‐ emitting diodes (PLEDs) are studied. The PLED investigated has a single emissive layer consisting of a copolymer in which a green and red dye are incorporated in a blue backbone. From single‐carrier devices the effect of the green‐ and red‐emitting dyes on the hole and electron transport is determined. The red dye acts as a deep electron trap thereby strongly reducing the electron transport. By incorporating trap‐assisted recombination for the red emission and bimolecular Langevin recombination for the blue emission, the current and light output of the white PLED can be consistently described. The color shift of single‐layer white‐emitting PLEDs can be explained by the different voltage dependencies of trap‐assisted and bimolecular recombination.     

Highly Efficient and Fully Solution‐Processed White Electroluminescence Based on Fluorescent Small Molecules and a Polar Conjugated Polymer as the Electron‐Injection Material

Highly efficient and fully solution‐processed white organic light‐emitting diodes (WOLEDs) based on fluorescent small molecules and a polar conjugated polymer as electron‐injection material are reported. The emitting layer in the WOLEDs is a blend of new blue‐, green‐, and red‐fluorescent small molecules, with a blending ratio of 100:0.4:0.8 (B/G/R) by weight, and a methanol/water soluble conjugated polymerpoly[(9,9‐bis(30‐(N,N‐dimethylamino)propyl)‐2,7‐fluorene)‐alt‐2,7‐(9,9‐dioctylfluorene)] (PFN) acts as the electron‐injection layer (EIL). All the organic layers are spin‐coated from solution. The device exhibits pure white emission with a maximum luminous efficiency of 9.2 cd A^?1 and Commission Internationale d'Eclairage Coordinates of (0.35, 0.36). PFN acting as the EIL material plays a key role in the improvement of the device performance when used in solution‐processed small‐molecule WOLEDs.     

The Fabrication and Characterization of Single‐Component Polymeric White‐Light‐Emitting Diodes

By using Ni⁰‐mediated polymerization, we have systematically synthesized a series of fluorene‐based copolymers composed of blue‐, green‐, and red‐light‐emitting comonomers with a view to producing polymers with white‐light emission. 2,7‐Dibromo‐9,9‐dihexylfluorene, {4‐(2‐[2,5‐dibromo‐4‐{2‐(4‐diphenylamino‐phenyl)‐vinyl}‐phenyl]‐vinyl)‐phenyl}‐diphenylamine (DTPA), and 2‐{2‐(2‐[4‐{bis(4‐bromo‐phenyl)amino}‐phenyl]‐vinyl)‐6‐tert‐butyl‐pyran‐4‐ylidene}‐malononitrile (TPDCM) were used as the blue‐, green‐, and red‐light‐emitting comonomers, respectively. It was found that the emission spectra of the resulting copolymers could easily be tuned by varying their DTPA and TPDCM content. Thus with the appropriate red/green/blue (RGB) unit ratio, we were able to obtain white‐light emission from these copolymers. A white‐light‐emitting diode using the polyfluorene copolymer containing 3 % green‐emitting DTPA and 2 % red‐emitting TPDCM (PG3R2) with a structure of indium tin oxide/poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonic acid)/PG3R2/Ca/Al was found to exhibit a maximum brightness of 820 cd m^–2 at 11 V with Commission Internationale de L'Eclairage (CIE) coordinates of (0.33,0.35), which are close to the standard CIE coordinates for white‐light emission (0.33,0.33).     

Stable Blue Emission from a Polyfluorene/Layered‐Compound Guest/Host Nanocomposite

In this study a blue‐light‐emitting conjugated polymer, poly(9,9‐dioctylfluorene), is confined to the interlayer space of inorganic, layered metal dichalcogenide materials, metallic MoS₂, and semiconducting SnS₂. The nanocomposites are prepared through Li intercalation into the inorganic compound, exfoliation, and restacking in the presence of the polymer. X‐ray diffraction and optical absorption measurements indicate that a single conjugated polymer monolayer, with an overall extended planar morphology conformation, is isolated between the inorganic sheets, so that polymer aggregation or π–π interchain interactions are significantly reduced. Photoluminescence (PL) measurements show that the appearance of the undesirable green emission observed in pristine polymer films is suppressed by incorporating the polymer into the inorganic matrix. The blue emission of the intercalated polymer is stable for extended periods of time, over two years, under ambient conditions. Furthermore, the green emission is absent in the PL spectra of nanocomposite films heated at 100 °C for 7 h in air with direct excitation of the keto defect. Finally, no green emission was observed in the electroluminescence spectrum of light‐emitting devices fabricated with a polymer‐intercalated SnS₂ nanocomposite film. These results support the proposed hypothesis that fluorenone defects alone are insufficient to generate the green emission and that interchain interactions are also required.     

Simple and Efficient Generation of White Light Emission From Organophosphorus Building Blocks

This paper describes a structure–property study using two dithieno[3,2‐b;2′,3′‐d]phosphole building blocks for the generation of white light emission and the incorporation of these units in a single polystyrene material. The emission of one of the light‐emitting organophosphorus building blocks can efficiently be switched from orange to green by simple protonation of the amino functional groups that are part of the π‐conjugated scaffold. The resulting three components (blue, green, and orange) exhibit photophysical properties that allow for an efficient fluorescence resonance energy transfer (FRET) in the mixture/polymer and provide intense white fluorescence upon excitation of the blue component; the fluorescence is close to pure white in solution and similar to the emission of an incandescent light bulb in the thin film. The results nicely illustrate the intriguing features that can be obtained by exclusively using organophosphorus‐based organic electronic materials.     

Simultaneous and Dual Emissive Imaging by Micro‐Contact Printing on the Surface of Electrostatically Assembled Water‐Soluble Poly(p‐phenylene) Using FRET

Poly{[2,5‐bis(3‐sulfonatobutoxy)‐1,4‐phenylene sodium salt]‐alt‐(1,4‐phenylene)}, which is an anionically charged, water‐soluble poly(para‐phenylene) derivative with aldehyde groups at both chain ends, is prepared via the Suzuki coupling reaction in order to develop a FRET energy donor, while simultaneously dual‐fluorescence‐patterning the protein. Regardless of the end‐capping, the synthesized polymer exhibits a good solubility in water with an absorption maximum at 338 nm and a photoluminescence maximum at 417 nm, similar to those of the the end‐capped polymer. The emission spectrum of the polymer overlaps the absorption spectrum of fluorescein, and therefore, the polymer can be used as an energy donor with fluorescein as the energy acceptor in the FRET mechanism. This polymer design not only takes advantage of the introduction of biotin at both chain ends (through a reaction with the aldehyde end groups) to realize the facile interaction with streptavidin, but also brings into play the electrostatic features of the anionic sulfonate groups to fabricate an electrostatic self‐assembly with polycation for the pattern substrate. The micropattern of fluorescein‐labeled streptavidin is fabricated on the polymer‐coated substrate through micro‐contact printing using a polydimethylsiloxane mold. As a result, the polymer substrate exhibits a dual fluorescence micropattern, which results from the blue emission color from the energy donor and the FRET‐amplified green emission from the energy acceptor. The high‐resolution patterning is carried out for the application of multiplexing by simultaneously imaging the patterned green‐emitting fluorescein by FRET and the surrounding blue‐emitting polymer according to an optical detection scheme.     

Binaphthyl‐Containing Green‐ and Red‐Emitting Molecules for Solution‐Processable Organic Light‐Emitting Diodes

Strong intermolecular interactions usually result in decreases in solubility and fluorescence efficiency of organic molecules. Therefore, amorphous materials are highly pursued when designing solution‐processable, electroluminescent organic molecules. In this paper, a non‐planar binaphthyl moiety is presented as a way of reducing intermolecular interactions and four binaphthyl‐containing molecules ( BNCM s): green‐emitting BBB and TBT as well as red‐emitting BTBTB and TBBBT , are designed and synthesized. The photophysical and electrochemical properties of the molecules are systematically investigated and it is found that TBT , TBBBT , and BTBTB solutions show high photoluminescence (PL) quantum efficiencies of 0.41, 0.54, and 0.48, respectively. Based on the good solubility and amorphous film‐forming ability of the synthesized BNCM s, double‐layer structured organic light‐emitting diodes (OLEDs) with BNCM s as emitting layer and poly(N‐vinylcarbazole) (PVK) or a blend of poly[N,N′‐bis(4‐butylphenyl)‐N,N′‐bis(phenyl)benzidine] and PVK as hole‐transporting layer are fabricated by a simple solution spin‐coating procedure. Amongst those, the BTBTB based OLED, for example, reaches a high maximum luminance of 8315 cd · m⁻² and a maximum luminous efficiency of 1.95 cd · A⁻¹ at a low turn‐on voltage of 2.2 V. This is one of the best performances of a spin‐coated OLED reported so far. In addition, by doping the green and red BNCM s into a blue‐emitting host material poly(9,9‐dioctylfluorene‐2,7‐diyl) high performance white light‐emitting diodes with pure white light emission and a maximum luminance of 4000 cd · m⁻² are realized.     

Dye Encapsulated Metal‐Organic Framework for Warm‐White LED with High Color‐Rendering Index

A strategy by encapsulating organic dyes into the pores of a luminescent metal‐organic framework (MOF) is developed to achieve white‐light‐emitting phosphor. Both the red‐light emitting dye 4‐(p‐dimethylaminostyryl)‐1‐methylpyridinium ( DSM ) and the green‐light emitting dye acriflavine ( AF ) are encapsulated into a blue‐emitting anionic MOF ZJU‐28 through an ion‐exchange process to yield the MOF?dye composite ZJU‐28?DSM/AF . The emission color of the obtained composite can be easily modulated by simply adjusting the amount and component of dyes. With careful adjustment of the relative concentration of the dyes DSM and AF , the resulting ZJU‐28?DSM/AF (0.02 wt% DSM , 0.06 wt% AF ) exhibits a broadband white emission with ideal CIE coordinates of (0.34, 0.32), high color‐rendering index value of 91, and moderate correlated color temperature value of 5327 K. Such a strategy can be easily expanded to other luminescent MOFs and dyes, thus opening a new perspective for the development of white light emitting materials.     

White Electroluminescence by Supramolecular Control of Energy Transfer in Blends of Organic‐Soluble Encapsulated Polyfluorenes

Here, it is demonstrated that energy transfer in a blend of semiconducting polymers can be strongly reduced by non‐covalent encapsulation of one constituent, ensured by threading of the conjugated strands into functionalized cyclodextrins. Such macrocycles control the minimum intermolecular distance of chromophores with similar alignment, at the nanoscale, and therefore the relevant energy transfer rates, thus enabling fabrication of white‐light‐emitting diodes (CIE coordinates: x = 0.282, y = 0.336). In particular, white electroluminescence in a binary blend of a blue‐emitting, organic‐soluble rotaxane based on a polyfluorene derivative and the green‐emitting poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole ( F8BT ) is achieved. Morphological and structural analyses by atomic force microscopy, fluorescence mapping, µ‐Raman, and fluorescence lifetime microscopy are used to complement optical and electroluminescence characterization, and to enable a deeper insight into the properties of the novel blend.     

White‐Emitting Conjugated Polymer/Inorganic Hybrid Spheres: Phenylethynyl and 2,6‐Bis(pyrazolyl)pyridine Copolymer Coordinated to Eu(tta)3

The synthesis of two cyan color (blue and green emission) displaying high molecular weight 2,6‐bis(pyrazolyl)pyridine‐co‐octylated phenylethynyl conjugated polymers (CPs) is presented. The conjugated polymers are solution‐processed to prepare spin coated thin films and self‐assembled nano/microscale spheres, exhibiting cyan color under UV. Additionally, the metal coordinating ability of the 2,6‐bis(pyrazolyl)pyridine available on the surface of the CP films and spheres is exploited to prepare red emitting Eu(III) metal ion containing conjugated polymer (MCCP) layer. The fabricated hybrid (CP/MCCP) films and spheres exhibit bright white‐light under UV exposure. The Commission Internationale de l'Eclairage (CIE) coordinates are found to be (x = 0.33, y = 0.37) for hybrid films and (x = 0.30, y = 0.35) for hybrid spheres. These values are almost close to the designated CIE coordinates for ideal white‐light color (x = 0.33, y = 0.33). This easy and efficient fabrication technique to generate white‐color displaying films and nano/microspheres signify an important method in bottom‐up nanotechnology of conjugated polymer based hybrid solid state assemblies.     

Microwave‐Assisted Preparation of White Fluorescent Graphene Quantum Dots as a Novel Phosphor for Enhanced White‐Light‐Emitting Diodes

Graphene quantum dots (GQDs) with white fluorescence are synthesized by a microwave‐assisted hydrothermal method using graphite as the precursor. A solution‐processed white‐light‐emitting diode (WLED) is fabricated using the as‐prepared white fluorescent GQDs (white‐light‐emitting graphene quantum dots, WGQDs) doped 4,4‐bis(carbazol‐9‐yl)biphenyl as the emissive layer. White‐light emission is obtained from the WLED with 10 wt% doping concentration of WGQDs, which shows a luminance of 200 cd m^?2 at the applied voltage of 11–14 V. Importantly, an external quantum efficiency of 0.2% is achieved, which is the highest among all the reported WLED based on GQDs or carbon dots. The results demonstrate that WGQDs as a novel phosphor may open up a new avenue to develop the environmentally friendly WLEDs for practical application in solid‐state lighting.     

Recent Progress in White Light‐Emitting Electrochemical Cells

Solid‐state white light‐emitting electrochemical cells (LECs) exhibit the following advantages: simple device structures, low operation voltage, and compatibility with inert metal electrodes. LECs have been studied extensively since the first demonstration of white LECs in 1997, due to their potential application in solid‐state lighting. This review provides an overview of recent developments in white LECs, specifically three major aspects thereof, namely, host–guest white LECs, nondoped white LECs, and device engineering of white LECs. Host–guest strategy is widely used in white LECs. Host materials are classified into ionic transition metal complexes, conjugated polymers, and small molecules. Nondoped white LECs are based on intra‐ or intermolecular interactions of emissive and multichromophore materials. New device engineering techniques, such as modifying carrier balance, color downconversion, optical filtering based on microcavity effect and localized surface plasmon resonance, light extraction enhancement, adjusting correlated color temperature of the output electroluminescence spectrum, tandem and/or hybrid devices combining LECs with organic light‐emitting diodes, and quantum‐dot light‐emitting diodes improve the device performance of white LECs by ways other than material‐oriented approaches. Considering the results of the reviewed studies, white LECs have a bright outlook.     

High‐Color‐Rendering and High‐Efficiency White Organic Light‐Emitting Devices Based on Double‐Doped Organic Single Crystals

Organic single crystals with much higher carrier mobility and stability compared to the amorphous organic materials have shown great potential in electronic and optoelectronic devices. However, their applications in white organic light‐emitting devices (WOLEDs), especially the three‐color‐strategy WOLEDs, have been hindered by the difficulties in fabricating complicated device structures. Here, double‐doped white‐emission organic single crystals are used as the active layers for the first time in the three‐color‐strategy WOLEDs by co‐doping the red and green dopants into blue host crystals. Precise control of the dopant concentration in the double‐doped crystals results in moderately partial energy transfer from the blue donor to the green and red dopants, and thereafter, simultaneous RGB emissions with balanced emission intensity. The highest color‐rendering index (CRI) and efficiency, to the best of the authors' knowledge, are obtained for the crystal‐based WOLEDs. The CRI of the WOLEDs varies between 80 and 89 with the increase of the driving current, and the luminance and current efficiency reach up to 793 cd m^?2 and 0.89 cd A^?1, respectively. The demonstration of the present three‐color organic single‐crystal‐based WOLED promotes the development of the single crystals in optoelectronics.     

Red,Green, and Blue Light‐Emitting Polyfluorenes Containing a Dibenzothiophene‐S,S‐Dioxide Unit and Efficient High‐Color‐Rendering‐Index White‐Light‐Emitting Diodes Made Therefrom

A series of blue (B), green (G) and red (R) light‐emitting, 9,9‐bis(4‐(2‐ethyl‐hexyloxy)phenyl)fluorene (PPF) based polymers containing a dibenzothiophene‐S,S‐dioxide (SO) unit (PPF‐SO polymer), with an additional benzothiadiazole (BT) unit (PPF‐SO‐BT polymer) or a 4,7‐di(4‐hexylthien‐2‐yl)‐benzothiadiazole (DHTBT) unit (PPF‐SO‐DHTBT polymer) are synthesized. These polymers exhibit high fluorescence yields and good thermal stability. Light‐emitting diodes (LEDs) using PPF‐SO25, PPF‐SO15‐BT1, and PPF‐SO15‐DHTBT1 as emission polymers have maximum efficiencies LE_max = 7.0, 17.6 and 6.1 cd A^?1 with CIE coordinates (0.15, 0.17), (0.37, 0.56) and (0.62, 0.36), respectively. 1D distributed feedback lasers using PPF‐SO30 as the gain medium are demonstrated, with a wavelength tuning range 467 to 487 nm and low pump energy thresholds (≥18 nJ). Blending different ratios of B (PPF‐SO), G (PPF‐SO‐BT) and R (PPF‐SO‐DHTBT) polymers allows highly efficient white polymer light‐emitting diodes (WPLEDs) to be realized. The optimized devices have an attractive color temperature close to 4700 K and an excellent color rendering index (CRI) ≥90. They are relatively stable, with the emission color remaining almost unchanged when the current densities increase from 20 to 260 mA cm^?2. The use of these polymers enables WPLEDs with a superior trade‐off between device efficiency, CRI, and color stability.     

Hybrid White Light Emitting Diode Based on Silicon Nanocrystals

A novel design of white light emitting diodes (WLEDs) emerges to meet the growing global demand for resource sustainability while preserving health and environment. To achieve this goal, a facile method is developed for the chemical synthesis of a luminescent silicon nanocrystal (ncSi) with a large Stokes shift between absorption and emission. The WLED is prepared by a simple spin‐coating method, and contains a hybrid‐bilayer of the ncSi and luminescent polymer in its device active region. Interestingly, a well‐controlled ultrathin ncSi layer on the polymer makes possible to recombine electrons and holes in both layers, respectively. Combining red and blue‐green lights, emitted from the ncSi and the polymer layers, respectively, produces the emission of white electroluminescence. Herein, a hybrid‐WLED with a sufficiently low turn‐on voltage (3.5 V), produced by taking advantages of the large Stokes shift inherent in ncSi, is demonstrated.