Cover Picture: White Electroluminescence from a Single‐Polymer System with Simultaneous Two‐Color Emission: Polyfluorene as the Blue Host and a 2,1,3‐Benzothiadiazole Derivative as the Orange Dopant (Adv. Funct. Mater. 7/2006)

New single‐polymer electroluminescent systems containing two individual emission species—polyfluorenes as a blue host and 2,1,3‐benzothiadiazole derivative units as an orange dopant on the main chain—have been designed and synthesized by Wang and co‐workers on p. 957. The resulting single polymers are found to have highly efficient white electroluminescence with simultaneous blue and orange emission from the corresponding emitting species. A single‐layer device has been fabricated that has performance characteristics roughly comparable to those of organic white‐light‐emitting diodes with multilayer device structures. New single‐polymer electroluminescent systems containing two individual emission species—polyfluorenes as a blue host and 2,1,3‐benzothiadiazole derivative units as an orange dopant on the main chain—have been designed and synthesized. The resulting single polymers are found to have highly efficient white electroluminescence with simultaneous blue (λ_max = 421 nm/445 nm) and orange emission (λ_max = 564 nm) from the corresponding emitting species. The influence of the photoluminescence (PL) efficiencies of both the blue and orange species on the electroluminescence (EL) efficiencies of white polymer light‐emitting diodes (PLEDs) based on the single‐polymer systems has been investigated. The introduction of the highly efficient 4,7‐bis(4‐(N‐phenyl‐N‐(4‐methylphenyl)amino)phenyl)‐2,1,3‐benzothiadiazole unit to the main chain of polyfluorene provides significant improvement in EL efficiency. For a single‐layer device fabricated in air (indium tin oxide/poly(3,4‐ethylenedioxythiophene): poly(styrene sulfonic acid/polymer/Ca/Al), pure‐white electroluminescence with Commission Internationale de l'Eclairage (CIE) coordinates of (0.35,0.32), maximum brightness of 12 300 cd m^–2, luminance efficiency of 7.30 cd A^–1, and power efficiency of 3.34 lm W^–1 can be obtained. This device is approximately two times more efficient than that utilizing a single polyfluorene containing 1,8‐naphthalimide moieties, and shows remarkable improvement over the corresponding blend systems in terms of efficiency and color stability. Thermal treatment of the single‐layer device before cathode deposition leads to the further improvement of the device performance, with CIE coordinates of (0.35,0.34), turn‐on voltage of 3.5 V, luminance efficiency of 8.99 cd A^–1, power efficiency of 5.75 lm W^–1, external quantum efficiency of 3.8 %, and maximum brightness of 12 680 cd m^–2. This performance is roughly comparable to that of white organic light‐emitting diodes (WOLEDs) with multilayer device structures and complicated fabrication processes.     

White Electroluminescence from a Single‐Polymer System with Simultaneous Two‐Color Emission: Polyfluorene Blue Host and Side‐Chain‐Located Orange Dopant

Four single polymers with two kinds of attachment of orange chromophore to blue polymer host for white electroluminescence (EL) were designed. The effect of the side‐chain attachment and main‐chain attachment on the EL efficiencies of the resulting polymers was compared. The side‐chain‐type single polymers are found to exhibit more efficient white EL than that of the main‐chain‐type single polymers. Based on the side‐chain‐type white single polymer with 4‐(4‐alkyloxy‐phenyl)‐7‐(4‐diphenylamino‐phenyl)‐2,1,3‐benzothiadiazoles as the orange‐dopant unit and polyfluorene as the blue polymer host, white EL with simultaneous orange (λ_max = 545 nm) and blue emission (λ_max = 432 nm/460 nm) is realised. A single‐layer device (indium tin oxide/poly(3,4‐ethylenedioxythiophene)/polymer/Ca/Al) made of these polymers emits white light with the Commission Internationale de l'Éclairage coordinates of (0.30,0.40), possesses a turn‐on voltage of 3.5 V, luminous efficiency of 10.66 cd A^–1, power efficiency of 6.68 lm W^–1, and a maximum brightness of 21 240 cd m^–2.     

Highly Efficient Pure‐White‐Light‐Emitting Diodes from a Single Polymer: Polyfluorene with Naphthalimide Moieties

Light‐emitting diodes exhibiting efficient pure‐white‐light electroluminescence have been successfully developed by using a single polymer: polyfluorene derivatives with 1,8‐naphthalimide chromophores chemically doped onto the polyfluorene backbones. By adjusting the emission wavelength of the 1,8‐naphthalimide components and optimizing the relative content of 1,8‐naphthalimide derivatives in the resulting polymers, white‐light electroluminescence from a single polymer, as opposed to a polymer blend, has been obtained in a device with a configuration of indium tin oxide/poly(3,4‐ethylenedioxythiophene)(50 nm)/polymer(80 nm)/Ca(10 nm)/Al(100 nm). The device exhibits Commission Internationale de l'Eclairage coordinates of (0.32,0.36), a maximum brightness of 11 900 cd m^–2, a current efficiency of 3.8 cd A^–1, a power efficiency of 2.0 lm W^–1, an external quantum efficiency of 1.50 %, and quite stable color coordinates at different driving voltages, even at high luminances of over 5000 cd m^–2.     

Pure and Saturated Red Electroluminescent Polyfluorenes with Dopant/Host System and PLED Efficiency/Color Purity Trade‐Offs

Three kinds of red electroluminescent (EL) polymers based on polyfluorene as blue host and 2,1,3‐benzothiadiazole derivatives with different emission wavelengths as red dopant units on the side chain are designed and synthesized. The influence of the photoluminescence (PL) efficiencies and emission wavelengths of red dopants on the EL efficiencies and color purities of the resulting polyfluorene copolymers of dopant/host system is investigated by adjusting the electron donating ability of the donor units in D‐π‐A‐D typed 2,1,3‐benzothiadiazole derivatives. The devices of these red‐emitting polymers realize remarkable EL efficiency/color purity trade‐offs. The single‐layer devices with the configuration of ITO/PEDOT:PSS/Polymer/Ca/Al show pure red emission at 624 nm with a luminous efficiency of 3.83 cd A^?1 and CIE of (0.63, 0.35) for PFR1, saturated red emission at 636 nm with a luminous efficiency of 2.29 cd A^?1 and CIE of (0.64, 0.33) for PFR2, respectively. By introduction of an additional electron injection layer PF‐EP(Ethanol soluble phosphonate‐functionalized polyfluorene), high performance pure and saturated red emission two‐layer devices (ITO/PEDOT:PSS/Polymer/PF‐EP/LiF/Al) were achieved with maximum luminous efficiencies of 5.50 cd A^?1 and CIE of (0.62, 0.35) for PFR1, 3.10 cd A^?1 and CIE of (0.63, 0.33) for PFR2, respectively, which are the best results for pure and saturated fluorescent red EL polymers reported so far.     

High‐Performance All‐Polymer White‐Light‐Emitting Diodes Using Polyfluorene Containing Phosphonate Groups as an Efficient Electron‐Injection Layer

We report an efficient non‐doped all‐polymer polymer white‐light‐emitting diode (PWLED) with a fluorescent three‐color, white single polymer as an emissive layer, an ethanol‐soluble phosphonate‐functionalized polyfluorene (PF‐EP) as an electron‐injection/electron‐transport layer, and LiF/Al as a cathode, respectively. The all‐polymer PWLED achieves a peak external quantum efficiency of 6.7%, a forward viewing luminous efficiency of 15.4 cd A^?1 and a power efficiency of 11.4 lm W^?1, respectively, at a brightness of 347 cd m^?2 with Commission Internationale d’Eclairage coordinates of (0.37, 0.42) and color rendering index of 85, which is the best results among the non‐doped PWLEDs. Moreover, this kind of PWLED not only shows excellent color stability, but also achieves high brightness at low voltages. The brightness reaches 1000, 10000, and 46830 cd m^?2 at voltages of 4.5, 5.4, and 7.5 V, respectively. The significant enhancement of white‐single‐polymer‐based PWLEDs with PF‐EP/LiF/Al to replace for the commonly used Ca/Al cathode is attributed to the more efficient electron injection at PF‐EP/LiF/Al interfaces, and the coordinated protecting effect of PF‐EP from diffusion of Al atoms into the emissive layer and exciton‐quenching near cathode interfaces. The developed highly efficient non‐doped all‐polymer PWLEDs are well suitable for solution‐processing technology and provide a huge potential of low‐cost large‐area manufacturing for PWLEDs.     

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.     

High‐Performance Organic Light‐Emitting Diodes Based on Intramolecular Charge‐Transfer Emission from Donor–Acceptor Molecules: Significance of Electron‐ Donor Strength and Molecular Geometry

Organic light‐emitting diodes based on intramolecular‐charge‐transfer emission from two related donor–acceptor (D–A) molecules, 3,7‐[bis(4‐phenyl‐2‐quinolyl)]‐10‐methylphenothiazine (BPQ‐MPT) and 3,6‐[bis(4‐phenyl‐2‐quinolyl)]‐9‐methylcarbazole (BPQ‐MCZ), were found to have electroluminescence (EL) efficiencies and device brightnesses that differ by orders of magnitude. High brightness (> 40 000 cd m^–2) and high efficiency (21.9 cd A^–1, 10.8 lm W^–1, 5.78 % external quantum efficiency (EQE) at 1140 cd m^–2) green EL was achieved from the BPQ‐MPT emitter, which has its highest occupied molecular orbital (HOMO) level at 5.09 eV and a nonplanar geometry. In contrast, diodes with much lower brightness (2290 cd m^–2) and efficiency (1.4 cd A^–1, 0.66 lm W^–1, 1.7 % EQE at 405 cd m^–2) were obtained from the BPQ‐MCZ emitter, which has its HOMO level at 5.75 eV and exhibits a planar geometry. Compared to BPQ‐MCZ, the higher‐lying HOMO level of BPQ‐MPT facilitates more efficient hole injection/transport and a higher charge‐recombination rate, while its nonplanar geometry ensures diode color purity. White EL was observed from BPQ‐MCZ diodes owing to a blue intramolecular charge‐transfer emission and a yellow–orange intermolecular excimer emission, enabled by the planar molecular geometry. These results demonstrate that high‐performance light‐emitting devices can be achieved from intramolecular charge‐transfer emission, while highlighting the critical roles of the electron‐donor strength and the molecular geometry of D–A molecules.     

A Multifunctional Iridium‐Carbazolyl Orange Phosphor for High‐Performance Two‐Element WOLED Exploiting Exciton‐Managed Fluorescence/Phosphorescence

By attaching a bulky, inductively electron‐withdrawing trifluoromethyl (CF₃) group on the pyridyl ring of the rigid 2‐[3‐ (N‐phenylcarbazolyl)]pyridine cyclometalated ligand, we successfully synthesized a new heteroleptic orange‐emitting phosphorescent iridium(III) complex [Ir( L ₁ )₂(acac)] 1 ( HL ₁ = 5‐trifluoromethyl‐2‐[3‐(N‐phenylcarbazolyl)]pyridine, Hacac = acetylacetone) in good yield. The structural and electronic properties of 1 were examined by X‐ray crystallography and time‐dependent DFT calculations. The influence of CF₃ substituents on the optical, electrochemical and electroluminescence (EL) properties of 1 were studied. We note that incorporation of the carbazolyl unit facilitates the hole‐transporting ability of the complex, and more importantly, attachment of CF₃ group provides an access to a highly efficient electrophosphor for the fabrication of orange phosphorescent organic light‐emitting diodes (OLEDs) with outstanding device performance. These orange OLEDs can produce a maximum current efficiency of ～40 cd A^?1, corresponding to an external quantum efficiency of ～12% ph/el (photons per electron) and a power efficiency of ～24 lm W^?1. Remarkably, high‐performance simple two‐element white OLEDs (WOLEDs) with excellent color stability can be fabricated using an orange triplet‐harvesting emitter 1 in conjunction with a blue singlet‐harvesting emitter. By using such a new system where the host singlet is resonant with the blue fluorophore singlet state and the host triplet is resonant with the orange phosphor triplet level, this white light‐emitting structure can achieve peak EL efficiencies of 26.6 cd A^?1 and 13.5 lm W^?1 that are generally superior to other two‐element all‐fluorophore or all‐phosphor OLED counterparts in terms of both color stability and emission efficiency.     

Stable and Efficient White Electroluminescent Devices Based on a Single Emitting Layer of Polymer Blends

An efficient orange‐light‐emitting polymer (PFTO‐BSeD5) has been developed through the incorporation of low‐bandgap benzoselenadiazole (BSeD) moieties into the backbone of a blue‐light‐emitting polyfluorene copolymer (PFTO poly{[9,9‐bis(4‐(5‐(4‐tert‐butylphenyl)‐[1,3,4]‐oxadiazol‐2‐yl)phenyl)‐9′,9′‐di‐n‐octyl‐[2,2′]‐bifluoren‐7,7′‐diyl]‐stat‐[9,9‐bis(4‐(N,N‐di(4‐n‐butylphenyl)amino)phenyl)‐9′,9′‐di‐n‐octyl‐[2,2′]‐bifluoren‐7,7′‐diyl]}) that contains hole‐transporting triphenylamine and electron‐transporting oxadiazole pendent groups. A polymer light‐emitting device based on this copolymer exhibits a strong, bright‐orange emission with Commission Internationale de L'Eclairage (CIE) color coordinates (0.45,0.52). The maximum brightness is 13 716 cd m^–2 and the maximum luminance efficiency is 5.53 cd A^–1. The use of blends of PFTO‐BSeD5 in PFTO leads to efficient and stable white‐light‐emitting diodes—at a doping concentration of 9 wt %, the device reaches its maximum external quantum efficiency of 1.64 % (4.08 cd A^–1). The emission color remains almost unchanged under different bias conditions: the CIE coordinates are (0.32,0.33) at 11.0 V (2.54 mA cm^–2, 102 cd m^–2) and (0.31,0.33) at 21.0 V (281 mA cm^–2, 7328 cd m^–2). These values are very close to the ideal CIE chromaticity coordinates for a pure white color (0.33,0.33).     

Organic Light‐Emitting Diodes with a White‐Emitting Molecule: Emission Mechanism and Device Characteristics

The unique and unprecedented electroluminescence behavior of the white‐emitting molecule 3‐(1‐(4‐(4‐(2‐(2‐hydroxyphenyl)‐4,5‐diphenyl‐1H‐imidazol‐1‐yl)phenoxy)phenyl)‐4,5‐diphenyl‐1H‐imidazol‐2‐yl)naphthalen‐2‐ol (W1), fluorescence emission from which is controlled by the excited‐state intramolecular proton transfer (ESIPT) is investigated. W1 is composed of covalently linked blue‐ and yellow‐color emitting ESIPT moieties between which energy transfer is entirely frustrated. It is demonstrated that different emission colors (blue, yellow, and white) can be generated from the identical emitter W1 in organic light‐emitting diode (OLED) devices. Charge trapping mechanism is proposed to explain such a unique color‐tuned emission from W1. Finally, the device structure to create a color‐stable, color reproducible, and simple‐structured white organic light‐emitting diode (WOLED) using W1 is investigated. The maximum luminance efficiency, power efficiency, and luminance of the WOLED were 3.10 cd A^?1, 2.20 lm W^?1, 1 092 cd m^?2, respectively. The WOLED shows white‐light emission with the Commission Internationale de l′Eclairage (CIE) chromaticity coordinates (0.343, 0.291) at a current level of 10 mA cm^?2. The emission color is high stability, with a change of the CIE chromaticity coordinates as small as (0.028, 0.028) when the current level is varied from 10 to 100 mA cm^?2.     

Solution‐Processible Multi‐component Cyclometalated Iridium Phosphors for High‐Efficiency Orange‐Emitting OLEDs and Their Potential Use as White Light Sources

The synthesis and photophysical studies of several multifunctional phosphorescent iridium(III) cyclometalated complexes consisting of the hole‐transporting carbazole and fluorene‐based 2‐phenylpyridine moieties are reported. All of them are isolated as thermally and morphological stable amorphous solids. Extension of the π‐conjugation through incorporation of electron‐pushing carbazole units to the fluorene fragment leads to bathochromic shifts in the emission profile, increases the highest occupied molecular orbital levels and improves the charge balance in the resulting complexes because of the propensity of the carbazole unit to facilitate hole transport. These iridium‐based triplet emitters give a strong orange phosphorescence light at room temperature with relatively short lifetimes in the solution phase. The photo‐ and electroluminescence properties of these phosphorescent carbazolylfluorene‐functionalized metalated complexes have been studied in terms of the coordinating position of carbazole to the fluorene unit. Organic light‐emitting diodes (OLEDs) using these complexes as the solution‐processed emissive layers have been fabricated which show very high efficiencies even without the need for the typical hole‐transporting layer. These orange‐emitting devices can produce a maximum current efficiency of ～ 30 cd A^–1 corresponding to an external quantum efficiency of ～ 10 % ph/el (photons per electron) and a power efficiency of ～ 14 lm W^–1. The homoleptic iridium phosphors generally outperform the heteroleptic counterparts in device performance. The potential of exploiting these orange phosphor dyes in the realization of white OLEDs is also discussed.     

A Multifunctional Bipolar Luminogen with Delayed Fluorescence for High‐Performance Monochromatic and Color‐Stable Warm‐White OLEDs

Increasing exciton utilization and reducing exciton annihilation are crucial to achieve high performance of organic light‐emitting diodes (OLEDs), which greatly depend on molecular engineering of emitters and hosts. A novel luminogen (SBF‐BP‐DMAC) is synthesized and characterized. Its crystal and electronic structures, thermal stability, electrochemical behavior, carrier transport, photoluminescence, and electroluminescence are investigated. SBF‐BP‐DMAC exhibits enhanced photoluminescence and promotes delayed fluorescence in solid state and bipolar carrier transport ability, and thus holds multifunctionality of emitter and host for OLEDs. Using SBF‐BP‐DMAC as an emitter, the nondoped OLEDs exhibit maximum electroluminescence (EL) efficiencies of 67.2 cd A^?1, 65.9 lm W^?1, and 20.1%, and the doped OLEDs provide maximum EL efficiencies of 79.1 cd A^?1, 70.7 lm W^?1, and 24.5%. A representative orange phosphor, Ir(tptpy)₂acac, is doped into SBF‐BP‐DMAC for OLED fabrication, giving rise to superior EL efficiencies of 88.0 cd A^?1, 108.0 lm W^?1, and 26.8% for orange phosphorescent OLEDs, and forward‐viewing EL efficiencies of 69.3 cd A^?1, 45.8 lm W^?1, and 21.0% for two‐color hybrid warm‐white OLEDs. All of these OLEDs can retain high EL efficiencies at high luminance, with very small efficiency roll‐offs. The outstanding EL performance demonstrates the great potentials of SBF‐BP‐DMAC in practical display and lighting devices.     

Organic Light‐Emitting Diodes: Organic Light‐Emitting Diodes with a White‐Emitting Molecule: Emission Mechanism and Device Characteristics (Adv. Funct. Mater. 4/2011)

The unique and unprecedented electroluminescence behavior of the white‐emitting molecule 3‐(1‐(4‐(4‐(2‐(2‐hydroxyphenyl)‐4,5‐diphenyl‐1H‐imidazol‐1‐yl)phenoxy)phenyl)‐4,5‐diphenyl‐1H‐imidazol‐2‐yl)naphthalen‐2‐ol (W1), fluorescence emission from which is controlled by the excited‐state intramolecular proton transfer (ESIPT) is investigated. W1 is composed of covalently linked blue‐ and yellow‐color emitting ESIPT moieties between which energy transfer is entirely frustrated. It is demonstrated that different emission colors (blue, yellow, and white) can be generated from the identical emitter W1 in organic light‐emitting diode (OLED) devices. Charge trapping mechanism is proposed to explain such a unique color‐tuned emission from W1. Finally, the device structure to create a color‐stable, color reproducible, and simple‐structured white organic light‐emitting diode (WOLED) using W1 is investigated. The maximum luminance efficiency, power efficiency, and luminance of the WOLED were 3.10 cd A^?1, 2.20 lm W^?1, 1 092 cd m^?2, respectively. The WOLED shows white‐light emission with the Commission Internationale de l′Eclairage (CIE) chromaticity coordinates (0.343, 0.291) at a current level of 10 mA cm^?2. The emission color is high stability, with a change of the CIE chromaticity coordinates as small as (0.028, 0.028) when the current level is varied from 10 to 100 mA cm^?2.     

Manipulation of Charge and Exciton Distribution Based on Blue Aggregation‐Induced Emission Fluorophors: A Novel Concept to Achieve High‐Performance Hybrid White Organic Light‐Emitting Diodes

The aggregation‐induced emission (AIE) phenomenon is important in organic light‐emitting diodes (OLEDs), for it can potentially solve the aggregation‐caused quenching problem. However, the performance of AIE fluorophor‐based OLEDs (AIE OLEDs) is unsatisfactory, particularly for deep‐blue devices (CIEy < 0.15). Here, by enhancing the device engineering, a deep‐blue AIE OLED exhibits low voltage (i.e., 2.75 V at 1 cd m^?2), high luminance (17 721 cd m^?2), high efficiency (4.3 lm W^?1), and low efficiency roll‐off (3.6 lm W^?1 at 1000 cd m^?2), which is the best deep‐blue AIE OLED. Then, blue AIE fluorophors, for the first time, have been demonstrated to achieve high‐performance hybrid white OLEDs (WOLEDs). The two‐color WOLEDs exhibit i) stable colors and the highest efficiency among pure‐white hybrid WOLEDs (32.0 lm W^?1); ii) stable colors, high efficiency, and very low efficiency roll‐off; or iii) unprecedented efficiencies at high luminances (i.e., 70.2 cd A^?1, 43.4 lm W^?1 at 10 000 cd m^?2). Moreover, a three‐color WOLED exhibits wide correlated color temperatures (10 690–2328 K), which is the first hybrid WOLED showing sunlight‐style emission. These findings will open a novel concept that blue AIE fluorophors are promising candidates to develop high‐performance hybrid WOLEDs, which have a bright prospect for the future displays and lightings.     

High‐Purity‐Blue and High‐Efficiency Electroluminescent Devices Based on Anthracene

Novel blue‐light‐emitting materials, 9,10‐bis(1,2‐diphenyl styryl)anthracene (BDSA) and 9,10‐bis(4′‐triphenylsilylphenyl)anthracene (BTSA), which are composed of an anthracene molecule as the main unit and a rigid and bulky 1,2‐diphenylstyryl or triphenylsilylphenyl side unit, have been designed and synthesized. Theoretical calculations on the three‐dimensional structures of BDSA and BTSA show that they have a non‐coplanar structure and inhibited intermolecular interactions, resulting in a high luminescence efficiency and good color purity. By incorporating these new, non‐doped, blue‐light‐emitting materials into a multilayer device structure, it is possible to achieve luminance efficiencies of 1.43 lm W^–1 (3.0 cd A^–1 at 6.6 V) for BDSA and 0.61 lm W^–1 (1.3 cd A^–1 at 6.7 V) for BTSA at 10 mA cm^–2. The electroluminescence spectrum of the indium tin oxide (ITO)/copper phthalocyanine (CuPc)/1,4‐bis[(1‐naphthylphenyl)‐amino]biphenyl (α‐NPD)/BDSA/tris(9‐hydroxyquinolinato)aluminum (Alq₃)/LiF/Al device shows a narrow emission band with a full width at half maximum (FWHM) of 55 nm and a λ_max = 453 nm. The FWHM of the ITO/CuPc/α‐NPD/BTSA/Alq₃/LiF/Al device is 53 nm, with a λ_max = 436 nm. Regarding color, the devices showed highly pure blue emission ((x,y) = (0.15,0.09) for BTSA, (x,y) = (0.14,0.10) for BDSA) at 10 mA cm^–2 in Commission Internationale de l'Eclairage (CIE) chromaticity coordinates.     

Novel Blue Bipolar Thermally Activated Delayed Fluorescence Material as Host Emitter for High‐Efficiency Hybrid Warm‐White OLEDs with Stable High Color‐Rendering Index

A novel thermally activated delayed fluorescence (TADF) molecule, PHCz2BP, is synthesized and used to construct high performance organic light‐emitting diodes (OLEDs) in this work. PHCz2BP is not only the neat emitting layer for efficient sky‐blue OLED, with very high peak external quantum efficiency/power efficiency (EQE/PE) values of 4.0%/6.9 lm W^?1, but also acts as a host to sensitize high‐luminance and high‐efficiency green, orange, and red electrophosphorescence with the universal high EQEs of >20%. More importantly, two hybrid white OLEDs based on the double‐layer emitting system of PHCz2BP:green phosphor/PHCz2BP:red phosphor are achieved. To the best of the knowledge, this is the first report for three‐color (blue–green–red) white devices that adopt a TADF blue host emitter and two phosphorescent dopants without any other additional host. Such simple emitting systems thus realized the best electroluminescent performance to date for the WOLEDs utilizing the hybrid TADF/phosphor strategy: forward‐viewing EQEs of 25.1/23.6% and PEs of 24.1/22.5 lm W^?1 at the luminance of 1000 cd m^?2 with the color rendering indexes of 85/87 and warm‐white Commission Internationale de L'Eclairage coordinates of (0.41, 0.46)/(0.42, 0.45), indicating its potential to be used as practical eye‐friendly solid‐state lighting in future.     

Improved Performance of White Phosphorescent Organic Light‐Emitting Diodes through a Mixed‐Host Structure

Highly efficient white phosphorescent organic light‐emitting diodes with a mixed‐host structure are developed and the device characteristics are studied. The introduction of a hole‐transport‐type host (N, N’‐dicarbazolyl‐3‐3‐benzen (mCP)) into an electron‐transport‐type host (m‐bis‐(triphenylsilyl)benzene (UGH3)) as a mixed‐host emissive layer effectively achieves higher current density and lower driving voltage. The peak external quantum and power efficiency with the mixed‐host structure improve up to 18.9% and 40.9 lm/W, respectively. Moreover, this mixed‐host structure device shows over 30% enhanced performance compared with a single‐host structure device at a luminance of 10,000 cd/m² without any change in the electroluminescence spectra.     

Electrophosphorescent Polyfluorenes Containing Osmium Complexes in the Conjugated Backbone

Electrophosphorescent copolymers have been synthesized by covalent bonding of a red‐emitting osmium complex Os(bpftz), which contains two 3‐trifluoromethyl‐5‐(4‐tert‐butyl‐2‐pyridyl)triazolate (bpftz) cyclometalated ligands, into the backbone of a bipolar polyfluorene (PF) copolymer. Employing these copolymers, a highly efficient red polymer light‐emitting diode has been realised that has an external quantum efficiency of 18.0%, a maximum brightness of 38 000 cd m^?2, and an emission centered at 618 nm. In addition, after incorporating appropriate amounts of green‐emitting benzothiadiazole (BT) and the aforementioned Os(bpftz) into the bipolar PF, an efficient white‐light electroluminescent polymer is obtained that displays simultaneous blue, green, and red emissions.     

Efficient White‐Electrophosphorescent Devices Based on a Single Polyfluorene Copolymer

An efficient white‐light‐emitting polymer ( W3 ) is realized by covalently attaching a green fluorophore and a red phosphor into the backbone and the side chains, respectively, of polyfluorene at a concentration of 0.04 mol %. In addition, charge‐transporting pendant units are included to improve carrier injection and transport. White‐electrophosphorescent devices with the structure ITO/PEDOT:PSS/ W3 /CsF/Al (ITO: indium tin oxide; PEDOT:PSS: poly(styrenesulfonate)‐doped poly(3,4‐ethylenedioxythiophene)) exhibit a low turn‐on voltage of 2.8 V and a luminance of ca. 10³ cd m^–2 at below 6 V. The peak luminance and power‐conversion efficiencies are 8.2 cd A^–1 and 7.2 lm W^–1, respectively. Furthermore, the device shows relatively stable white emission: the Commission Internationale d'Éclairage (CIE) chromaticity coordinates of the devices change only slightly from (0.35,0.38) at 10 mA cm^–2 to (0.33,0.36) at 100 mA cm^–2, with an almost constant color render index (CRI) value of 82 at all measured current densities.     

High‐Performance Hybrid White Organic Light‐Emitting Diodes with Superior Efficiency/Color Rendering Index/Color Stability and Low Efficiency Roll‐Off Based on a Blue Thermally Activated Delayed Fluorescent Emitter

Thermally activated delayed fluorescence (TADF)‐based white organic light‐emitting diodes (WOLEDs) are highly attractive because the TADF emitters provide a promising alternative route to harvest triplet excitons. One of the major challenges is to achieve superior efficiency/color rendering index/color stability and low efficiency roll‐off simultaneously. In this paper, high‐performance hybrid WOLEDs are demonstrated by employing an efficient blue TADF emitter combined with red and green phosphorescent emitters. The resulting WOLED shows the maximum external quantum efficiency, current efficiency, and power efficiency of 23.0%, 51.0 cd A^?1, and 51.7 lm W^?1, respectively. Moreover, the device exhibits extremely stable electroluminescence spectra with a high color rendering index of 89 and Commission Internationale de L'Eclairage coordinates of (0.438, 0.438) at the practical brightness of 1000 cd m^?2. The achievement of these excellent performances is systematically investigated by versatile experimental and theoretical evidences, from which it is concluded that the utilization of a blue‐green‐red cascade energy transfer structure and the precise manipulation of charges and excitons are the key points. It can be anticipated that this work might be a starting point for further research towards high‐performance hybrid WOLEDs.