Achieving Highly Efficient Fluorescent Blue Organic Light‐Emitting Diodes Through Optimizing Molecular Structures and Device Configuration

Based on the results of first‐principles calculations of the electronic properties of blue light‐emitting materials, the molecular structures of oligofluorenes are optimized by incorporating electron‐withdrawing groups into the molecules to balance hole and electron injection and transport for organic light‐emitting diodes (OLEDs). The result is a remarkable improvement in the maximum external quantum efficiency (EQE) of the undoped device from 2.0% to 4.99%. Further optimization of the device configurations and processing procedures, e.g., by changing the thickness of the emitting layer and through thermal annealing treatments, leads to a very high maximum EQE of 7.40% for the undoped sky‐blue device. Finally, by doping the emitter in a suitable host material, 4,4’‐bis(carbazol‐9‐yl)biphenyl (CBP), at the optimal concentration of 6%, pure blue emission with extremely high maximum EQE of 9.40% and Commission Internationale de l’Eclairage (CIE) coordinates of (0.147, 0.139) is achieved.     

Organic Light‐Emitting Diodes with 30% External Quantum Efficiency Based on a Horizontally Oriented Emitter

High‐efficiency phosphorescent organic light‐emitting diodes (OLEDs) doped with Ir(ppy)₂(acac) [bis(2‐phenylpyridine)iridium(III)‐acetylacetonate] in an exciplex forming co‐host have been optically analyzed. This emitter has a preferred orientation with the horizontal to vertical dipole ratio of 0.77:0.23 as compared to 0.67:0.33 in the isotropic case. Theoretical analysis based on the orientation factor (Θ, the ratio of the horizontal dipoles to total dipoles) and the photoluminescence quantum yield (q_PL) of the emitter predicts that the maximum external quantum efficiency (EQE) of the OLEDs with this emitter is about 30%, which matches very well with the experimental data, indicating that the electrical loss of the OLEDs is negligible and the device structure can be utilized as a platform to demonstrate the validity of optical modeling. Based on the results, the maximum EQE achievable for a certain emitting dye in a host can be predicted by just measuring q_PL and Θ in a neat film on glass without the need to fabricate devices, which offers a universal plot of the maximum EQE as a function of q_PL and Θ.     

Extremely Efficient White Organic Light‐Emitting Diodes for General Lighting

Highly power‐efficient white organic light‐emitting diodes (OLEDs) are still challenging to make for applications in high‐quality displays and general lighting due to optical confinement and energy loss during electron‐photon conversion. Here, an efficient white OLED structure is shown that combines deterministic aperiodic nanostructures for broadband quasi‐omnidirectional light extraction and a multilayer energy cascade structure for energy‐efficient photon generation. The external quantum efficiency and power efficiency are raised to 54.6% and 123.4 lm W^?1 at 1000 cd m^?2. An extremely small roll‐off in efficiency at high luminance is also obtained, yielding a striking value of 106.5 lm W^?1 at 5000 cd m^?2. In addition to a substantial increase in efficiency, this device structure simultaneously offers the superiority of angular color stability over the visible wavelength range compared to conventional OLEDs. It is anticipated that these findings could open up new opportunities to promote white OLEDs for commercial applications.     

Engineering Efficiency Roll‐Off in Organic Light‐Emitting Devices

Previous studies have identified triplet‐triplet annihilation and triplet‐polaron quenching as the exciton density‐dependent mechanisms which give rise to the efficiency roll‐off observed in phosphorescent organic light‐emitting devices (OLEDs). In this work, these quenching processes are independently probed, and the impact of the exciton recombination zone width on the severity of quenching in various OLED architectures is examined directly. It is found that in devices employing a graded‐emissive layer (G‐EML) architecture the efficiency roll‐off is due to both triplet‐triplet annihilation and triplet‐polaron quenching, while in devices which employ a conventional double‐emissive layer (D‐EML) architecture, the roll‐off is dominated by triplet‐triplet annihilation. Overall, the efficiency roll‐off in G‐EML devices is found to be much less severe than in the D‐EML device. This result is well accounted for by the larger exciton recombination zone measured in G‐EML devices, which serves to reduce exciton density‐driven loss pathways at high excitation levels. Indeed, a predictive model of the device efficiency based on the quantitatively measured quenching parameters shows the role a large exciton recombination zone plays in mitigating the roll‐off.     

High‐Efficiency Deep‐Blue Light‐Emitting Diodes Based on Phenylquinoline/Carbazole‐Based Compounds

Highly efficient deep‐blue fluorescent materials based on phenylquinoline–carbazole derivatives (PhQ‐CVz, MeO‐PhQ‐CVz, and CN‐PhQ‐CVz) are synthesized for organic light‐emitting diodes (OLEDs). The materials form high‐quality amorphous thin films by thermal evaporation and the energy levels can be easily adjusted by the introduction of different electron‐donating and electron‐withdrawing groups on carbazoylphenylquinoline. Non‐doped deep‐blue OLEDs that use PhQ‐CVz as the emitter show bright emission (Commission Internationale de L'Éclairage (CIE) coordinates, x = 0.156, y = 0.093) with an external quantum efficiency of 2.45%. Furthermore, the material works as an excellent host material for 4,4′‐bis(9‐ethyl‐3‐carbazovinylene)‐1,1′‐biphenyl dopant to get high‐performance OLEDs with excellent deep‐blue CIE coordinates (x = 0.155, y = 0.157), high power efficiency (5.98 lm W^?1), and high external quantum efficiency (5.22%).     

Study of Configuration Differentia and Highly Efficient,Deep‐Blue,Organic Light‐Emitting Diodes Based on Novel Naphtho[1,2‐d]imidazole Derivatives

Two novel naphtho[1,2‐d]imidazole derivatives are developed as deep‐blue, light‐emitting materials for organic light‐emitting diodes (OLEDs). The 1H‐naphtho[1,2‐d]imidazole based compounds exhibit a significantly superior performance than the 3H‐naphtho[1,2‐d]imidazole analogues in the single‐layer devices. This is because they have a much higher capacity for direct electron‐injection from the cathode compared to their isomeric counterparts resulting in a ground‐breaking EQE (external quantum efficiency) of 4.37% and a low turn‐on voltage of 2.7 V, and this is hitherto the best performance for a non‐doped single‐layer fluorescent OLED. Multi‐layer devices consisting of both hole‐ and electron‐transporting layers, result in identically excellent performances with EQE values of 4.12–6.08% and deep‐blue light emission (Commission Internationale de l'Eclairage (CIE) y values of 0.077–0.115) is obtained for both isomers due to the improved carrier injection and confinement within the emissive layer. In addition, they showed a significantly better blue‐color purity than analogous molecules based on benzimidazole or phenanthro[9,10‐d]imidazole segments.     

Over 100 cd A−1 Efficient Quantum Dot Light‐Emitting Diodes with Inverted Tandem Structure

Quantum dot light‐emitting diodes (QLEDs) with tandem structure are promising candidates for future displays because of their advantages of pure emission color, long lifetime, high brightness, and high efficiency. To obtain efficient QLEDs, a solution‐processable interconnecting layer (ICL) based on poly(3, 4‐ethylenedioxythiophene)/polystyrene sulfonate/ZnMgO is developed. With the proposed ICL, all‐solution‐processed, inverted, tandem QLEDs are demonstrated with high current efficiency (CE) of 57.06 cd A^?1 and external quantum efficiency (EQE) of 13.65%. By further optimizing the fabrication processes and using a hybrid deposition technique, the resultant tandem QLEDs exhibit a very high CE over 100 cd A^?1 and an impressive EQE over 23%, which are the highest values ever reported and are comparable with those of the state‐of‐the‐art phosphorescent organic LEDs. Moreover, the efficiency roll‐off, a notorious phenomenon in phosphorescent LEDs, is significantly reduced in the developed QLEDs. For example, even at a very high brightness over 200 000 cd m^?2, the tandem QLEDs can still maintain a high CE of 96.47 cd A^?1 and an EQE of 22.62%. The proposed ICL and the developed fabrication methods allow for realization of very efficient tandem QLEDs for next generation display and lighting applications.     

Exciplex‐Forming Co‐host for Organic Light‐Emitting Diodes with Ultimate Efficiency

Phosphorescent organic light‐emitting diodes (OLEDs) with ultimate efficiency in terms of the external quantum efficiency (EQE), driving voltage, and efficiency roll‐off are reported, making use of an exciplex‐forming co‐host. This exciplex‐forming co‐host system enables efficient singlet and triplet energy transfers from the host exciplex to the phosphorescent dopant because the singlet and triplet energies of the exciplex are almost identical. In addition, the system has low probability of direct trapping of charges at the dopant molecules and no charge‐injection barrier from the charge‐transport layers to the emitting layer. By combining all these factors, the OLEDs achieve a low turn‐on voltage of 2.4 V, a very high EQE of 29.1% and a very high power efficiency of 124 lm W^?1. In addition, the OLEDs achieve an extremely low efficiency roll‐off. The EQE of the optimized OLED is maintained at more than 27.8%, up to 10 000 cd m^?2.     

Blue Single‐Layer Organic Light‐Emitting Diodes Using Fluorescent Materials: A Molecular Design View Point

Since the beginning of organic light‐emitting diodes (OLEDs), blue emission has attracted the most attention and many research groups worldwide have worked on the design of materials for stable and highly efficient blue OLEDs. However, almost all the high‐efficiency blue OLEDs using fluorescent materials are multilayer devices, which are constituted of a stack of organic layers to improve the injection, transport, and recombination of charges within the emissive layer. Although the technology has been mastered, it suffers from real complexity and high cost and is time‐consuming. Simplifying the multilayer structure with a single‐layer one, the simplest devices made only of electrodes and the emissive layer have appeared as an appealing strategy for this technology. However, removing the functional organic layers of an OLED stack leads to a dramatic decrease of the performance and achieving high‐efficiency blue single‐layer OLEDs requires intense research especially in terms of materials design. Herein, an exhaustive review of blue emitting fluorophores that have been incorporated in single‐layer OLEDs is reported, and the links between their electronic properties and the device performance are discussed. Thus, a structure/properties/device performance relationship map is drawn, which is of interest for the future design of organic materials.     

Recent Progress in High‐Efficiency Blue‐Light‐Emitting Materials for Organic Light‐Emitting Diodes

Organic light‐emitting diodes (OLEDs) are increasingly used in displays replacing traditional flat panel displays; e.g., liquid crystal displays. Especially, the paradigm shifts in displays from rigid to flexible types accelerated the market change from liquid crystal displays to OLEDs. However, some critical issues must be resolved for expansion of OLED use, of which blue device performance is one of the most important. Therefore, recent OLED material development has focused on the design, synthesis and application of high‐efficiency and long‐life blue emitters. Well‐known blue fluorescent emitters have been modified to improve their efficiency and lifetime, and blue phosphorescent emitters are being investigated to overcome the lifetime issue. Recently, thermally activated delayed fluorescent emitters have received attention due to the potential of high‐efficiency and long‐living emitters. Therefore, it is timely to review the recent progress and future prospects of high‐efficiency blue emitters. In this feature article, we summarize recent developments in blue fluorescent, phosphorescent and thermally activated delayed fluorescent emitters, and suggest key issues for each emitter and future development strategies.     

A Pyridine‐Containing Anthracene Derivative with High Electron and Hole Mobilities for Highly Efficient and Stable Fluorescent Organic Light‐Emitting Diodes

A pyridine‐containing anthracene derivative, 9,10‐bis(3‐(pyridin‐3‐yl)phenyl)anthracene (DPyPA), which comprehensively outperforms the widely used electron‐transport material (ETM), tris(8‐quinolinolato) aluminum (Alq₃), is synthesized. DPyPA exhibits ambipolar transport properties, with both electron and hole mobilities of around 10^?3 cm^?2 V^?1 s^?1; about two orders of magnitude higher than that of Alq₃. The nitrogen atom in the pyridine ring of DPyPA coordinates to lithium cations, which leads to efficient electron injection when LiF/Al is used as the cathode. Electrochemical measurements demonstrate that both the cations and anions of DPyPA are stable, which may improve the stability of devices based on DPyPA. Red‐emitting, green‐emitting, and blue‐emitting fluorescent organic light emitting diodes with DPyPA as the ETM display lower turn‐on voltages, higher efficiencies, and stronger luminance than the devices with Alq₃ as the ETM. The power efficiencies of the devices based on DPyPA are greater by 80–140% relative to those of the Alq₃‐based devices. The improved performance of these devices is attributed to the increased carrier balance. In addition, the device employing DPyPA as the ETM possesses excellent stability: the half‐life of the DPyPA‐based device is 67 000 h—seven times longer than that of the Alq₃‐based device—for an initial luminance of 5000 cd m^?2.     

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.     

Highly Efficient Non‐Doped Blue Organic Light‐Emitting Diodes Based on Fluorene Derivatives with High Thermal Stability

A new series of blue‐light‐emitting fluorene derivatives have been synthesized and characterized. The fluorene derivatives have high fluorescence yields, good thermal stability, and high glass‐transition temperatures in the range 145–193 °C. Organic light‐emitting diodes (OLEDs) fabricated using the fluorene derivatives as the host emitter show high efficiency (up to 5.3 cd A^–1 and 3.0 lm W^–1) and bright blue‐light emission (Commission Internationale de L'Eclairage (CIE) coordinates of x = 0.16, y = 0.22). The performance of the non‐doped fluorene‐based devices is among the best fluorescent blue‐light‐emitting OLEDs. The good performance of the present blue OLEDs is considered to derive from: 1) appropriate energy levels of the fluorene derivatives for good carrier injection; 2) good carrier‐transporting properties; and 3) high fluorescence efficiency of the fluorene derivatives. These merits are discussed in terms of the molecular structures.     

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.     

Built‐In Haze Glass‐Fabric Reinforced Siloxane Hybrid Film for Efficient Organic Light‐Emitting Diodes (OLEDs)

Substrates with high transmittance and high haze are desired for increasing the light outcoupling efficiency of organic light‐emitting diodes (OLEDs). However, most of the polymer films used as substrate have high transmittance and low haze. Herein, a facile route to fabricate a built‐in haze glass‐fabric reinforced siloxane hybrid (GFRH) film having high total transmittance (≈89%) and high haze (≈89%) is reported using the scattering effect induced by refractive index contrast between the glass fabric and the siloxane hybrid (hybrimer). The hybrimer exhibiting large refractive index contrast with the glass fabric is synthesized by removing the phenyl substituents. Besides its optical properties, the hazy GFRH films exhibit smooth surface (R_sq = 0.2 nm), low thermal expansion (13 ppm °C⁻¹), high chemical stability, and dimensional stability. Owing to the outstanding properties of the GFRH film, OLED is successfully fabricated onto the film exhibiting 74% external quantum efficiency enhancement. The hazy GFRH's unique optical properties, excellent thermal stability, outstanding dimensional stability, and the ability to perform as a transparent electrode enable them as a wide ranging substrate for the flexible optoelectronic devices.     

Triplet Harvesting in Hybrid White Organic Light‐Emitting Diodes

White organic light‐emitting diodes (OLEDs) are highly efficient large‐area light sources that may play an important role in solving the global energy crisis, while also opening novel design possibilities in general lighting applications. Usually, highly efficient white OLEDs are designed by combining three phosphorescent emitters for the colors blue, green, and red. However, this procedure is not ideal as it is difficult to find sufficiently stable blue phosphorescent emitters. Here, a novel approach to meet the demanding power efficiency and device stability requirements is discussed: a triplet harvesting concept for hybrid white OLED, which combines a blue fluorophor with red and green phosphors and is capable of reaching an internal quantum efficiency of 100% if a suitable blue emitter with high‐lying triplet transition is used is introduced. Additionally, this concept paves the way towards an extremely simple white OLED design, using only a single emitter layer.     

Over 30% External Quantum Efficiency Light‐Emitting Diodes by Engineering Quantum Dot‐Assisted Energy Level Match for Hole Transport Layer

In the study of hybrid quantum dot light‐emitting diodes (QLEDs), even for state‐of‐the‐art achievement, there still exists a long‐standing charge balance problem, i.e., sufficient electron injection versus inefficient hole injection due to the large valence band offset of quantum dots (QDs) with respect to the adjacent carrier transport layer. Here the dedicated design and synthesis of high luminescence Zn_1?x Cd_xSe/ZnSe/ZnS QDs is reported by precisely controlled shell growth, which have matched energy level with the adjacent hole transport layer in QLEDs. As emitters, such Zn_1?xCd_xSe‐ based QLEDs exhibit peak external quantum efficiencies (EQE) of up to 30.9%, maximum brightness of over 334 000 cd m^?2, very low efficiency roll‐off at high current density (EQE ≈25% @ current density of 150 mA cm^?2), and operational lifetime extended to ≈1 800 000 h at 100 cd m^?2. These extraordinary performances make this work the best among all solution‐processed QLEDs reported in literature so far by achieving simultaneously high luminescence and balanced charge injection. These major advances are attributed to the combination of an intermediate ZnSe layer with an ultrathin ZnS outer layer as the shell materials and surface modification with 2‐ethylhexane‐1‐thiol, which can dramatically improve hole injection efficiency and thus lead to more balanced charge injection.