Efficient Red Electroluminescence Research Articles

Circularly polarized thermally activated delayed fluorescence OLEDs with nearly BT.2020 red emission

Thermally activated delayed fluorescence (TADF) materials with saturated red emission anda large x CIE coordinate hold significant potential for application in high-efficiency, high-contrast and wide-gamut display techniques. Recently, circularly polarized organic light-emitting diodes (CP-OLEDs) have attracted increasing attention for the enhancement of electroluminescence (EL) performance via minimizing the light loss caused by the integrated polarizer. However, the efficient red CP-OLEDs have been rarely reported by now. In this work, a simple approach to CP-OLEDs towards BT.2020 red emission was applied via the integration of polycyclic aromatic chromophore and chiral octahydrobinaphthol, which consequently resulted in the intense circularly polarized luminescence (CPL), long-wavelength emission and excellent photoluminescence quantum yields (PLQY). The CP-OLEDs based on (R)-OBN-3CN demonstrated the state-of-the-art EL performance with the maximum external quantum efficiencies (EQEmax) of 19.7 % at 646 nm and the dissymmetry factor (gEL) of −1.1 × 10−3. Furthermore, CP-OLEDs doped with 12 wt% (R/S)-OBN-3CN exhibited efficient red electroluminescence with the CIE coordinates of (0.70, 0.30), which was very close to the saturated red emission specified in BT.2020.

Efficient Red Electroluminescence From Phenanthro[9,10-d]imidazole-Naphtho[2,3-c][1,2,5]thiadiazole Donor-Acceptor Derivatives.

Red emission is one of the three primary colors and is indispensable for full color displays. Fluorescent materials that can generate efficient red electroluminescence (EL) are limited and need to be developed. In this work, we report efficient red emitters based on phenanthro[9,10-d]imidazole-naphtho[2,3-c][1,2,5]thiadiazole donor-acceptor derivatives. The molecules, abbreviated as PINzP and PINzPCN, exhibited high photoluminescence quantum yield (PLQY) up to unity in doped films. They can also reach a relatively high PLQY of ∼30% in neat films. PINzP and PINzPCN were capable of generating efficient red EL in doped devices with a maximum external quantum efficiency (EQE) of 6.96% and 5.92%, respectively.

Rational design of perfectly oriented thermally activated delayed fluorescence emitter for efficient red electroluminescence

How to control the dipole orientation of organic emitters is a challenge in the field of organic light-emitting diodes (OLEDs). Herein, a linear thermally activated delayed fluorescence (TADF) molecule, PhNAI-PMSBA, bearing a 1,8-naphthalimide-acridine framework was designed by a double-site long-axis extension strategy to actively control the dipole orientation. The horizontal ratio of emitting dipole orientation of PhNAI-PMSBA reaches 95%, substantially higher than that of isotropic emitters (67%). This unique feature is associated with the intrinsically horizontal molecular orientation of PhNAI-PMSBA and the good agreement between its transition dipole moment direction and molecular long axis. The PhNAI-PMSBA-based OLED achieves an ultrahigh optical outcoupling efficiency of 43.2% and thus affords one of the highest red electroluminescence with an external quantum efficiency of 22.3% and the Commission International de l’Eclairage 1931 coordinates at around (0.60, 0.40).

Optimizing carrier balance of a red quantum-dot light-emitting electrochemical cell with a carrier injection layer of cationic Ir(III) complex

Abstract Solid-state light-emitting electrochemical cells (LECs) with promising features of solution processability, low-voltage operation and compatibility with inert cathode metals have shown great potential in display and lighting applications in recent years. Among the reported emissive materials for LECs, ionic transition metal complexes (iTMCs) have relatively higher electroluminescence (EL) efficiencies due to their phosphorescent property. However, the red iTMCs generally exhibit moderate color saturation and low emission efficiency, limiting their display applications. To improve color saturation and device efficiency of red LECs, efficient quantum dots (QDs) with narrow emission bandwidth are good alternative emissive materials. In this work, efficient and saturated red QD LECs employing iTMC carrier injection layers to provide in situ electrochemical doping are demonstrated. The thicknesses of iTMC and red-QD layers are systematically adjusted to achieve the best carrier balance. In the optimized device, the iTMC carrier injection layer facilitates hole injection into the red-QD layer while electrons are injected from the cathode into the red-QD layer directly since the electron injection barrier is low. The Commission Internationale de I'Eclairage (CIE) coordinates of the EL spectra approach the red standard point of National Television System Committee (NTSC). High external quantum efficiency and current efficiency reaching 9.7% and 16.1 cd A−1, respectively. These results confirm superior carrier balance in such a simple iTMC/QD bilayer device structure. Furthermore, compared with iTMC LECs, less degree of device efficiency roll-off upon increasing device current is observed in QD LECs since a shorter excited-state lifetime of fluorescent QDs reduces the probability of collision exciton quenching. Saturated and efficient red EL with mitigated efficiency roll-off from red-QD LECs employing iTMC carrier injection layers confirms that they are good candidates of saturated light sources for displays.

Anthraquinone-based intramolecular charge-transfer compounds: computational molecular design, thermally activated delayed fluorescence, and highly efficient red electroluminescence.

Red fluorescent molecules suffer from large, non-radiative internal conversion rates (k(IC)) governed by the energy gap law. To design efficient red thermally activated delayed fluorescence (TADF) emitters for organic light-emitting diodes (OLEDs), a large fluorescence rate (k(F)) as well as a small energy difference between the lowest singlet and triplet excited states (ΔE(ST)) is necessary. Herein, we demonstrated that increasing the distance between donor (D) and acceptor (A) in intramolecular-charge-transfer molecules is a promising strategy for simultaneously achieving small ΔE(ST) and large k(F). Four D-Ph-A-Ph-D-type molecules with an anthraquinone acceptor, phenyl (Ph) bridge, and various donors were designed, synthesized, and compared with corresponding D-A-D-type molecules. Yellow to red TADF was observed from all of them. The k(F) and ΔE(ST) values determined from the measurements of quantum yield and lifetime of the fluorescence and TADF components are in good agreement with those predicted by corrected time-dependent density functional theory and are approximatively proportional to the square of the cosine of the theoretical twisting angles between each subunit. However, the introduction of a Ph-bridge was found to enhance k(F) without increasing ΔE(ST). Molecular simulation revealed a twisting and stretching motion of the N-C bond in the D-A-type molecules, which is thought to lower ΔE(ST) and k(F) but raise k(IC), that was experimentally confirmed in both solution and doped film. OLEDs containing D-Ph-A-Ph-D-type molecules with diphenylamine and bis(4-biphenyl)amine donors demonstrated maximum external quantum efficiencies of 12.5% and 9.0% with emission peaks at 624 and 637 nm, respectively.

Enhancement of red organic light-emitting diodes via cascade energy transfer

We have demonstrated that efficient red electroluminescence is obtained via cascade energy transfer from Alq to fluorescent dye Coumarin(C545) and then from C545 to 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB). The cell structure was indium tin oxide (ITO)/ N,N′-bis-(1-naphenyl)-N,N′-biphenyl-1,1′-bipheny1-4-4′-diamine (NPB)/ tris (8-hydroxyquinoline) aluminum (Alq): C545: DCJTB/Alq/LiF/Al. An additional dopant, C545, was used to assist the energy transfer from Alq to the red dopant. Compared with the devices where the emitting layer is only composed of Alq and DCJTB, the emission efficiency and color purity were improved. We attribute these improvements to the assistant dopant C545 which leads to the more efficient energy transfer from Alq to DCJTB. The co-doping system is a promising method for red organic light-emitting diodes.

Highly efficient red electroluminescence induced by efficient electron injection and exciton confinement

Highly efficient DCJTB-doped device was realized by enhanced electron injection and exciton confinement. A fluorine end-capped linear phenylene/oxadiazole oligomer 2,5-bis(4-fluorobiphenyl-4′-yl)-1,3,4-oxadiazole ( 1) and a trifluoromethyl end-capped oligomer 2,5-bis(4-trifluoromethylbiphenyl-4′-yl)-1,3,4-oxadiazole ( 2) were designed and incorporated as an electron transporting/hole blocking material in the device structure ITO/NPB (60 nm)/DCJTB:Alq 3 (0.5%, 10 nm)/ 1 or 2 (20 nm)/Alq 3 (30 nm)/LiF (1 nm)/Al (100 nm). The devices showed highly efficient red luminescence. In particular, the device based on 1 achieved pure red luminescence at 620 nm originating from DCJTB, with a narrow FWHI of 65 nm, maximal brightness of 13,300 cd/m 2 at voltage of 20.8 V and current density of ca. 355 mA/cm 2. High current and power efficiencies (>3.6 cd/A, 1.0 lm/W) were retained within a wide range of current densities. Our results show efficient and stable DCJTB-doped red electroluminescence could be anticipated for practical applications by taking advantage of the present approaches. The control experiments using BCP were also studied.

Highly efficient red electroluminescence from stacked organic light-emitting devices based on a europium complex

Stacked organic light-emitting devices (OLEDs) based on a europium complex Eu(TTA) 3(Tmphen) (TTA = thenoyltrifluoroacetone,Tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline) were fabricated. In this stacked OLEDs, Li:BCP/V 2O 5 was used the intermediate charge generation layer sandwiched between two identical emissive units consisting of TPD/CBP:DCJTB:Eu(TTA) 3(Tmphen)/BCP. As expected, the brightness and electroluminescent (EL) current efficiency were approximately enhanced by double times that of conventional single-unit devices. The stacked OLEDs showed the maximum luminance up to 3000 cd/m 2 at a current density of 190 mA/cm 2 and a current efficiency of 14.5 cd/A at a current density of 0.08 mA/cm 2. At the brightness of 100 cd/m 2, the current efficiency reached 10 cd/A at a current density of 1.6 mA/cm 2.

High-efficiency red electrophosphorescence based on neutral bis(pyrrole)-diimine platinum(ii) complex

Efficient red electroluminescence from the excimer or oligomer of neutral phosphorescent bis(pyrrole)-diimine Pt(II) complex has been achieved with maximum external quantum efficiency, luminous efficiency, power efficiency and brightness of 6.5%, 9.0 cd A(-1), 4.0 lm W(-1) and 11 100 cd m(-2), respectively.

Efficient red electroluminescence from organic devices using dye-doped rare earth complexes

Using rare earth complexes with electron-transporting properties as host materials, and 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) as a dopant, bilayer doped electroluminescent (EL) devices with efficient red light emission were fabricated. When a europium complex was adopted, the EL spectrum consisted of emissions from DCJTB and Eu3+ ions. At optimal dopant concentration, an EL efficiency of 5.7 cd/A at 0.04 mA/cm2 was observed. Although the EL efficiency decreased with an increase in current density, it remained higher than 2.0 cd/A with brightness of 347.0 cd/m2 at 5.7 V bias. DCJTB and Eu3+ ions collected the energy of singlet and triplet excitons, respectively, and then gave rise to pure red color emission, suggesting a promising way by which to utilize both singlet and triplet excited states in EL devices.

Efficient red electroluminescence from devices having multilayers of a europium complex

In order to get red electroluminescence from a europium (Eu) complex with high efficiency, a hole-injection layer was inserted between the Eu-complex layer and an indium–tin–oxide electrode, and a hole-blocking layer was inserted between the Eu-complex and electron-transporting layers. To further improve the efficiency, devices having multiple-stacked Eu-complex (2.5 nm)/hole blocking (2.5 nm) units were fabricated. By stacking six units, the maximal luminance and emission efficiency of the red emission were increased to more than twice that from a device with a single Eu-complex layer.

Surface-emitting polymer light-emitting diodes

We report the fabrication of polymer electroluminescent devices on doped silicon substrates. An electron-injecting aluminium electrode is thermally evaporated onto the silicon substrate, and a layer of a high electron affinity conjugated polymer, poly(cyanoterephthalylidene), CN-PPV, is then spin-coated onto this. A hole-transporting layer of poly( p-phenylene vinylene), PPV, is formed on top of the CN-PPV layer by thermal conversion of a spin-coated layer of the sulphonium precursor polymer. Finally, a transparent hole-injecting top electrode of indium-tin oxide is formed by RF sputtering. These devices show efficient red electroluminescence, with emission from the CN-PPV layer.

Conjugated polymer light-emitting diodes on silicon substrates

We report the fabrication of polymer electroluminescent devices on doped silicon substrates. An electron-injecting aluminum electrode is thermally evaporated onto the silicon substrate, and a layer of a high electron affinity conjugated polymer, poly(cyanoterephthalylidene), CN-PPV, is then spin coated onto this. A hole-transporting layer of poly(p-phenylene vinylene), PPV, is formed on top of the CN-PPV layer by thermal conversion of a spin-coated layer of the sulphonium precursor polymer. Finally, a transparent hole-injecting top electrode of indium-tin oxide is formed by rf sputtering. These devices show efficient red electroluminescence, with emission from the CN-PPV layer.