Single emitting layer white organic light-emitting device with high color stability to applied voltage

A new type of white organic light-emitting device has been fabricated incorporating a single light-emitting layer of bis(2-methyl-8-quinolinolato) (para-phenylphenolato) aluminum (III) (BAlq) doped with 2,5,8,11-tetra-tert-butylperylene (TBPe) and 5,6,11,12-tetraphenyl-naphthacene (rubrene). The configuration of the device was ITO/PVK:TPD/BAlq:TBPe:rubrene/Alq₃/Mg:Ag. By adjusting the proportion of the dopants (TBPe and rubrene) in the light-emitting layer, white light with Commission Internationale de l’Eclairage (CIE) coordinates of (0.34, 0.39) was obtained at an applied voltage of 8 V; the change of emission spectra was minimal when the voltage increased from 5 to 20 V. The device exhibited a maximum external quantum efficiency of 0.68% and a brightness of 1020 cd/m² at 8 V, the brightness increasing to 5723 cd/m² at 17 V.     

Electroluminescence in organic single-layer light-emitting diodes at high fields

By considering the interaction between Fowler-Nordheim tunneling injection theory and charge carriers transporting through the bulk, an electroluminescence model for organic single-layer diodes is presented. The expressions of the recombination current density, recombination efficiency and conductivity of the diodes are provided, which elucidate the controlling role of the electric field on mobility and recombination zone. The equilibrium of two opposite charge carriers injection and the cen-tral position of recombination zone are two important preconditions for reducing the leakage current. Space-charge-limited current occurs only over a certain high bias, meanwhile, the quantity of injection carriers increases over the transport capacity of the bulk.     

Efficient organic light-emitting diodes with C60 buffer layer

Organic light-emitting diodes (OLEDs) with C₆₀ buffer layer were fabricated. The effect of C₆₀ buffer layer on the performance of the devices was investigated by inserting C₆₀ buffer layer at the interface between the electrode and organic layers. The device structures were (1) ITO/C₆₀ (0.0, 0.4, 0.7 and 1.0 nm)/NPB/Alq₃/LiF/Al and (2) ITO/NPB/Alq₃/C₆₀ (0.0, 0.4, 0.7 and 1.0 nm)/LiF/Al. The highest brightness and efficiency of the device (1) with 0.7 nm-thick C₆₀ layer reached 6439 cd/m² at 16 V and 1.80 cd/A at 6.4 V, respectively. The enhancements in brightness and efficiency are attributed to an improved balance of hole and electron injections due to C₆₀ layer blocking parts of the injected holes. On the contrary, the brightness and efficiency of the devices with the structure (2) had been hardly enhanced.     

Novel carbazole/anthracene hybrids for efficient blue organic light-emitting diodes

Two novel carbazole/anthracene hybrided molecules, namely 2-(anthracen-9-yl)-9-ethyl-9H-carbazole (AnCz) and 2,7-di(anthracen-9-yl)-9-ethyl-9H-carbazole (2AnCz), were designed and synthesized via palladium catalyzed coupling reaction. The anthracene was attached either at the 2-site (AnCz) or at both 2,7-sites (2AnCz) of the central carbazole core to tune the conjugation state and the optoelectronic properties of the resultant molecules. Both of them show good solubility in common organic solvents. They also possess relatively high HOMO levels (−5.39 eV, −5.40 eV) that would facilitate efficient hole injection and be favorable for high power efficiencies when used in organic light-emitting devices (OLEDs). AnCz and 2AnCz were used as non-doped emitter to fabricate OLEDs by vacuum evaporation. Good performance was achieved with maximum luminance efficiency of 2.61 cd A⁻¹ and CIE coordinates of (0.15, 0.12) for AnCz, and 9.52 cd A⁻¹ and (0.22, 0.37) for 2AnCz.     

Effects of emitting layer host composition profile on the recombination zone of blue phosphorescent organic light emitting diodes

The spatial distribution of charge recombination in blue phosphorescent organic light‐emitting diodes with linearly graded, step‐graded and uniformly mixed host architectures was identified by selective doping of the emissive layer. Using TCTA and UGH‐3 for hole and electron transporting hosts in the emitting layer, the recombination zones were found to be near the interface with electron transport layer due to the relatively high hole mobility of TCTA. For linearly graded host concentration profiles, however, the recombination region extends much further into the emissive layer. Expansion of the recombination region increases device efficiency by reducing quenching at the interface with the electron transport layer or by reducing triplet diffusion into the electron transport layer.     

Modeling the width of exciton formation zone in organic light emitting diode

The width of exciton formation zone has been simulated in a single-layer organic light emitting diode (OLED) based on the model of carrier device lifetimes. The width of exciton formation zone decreases with device current density increasing. Increasing the thickness of emissive layer (EML) is able to widen exciton formation zone. There is an optimal value of EML’s charge carrier mobility to maximize the width of exciton formation zone. In addition, the width of exciton formation zone increases markedly with the trap density of EML increasing, and shows certain correlation with the dielectric constant of EML. The current modeling provides novel insights on optimizing EMLs towards wide exciton formation zone, helpful for extending operational stability of OLED.     

Efficient bluish-green phosphorescent iridium complex for both solution-processed and vacuum-deposited organic light-emitting diodes

Iridium(III)bis(4,6-(difluorophenyl)pyridinato-N,C^2′)picolinate (Firpic) is one typical bluish-green phosphor widely used in phosphorescent organic light-emitting diodes (PhOLEDs). In order to optimize its electroluminescent performance, 3,6-(di-tert-butyl)carbazolyl was introduced into the pyridine ring of the 2,4-difluorophenyl-pyridine ligand via a non-conjugated CH₂ linkage. The generated 3,6-di-tert-butyl-9-((6-(2,4-difluorophenyl)pridine-3-yl)methyl)-9H-carbazole (Cz-CH₂-dfppy) was used as cyclometalating ligand to prepare iridium complex 1, (Cz-CH₂-dfppy)₂Ir(pic). In comparison with the case to attach carbazole directly on pyridine, this non-conjugated CH₂ linking strategy avoids the unwanted bathochromic shift of the phosphorescence and improves the solubility of the iridium complex. (Cz-CH₂-dfppy)₂Ir(pic) (1) was used as doped emitter to fabricate OLEDs by both spin-coating and vacuum evaporation methods. Efficient bluish-green electrophosphorescence was obtained with maximum luminance efficiency of 22 cd/A (14 lm/W, 8.7%) and 26 cd/A (12 lm/W, 9.5%) for the solution-processed and vacuum-deposited devices, respectively, which far exceed those of the parent Firpic based device. The improved performance for (Cz-CH₂-dfppy)₂Ir(pic) was interpreted in terms of improved charge balance brought by the presence of the carbazole groups in the ligands.     

Blue electroluminescent materials based on phenylanthracene-substituted fluorene derivatives for organic light-emitting diodes

Two phenylanthracene-substituted fluorene derivatives, 10-(9,9′-dimethyl-2-(10-phenylanthracen-9-yl)-9H-fluoren-7-yl)-phenylanthracene (1) and 2′,7′-di-(10-phenylanthracen-9-yl)-9,9′-spirobi[9H-fluorene] (2) have been designed, synthesized, and characterized. A device using compound 1 as an emitting material exhibited luminous efficiency, power efficiency, external quantum efficiency and CIE coordinates of 3.37 cd/A, 1.50 lm/W, 1.87% at 20 mA/cm² and (0.18, 0.25) at 7 V, respectively. Furthermore, by exploiting this efficient blue fluorescent material as a blue emitting material with the combination of red phosphorescent bis(2-phenylquinoline)acetylacetonate [(pq)₂Ir(acac)], an efficient white OLED (WOLED) with a external quantum efficiency of 1.70%, luminous efficiency of 1.38 cd/A, power efficiency of 0.94 lm/W at 20 mA/cm² and the color coordinates of (0.33, 0.36) at 14 V is demonstrated.     

Improved efficiency and lifetime for green phosphorescent organic light-emitting diodes using charge control layer

We investigated green phosphorescent organic light-emitting diodes (PHOLEDs) with charge control layer (CCL) to produce high efficiency and improve operational lifetime. Three types of devices were fabricated following the number of CCL within emitting layer (EML), maintaining the thickness of whole EML. The CCL and host material, which was 4,4′-bis (carbazol-9-yl)biphenyl (CBP) with bipolar property, can control carrier movement in EML. Therefore, the electroluminescent (EL) performance improvement as efficiency and lifetime was realized with a good charge balance, an effective triplet exciton confinement, and the reduced triplet exciton quenching effect in EML. Device 2 with a CCL as exciton distribution structure exhibits the remarkable EL performances for the maximum luminous and external quantum efficiency of 65.34 cd/A and 20.42%, respectively. Moreover, operational lifetime is nearly improved 2.5 times than the conventional device.     

White organic light-emitting diodes with improved performance using phosphorescent sensitizer and ultrathin fluorescent emitter

White organic light-emitting diodes (WOLEDs) have been fabricated by using a novel phosphor of bis(1,2-dipheny1-1H-benzoimidazole) iridium (acetylacetonate) [(pbi)₂Ir(acac)] as a sensitizer doped into a carbazole polymer of poly(N-vinylcarbazole) (PVK), and a fluorescent dye of 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran(DCJTB) as an ultrathin red light-emitting layer. By characterizing the UV–Vis absorption spectra, photoluminescence (PL) spectra of (pbi)₂Ir(acac) and DCJTB, and the electroluminescence (EL) properties of the WOLEDs, the effect of ultrathin fluorescent light-emitting layer on the injection and transport characteristics of charge carrier and EL performance of WOLEDs was investigated. The results demonstrated that the device has stable EL spectra and Commissions Internationale de 1’Eclairage (CIE) coordinates in a wide bias range. Both energy transfer and charge trapping play the role on the performance of WOLEDs, and the device luminance was enhanced significantly with a maximum luminance of 9260 cd/m², which is about 70% higher than the codoped device.     

Effect of electron transport layer engineering based on blue phosphorescent organic light-emitting diodes

A main requirement for achieving high efficiency in organic light-emitting diodes (OLEDs) is that all charges and electrically generated excitons should be employed for emission. We fabricated blue phosphorescent OLEDs with four types electron transporting layers, which were doped with lithium quinolate (Liq) from 0% to 10%. A series of blue devices consisted of indium tin oxide (ITO, 180 nm)/4,4-bis[N-(naphthyl)-N-phenyl-amino]biphenyl (NPB, 50 nm)/N,N′-dicarbazolyl-3,5-benzene (mCP, 10 nm)/iridium(III)bis[(4,6-di-fluoropheny)-pyridinato-N,C²] picolinate (FIrpic) doped in mCP (8%, 30 nm)/1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi, 20 nm)/TPBi mixed with Liq (20 nm)/Liq (2 nm)/aluminum (Al, 100 nm). The blue OLED doped with 5% Liq, which demonstrated a maximum luminous efficiency and external quantum efficiency of 17.64 cd/A and 8.78%, respectively, were found to be superior to the other blue devices.     

The reduction of driving voltage in organic light-emitting devices by inserting step barrier layer

The electron injection and transport in OLEDs have been improved by using a tris-[8-hydroxyquinoline] gallium (Gaq) layer as step barrier between tris-[8-hydroxyquinoline]aluminum (Alq₃) (or 4,7-diphyenyl-1,10-phenanthroline (Bphen)) and 2-t-butyl-9,10-di-(2-naphthyl)anthracene (TBADN). Since the LUMO (lowest unoccupied molecular orbital) of Gaq (2.9 eV) lies in between that of Alq₃ (3.1 eV) (or Bphen (3.0 eV)) and TBADN (2.8 eV), step barrier from Alq₃ (or BPhen) though Gaq to TBADN can be formed. The experimental results indicate that the J–V characteristics of both the electron-only and the complete devices show the increase of the current density in devices with step barrier compared with the devices without step barrier. For electron-only devices, the driving voltage at the current density of 20 mA/cm² is reduced from 7.9 V to 4.9 V for devices with Alq₃, and from 4.2 V to 3.1 V for devices with BPhen, respectively, owing to the introduction of step barrier. For the complete devices, when Gaq step barrier is introduced, at 20 mA/cm², the driving voltage is reduced from 7 V to 5.8 V for devices with Alq₃ and from 6.2 V to 5.1 V for devices with BPhen. It has also been observed that for devices with step barrier layer, the luminance at 200 mA/cm² is increased from 1992 cd/m² to 3281 cd/m² for device with Alq₃, and from 1745 cd/m² to 2876 cd/m² for devices with BPhen, respectively. The highest luminance reaches 3420 cd/m² in devices with Alq₃ as ETL and 3176 cd/m² in devices with BPhen as ETL after the introduction of step barrier. The phenomena are explained by using tunnel theory.     

Solution-processed high-performance organic light-emitting diodes containing a green-emitting multiresonant thermally activated delayed fluorescent dendrimer

A multi-resonance thermally activated delayed fluorescence (MR-TADF) dendrimer emitter and a related reference MR-TADF compound were designed, synthesized, and characterized for use as narrowband emitters in solution-processed OLEDs. The 1 wt% doped films in PMMA film revealed that the compounds MR-D1 and MR-D2 showed narrowband green emission at λ_PL of 490 and 495 nm and with FWHM of 23 and 29 nm, respectively. The 50 wt% doped films in mCP still show narrowband green emission at λ_PL of 495 and 499 nm and with FWHM of 28 nm for MR-D1 and MR-D2 , respectively, while conserving the small ΔE_ST of 0.14 and 0.13 eV, respectively. OLEDs containing an emissive layer consisting of 50 wt% MR-D1 and MR-D2 in mCP showed high EQE_max of 27.7% and 21.0%, respectively, and low efficiency roll-off of 19% and 30% at a luminance of 2000 cd/m⁻².     

Fabrication and characterization of double-sided organic light-emitting diodes using silver and nickel as the metal linking layer

In this work, a novel double-sided emitting organic light-emitting diode (DE-OLED) was developed, and the effect of several metal linking layers on such DE-OLEDs was investigated. To form a DE-OLED, a metal linking layer was thermally deposited between a bottom-emitting organic light-emitting diode (BE-OLED) and a top-emitting organic light-emitting diode (TE-OLED). A series of metal films of 10-nm Ag, 20-nm Ag, 10-nm Ag/10-nm Ni, and 20-nm Ni were used as the linking layers, and 20-nm Ag based DE-OLEDs show the highest current efficiency and luminance. Two types of DE-OLEDs, including monochromatic and dichromatic, have been fabricated successfully. The EL spectrum of the TE-OLED is significantly narrower than that of BE-OLED because of the microcavity effect in both monochromatic and dichromatic DE-OLEDs. The results indicate that the electroluminescent characteristics of the OLED devices at both (bottom and top) sides can be modulated independently by using the metal linking layer without sacrificing the brightness and color stability of the device.     

Optimal design of ITO/organic photonic crystals in polymer light-emitting diodes with sidewall reflectors for high efficiency

This study aims to achieve large extraction of light emission from polymer light emitting diodes (PLEDs) via optimizing photonic crystals (PCs) and sidewall angle reflectors. Both PCs and sidewall reflectors can be resulting in increasing light emission in useful directions and reducing refection loss. The optimization is achieved through the optical modeling using a 3D finite-difference time-domain (FDTD) method and the intelligent numerical optimization technique, genetic algorithm (GA). The optimal design of PCs and sidewall angle reflectors are presented in details. To accurately predict light extraction of the PLED, the numerical simulation tool, the FDTD method is employed. Based on the FDTD simulation, the optimal sidewall angle which can increase maximum light extraction efficiency (LEE) in our designed PLED structure is 35°. With the optical modeling of optimal sidewall angle reflectors via FDTD computation and the next step is using GA optimization to seek optimal pitch and radius of photonic crystals. According to the GA optimal result, the ratio of pitch to wavelength is 0.47 times and the ratio of radius to pitch is 0.25 times. GA is a powerful tool to cope with a complicated optimization problem with multiple variables to optimize. The PLEDs with optimized PCs and angle of sidewall reflectors would increase extraction of light emission from 20 to 26?% and the 3D FDTD calculation was conducted to explain this result.     

Utilization of a composite hole transporting layer and novel homogeneous double emitting layers for performance improvement and low efficiency roll-off in organic light-emitting diodes

Blue organic light-emitting devices (OLEDs) combing a composite hole transporting layer (c-HTL) and novel homogeneous double emitting layers (DELs) have been fabricated. The c-HTL plays a significant role of rectification in balancing the carriers’ injection concentration which matches well with the DELs structure. The DELs is consisted of two homogeneous hosts, such as 2-methyl-9,10-di(2-naphthyl) anthracene (MADN) and 9,10-di(2-naphthyl) anthracene (ADN). The optimal device presents the maximal current efficiency of 15.9 cd/A at 4.9 mA/cm² and the minor efficiency roll-off of 13.4% under the driving voltage varying from 5 V to 10 V, respectively. Meanwhile, the device’s maximal current efficiency and the corresponding efficiency roll-off have been obviously improved by 55.9% and 63.9% compared with those of the conventional device. These results indicate that the homogeneous DELs not only greatly facilitate carriers’ injection into the emitting layer (EML), but also evenly modulate carriers’ distribution due to natural energy barrier of the interface. The transient photoluminescence decay of double hosts further illustrates that the DELs structure can increase the recombination ratio of electron–hole pairs and improve the exciton’s utilization. Additionally, the optimal device’ current density is reduced by 44.1% under the same luminance of 25,780 cd/m² compared with that of the conventional device.     

Turn-on voltage reduction of organic light-emitting diode using a nickel-doped indium tin oxide anode prepared by single target sputtering

Organic light-emitting diodes (OLEDs) with a nickel (Ni)-doped indium tin oxide (ITO) anode were fabricated. The Ni-doped ITO anode was prepared using sputter deposition of Ni–ITO single targets consisting of 1, 3 and 5 wt% of nickel. Turn-on voltage of OLED devices with the Ni-doped ITO anode was reduced by 2.5, 4 and 3.8 V for 1, 3 and 5 wt% targets, respectively. Half-luminance lifetime was improved by 2.5 times with a Ni(3 wt%)-ITO single target. The successful development in preparing Ni-doped ITO films by Ni–ITO single target sputtering allows this approach to be adopted for OLED manufacturing.     

Improvement of brightness and efficiency in organic light-emitting diodes using 1,3-bis(4-tert-butylphenyl-1,3,4-oxadiazoyl) phenylene as the hole buffer layer

We have reported the efficient Poly (2-methoxy, 5-(2-ethylhexoxy)-1,4-phenylene vinylene) (MEHPPV) organic light-emitting diodes based on 3-bis(4-tert-butylphenyl-1,3,4-oxadiazoyl) phenylene (OXD-7) and ITO bilayer anodes. OXD-7 was inserted between ITO anode and MEHPPV and acted as a hole buffer layer. The results demonstrated that the efficiency and brightness of the devices with ITO/OXD-7/MEHPPV/LiF/Al were four factors higher than that of devices with ITO/MEHPPV/LiF/Al. However, the efficiency and brightness of devices with ITO/OXD-7/MEHPPV/Al were worse than that of devices with ITO/MEHPPV/Al. Our results suggested that the better performance of devices should have not only the balance of hole and electron injection, but also higher hole and electron density in the emissive layer. OXD-7, as an electron-transporting material, was demonstrated to be a good candidate for hole injection buffer layer.     

Effective burn-in reduction of organic light-emitting diode display using perceptual color difference between logo and background

The paper proposes a method to delay the burn-in effect of an organic light-emitting diode (OLED) display. Usually, the burn-in occurs in broadcasting company logo regions. In previous studies, logos were compensated by reducing the logo luminance by a specific ratio. This paper proposes a novel logo-compensation method regarding the perceptual color difference between a logo and background areas. The proposed method has the advantage of being less affected by changes in the surrounding area than the existing compensation method, minimizing the deterioration of the recognition of the logo, and delaying the burn-in as much as possible.     

Organic light-emitting diodes with a nanostructured TiO2 layer at the interface between ITO and NPB layers

Organic light-emitting diodes (OLEDs) with a nanostructured TiO₂ layer at the interface between indium tin oxide and a-naphtylphenyliphenyl diamine layers were fabricated using a vacuum evaporation method. The nanostructured TiO₂ layer was achieved by the Sol–Gel method. Compared to the different thickness of the buffer layer, the OLEDs with the 6 nm buffer layer showed the highest efficiency. The enhancements in efficiency result from an improved balance of hole and electron injections and a more homogeneous adhesion of the buffer layer inserted.