Improved stability in organic light‐emitting devices by mixing ambipolar and wide energy gap hosts

We demonstrate improved stability in phosphorescent organic light‐emitting devices (OLEDs) by incorporating a wide energy gap host material into an ambipolar emissive layer. Unlike conventional mixed‐host OLEDs that combine hole‐ and electron‐transporting hosts, charge transport in this device occurs primarily along the ambipolar host and the emitter, while the wide energy gap host serves to modify the charge injection and transport characteristics of the emissive layer. This approach allows both the width and position of the exciton recombination zone to be tuned without introducing exciplex states. Whereas overall device stability improves with increasing recombination zone width in conventional mixed‐host OLEDs, mixing in this system reduces the recombination zone extent yet still increases device lifetime. By decoupling luminance losses into the photostability of the emitter and the exciton formation efficiency, we show that this enhancement arises from a trade‐off between bulk and interfacial degradation. The addition of the wide energy gap host moves the recombination zone away from the interface between the hole‐transport layer and the emissive layer, sacrificing a modest increase in bulk degradation to substantially reduce interfacial degradation. We find that the lifetime can be improved by 50% by balancing these competing degradation pathways.     

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.     

High‐efficiency red‐phosphorescent organic light‐emitting diode with the organic structure of 2‐TNATA/Bebq2:SFC‐411/SFC‐137

Abstract— High‐efficiency and simple‐structured red‐emitting phosphorescent devices based on the hole‐injection layer of 4,4′,4″‐tris(2‐naphthylphenyl‐phenylamino)‐triphenylamine [2‐TNATA] and the emissive layer of bis(10‐hydroxybenzo[h] quinolinato)beryllium complex [Bebq²] doped with SFC‐411 (proprietary red phosphorescent dye) have been researched. The fabricated devices are divided into three types depending on whether or not the hole‐transport layer of N,N′‐bis(1 ‐naphthyl)‐N, N'‐diphenyl‐1,1′‐biphenyl‐4,4′‐diamine [NPB] or the electron‐transport layer of SFC‐137 (proprietary electron transporting material) is included. Among the experimental devices, the best electroluminescent characteristics were obtained for the device with an emission structure of 2‐TNATA/Bebq²:SFC‐411/SFC‐137. In this device, current density and luminance were found to be 200 mA/cm² and 15,000 cd/m² at an applied voltage of 7 V, respectively. Current efficiencies were 15 and 11.6 cd/A under a luminance of 500 and 5000 cd/m². The peak wavelength in the electroluminescent spectral distribution and color coordinates on the Commission Internationale de I'Eclairage (CIE) chart were 628 nm and (0.67, 0.33), respectively.     

Highly efficient blue organic light‐emitting diode with high color purity using 4,4′‐N,N′‐dicarbazole‐biphyenyl (CBP) doped with 1,4‐bis[2‐(3‐N‐ethylcarbazoryl) vinyl]benzene (BCzVB)

Abstract— An efficient pure blue multilayer organic light‐emitting diode employing 1,4‐bis[2‐(3‐N‐ethylcarbazoryl)vinyl]benzene (BCzVB) doped into 4,4′‐N,N′‐dicarbazole‐biphyenyl (CBP) is reported. The device structure is ITO (indium tin oxide)/TPD (N,N′‐diphenyl‐N,N′‐bis (3‐methylphenyl)‐1,1′biphenyl‐4,4′diamine)/CBP:BCzVB/Alq₃ (tris‐(8‐hydroxy‐quinolinato) aluminum)/Liq (8‐hydroxy‐quinolinato lithium)/Al; here TPD was used as the hole‐transporting layer, CBP as the blue‐emitting host, BCzVB as the blue dopant, Alq₃ as the electron‐transporting layer, Liq as the electron‐injection layer, and Al as the cathode, respectively. A maximum luminance of 8500 cd/m² and a device efficiency of 3.5 cd/A were achieved. The CIE co‐ordinates were x = 0.15, y = 0.16. The electroluminescent spectra reveal a dominant peak at 448 nm and additional peaks at 476 nm with a full width at half maximum of 60 nm. The Föster energy transfer and, especially, carrier trapping models were considered to be the main mechanism for exciton formation on BCzVB molecules under electrical excitation.     

Efficiency enhancement of solution‐processed single‐layer blue‐phosphorescence organic light‐emitting devices having co‐host materials of polymer (PVK) and small‐molecule (SimCP2)

Abstract— A new type of single‐layer blue‐phosphorescence organic light‐emitting devices (OLEDs) containing poly(9‐vinylcarbazole) (PVK) and small‐molecule‐based amorphous ambipolar bis(3,5‐di(^9H‐carbazol‐9‐yl)phenyl) diphenylsilane (SimCP2) as the co‐host material have been demonstrated. All active materials [PVK, SimCP2, Flrpic (blue‐phosphorescence dopant), and OXD‐7 (electron transport)] were mixed in a single layer for solution processing in the fabrication of OLEDs. The SimCP2 small‐molecule host has adequate high electron and hole‐carrier mobiltieis of ～10^?4 cm²/V‐sec and a sufficiently large triplet state energy of ～2.70 eV in confining emission energy on FIrpic. Based on such an architecture for single‐layer devices, a maximum external quantum efficiency of 6.2%, luminous efficiency of 15.8 cd/A, luminous power efficiency of 11 lm/W, and Commision Internale de l'Eclairage (CIE^x,y) coordinates of (0.14,0.32) were achieved. Compared with those having PVK as the single‐host material, the improvement in the device performance is attributed to the balance of hole and electron mobilities of the co‐host material, efficient triplet‐state energy confinement on FIrpic, and the high homogeneity of the thin‐film active layer. Flexible blue‐phosphorescence OLEDs based on solution‐processed SimCP2 host material (withou PVK) have been demonstrated as well.     

Surface‐light‐emitting transistors based on vertical‐type metal‐base organic transistors

Abstract— Light‐emitting transistors having a metal‐base organic transistor (MBOT) structure demonstrate both the function of an organic thin‐film transistor (OTFT) and organic light‐emitting diode (OLED). The MBOT is a vertical‐type organic transistor having a simple structure composed of organic/metal/organic layers demonstrating high‐current and low‐voltage operation. The light‐emitting MBOT was fabricated simply by inserting additional layers of hole‐transporting and emissive materials used in the OLED into the col lector layer. The device showed perfect surface emission similar to an OLED. A luminance modulation of 370 cd/m² was observed at a collector voltage of 20 V and a base voltage of 3 V. This device can be applied to an OLED display device to increase the numerical aperture or reduce the required current of the TFT backplane.     

Improved performance of organic light‐emitting devices with ultra‐thin hole‐blocking layers

Abstract— Tris‐(8‐hydroxyqunoline) aluminum (Alq₃)‐based organic light‐emitting devices (OLEDs) using different thickness of 2,9‐Dimethyl‐4,7‐diphenyl‐1,110‐phenanthorline (BCP) as a hole‐blocking layer inserted both in the electron‐ and hole‐transport layers have been fabricated. The devices have a configuration of indium tin oxide (ITO)/m‐MTDATA (80 nm)/BCP (X nm)/NPB (20 nm)/Alq₃ (40 nm)/BCP (X nm)/Alq₃ (60 nm)/Mg: Ag (200 nm), where m‐MTDATA is 4, 4′, 4″‐Tris(N‐3‐methylphenyl‐N‐phenyl‐amino) triphenylamine, which is used to improve hole injection and NPB is N,N′‐Di(naphth‐2‐yl)‐N,N′‐diphenyl‐benzidine. X varies between 0 and 2 nm. For a device with an optimal thickness of 1‐nm BCP, the current and power efficiencies were significantly improved by 47% and 43%, respectively, compared to that of a standard device without a BCP layer. The improved efficiencies are due to a good balance between the electron and hole injection, exciton formation, and confinement within the luminescent region. Based on the optimal device mentioned above, the NPB layer thickness influences the properties of the OLEDs.     

The role of LiF buffer layer in tris‐(8‐hydroxyquinoline) aluminum‐based organic light‐emitting devices with Mg:Ag cathode

Abstract— The device characteristics of organic light‐emitting devices based on tris‐(8‐hydroxyqunoline) aluminum with a thin layer of LiF inserted at the ITO and organic interface or organic and Mg:Ag cathode interface were investigated. A thin layer of LiF can enhance the electron injection when it was inserted only between the organic electron‐transporting layer and the Mg:Ag alloy cathode, but can block hole injection when inserted between the ITO anode and the organic hole‐transport layer. By inserting both a 1.0‐nm LiF layer at side of the ITO anode and a 0.5‐nm LiF layer under the Mg:Ag cathode, the device, at a current injection of 10 mA/cm², exhibited the highest current efficiency of 8.2 cd/A and power efficiency of 1.93 lm/W for all the types of devices investigated in this study. Both the current efficiency and power efficiency of the device were improved by 1.2 times at a current injection of 10 mA/cm², compared to the standard device without any LiF buffer layer. This is due to the increased electron injection and decreased hole injection that off‐sets the imbalance of electron and hole injection and brings it towards the balanced injection of electrons and holes, thus reducing the non‐productive hole current.     

Highly efficient deep‐blue organic light‐emitting devices

Abstract— A highly efficient deep‐blue organic light‐emitting device (OLED) incorporating a novel composite hole‐transport layer (c‐HTL) and an emitter based on the new non‐symmetrical mono(styryl)amine fluorescent dopant in the stable host MADN, which achieved a luminance efficiency of 5.4 cd/A with a Commission Internationale d'Eclairage (CIEx,y) of (0.14, 0.13) and an external quantum efficiency of 5.1% at 20 mA/cm² and 6.8 V, is reported. The increased device efficiency is attributed to an improved balance between hole and electron currents in the recombination zone.     

Detailed analysis of exciton decay time change in organic light‐emitting devices caused by optical effects

Abstract— The exciton decay time in organic light‐emitting devices (OLEDs) depends on the optical environment, i.e., the thicknesses and refractive indices of all layers in a device. The decay of an exciton can occur through a radiative or a non‐radiative channel. Each of these channels has a probability, which is expressed by, respectively, the radiative and the non‐radiative decay rate. The radiative decay rate is influenced by the optical environment, i.e., the OLED's thin‐film layer structure. In this paper, a model for estimating the change of the exciton decay time (inverse of the decay rate) is presented. In addition, the decay time change in both top‐ and bottom‐emitting OLEDs as a function of the charge‐transport layer thicknesses has been investigated. Furthermore, the most important mechanism responsible for the exciton decay time change is outlined.     

Charge‐recombination processes in oligomer‐ and polymer‐based light‐emitting diodes: A molecular picture

Abstract— An overview of our recent work on the mechanisms of singlet and triplet exciton formation in electroluminescent π‐conjugated materials will be presented. According to simple spin statistics, only one‐fourth of the excitons are formed as singlets. However, deviations from that statistics can occur if the initially formed triplet charge‐transfer (CT) excited states are amenable to intersystem crossing or dissociation. Although the electronic couplings between the CT states and the neutral exciton states are expected to be largest for the lowest singlet and triplet excitons (S¹ and T¹, respectively), the possibility for direct recombination into T¹ is always very small due to the large exchange energy. In small molecules, spin statistics is expected to be observed because both singlet and triplet exciton formations proceed via higher‐lying S_n/T_n states with similar electronic couplings and fast formation rates. In extended conjugated chains, however, that the ¹CT → S¹ pathway is faster while the ³CT → ^Tn channels become much slower, opening the route to intersystem crossing or dissociation among the ³CT states.     

High‐efficiency organic light‐emitting diodes based on the gradient doping and nonlinear cross‐fading doping in transporting layers

We have demonstrated that carrier injection and transporting can be fine‐tuned via gradient p‐doping and n‐doping in organic light‐emitting diodes. The doping profile of gradient doping in transporting layer is ultrahigh at the electrode side, declining gradually with the depth into the device until the emission layer. This not only ensures perfect charge injection from electrode to organic transporting layer but also proves an efficient charge transport for light emission. It is proposed that low doping ratio close to the emission layer may avoid possible quenching of excitons by the diffusion of dopant as well. A device based on gradient doping has been proved to obtain better carrier injection and achieve higher external quantum efficiency. To get smoother charge injection and transporting, and simplify the fabrication process, we have developed a nonlinear cross‐fading doping in transporting layer, which has been demonstrated to further enhance the current density characteristics.     

Solution‐processable double‐layered ionic p‐i‐n organic light‐emitting diodes

Abstract— Solution‐processed double‐layered ionic p‐i‐n organic light‐emitting diodes (OLEDs), comprised of an emitting material layer doped with an organometallic green phosphor and a photo‐cross‐linked hole‐transporting layer doped with photo‐initiator is reported. The fabricated OLEDs were annealed using simultaneous thermal and electrical treatments to form a double‐layered ionic p‐i‐n structure. As a result, an annealed double‐layered OLED with a peak brightness over 20,000 cd/m² (20 V, 390 mA/cm²) and a peak efficiency of 15 cd/A (6 V, 210 cd/m²) was achieved.     

Thermally cross‐linkable copolymer and its evaluation as a hole transport layer in organic light‐emitting diode devices

This study reports on the synthesis of new thermally cross‐linkable copolymers containing a reactive cross‐linking comonomer. Synthesized polymers showed narrow molecular weight distribution (polydispersity) between 1.18 to 1.22 and 54 to 67% monomer conversion and incorporation of 2 to 7 mol% vinylbenzylcyclobutene comonomer. The polymer was soluble in nonpolar organic solvents such as chloroform, dichloromethane, toluene, and chlorobenzenes, and when cross‐linked, showed resistance to solubility in the previously listed solvents. The cross‐linked films exhibited uniform surface roughness below 1 nm. A polymer containing ~3.6 mol% vinylbenzylcyclobutene was thermally cross‐linked and evaluated as a hole‐transporting layer in green organic light‐emitting diode devices. The devices showed a maximum current efficiency of 39.5 cd/A at a current density of 2.7 mA/cm² and a brightness of 1000 cd/m² with an International Commission on Illumination coordinate (0.33, 0.62). The device performances are found comparable with the ones with the conventional hole‐transporting layer material, NPD.     

Printable phosphorescent organic light‐emitting devices

Abstract— A new approach to full‐color printable phosphorescent organic light‐emitting devices (P²OLEDs) is reported. Unlike conventional solution‐processed OLEDs that contain conjugated polymers in the emissive layer, the P²OLED's emissive layer consists of small‐molecule materials. A red P²OLED that exhibits a luminous efficiency of 11.6 cd/A and a projected lifetime of 100,000 hours from an initial luminance of 500 cd/m², a green P²OLED with a luminous efficiency of 34 cd/A and a projected lifetime of 63,000 hours from an initial luminance of 1000 cd/m², a light‐blue P²OLED with a luminous efficiency of 19 cd/A and a projected lifetime 6000 hours from an initial luminance of 500 cd/m², and a blue P²OLED with a luminous efficiency of 6.2 cd/A and a projected lifetime of 1000 hours from an initial luminance of 500 cd/m² is presented.     

Ink‐jet‐printable phosphorescent organic light‐emitting‐diode devices

Abstract— A novel method for the fabrication of ink‐jet‐printed organic light‐emitting‐diode devices is discussed. Unlike previously reported solution‐processed OLED devices, the emissive layer of OLED devices reported here does not contain polymeric materials. The emission of the ink‐jet‐printed P²OLED (IJ‐P²OLED) device is demonstrated for the first time. It shows good color and uniform emission although it uses small‐molecule solution. Ink‐jet‐printed green P²OLED devices possess a high luminous efficiency of 22 cd/A at 2000 cd/m² and is based on phosphorescent emission. The latest solution‐processed phosphorescent OLED performance by spin‐coating is disclosed. The red P²OLED exhibits a projected LT50 of >53,000 hours with a luminous efficiency of 9 cd/A at 500 cd/m². The green P²OLED shows a projected LT50 of >52,000 hours with a luminous efficiency of 35 cd/A at 1000 cd/m². Also discussed is a newly developed sky‐blue P²OLED with a projected LT50 of >3000 hour and a luminous efficiency of 18 cd/A at 500 cd/m².     

Highly efficient solution‐processible phosphorescent dendrimers for organic light‐emitting diodes

Abstract— Currently, most research into organic light‐emitting diodes (OLEDs) has focused on two main classes of materials: small organic molecules and conjugated polymers. An alternative approach is to use conjugated dendrimers. We show that conjugated dendrimers are a promising new class of solution‐processible materials for use as the active layer in highly efficient organic LEDs. By optimizing the choice of device structure, host material, and electron transport layer, we can obtain efficiencies of 55 cd/A and power efficiencies of 40 lm/W. This is an excellent result for a spin‐coated emissive layer.     

A new wettability‐control technique for fabricating color OLED panels by an ink‐jet‐printing method

Abstract— The fabrication technique for color OLED panels by means of wettability‐controllable hole‐injection material (HIM) and a photocatalytic lithography method achieves both precise ink‐jet printing and long‐lifetime devices. The technique enables us to selectively change the non‐wetting surface of a hole‐injection layer (HIL) of metal‐oxide nanoparticles (MONPs) into a wetting surface without damage to the device performance. Wetting patterns formed by this method with photocatalyst‐coated photomasks made it possible to print emission material with patterns of precisely 98‐μm widths on the hole‐injection layer. A fluorescent green‐emitting device fabricated with an HIM of MONPs by the photocatalytic treatment exhibited a long lifetime of 365 hours at30,000 cd/m², which can be extrapolated to a lifetime of more than 110,000 hours at 1000 cd/m², assuming an acceleration coefficient of 1.7. A two‐color device and a monochrome passive‐matrix panel were also successfully fabricated. The two‐color device emitted light without the mixing of colors. The monochrome panel displayed alphabetical characters with good uniformity and no flaws.     

Deep‐UV exciton emission from Mg1 − xCaxO—A first‐principle investigation

In order to investigate the ultraviolet and electron emission properties of rock‐salt Mg_{1 ? x}Ca_xO, first‐principle calculations are carried out with x ranging from 0 to 0.5. The electron–hole interaction is taken into account by solving the Bethe–Salpeter equation. In pure MgO, the calculated exciton binding‐energy value of 83 ± 3 meV is quite close to the published experimental values. The electronic properties of doped MgO are investigated based on the super‐cell model. Both the optical bandgap and the exciton binding energy of the super cell of Mg_{1 ? x}Ca_xO are calculated for five discrete compositions, with an increasing amount x of CaO. The results show that there are strong excitonic effects in all of these materials. A rapid reduction in bandgap value is observed for Mg_{1 ? x}Ca_xO, when increasing the x value. The exciton binding energy of Mg_{1 ? x}Ca_xO shows a minimum value of about 40 meV around x = 0.2. In all of these cases, the exciton is stable at room temperature. The lowest excitation levels in Mg_{1 ? x}Ca_xO are determined by dark excitons. The occurrence of these dark excitons might explain why Mg_{1 ? x}Ca_xO is a strong source of delayed electron emission, after being bombarded by ions in a plasma.     

Highly efficient white organic light‐emitting diodes with over 100 lm/W for next‐generation solid‐state lighting

High‐performance two‐unit all‐phosphorescent white devices on a built‐up light extraction substrate that comprised high‐index materials were studied. As a result of suitable optical and electrical design, the device showed an extremely high efficacy of 114 lm/W at 1000 cd/m². The device also showed 102 lm/W with long lifetime (LT70) of over 10,000 h at 3000 cd/m². Outstanding external quantum efficiency of almost 50% was also achieved in a flat panel with an emissive area of 25 cm². Color coordinates of the panel met the Energy Star ® criteria of solid‐state lighting with CIE (Commission Internationale de l'Éclairage) 1931 (x, y) = (0.477, 0.423), and the color rendering index was 81.