Preparation and characterization of highly efficient blue TPBI:FIrpic organic light‐emitting devices

Abstract— Blue phosphorescent organic light‐emitting devices (PhOLEDs) using 1,3,5‐tris(N‐phenyl‐ benzimiazole‐2‐yl)benzene [TPBI] as the host and bis((4,6‐difluorophenyl)‐pyridinate‐N,C2′)picolinate [FIrpic] as the dopant in the emitter were fabricated with different treatments of the hole‐transport layers and doping levels. Among the experimental devices, the best electroluminescent characteristics were obtained in the device with the combined hole‐transport layer of N,N′‐diphenyl‐N,N′‐bis‐[4‐ (phenyl‐m‐tolylamino)‐phenyl]‐biphenyl‐4,4′‐diamine [DNTPD]/1, 1‐bis‐(di‐4‐polyaminophen yl)‐ cyclo‐hexane [TAPC] and a doping level of 10‐vol.% FIrpic. The device with a structure of DNTPD/TAPC/TPBI:Firpic (10%) showed a luminance of 1300 cd/m² at an applied voltage of 10 V, a maximum current efficiency of 18 cd/A, and color coordinates of (0.17, 0.43) on the Commission Internationale de I'Eclairage (CIE) chart.     

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.     

Efficient and bright blue‐emitting phosphorescent materials

Abstract— A new type of ancillary ligand for blue‐emitting heteroleptic iridium complexes has been successfully developed. New ligands, 3‐(trifluoromethyl)‐5‐(pyridin‐2‐yl)‐1,2,4‐triazolate and 5‐(pyridin‐2‐yl)‐tetrazolate, show stronger blue‐shifting power than that of the picolate of FIrpic [iridium (III) bis(4,6‐difluorophenylpyridinato)picolate]. Organic light‐emitting diodes (OLEDs) fabricated with a new complex, FIrtaz [iridium (III) bis(4,6‐difluorophenylpyridinato)(5‐(pyridine‐2‐yl)‐1,2,4‐triazolate) or FIrN4 [(iridium (III) bis(4,6‐difluorophenylpyridinato)(5‐(pyridin‐2‐yl)‐tetrazolate], as the blue dopant in the host of mCP [1,3‐ bis(9‐carbazolyl)benzene], exhibit near‐saturated blue electrophosphorescence with Commision Internale de l'Eclairage (CIE^x,y) coordinates of (0.14, 0.18) and (0.15, 0.24), respectively.     

Tuning of electrical and optical properties of organic LEDs using combinatorial device fabrication

The capabilities of combinatorial methods are presented in order to get a detailed understanding of the electrical and optical properties of organic light‐emitting devices (OLEDs), to optimize their performance, and to provide reliable data for device modeling. We show results on multilayer OLEDs ranging from the conventional copper‐phthalocyanine (CuPc)/N,N′di‐(naphtalene‐1‐yl)‐N,N′‐diphenyl‐benzidine (NPB) and tris‐(8‐hydroxy‐quinolinato)aluminum (Alq) tri‐layer device to double‐doped deep‐red‐emitting OLEDs.     

OLEDs based on an europium(IIIrpar; complex: {Tris(thenoyltrifluoroacetonate)[1,2,5]‐thiadiazolo[3,4‐f] [1,10]phenanthroline}europium(III)

Abstract— The contribution of radiative and non‐radiative processes to the electroluminescence emission of OLEDs based on Eu‐complex, {tris(thenoyltrifluoroacetone)[1,2,5]thiadiazolo[3,4‐f][1,10]phenanthroline} europium(III), [Eu(TTA)³TDZP], which acts as transporting and emitting layers, is investigated. The Eu‐complex presented an intense photoluminescence with high color purity in the red region, characteristic of the Eu(III) ⁵D⁰ → ⁷F² narrow line transition. However, when used in a double‐layered OLED its electroluminescence showed additional undesired broad bands, which can be attributed to the possible electrophosphorescence of the ligand and to an inefficient energy transfer from the organic ligand to the Eu(III). The characteristic narrow lines could be achieved using a co‐deposited active layer with the Eu‐complex acting as a dopant in a matrix comprised of 4,4’‐bis(carbazol‐9‐yl)biphenyl (CBP).     

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.     

Correlation between host material compositions and performances in organic white-light-emitting diodes with blue/orange/blue emitting stacked structure

Quantum efficiency, driving voltage, color stability and recombination zone of organic white-light-emitting diodes (OWLEDs) with blue/orange/blue stacked emitting structure were correlated with host structure of emitting layer. A mixed host structure of 4,4′,4″-tris(N-carbazolyl)triphenylamine (TCTA) and 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBI) was used in orange emitting layer and host composition was critical to device performances of OWLEDs. TPBI host structure was better than other host structures in terms of quantum efficiency and color stability, while TCTA shows poor quantum efficiency in spite of good color stability. TCTA:TPBI mixed host structure showed better driving voltage than other host structures. In addition, recombination zone in blue/orange/blue stacked OWLEDs could be controlled by changing host structure in orange light-emitting layer. OWLEDs with TPBI host in orange emitting layer showed high quantum efficiency of 10.3% at 1000 cd/m² with little change of Commission international De L’Eclairage coordinates of (0.32, 0.34) from 100 cd/m² to 10,000 cd/m².     

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.     

Materials optimization for semiconducting polymer LEDs and FETs

Dye‐doped semiconducting polymers are used as active layers in polymer light‐emitting diodes (polyLEDs). The emission color can be tuned by doping the active polymer with certain dyes. This concept of energy transfer is demonstrated for a green matrix doped with a red‐emitting dye, suitable for use in LEDs. An absolute PL efficiency of 39% is observed for this system. Another very attractive development is taking place in the area of all‐polymer transistors. This may lead to a (partial) replacement of the driving electronics by all‐plastic circuits. A new precursor route toward poly(thienylenevinylene)s (PTVs), suitable as active material in all‐polymer integrated circuits, is presented. Synthesis of the precursors is reproducible and fast, and can readily be scaled for manufacture. Quantitative conversion of the precursor polymer can be accomplished by heating at 150°C for 20 min. The resulting mobility (6 × 10^?3 cm²/V‐sec) and ON‐OFF ratio (4 × 10⁴) makes this material a suitable candidate for the development and large‐scale manufacturing of all‐polymer integrated circuits.     

Passive‐matrix polymer light‐emitting displays

Abstract— Organic light‐emitting device research focuses on the use of small‐molecule and polymer materials to make organic electroluminescent displays, with both passive‐ and active‐matrix technologies. This paper will focus on the characteristics of red, green, and blue electroluminescent polymers suitable for fabricating monochrome and full‐color passive‐matrix displays. The stability of polymer OLEDs, and the use of ink‐jet printing for direct high‐resolution patterning of the light‐emitting polymers will also be discussed. It will be shown that the performance of light‐emitting polymers is at the brink of being acceptable for practical applications.     

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.     

Polysilicon TFT technology on flexible metal foil for AMPLED displays

Abstract— A top‐emitting 230‐dpi active‐matrix polymer light‐emitting diode (AMPLED) display, having a VGA format and a 3.3‐in.‐diagonal size, on a flexible stainless‐steel‐foil substrate is reported. The active‐matrix array was fabricated with laser‐crystallized polysilicon TFTs at a maximum process temperature of 700°C. The top‐emitting PLED diodes were prepared by spin‐casting organic light‐emitting polymers. This work demonstrates the compatibility of polysilicon‐TFT technology with flexible metal‐foil substrates for active‐matrix organic light‐emitting‐diode (AMOLED) display applications.     

The influence of the hole transport layers on the performance of blue and color tunable quantum dot light‐emitting diodes

The performance of the blue quantum dot light‐emitting diodes (QLEDs) is largely affected by the hole transport layers (HTLs). As a consequence of the deep valance band level of blue quantum dots (QDs), hole injection is relatively difficult in blue QLEDs. To favor the hole injection, HTLs with high hole mobility and deep‐lying highest occupied molecular orbital level are desired. In this work, various HTLs and their influence on the performance of blue QLEDs are demonstrated. Devices with poly(N‐vinylcarbazole) (PVK) HTL exhibit the highest external quantum efficiency while devices with poly[9,9‐dioctylfluorene‐co‐N‐(4‐(3‐methylpropyl))‐diphenylamine] (TFB) exhibit the lowest driving voltage. By combining the advantages of PVK and TFB, the blue QLEDs with TFB/PVK bilayered HTL simultaneously exhibit a low driving voltage of 2.6 V and a high external quantum efficiency of 5.9%. Moreover, the exciplex emission at the interface of HTL/QDs is also observed, and the emission intensity can be tuned by modulating the hole injection. By utilizing PVK doped with 25% poly(3‐hexylthiophene) (P3HT) as HTL, exciplex emission is significantly enhanced at low driving voltage while QD emission is dominant at high driving voltage. By combining the exciplex emission and the QD emission, the emission color can be effectively tuned from red to blue as the driving voltage changing from 2 to 10 V.     

Development of 8‐in. oxide‐TFT‐driven flexible AMOLED display using high‐performance red phosphorescent OLED

An 8‐in. flexible active‐matrix organic light‐emitting diode (AMOLED) display driven by oxide thin‐film transistors (TFTs) has been developed. In‐Ga‐Zn‐O (IGZO)‐TFTs used as driving devices were fabricated directly on a plastic film at a low temperature below 200 °C. To form a SiO_x layer for use as the gate insulator of the TFTs, direct current pulse sputtering was used for the deposition at a low temperature. The fabricated TFT shows a good transfer characteristic and enough carrier mobility to drive OLED displays with Video Graphic Array pixels. A solution‐processable photo‐sensitive polymer was also used as a passivation layer of the TFTs. Furthermore, a high‐performance phosphorescent OLED was developed as a red‐light‐emitting device. Both lower power consumption and longer lifetime were achieved in the OLED, which used an efficient energy transfer from the host material to the guest material in the emission layer. By assembling these technologies, a flexible AMOLED display was fabricated on the plastic film. We obtained a clear and uniform moving color image on the display.     

A 5.8‐in. phosphorescent color AMOLED display fabricated by ink‐jet printing on plastic substrate

Abstract— A flexible phosphorescent color active‐matrix organic light‐emitting‐diode (AMOLED) display on a plastic substrate has been fabricated. Phosphorescent polymer materials are used for the emitting layer, which is patterned using ink‐jet printing. A mixed solvent system with a high‐viscosity solvent is used for ink formulation to obtain jetting reliability. The effects of evaporation and the baking condition on the film profile and OLED performances were investigated. An organic thin‐film‐transistor (OTFT) backplane, fabricated using pentacene, is used to drive the OLEDs. The OTFT exhibited a current on/off ratio of 10⁶ and a mobility of 0.1 cm²/V‐sec. Color moving images were successfully shown on the fabricated display.     

Efficient electron injection from bilayer cathode consisting of aluminum and alcohol/water‐soluble conjugated polymers

Abstract— A novel bilayer cathode has been developed that forms an effective electron‐injecting contact to a variety of red‐, green‐, and blue‐emitting polymers. An efficient electron injection directly from Al can be realized by incorporation of a thin layer of water‐ or alcohol‐soluble copolymer between the metal and emitting layer. The device performance of polymer LEDs fabricated with such bilayer cathodes is comparable to or even higher than that obtained by using typical low‐work‐function Ca and Ba cathodes. The utilization of alcohol or water‐soluble conjugated polymer as an electron‐injection layer in combination with air‐stable metals is of special importance for PLEDs, since it enables the fabrication of multilayer devices by spin‐coating, without intermixing between the emitting layer and the injection layers.     

A novel electron‐transport material for organic light‐emitting devices

Abstract— A nanocrystalline electron‐transport material [ET68] was introduced into organic light‐emitting devices (OLEDs). By integrating a p‐doped transport system and phosphorescent emitters, a very bright and stable device could be obtained. Furthermore, 40% saving in power consumption can be achieved when the efficient pixels with ET68 were applied to AMOLEDs.     

Light‐emitting polymers for full‐color display applications

The latest developments in light‐emitting‐polymer (LEP) technology at CDT continue to show steady progress. Device performance for blue, green, and red systems as well as a high‐performance yellow system in terms of device efficiency and stability will be described. Some of the issues associated with the commercialization of LEP technology including the development of direct‐patterning techniques enabling full‐color passive‐ and active‐matrix display will be discussed.     

Highly efficient green polymer light‐emitting diodes through interface engineering

Abstract— By interface engineering, we have improved the quantum efficiency for green polyfluorene polymer light‐emitting diodes by three fold: the efficiency improved from 9 to 28 cd/A. This interface engineering was achieved by inserting a thin layer of calcium (2) acetylacetonate, denoted as Ca(acac)², at the polymer/metal cathode interface. The Ca(acac)² layer behaves in a multifunctional way. It assists the electron injection by lowering the electron‐injection barrier. In the meanwhile, hole injection is enhanced by the accumulated electrons in the polymer layer. This effect is believed to lower the barrier height for hole‐injection through the Schottky effect. Finally, the Ca(acac)² layer works as a hole block layer to block the holes. As a result of the charge balance and charge confinement, the device quantum efficiency increases dramatically.     

Influence of intermolecular interactions on the formation of spontaneous orientation polarization in organic semiconducting films

Spontaneous orientation polarization (SOP) has been frequently observed in the evaporated films of organic light‐emitting diode materials. Because SOP modifies the charge injection and the accumulation properties of the device, understanding and controlling SOP is crucial in optimizing the performance of the device. In this study, we investigated the dominant factors for SOP formation by focusing on intermolecular interactions. We examined the giant surface potential characteristics of coevaporated films incorporating 1,3,5‐tris(1‐phenyl‐1H‐benzimidazol‐2‐yl)benzene (TPBi) that is a typical polar molecule exhibiting SOP. In the coevaporated films of TPBi and nonpolar molecules such as 4,4′‐bis(N‐carbazolyl)‐1,1′‐biphenyl and 4,4′,4″‐tris (carbazol‐9‐yl)triphenylamine, the orientation degree of the permanent dipole moment (PDM) of TPBi is significantly enhanced with diluted TPBi density, though the enhancement is weak on the film with N,N′‐bis(1‐naphthyl)‐N,N′‐diphenyl‐1,1′‐biphenyl‐4,4′‐diamine. The results indicate that the PDM interaction between polar molecules results as a negative factor for SOP formation. Furthermore, we found that SOP formation is suppressed by the surface treatment of the self‐assembled monolayer on the gold substrate, indicating a positive effect of the van der Waals interaction between the molecule and the substrate surface.