Efficient orange-red organic light-emitting diodes using 9,10-bis[4-(di-4-tert-butylphenylamino)styryl] anthracene as a fluorescent orange-red emitter

The authors have demonstrated efficient orange-red organic lighting diodes (OLEDs) using a new fluorescent orange-red material, 9,10-bis[4-(di-4-tert-buthylphenylamino)styryl]anthracene (ATBTPA). The optimized orange-red OLED using ATBTPA achieved a maximum external quantum efficiency (EQE) of 3.78%, a current efficiency (CE) of 9.47 cd/A, and Commision Internationale de L'Eclairage (CIE_x,y) coordinates of (0.51, and 0.48) at 1.61 mA/cm² in comparison with orange-red OLED using (5,6,11,12)-tetraphenyl-naphthacene (rubrene) which showed a maximum EQE of 1.65%, CE of 4.94 cd/A, and CIE_x,y coordinates of (0.50, and 0.49) at 0.61 mA/cm², respectively. The optimized orange-red device using ATBTPA showed higher efficiency of two times the orange-red device using rubrene due to the efficient Förster singlet energy transfer from MADN to ATBTPA in comparison with that from MADN to rubrene. This study clearly suggests that ATBTPA is an excellent fluorescent orange-red material for efficient WOLEDs.     

Synthesis and electroluminescent properties of blue-emitting t-butylated bis(diarylaminoaryl)anthracenes for OLEDs

A new series of blue fluorescent emitters based on t-butylated bis(diarylaminoaryl) anthracenes were synthesized and their electroluminescent properties investigated. Into these blue materials, t-butyl groups were introduced to both prevent molecular aggregation between the blue emitters through steric hindrance and reduce self-quenching. As such, this would contribute to overall improvement in OLED efficiency. To explore the electroluminescent properties of these materials, multilayered OLEDs were fabricated into a device structure of: ITO/NPB(50 nm)/blue emitters doped in ADN(30 nm)/Alq₃(20 nm)/Liq(2 nm)/Al(100 nm). All devices showed efficient blue emissions. In particular, one device exhibited highly efficient sky blue emissions with a maximum luminance of 11,060 cd/m² at 12.0 V and respective luminous and power efficiencies of 6.59 cd/A and 2.58 lm/W at 20 mA/cm². The peak wavelength of the electroluminescence was 468 nm with CIEx,y coordinates of (0.159, 0.198) at 12.0 V. In addition, a deep blue device with CIEx,y coordinates of (0.159, 0.151) at 12.0 V showed a luminous efficiency of 4.2 cd/A and power efficiency of 1.66 lm/W at 20 mA/cm².     

Influence of heat treatment on indium-tin-oxide anodes and copper phthalocyanine hole injection layers in organic light-emitting diodes

Modifications of indium-tin-oxide (ITO) and copper phthalocyanine (CuPc) layers by heat treatment aimed at lowering driving voltage in organic light-emitting diodes (OLEDs) are examined. Significant changes were observed in the surface morphology and carrier injection properties of ITO and CuPc layers after annealing at T = 250 °C for 0-60 min in a glove box. In the case of ITO annealing, although the ITO work function gradually decreased and the surface of the ITO layer became smoother than that of an unannealed ITO layer, we observed an appreciable decrease in the driving voltage with an increase in annealing time. In the case of CuPc annealing, on the other hand, we observed deterioration of the OLED's characteristics. All devices demonstrated an increase in driving voltage due to the pronounced crystallization of the CuPc layer.     

The relationship between the host structure and optimum doping concentration in red phosphorescent organic light-emitting diodes

The effect of the host structure on the optimum doping concentration in red phosphorescent organic light-emitting diodes (PHOLEDs) was investigated. A mixed host of the phosphine oxide derivative (SPPO21) and 4,4′,4″-tris(N-carbazolyl)triphenylamine (TCTA) was used as the host in the red PHOLED and the device performances were studied according to the doping concentration. The device performance of the red PHOLED was optimized at a doping concentration of 2% in the SPPO21 rich red PHOLED, while the device performance was optimized at a doping concentration of 6% in the device with 50% TCTA content. It was found that the optimum doping concentration was determined by the energy transfer and charge trapping.     

Admittance spectroscopic analysis of organic light emitting diodes with the CFX plasma treatment on the surface of indium tin oxide anodes

Admittance spectroscopic analysis was used to examine the effect of a CF_X plasma surface treatment on indium tin oxide (ITO) anodes using CF₄ gas and model the equivalent circuit for organic light emitting diodes (OLEDs) with the of ITO anode surface treated with CF_X plasma. This device with the ITO/N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-diphenyl-4,4′-diamine/tris-(8-hydroxyquinoline) aluminum/lithium fluoride/Al structure was modeled as a simple combination of two resistors and a capacitor. The ITO anode surface treated with the CF_X plasma showed a shift in the vacuum level of the ITO, which resulted in a decrease in the barrier height for hole injection at the ITO/organic interface. Admittance spectroscopy measurements of the devices with the CF_X plasma treatment on the surface of the ITO anodes showed a change in the contact resistance, bulk resistance and bulk capacitance.     

Correlation of recombination zone and device performances of phosphorescent white organic light-emitting diodes

We demonstrate a general method for tuning the color performance of white organic light-emitting diodes (WOLEDs) by inserting 0.5 nm thick red emitting layer in different location of blue phosphorescent emitting layer. The Commission Internationale de L'Eclairage (CIE) coordinates of WOLEDs were dependent on the position of red emitting layer and they were correlated with recombination zone of the blue phosphorescent emitting layer. Red shift of white CIE was observed as the location of red emitting layer get close to recombination zone of blue emitting layer. In addition, CIE of WOLEDs was kept stable between 100 cd/m² and 10,000 cd/m².     

Efficient top-emitting organic light-emitting diodes with Sm/Ag bilayer cathode

A bilayer is used as a semitransparent cathode for top-emitting organic light emitting devices (top-emitting OLEDs). The bilayer cathode consists of samarium (Sm) and silver (Ag). Top-emitting OLEDs with the bilayer cathode showed enhanced current injection and high electroluminescence efficiency as compared with a Sm cathode. The maximum current efficiency of the top-emitting OLEDs with a Sm/Ag cathode is 9.9 cd/A, much greater than 4.9 cd/A obtained from the top-emitting OLEDs with a Sm cathode. The improved performance can be attributed to the balance between optical transparency and electrical conductivity of the Sm/Ag cathode.     

Efficiency optimization of green phosphorescent organic light-emitting device

Using a narrow band gap host of bis[2-(2-hydroxyphenyl)-pyridine]beryllium (Bepp₂) and green phosphorescent Ir(ppy)₃ [fac-tris(2-phenylpyridine) iridium III] guest concentration as low as 2%, high efficiency phosphorescent organic light-emitting diode (PHOLED) is realized. Current and power efficiencies of 62.5 cd/A (max.), 51.0 lm/W (max.), and external quantum efficiency (max.) of 19.8% are reported in this green PHOLED. A low current efficiency roll-off value of 10% over the brightness of 10,000 cd/m² is noticed in this Bepp₂ single host device. Such a high efficiency is obtained by the optimization of the doping concentration with the knowledge of the hole trapping and the emission zone situations in this host-guest system. It is suggested that the reported device performance is suitable for applications in high brightness displays and lighting.     

Recent progresses on materials for electrophosphorescent organic light-emitting devices

Although organic light-emitting devices have been commercialized as flat panel displays since 1997, only singlet excitons were emitted. Full use of singlet and triplet excitons, electrophosphorescence, has attracted increasing attentions after the premier work made by Forrest, Thompson, and co-workers. In fact, red electrophosphorescent dye has already been used in sub-display of commercial mobile phones since 2003. Highly efficient green phosphorescent dye is now undergoing of commercialization. Very recently, blue phosphorescence approaching the theoretical efficiency has also been achieved, which may overcome the final obstacle against the commercialization of full color display and white light sources from phosphorescent materials. Combining light out-coupling structures with highly efficient phosphors (shown in the table-of-contents image), white emission with an efficiency matching that of fluorescent tubes (90 lm/W) has now been realized. It is possible to tune the color to the true white region by changing to a deep blue emitter and corresponding wide gap host and transporting material for the blue phosphor. In this article, recent progresses in red, green, blue, and white electrophosphorescent materials for OLEDs are reviewed, with special emphasis on blue electrophosphorescent materials.     

Surface conditioning of indium-tin oxide anodes for organic light-emitting diodes

Oxygen-plasma treatment of indium-tin oxide (ITO) anodes is now widely used as one of the most effective ways to improve the device performance of organic light-emitting diodes (LEDs). However, the role of oxygen-plasma treatment has not been clearly understood. We have performed detailed studies of the surface and bulk of the ITO thin films exposed to oxygen-plasma. We employed a multitude of experimental techniques, including X-ray and ultraviolet photoelectron spectroscopies, atomic force microscopy, dynamic contact angle measurement, four-point probe and Hall measurements to investigate the changes induced by the plasma. We have also analyzed the device characteristics of polymer LEDs fabricated with these anodes. We found significant modifications of the physico-chemical, morphological, transport and optical properties of the oxygen-plasma treated ITO. Although oxygen-plasma does not show any measurable etching effect, it induces considerable changes leading to an increase in work function, electron carrier concentration and conductivity. It also increases the surface energy and polarity. We relate these modifications to enhancement of the device performance, such as electroluminescence efficiency and lifetime, through their effects on hole injection, and interface structure and stability. Finally, we show that even in the presence of a hole-transport layer such as a poly(styrene sulphonate)-doped poly(3,4-ethylene dioxythiophene) (PEDOT:PSS) inserted between the anode and the emissive polymer layer, oxygen-plasma treatment of the ITO anodes is still beneficial for the devices.     

Electrically driven quantum dot/wire/well hybrid light-emitting diodes

Electrically driven quantum dot, wire, and well hybrid light-emitting diodes are demonstrated by using nanometer-sized pyramid structures of GaN. InGaN quantum dots, wires, and wells are formed at the tops, edges, and sidewalls of pyramids, respectively. The hybrid light-emitting diodes containing low-dimensional quantum structures are good candidates for broad-band highly efficient visible lighting sources.     

Highly flexible silver nanowire electrodes for shape-memory polymer light-emitting diodes

Shape-memory polymer light-emitting diodes (PLEDs) using a new silver nanowire/polymer electrode are reported. The electrode can be stretched by up to 16% with only a small increase in sheet resistance. Large deformation shape change and recovery of the PLEDs to various bistable curvatures result in minimal loss of electroluminescence performance.     

High efficiency red organic light-emitting diodes using a phosphorescent iridium complex doped into a hole-blocking material

We demonstrate high-efficiency red electrophosphorescent organic light-emitting devices (OLEDs) by doping a red-emitting iridium complex, Bis[7-methyl-1-p- tolyisoquinolinato-N,C²′]-iridium(III)(acetylacetonate) [(7-mtiq)₂Ir(acac)], into a hole-blocking material, 4-biphenyloxolato aluminum(III)bis(2-methyl-8- quinolinato)4-phenylphenolate. Both the phosphorescent characteristics of (7-mtiq)₂Ir(acac) and the electroluminescence mechanisms of OLEDs are investigated in this study. The Commission Internationale de L'Eclairage coordinates of (0.66, 0.34) is very close to the National Television System Committee standard red point (0.66, 0.33). With a dopant concentration of about 4%, a maximum luminance of 31317 cd/m² and a luminous efficiency of 21.6 cd/A have been obtained.