Kevin P. Klubek

High-efficiency tandem organic light-emitting diodes

Tandem organic light-emitting diodes (OLEDs), with multiple electroluminescent (EL) units connected electrically in series, have been fabricated. Using an optically transparent doped organic “p-n” junction as the connecting unit between adjacent EL units, excellent light out-coupling and carrier-injection properties have been realized. The luminous efficiency is found to scale almost linearly with the number of EL units in the stack, giving values as high as 32 or 136 cd/A for a three-unit tandem OLED using a fluorescent or a phosphorescent emitter, respectively.

Recent developments in the synthesis of red dopants for Alq3 hosted electroluminescence

Abstract An isopropyl substituted red fluorescent dye, 4-(dicyanomethylene)-2- i - propyl -6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4 H -pyran (DCJTI), has been found to be a good dopant in a host MQ 3 matrix which produces organic EL devices with essentially identical luminance efficiency and chromaticity to that of the t-butyl derivative, DCJTB. The key intermediate, 4-(dicyanomethylene)-2-( I-propyl )6-methyl 4 H -pyran, involved in the synthesis of DCJTI is much easier to manufacture than that of DCJTB. In the interest of driving down the cost of OLED materials, DCJTI may prove to be a cheaper alternative for the red dopant DCJTB.

High-efficiency, low-voltage phosphorescent organic light-emitting diode devices with mixed host

We report high-efficiency, low-voltage phosphorescent green and blue organic light-emitting diode (PHOLED) devices using mixed-host materials in the light-emitting layer (LEL) and various combinations of electron-injecting and electron-transporting layers. The low voltage does not rely on doping of the charge-transport layers. The mixed LEL architecture offers significantly improved efficiency and voltage compared to conventional PHOLEDs with neat hosts, in part by loosening the connection between the electrical band gap and the triplet energy. Bulk recombination in the LEL occurs within ∼10 nm of the interface with an electron-blocking layer. A “hole-blocking layer” need not have hole- or triplet-exciton-blocking properties. Optical microcavity effects on the spectrum and efficiency were used to locate the recombination zone. The effect of layer thickness on drive voltage was used to determine the voltage budget of a typical device. The behavior of undoped devices was investigated, and the electrolumines...

Power efficiency improvement in a tandem organic light-emitting diode

When a tandem light-emitting diode (OLED) utilizes unoptimized electroluminescent (EL) units, it is fairly easy to improve the power efficiency of such a device. However, when a tandem OLED utilizes optimized EL units, improved power efficiency can only be achieved if each intermediate connector has excellent carrier injection capabilities along with a negligible voltage drop across it. Four organic intermediate connectors were studied in this work, one of which consisting of a Li-doped 4,7-diphenyl-1,10-phenanthroline layer and a 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile layer, exhibits the best power efficiency improvement for tandem OLEDs.

Operating lifetime recovery in organic light-emitting diodes having an azaaromatic hole-blocking/electron-transporting layer

Azaaromatic compounds (AACs) are widely used in organic light-emitting diodes (OLEDs), especially as efficient electron transporters. Yet, the operating lifetime of OLEDs is always compromised when AACs are involved in anything more than electron transport (e.g., hole blocking). We show (i) the operating lifetime of OLEDs incorporating AACs as a hole-blocking/electron-transporting layer (HBETL) depends strongly on the charge-conducting ability and excited state energy of the light-emitting layer (LEL) materials and (ii) shifting the charge recombination zone away from the LEL∣HBETL interface deeper into the LEL can recover the lost lifetime. Thus, a pure red fluorescent OLED is demonstrated having 5.3 V drive voltage, 6.5% external quantum efficiency, 6.6 cd/A electroluminescent yield, and ∼125 000 h half-life, all at 20 mA/cm2. This device utilizes an AAC as HBETL followed by an aluminum triquinolate (Alq) ETL doped with Li metal. Alternatively, the lifetime recovery might be assigned to the presence of ...

23.3: Distinguished Paper: High‐Efficiency Tandem Blue OLEDs

Tandem blue OLEDs using different fluorescent dopants have achieved 20 cd/A (CIEx,y = 0.14, 0.18) and 38 cd/A (CIEx,y = 0.15, 0.42) with external quantum efficiencies higher than 11%. The tandem blue OLEDs include two electroluminescent units that are connected in series with an organic intermediate connector. At an initial brightness of 1,000 cd/m2, the drive voltage of the devices can be less than 6.5 V, and the operational lifetime (T 50) of the devices is estimated to be greater than 10,000 h. We also demonstrate that the selection of electron-transporting material and an intermediate connector have significant impact on the electroluminescence performance of the tandem blue OLEDs.

36.3: Improving Operating Lifetime of Blue OLEDs with Phenanthroline-Based Electron-Transport Materials

We report new phenanthroline-based electron-transport materials that greatly improve the operating lifetime of blue OLEDs relative to 4,7-diphenyl-1,10-phenanthroline (Bphen). Use of these new materials results in efficient electron injection and transport, allowing for low voltage devices. Excellent performance is also observed for red and green OLEDs.

P-169: Efficient, Long-Lifetime OLED Host and Dopant Formulations for Full-Color Displays

We report developments in materials and formulations for blue, green, and red fluorescent OLEDs that provide lifetimes exceeding 15,000 h for a model display operating at 200 cd/m2 with a polarizer. In addition, we describe improvements in electron transport and injection that result in a reduction in display power consumption of up to 55%.

30.2: Improving Operating Lifetime of Organic Light‐Emitting Diodes with Perylene and Derivatives as Aggregating Light‐Emitting‐Layer Additives

Aggregating PAHs of the perylene class are useful as LEL additives. They greatly improve the device operating half-life up to 1,000,000 h at the initial luminance of 1540 cd/m2 (20 mA/cm2). There appear to be at least three factors in the lifetime increase: takeover of the LEL functions by the PAHs, aggregation, and an expansion of the recombination and/or emission zone.

Glassy nematic conjugated oligomers: materials for organic light-emitting diodes

Conjugated organic molecules and polymers are of great interest for applications as the active element in new types of electroluminescent, photovoltaic, and electronic devices. Control of the molecular order of these materials can have a significant impact on their optical and electronic properties. In particular, alignment of conjugated organics that exhibit an intrinsic anisotropy in a dipole moment permits generation of thin films that emit linearly polarized light in an orientation specific to the alignment direction of the molecule. Polarized photoluminescence and electroluminescence from ordered conjugated oligomers and polymers has been demonstrated using many alignment methods, but particular success in integrating these materials into electrical devices has been achieved by utilizing conjugated materials that exhibit a glassy nematic liquid-crystalline phase. We report the status of our ongoing development of OLED devices based on glassy nematic oligofluorenes. These devices exhibit electroluminescent peak polarization ratios as high as 31:1 and color coordinates spanning the entire visible spectrum.