Marina E. Kondakova

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...

Highly efficient fluorescent-phosphorescent triplet-harvesting hybrid organic light-emitting diodes

We demonstrate highly efficient white and nonwhite hybrid organic light-emitting diodes (OLEDs) in which singlet and triplet excited states, generated in the recombination zone, are utilized by fluorescence and phosphorescence, respectively. The excited states are formed at a blue fluorescent light-emitting layer (LEL), and the triplets diffuse through a spacer layer to one or more phosphorescent LEL(s). A key feature enabling the triplet diffusion in such OLEDs is the use of a blue fluorescent emitter with triplet energy above, or not much below, that of the fluorescent host. Additional material properties required for triplet harvesting are outlined. At 1000 cd/m2 a blue and yellow harvesting OLED shows 13.6% external quantum efficiency, 3.8 V, 30.1 lm/W, and color characteristics suitable for display application. High-efficiency harvesting R+G+B white, and B+G and B+R nonwhite OLEDs are also demonstrated. The triplet-harvesting mechanism was verified in all devices by physical methods including spectra...

Coumarin-Based, Electron-Trapping Iridium Complexes as Highly Efficient and Stable Phosphorescent Emitters for Organic Light-Emitting Diodes

A new class of coumarin-based iridium tris-cyclometalated complexes has been developed. These complexes are highly emissive, with emission colors ranging from green to orange-red. Besides modification of ligand structures, color tuning was realized by incorporation of ligands with different electrochemical properties in a heteroleptic structure. The organic light-emitting diodes (OLEDs) using these compounds as emissive dopants are highly efficient and stable. Unlike other Ir(III) phosphorescent dopants, these coumarin-based Ir(III) dopants can effectively trap and transport electrons in the emissive layer.

52.2: Hybrid Tandem White OLEDs with High Efficiency and Long Life‐time for AMOLED Displays and Solid‐State Lighting

Excellent performance (61 cd/A efficiency, 27% EQE, 6.2 V, 6500 K white point, 35,000 h half-life) was obtained using 2-stack hybrid tandem white OLED. This is >70% EQE improvement over the all-fluorescent tandem white resulting in >30% reduction in power consumption for a 32″ AMOLED HDTV. Hybrid Tandems for lighting show EQE between 28.4 and − 30.6%, and lifetime between 70,000 and 125,000 h without outcoupling.

P-171: High-Efficiency Low-Voltage Phosphorescent OLED Devices with Mixed Host

We demonstrate high-efficiency, low-voltage phosphorescent OLED devices (PHOLEDs) using mixed host materials in the light-emitting layer (LEL) and novel formulations in the electrontransporting/ electron-injecting layers (ETL/EIL). The LEL architecture offers significant improvement in efficiency and voltage compared to conventional phosphorescent OLEDs with carbazole-based hosts. Further voltage reduction in our PHOLEDs is achieved through use of a novel material formulation in the ETL and EIL.

Charge carriers and triplets in OLED devices studied by electrically detected electron paramagnetic resonance

Abstract— Organic light-emitting diodes (OLEDs) were investigated by an electron paramagnetic resonance (EPR) technique that uses the effective device conductance as the detection channel. This technique enables us to identify and study charge carriers and triplet excitons with high sensitivity. By using a series of model devices, it was demonstrated that this type of spectroscopy provides information regarding triplet energy transfer and the location of the recombination zone. The fundamental understanding about the extent of the recombination zone in various OLED architectures helps in the design of devices with improved performance.

17.3: Highly Efficient Fluorescent/Phosphorescent OLED Devices Using Triplet Harvesting

We demonstrate efficient white and non-white hybrid OLED devices operating by a triplet harvesting mechanism to create light. Triplet excited states are generated in a blue fluorescent light-emitting layer (LEL) and utilized upon their diffusion to the phosphorescent LEL(s). At 1000 cd/m2 a blue/yellow hybrid OLED device shows external quantum efficiency (EQE) of 13.6%, 3.8 V, 30.1 lm/W, and excellent color characteristics suitable for display application. Performance of non-white-emitting hybrids, RGB white, and a tandem hybrid device is discussed. The triplet harvesting mechanism in all hybrid devices was verified by several experimental methods (spectral analysis, time-resolved electroluminescence (EL), magnetic field effect on EL).

P‐174: Triplet Exciton Diffusion in Hybrid Fluorescent/Phosphorescent OLEDs

A hybrid OLED comprising blue fluorescent and red phosphorescent emitters is reported. External quantum efficiencies for the blue and the red components of the electroluminescence along with time-resolved measurements, magnetic field effect, and ED-EPR experiments suggest that the recombination occurs in the blue emissive layer producing fluorescence with the singlet excitons, while the triplet excitons are utilized upon migration to the red layer.

P-149: Effect of Organic Hole-Injecting Buffer Layer on Stability of Organic Light-Emitting Diodes

Lifetimes of blue fluorescent and red phosphorescent OLEDs are improved by the presence of an organic buffer layer between the oxygen-plasma-treated ITO and a hole-injecting layer of HAT-CN. Possible explanations are discussed.

P-146: Optical Measurement of the Emission Zone in Organic Light-Emitting Diodes

Optical interference (weak microcavity) effects are used to measure the location and width of the emission zone in mixed-host phosphorescent organic light-emitting diodes. The consequences of varying composition and varying current density are evaluated quantitatively for the width of the emission zone, the internal quantum efficiency, and the outcoupling efficiency. The devices that are used here are optimized for optical interference effects. The conclusions should carry over to devices that are optimized for practical performance.