Michael S. Weaver

High operational stability of electrophosphorescent devices

Electrophosphorescent devices with fac-tris(2-phenylpyridine)iridium as the green emitting dopant have been fabricated with a variety of hole and exciton blocking materials. A device with aluminum(III)bis(2-methyl-8-quinolinato)4-phenylphenolate (BAlq) demonstrates an efficiency of 19 cd/A with a projected operational lifetime of 10 000 h, operated at an initial brightness of 500 cd/m2; or 50 000 h normalized to 100 cd/m2. An orange-red electrophosphorescent device with iridium(III) bis(2-phenylquinolyl-N,C2′)acetylacetonate as the dopant emitter and BAlq as the hole blocker demonstrates a maximum efficiency of 17.6 cd/A with a projected operational lifetime of 5000 h at an initial brightness of 300 cd/m2; or 15 000 h normalized to 100 cd/m2. The average voltage increase for both devices is <0.3 mV/h. The device operational lifetime is found to be inversely proportional to the initial brightness, typical of fluorescent organic light emitting devices.

Organic light-emitting devices with extended operating lifetimes on plastic substrates

We fabricate long-lived organic light-emitting devices using a 175 μm thick polyethylene terephthalate substrate coated with an organic–inorganic multilayered barrier film and compare the rate of degradation to glass-based devices. The observed permeation rate of water vapor through the plastic substrate was estimated to be 2×10−6 g/m2/day. Driven at 2.5 mA/cm2, we measure a device lifetime of 3800 h from an initial luminance of 425 cd/m2.

Thin film encapsulated flexible organic electroluminescent displays

We describe encapsulated passive matrix, video rate organic light-emitting diode (OLED) displays on flexible plastic substrates using a multilayer barrier encapsulation technology. The flexible OLED (FOLED™) displays are based on highly efficient electrophosphorescent OLED (PHOLED™) technology deposited on barrier coated plastic (Flexible Glass™ substrate) and are hermetically sealed with an optically transmissive multilayer barrier coating (Barix™ encapsulation). Preliminary lifetime to half initial luminance (L0∼100 cd/m2) of order 200 h is achieved on the passive matrix driven encapsulated 80 dpi displays; 2500 h lifetime is achieved on a dc tested encapsulated 5 mm2 FOLED test pixel. The encapsulated displays are flexed 1000 times around a 1 in. diameter cylinder and show minimal damage.

Saturated deep blue organic electrophosphorescence using a fluorine-free emitter

We demonstrate saturated, deep blue organic electrophosphorescence using the facial- and meridianal- isomers of the fluorine-free emitter tris(phenyl-methyl-benzimidazolyl)iridium(III)(f-Ir(pmb)3 and m-Ir(pmb)3, respectively) doped into the wide energy gap host, p-bis(triphenylsilyly)benzene (UGH2). The highest energy electrophosphorescent transition occurs at a wavelength of λ=389nm for the fac- isomer and λ=395nm for the mer- isomer. The emission chromaticity is characterized by Commission Internationale de l’Eclairage coordinates of (x=0.17,y=0.06) for both isomers. Peak quantum and power efficiencies of (2.6±0.3)% and (0.5±0.1)lm∕W and (5.8±0.6)% and (1.7±0.2)lm∕W are obtained using f-Ir(pmb)3 andm-Ir(pmb)3 respectively. This work represents a departure from previously explored, fluorinated blue phosphors, and demonstrates an efficient deep blue/near ultraviolet electrophosphorescent device.

High-efficiency top-emitting organic light-emitting devices

Based on theoretical arguments that top-emitting organic light-emitting devices (TOLEDs) can be more efficient than equivalent bottom-emitting devices, we fabricated TOLEDs comprising reflective anodes and transparent compound cathodes that emit 20.8% more photons in the forward 120° cone than equivalent bottom-emitting OLEDs. Device optimization by tuning the thickness of the top indium–tin–oxide layer according to a microcavity model is also reported.

Gas permeation and lifetime tests on polymer-based barrier coatings

We describe a flexible, transparent plastic substrate for OLED display applications. A flexible, composite thin film barrier is deposited under vacuum onto commercially available polymers, restricting moisture and oxygen permeation rates to undetectable levels using conventional permeation test equipment. The barrier is deposited under vacuum in a process compatible with conventional roll- coating technology. The film is capped with a thin film of transparent conductive oxide yielding an engineered substrate (BarixTM) for next generation, rugged, lightweight or flexible OLED displays. Preliminary tests indicate that the substrate is sufficiently impermeable to moisture and oxygen for application to moisture-sensitive display applications, such as organic light emitting displays, and is stable in pure oxygen to 200 degrees Celsius.

Intrinsic luminance loss in phosphorescent small-molecule organic light emitting devices due to bimolecular annihilation reactions

Operational degradation of blue electrophosphorescent organic light emitting devices (OLEDs) is studied by examining the luminance loss, voltage rise, and emissive layer photoluminescence quenching that occur in electrically aged devices. Using a model where defect sites act as deep charge traps, nonradiative recombination centers, and luminescence quenchers, we show that the luminance loss and voltage rise dependence on time and current density are consistent with defect formation due primarily to exciton-polaron annihilation reactions. Defect densities ∼1018cm−3 result in >50% luminance loss. Implications for the design of electrophosphorescent OLEDs with improved lifetime are discussed.

Direct evidence for degradation of polaron excited states in organic light emitting diodes

We investigate the intrinsic degradation mechanisms of the prototypical phosphorescent emissive material fac-tris(2-phenylpyridine) iridium [Ir(ppy)3] doped into the host 4, 4′-bis(3-methylcarbazol-9-yl)-2,2′-biphenyl (mCBP) by separately evaluating the effects of unipolar current, optical excitation, and their combination. We find that the mCBP anion is unstable and becomes more so in its excited state. Degradation due to the formation of defect states is evident from changes in the capacitance-voltage characteristics and from increasing drive voltage over time of a unipolar test device. These changes are understood within the framework of trapped-charge-limited transport, allowing for the determination of rate constants for each degradation mechanism. We also observe degradation of the hole transport material 4, 4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl under sub-energy-gap illumination and suggest that this instability may proceed through excitation of its cationic state. These results provide dir...

Improved initial drop in operational lifetime of blue phosphorescent organic light emitting device fabricated under ultra high vacuum condition

A blue phosphorescent organic light emitting device fabricated under the ultra high vacuum (UHV) condition of 6.5 × 10−7 Pa decreases the initial luminance drop upon lifetesting under a constant dc current of 15 mA/cm2 by 3 times compared to a device fabricated under a high vacuum (HV) condition of 7.6 × 10−6 Pa resulting in a 23% increase in half lifetime. We calculate a water content of 10−4 mol. % in the UHV device emissive layer (EML) and 10−2 mol. % in the HV device EML. We discuss the effects of water on luminance loss and voltage rise for the devices.

Power Consumption and Temperature Increase in Large Area Active-Matrix OLED Displays

We model and analyze the power consumption and resulting temperature rise in active-matrix organic-light-emitting device (AMOLED) displays as a function of the OLED efficiency, display resolution and display size. Power consumption is a critical issue for mobile display applications as it directly impacts battery requirements, and it is also very important for large area applications where it affects the display temperature rise, which directly impacts the panel lifetime. Phosphorescent OLEDs (PHOLEDs) are shown to offer significant advantage as compared to conventional fluorescent OLEDs due to high luminous efficiency resulting in lower pixel currents, reducing both the power consumed in the OLED devices and the series connected driving thin-film transistor (TFT). The power consumption and temperature rise of OLED displays are calculated as a function of the device efficiency, display size, display luminance and the type of backplane technology employed. The impact of using top-emission OLEDs is also discussed.