Julie J. Brown

Blue organic electrophosphorescence using exothermic host–guest energy transfer

We demonstrate efficient blue electrophosphorescence using exothermic energy transfer from a host consisting of N,N′-dicarbazolyl-3,5-benzene (mCP) to the phosphorescent iridium complex iridium(III)bis[(4,6-difluorophenyl)-pyridinato-N,C2′]picolinate (FIrpic). By examining the temperature dependence of the radiative lifetime and the photoluminescence of a film of mCP doped with FIrpic, we confirm the existence of exothermic energy transfer in contrast to the endothermic transfer characteristic of the N,N′-dicarbazolyl-4-4′-biphenyl and FIrpic system. In employing exothermic energy transfer between mCP and FIrpic, a maximum external electroluminescent quantum efficiency of (7.5±0.8)% and a luminous power efficiency of (8.9±0.9)lm/W are obtained, representing a significant increase in performance over previous endothermic blue electrophosphorescent devices.

100% phosphorescence quantum efficiency of Ir(III) complexes in organic semiconductor films

We demonstrate that three Ir(III) complexes used as principal dopants in organic electrophosphorescent diodes have very high photoluminescence quantum efficiency (ηPL) in a solid-state film. The green emitting complex, fac-tris(2-phenylpyridinato)iridium(III) [Ir(ppy)3], the red-emitting bis[2-(2′-benzothienyl)pyridinato-N,C3′] (acetylacetonato)iridium(III) [Btp2Ir(acac)], and the blue complex bis[(4,6-difluorophenyl)pyridinato-N,C2](picolinato)iridium(III) (FIrpic) were prepared as codeposited films of varying concentration with 4,4′-bis(N-carbazolyl)-2,2′-biphenyl, a commonly used host material. The maximum ηPL values for Ir(ppy)3, Btp2Ir(acac), and FIrpic were, respectively, 97%±2% (at 1.5mol%), 51%±1% (at 1.4mol%), and 78%±1% (at 15mol%). Furthermore, we also observed that the maximum ηPL of FIrpic reached 99%±1% when doped into the high triplet energy host, m-bis(N-carbazolyl)benzene, at an optimal concentration of 1.2mol%.

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.

Triplet exciton confinement and unconfinement by adjacent hole-transport layers

To understand confinement of the triplet exciton of Ir(ppy)3 by hole-transport layers, we compared energy-dissipative processes of the triplet exciton of Ir(ppy)3 which is doped into 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (α-NPD), 4,4′-bis [N-(p-tolyl)-N- phenyl-amino]biphenyl (TPD), 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC), and 4,4′-N,N′-dicarbazole-biphenyl hosts. Significant energy transfer from Ir(ppy)3 into the triplet levels of α-NPD was observed. In the case of the TPD host, however, partial confinement of the Ir(ppy)3 triplet exciton was observed. This result suggests both forward and backward energy transfer from Ir(ppy)3 to the TPD triplet levels. Furthermore, employing TAPC as a hole-transport layer achieved strong confinement of the Ir(ppy)3 triplet exciton. One conclusion from these results is that electrophosphorescence efficiency is well correlated with the triplet energy level of the hole-transport layer host materials.

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.

Graded mixed-layer organic light-emitting devices

We describe the performance of graded, mixed-layer organic light- emitting devices (OLEDs). The devices are step graded from a mostly hole transporting layer (HTL) to a mostly electron transporting layer (ETL) from anode side to cathode side, respectively. Luminous efficiencies of >4.5 lm/W and 10 cd/A are obtained at 1000 cd/m2 for green, electrofluorescent, graded mixed OLEDs. These efficiencies are significantly higher than those of a uniformly mixed device, i.e., a device in which the HTL and ETL are uniformly mixed, but lower than those of a conventional heterostructure device employing the same dopant material. The operating lifetime of the graded mixed OLEDs, however, is much improved over the heterostructure device. The results of our work suggest that the graded mixed OLED device structure represents a path to achieving extended lifetimes with sufficient efficiency for flat panel display applications in which this parameter is critical to market acceptance.

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.