Raymond Kwong

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.

Endothermic energy transfer: A mechanism for generating very efficient high-energy phosphorescent emission in organic materials

Intermolecular energy transfer processes typically involve an exothermic transfer of energy from a donor site to a molecule with a substantially lower-energy excited state (trap). Here, we demonstrate that an endothermic energy transfer from a molecular organic host (donor) to an organometallic phosphor (trap) can lead to highly efficient blue electroluminescence. This demonstration of endothermic transfer employs iridium(III)bis(4,6-di-fluorophenyl)-pyridinato-N,C2′)picolinate as the phosphor. Due to the comparable energy of the phosphor triplet state relative to that of the 4,4′-N,N′-dicarbazole-biphenyl conductive host molecule into which it is doped, the rapid exothermic transfer of energy from phosphor to host, and subsequent slow endothermic transfer from host back to phosphor, is clearly observed. Using this unique triplet energy transfer process, we force emission from the higher-energy, blue triplet state of the phosphor (peak wavelength of 470 nm), obtaining a very high maximum organic light-emi...

High-efficiency red electrophosphorescence devices

We demonstrate high-efficiency red electrophosphorescent organic light-emitting devices employing bis(2-(2′-benzo[4,5-a]thienyl)pyridinato-N,C3′) iridium(acetylacetonate) [Btp2Ir(acac)] as a red phosphor. A maximum external quantum efficiency of ηext=(7.0±0.5)% and power efficiency of ηp=(4.6±0.5) lm/W are achieved at a current density of J=0.01 mA/cm2. At a higher current density of J=100 mA/cm2, ηext=(2.5±0.3)% and ηp=(0.56±0.05) lm/W are obtained. The electroluminescent spectrum has a maximum at a wavelength of λmax=616 nm with additional intensity peaks at λsub=670 and 745 nm. The Commission Internationale de L’Eclairage coordinates of (x=0.68, y=0.32) are close to meeting video display standards. The short phosphorescence lifetime (∼4 μs) of Btp2Ir(acac) leads to a significant improvement in ηext at high currents as compared to the previously reported red phosphor, 2,3,7,8,12,13,17,18-octaethyl-12H, 23H-prophine platinum (II) PtOEP with a lifetime of ∼50 μs.

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.

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.

Efficient electrophosphorescence using a doped ambipolar conductive molecular organic thin film

Abstract We demonstrate a high efficiency organic electrophosphorescent device comprised of a 4,4′,4′′-tris(3-methylphenylphenylamino)triphenylamine (m-MTDATA) hole transport layer and a 4,4′-N,N′-dicarbazole-biphenyl (CBP) host doped with the metallorganic phosphor, fac-tris(2-phenylpyridine)iridium (Ir(ppy)3) as the green light-emitting layer. The device exhibits peak external quantum and power efficiencies of (12.0±0.6)% and (45±2) lm/W, respectively, corresponding to ∼60% internal quantum efficiency. A luminance of 1850 cd/m2 is observed at a current density of 10 mA/cm2. The device operating properties are controlled by electron injection into, and transport by the CBP layer along with hole injection from m-MTDATA directly into the Ir(ppy)3 highest occupied molecular level, leading to direct carrier recombination and exciton formation on the phosphor dopant. Ambipolar conduction properties of the Ir(ppy)3:CBP layer are established by analysis of triplet–triplet annihilation, exciton formation and the luminance–current–voltage characteristics.

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.

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.

Improving the performance of conjugated polymer-based devices by control of interchain interactions and polymer film morphology

Interchain interactions in conjugated polymer films promote good carrier transport but also reduce the luminescence quantum yield, leading to a fundamental trade-off in optimizing film morphology for device performance. We present two methods to improve the efficiency of light-emitting diodes (LEDs) based on poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) by altering film morphology without changing device architecture. First, “trilayer” LEDs, which use a central MEH-PPV layer with reduced interchain interactions between layers of highly aggregated MEH-PPV near the electrodes, have a higher efficiency than single-layer devices. Second, device efficiency can be improved by annealing MEH-PPV films, so that the reduced emission upon increasing interchain interactions is overcome by more balanced charge injection.