Marc A. Baldo

Highly efficient phosphorescent emission from organic electroluminescent devices

The efficiency of electroluminescent organic light-emitting devices, can be improved by the introduction of a fluorescent dye. Energy transfer from the host to the dye occurs via excitons, but only the singlet spin states induce fluorescent emission; these represent a small fraction (about 25%) of the total excited-state population (the remainder are triplet states). Phosphorescent dyes, however, offer a means of achieving improved light-emission efficiencies, as emission may result from both singlet and triplet states. Here we report high-efficiency ([gsims]90%) energy transfer from both singlet and triplet states, in a host material doped with the phosphorescent dye 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum(II) (PtOEP). Our doped electroluminescent devices generate saturated red emission with peak external and internal quantum efficiencies of 4% and 23%, respectively. The luminescent efficiencies attainable with phosphorescent dyes may lead to new applications for organic materials. Moreover, our work establishes the utility of PtOEP as a probe of triplet behaviour and energy transfer in organic solid-state systems.

Very high-efficiency green organic light-emitting devices based on electrophosphorescence

We describe the performance of an organic light-emitting device employing the green electrophosphorescent material, fac tris(2-phenylpyridine) iridium [Ir(ppy)3] doped into a 4,4′-N,N′-dicarbazole-biphenyl host. These devices exhibit peak external quantum and power efficiencies of 8.0% (28 cd/A) and 31 lm/W, respectively. At 100 cd/m2, the external quantum and power efficiencies are 7.5% (26 cd/A) and 19 lm/W at an operating voltage of 4.3 V. This performance can be explained by efficient transfer of both singlet and triplet excited states in the host to Ir(ppy)3, leading to a high internal efficiency. In addition, the short phosphorescent decay time of Ir(ppy)3 (<1 μs) reduces saturation of the phosphor at high drive currents, yielding a peak luminance of 100u200a000 cd/m2.

Nearly 100% internal phosphorescence efficiency in an organic light-emitting device

We demonstrate very high efficiency electrophosphorescence in organic light-emitting devices employing a phosphorescent molecule doped into a wide energy gap host. Using bis(2-phenylpyridine)iridium(III) acetylacetonate [(ppy)2Ir(acac)] doped into 3-phenyl-4(1′-naphthyl)-5-phenyl-1,2,4-triazole, a maximum external quantum efficiency of (19.0±1.0)% and luminous power efficiency of (60±5) lm/W are achieved. The calculated internal quantum efficiency of (87±7)% is supported by the observed absence of thermally activated nonradiative loss in the photoluminescent efficiency of (ppy)2Ir(acac). Thus, very high external quantum efficiencies are due to the nearly 100% internal phosphorescence efficiency of (ppy)2Ir(acac) coupled with balanced hole and electron injection, and triplet exciton confinement within the light-emitting layer.

High-efficiency fluorescent organic light-emitting devices using a phosphorescentsensitizer

To obtain the maximum luminous efficiency from an organic material, it is necessary to harness both the spin-symmetric and anti-symmetric molecular excitations (bound electron–hole pairs, or excitons) that result from electrical pumping. This is possible if the material is phosphorescent, and high efficiencies have been observed in phosphorescent organic light-emitting devices. However, phosphorescence in organic molecules is rare at room temperature. The alternative radiative process of fluorescence is more common, but it is approximately 75% less efficient, due to the requirement of spin-symmetry conservation. Here, we demonstrate that this deficiency can be overcome by using a phosphorescent sensitizer to excite a fluorescent dye. The mechanism for energetic coupling between phosphorescent and fluorescent molecular species is a long-range, non-radiative energy transfer: the internal efficiency of fluorescence can be as high as 100%. As an example, we use this approach to nearly quadruple the efficiency of a fluorescent red organic light-emitting device.

High-efficiency organic electrophosphorescent devices with tris(2-phenylpyridine)iridium doped into electron-transporting materials

We demonstrate high-efficiency organic light-emitting devices employing the green electrophosphorescent molecule, fac tris(2-phenylpyridine)iridium [Ir(ppy)3], doped into various electron-transport layer (ETL) hosts. Using 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole as the host, a maximum external quantum efficiency (ηext) of 15.4±0.2% and a luminous power efficiency of 40±2u200aIm/W are achieved. We show that very high internal quantum efficiencies (approaching 100%) are achieved for organic phosphors with low photoluminescence efficiencies due to fundamental differences in the relationship between electroluminescence from triplet and singlet excitons. Based on the performance characteristics of single and double heterostructures, we conclude that exciton formation in Ir(ppy)3 occurs within close proximity to the hole-transport layer/ETL:Ir(ppy)3 interface.

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...

Improved energy transfer in electrophosphorescent devices

External quantum efficiencies of up to (5.6±0.1)% at low brightness and (2.2±0.1)% at 100u2002cd/m2 are obtained from a red electrophosphorescent device containing the luminescent dye 2,3,7,8, 12,13,17,18-octaethyl-21H23H-phorpine platinum(II) (PtOEP) doped in a 4,4′-N,N′-dicarbazolebiphenyl (CBP) host. Due to weak overlap between excitonic states in PtOEP and CBP, efficiency losses due to nonradiative recombination are low. However, energy transfer between the species is also poor. In compensation, a thin layer of 2,9-dimethyl-4,7 diphenyl-1,10-phenanthroline is used as a barrier to exciton diffusion in CBP, improving the energy transfer to PtOEP. This technique may be applied to improve the efficiency of other electrophosphorescent devices.

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)u200alm/W are achieved at a current density of J=0.01u200amA/cm2. At a higher current density of J=100u200amA/cm2, ηext=(2.5±0.3)% and ηp=(0.56±0.05)u200alm/W are obtained. The electroluminescent spectrum has a maximum at a wavelength of λmax=616u200anm 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.

Phosphorescent materials for application to organic light emitting devices

Organic phosphors have demanded the attention of the organic electroluminescence community because they enable efficiencies quadruple that of fluorescent materials. In this work, we review the categories of organic phosphors: lanthanide complexes, organic phosphors and metal-organic complexes. The characteristics necessary for efficient phosphor- escence are considered and conclusions are drawn as to the most promising materials.

Electroluminescence mechanisms in organic light emitting devices employing a europium chelate doped in a wide energy gap bipolar conducting host

The mechanism for energy transfer leading to electroluminescence (EL) of a lanthanide complex, Eu(TTA)3phen (TTA=thenoyltrifluoroacetone,phen=1,10-phenanthroline), doped into 4,4′-N,N′-dicarbazole-biphenyl (CBP) host is investigated. With the device structure of anode/hole transport layer/Eu(TTA)3phen(1%):CPB/electron transport layer/cathode, we achieve a maximum external EL quantum efficiency (η) of 1.4% at a current density of 0.4 mA/cm2. Saturated red Eu3+ emission based on 5Dx–7Fx transitions is centered at a wavelength of 612 nm with a full width at half maximum of 3 nm. From analysis of the electroluminescent and photoluminescent spectra, and the current density–voltage characteristics, we conclude that direct trapping of holes and electrons and subsequent formation of the excitons occurs on the dopant, leading to high quantum efficiencies at low current densities. With increasing current between 1 and 100 mA/cm2, however, a significant decrease of η along with an increase in CBP host emission is ob...