The efficiency of electroluminescent organic light-emitting devices1,2 can be improved by the introduction3 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 (≳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.

Highly efficient phosphorescent emission from organic electroluminescent devices

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 100 000 cd/m2.

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

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.

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

During the past decade, enormous progress has been made in growing ultrathin organic films and multilayer structures with a wide range of exciting optoelectronic properties. This progress has been made possible by several important advances in our understanding of organic films and their modes of growth. Perhaps the single most important advance has been the use of ultrahigh vacuum (UHV) as a means to achieve, for the first time, monolayer control over the growth of organic thin films with extremely high chemical purity and structural precision.1-3 Such monolayer control has been possible for many years using well-known techniques such as Langmuir-Blodgett film deposition,4 and more recently, self-assembled monolayers from solution have also been achieved.5 However, ultrahighvacuum growth, sometimes referred to as organic molecular beam deposition (OMBD) or organic molecular beam epitaxy (OMBE), has the advantage of providing both layer thickness control and an atomically clean environment and substrate. When combined with the ability to perform in situ highresolution structural diagnostics of the films as they are being deposited, techniques such as OMBD have provided an entirely new prospect for understanding many of the fundamental structural and optoelectronic properties of ultrathin organic film systems. Since such systems are both of intrinsic as well as practical interest, substantial effort worldwide has been invested in attempting to grow and investigate the properties of such thin-film systems. This paper is a review of recent progress made in organic thin films grown in ultrahigh vacuum or using other vapor-phase deposition methods. We will describe the most important work which has been published in this field since the emergence of OMBD in the mid-1980s. Both the nature of thin-film growth and structural ordering will be discussed, as well as some of the more interesting consequences to the physical properties of such organic thin-film systems will be considered both from a theoretical as well as an experimental viewpoint. Indeed, it will 1793 Chem. Rev. 1997, 97, 1793−1896

Ultrathin Organic Films Grown by Organic Molecular Beam Deposition and Related Techniques.

Simple rules for the understanding of Heusler compounds

We report the effects of dyes doped in the emitting layer on the electroluminescent characteristics of multilayer organic light‐emitting diodes (LEDs) using a polysilane polymer, poly(methylphenylsilane) (PMPS), as the hole transporting material. We formed the emitting layer by dispersing in poly(styrene) (PS), one of four dyes whose fluorescence ranged from blue to orange. Two‐ or three‐layer LEDs were prepared by combining PMPS and dye doped PS layers with the indium tin oxide and aluminum used for the hole and electron injecting electrodes, respectively. The three‐layer LEDs had an additional vacuum‐deposited tris‐(8‐hydroxyquinoline) aluminum layer. The electroluminescent (EL) characteristics of these multilayer organic LEDs, such as the current‐voltage–EL intensity curve, the relative EL efficiency, and the EL emitting species, exhibit a marked dependence on the emitting dye. The observed dependence can be described consistently in terms of the dependence of the charge carrier trapping efficiency on ...

Effects of doping dyes on the electroluminescent characteristics of multilayer organic light‐emitting diodes

We report the electroluminescent characteristics of organic multilayer light‐emitting diodes (LEDs) with a benzophenone (BP) dispersed poly(methylmethacrylate) (PMMA) film as an emitting layer. The electroluminescence (EL) intensity of these LEDs increases with decreasing temperature from 273 to 100 K when they are operated at the same voltages or the same current densities. The EL spectrum of the LEDs, which peaks at around 450 nm, is identical to the phosphorescence spectrum of BP in PMMA. In addition, the EL decay time was determined as 46.8 μs at 100 K by applying a rectangular voltage pulse. These results indicate that the EL of the LED originates from the triplet excited states of BP.

Electroluminescence from triplet excited states of benzophenone

We report the ac magnetic susceptibility χ ac and resistivity ρ measurements of EuFe 2 As 2 under high pressure P . By observing nearly 100% superconducting shielding and zero resistivity at P =28 kbar, we establish that P -induced superconductivity occurs at T c ∼30 K in EuFe 2 As 2 . ρ shows an anomalous nearly linear temperature dependence from room temperature down to T c at the same P . χ ac indicates that an antiferromagnetic order of Eu 2+ moments with T N ∼20 K persists in the superconducting phase. The temperature dependence of the upper critical field is also determined.

EuFe2As2 under High Pressure: An Antiferromagnetic Bulk Superconductor

Neutron diffraction measurements have been performed on the cubic compound ${\mathrm{PrPb}}_{3}$ in a [001] magnetic field to examine the quadrupolar ordering. Antiferromagnetic components with $q=(\frac{1}{2}\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta}\text{ }\text{ }\frac{1}{2}\text{ }\text{ }0)$, ($\frac{1}{2}\text{ }\text{ }\frac{1}{2}\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta}\text{ }\text{ }0$) ($\ensuremath{\delta}\ensuremath{\approx}\frac{1}{8}$) are observed below the transition temperature ${T}_{\mathrm{Q}}$ (0.4 K at $H=0$) whose amplitudes vary linear with $H$ and vanish at zero field, providing the first evidence for a modulated quadrupolar phase. For $Hl1\text{ }\text{ }\mathrm{T}$, a nonsquare modulated state persists even below 100 mK suggesting quadrupole moments associated with a ${\ensuremath{\Gamma}}_{3}$ doublet ground state to be partially quenched by hybridization with conduction electrons.

https://arxiv.org/pdf/cond-mat/0504625.pdf

Observation of modulated quadrupolar structures in PrPb3.

We report the ac magnetic susceptibility $\chi_{ac}$ and resistivity $\rho$ measurements of EuFe$_2$As$_2$ under high pressure $P$. By observing nearly 100% superconducting shielding and zero resistivity at $P$ = 28 kbar, we establish that $P$-induced superconductivity occurs at $T_c \sim$~30 K in EuFe$_2$As$_2$. $\rho$ shows an anomalous nearly linear temperature dependence from room temperature down to $T_c$ at the same $P$. $\chi_{ac}$ indicates that an antiferromagnetic order of Eu$^{2+}$ moments with $T_N \sim$~20 K persists in the superconducting phase. The temperature dependence of the upper critical field is also determined.

Hiroyuki Suzuki

Papers

Effects of doping dyes on the electroluminescent characteristics of multilayer organic light‐emitting diodes

Electroluminescence from triplet excited states of benzophenone

EuFe2As2 under High Pressure: An Antiferromagnetic Bulk Superconductor

Observation of modulated quadrupolar structures in PrPb3.

EuFe$_2$As$_2$ under high pressure: an antiferromagnetic bulk superconductor