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 present a comprehensive, up-to-date compilation of band parameters for the technologically important III–V zinc blende and wurtzite compound semiconductors: GaAs, GaSb, GaP, GaN, AlAs, AlSb, AlP, AlN, InAs, InSb, InP, and InN, along with their ternary and quaternary alloys. Based on a review of the existing literature, complete and consistent parameter sets are given for all materials. Emphasizing the quantities required for band structure calculations, we tabulate the direct and indirect energy gaps, spin-orbit, and crystal-field splittings, alloy bowing parameters, effective masses for electrons, heavy, light, and split-off holes, Luttinger parameters, interband momentum matrix elements, and deformation potentials, including temperature and alloy-composition dependences where available. Heterostructure band offsets are also given, on an absolute scale that allows any material to be aligned relative to any other.

Band parameters for III–V compound semiconductors and their alloys

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

White organic light-emitting devices (WOLEDs) have advanced over the last twelve years to the extent that these devices are now being considered as efficient solid-state lighting sources. Initially, WOLEDs were targeted towards display applications for use primarily as liquid-crystal display backlights. Now, their power efficiencies have surpassed those of incandescent sources due to improvements in device architectures, synthesis of novel materials, and the incorporation of electrophosphorescent emitters. This review discusses the advantages and disadvantages of several WOLED architectures in terms of efficiency and color quality. Hindrances to their widespread acceptance as solid-state lighting sources are also noted.

White Organic Light‐Emitting Devices for Solid‐State Lighting

An organic light emitting device (OLED) structure having reflective surfaces is fabricated in a pit formed in a substrate. The pit has slanted reflective side walls which redirect light that is waveguided in the organic layers of the OLED to a direction substantially normal to the plane of the substrate. The reflective structure can also be formed with a planar configuration over interconnects covered with a polyimide.

High efficiency organic light emitting devices with light directing structures

A multicolor organic light emitting display device employs angle-walled blue, green and red emitting mesas, with optional metal reflectors on the angled walls, in a plurality of pixels. The angle-walled mesas, which resemble truncated pyramids, direct light out of the mesa by reflection from the mesa side walls or by mirror reflection. The device of the present invention reduces waveguiding, thus simultaneously increasing both display brightness and resolution.

Displays having mesa pixel configuration

We present an integrated classical and quantum-mechanical theory of weak microcavity effects in layered media that treats both radiative and waveguided modes. The electromagnetic field of radiative modes is determined using classical field quantization, with the transition probability into each mode given by Fermi's ``golden rule.'' We apply this theory to model the dependence of the electroluminescence spectral intensity and polarization of organic light-emitting devices (OLED's) on emission angle, organic layer thickness, and applied voltage. Light propagation in the OLED layers and the substrate is described by both ray and wave optics. Theoretical predictions are compared to experimental observations on single heterostructure, and multiple layer stacked red-green-blue OLEDs. Analysis of the polarization, spectral shape, and intensity of the electroluminescence spectrum in the forward-scattered half plane accurately fits the experimental data. The theory predicts, and the experimental measurements confirm, that the in-plane emission from conventional OLED structures is strongly TM polarized, and can be redshifted by as much as 60 nm with respect to the peak emission in the normal direction. Measurements coupled to our analysis also indicate that the efficiency of generating singlet excitons in aluminum tris(8-hydroxyquinoline) $({\mathrm{Alq}}_{3})$-based OLED's is $5\ifmmode\pm\else\textpm\fi{}1%,$ with a \ensuremath{\sim}500-\AA{}-thick ${\mathrm{Alq}}_{3}$ layer corresponding to the highest external power efficiency.

Weak microcavity effects in organic light-emitting devices

We study the internal and external quantum efficiencies of vacuum-deposited organic light-emitting devices (OLED's). The internal quantum efficiency of OLED's based on tris-(8-hydroxyquinoline) aluminum is calculated to be 5.7 times the observed external quantum efficiency ?(e), consistent with measurements. We demonstrate a shaped substrate that increases ?(e) by a factor of 1.9+/-0.2 over similar OLED's fabricated upon flat glass substrates and leads to a 100%-emissive aperture, i.e., the emitting area completely occupies the display area even in the presence of metal interconnects. We also discuss a substrate structure that increases ?(e) by an additional factor of 2. The high device efficiencies are promising for developing OLED-based displays with extremely low power consumption and increased operational lifetime.

High-external-quantum-efficiency organic light-emitting devices

By incorporating a broad transverse waveguide (1.3 μm) in 0.97-μm-emitting InGaAs(P)/InGaP/GaAs separate-confinement-heterostructure quantum-well diode-laser structures we obtain record-high continuous-wave (cw) output powers for any type of InGaAs-active diode lasers: 10.6–11.0 W from 100-μm-wide-aperture devices at 10 °C heatsink temperature, mounted on either diamond or Cu heatsinks. Built-in discrimination against the second-order transverse mode allows pure fundamental-transverse-mode operation (θ⊥=36°) to at least 20-W-peak pulsed power, at 68×threshold. The internal optical power density at catastrophic optical mirror damage (COMD) PCOMD is found to be 18–18.5 MW/cm2 for these conventionally facet-passivated diodes. The lasers are 2-mm-long with 5%/95% reflectivity for front/back facet coating. A low internal loss coefficient (αi=1 cm−1) allows for high external differential quantum efficiency ηd (85%). The characteristic temperatures for the threshold current T0 and the differential quantum effic...

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Dmitri Z. Garbuzov

Papers

High efficiency organic light emitting devices with light directing structures

Displays having mesa pixel configuration

Weak microcavity effects in organic light-emitting devices

High-external-quantum-efficiency organic light-emitting devices

High-power (>10 W) continuous-wave operation from 100-μm-aperture 0.97-μm-emitting Al-free diode lasers