Phosphors in LED packages can experience much higher temperatures (>100°C) and light fluxes (>10 W/cm2) versus
traditional phosphors in fluorescent lighting. These conditions place stringent restrictions on LED phosphor selections
and requires, to some extent, an understanding of the potential quenching mechanisms that occur within LED packages.
In this report, we discuss flux-based and temperature-based quenching of LED phosphors, the measurements used to
analyze these quenching processes, and some of the basic mechanisms behind this. It is shown that flux-based
quenching in LEDs can be reasonably anticipated through simple design parameters. However, while it is more difficult
to a priori predict the thermal quenching of new phosphors and their modifications, it is possible to make initial
conclusions about phosphor design through a combination of spectroscopic measurements and chemical inference. This
is specifically demonstrated within the Ce3+-doped garnet family of phosphors, where there is significant flexibility to
modify compositions, leading to initial relationships between composition, emission color, and high temperature
quenching.
A series of phenol-capped, oligofluorenes having 2,3,5 and 7 fluorene units and a statistical oligomer with an average of about 10 fluorene units was prepared. In a similar fashion, phenol-capped oligomers having various charge-transporting moieties incorporated into the oligomeric structures were prepared. Polymers were prepared from the oligomers by various linking reactions involving the phenol groups. Trends in the optical and electrical properties as a function of oligomer length will be reported. Device data for this family of emissive copolymers indicates that charge mobility increases with conjugation length, and can be as good as or better than that of an analogous fluorene homopolymer.
Bilayers of aluminum (Al) and alkali fluoride (such as sodium fluoride) are well-known top cathode contacts for organic light-emitting devices (OLEDs) in which the alkali fluoride is inserted in between the Al and organic materials. However, the configuration, to date, has never been successfully applied as bottom cathode contacts. In this article, we describe a novel bilayer bottom cathode contact for OLEDs utilizing the same materials but with a reversed structure, i.e. the Al rather than the alkali fluoride contacts the organic material. Electron-only devices were fabricated showing enhanced electron injection from this bottom contact with respect to an Al-only contact. Kelvin probe, X-ray photoelectron spectroscopy, Auger electron spectroscopy experiments and thermodynamic calculations suggest that the enhancement results from n-doping of the organic material by dissociated alkali metals.
One of the limitations on OLED performance is the optical extraction efficiency, ηex, which is the ratio of light generated within the device to light emitted into the ambient. Ideally ηex is equal to unity. Typical estimates for this efficiency factor in OLEDs range between 0.17-0.5. We present a simple radiative transport model that quantifies the effect of volumetric light scattering on light output in terms of a small set of readily measured parameters. Our methodology is sufficiently general to parameterize and describe many of the light extraction schemes found in the literature. We will present a set of model calculations using parameters typical of many OLEDs, and show that the introduction of light scattering sites within the otherwise transparent substrate can increase light extraction efficiencies by at least a factor of 1.4. We also present experimental data to validate our analysis and demonstrate a high level of agreement between model and experiment.
OLED technology has improved to the point where it is now possible to envision developing OLEDs as a low cost solid state light source. In order to realize this, significant advances have to be made in device efficiency, lifetime at high brightness, high throughput fabrication, and the generation of illumination quality white light. In this talk, a down conversion method of generating white light is demonstrated and shown to be capable of generating illumination quality white light over the full range of color temperatures required for lighting. It is also demonstrated that, due to the presence of light scattering, the down-conversion method can actually increase the overall device power efficiency.
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