Joseph John Shiang

Organic light-emitting devices for illumination quality white light

A method of generating white light by combining a blue organic light-emitting diode with a down-conversion phosphor system is presented. It is demonstrated that the use of the down-conversion phosphor system actually leads to an overall power efficiency increase, an effect attributed to the high quantum efficiency of phosphor materials and the presence of light scattering in the phosphor layers. It is also shown that this approach permits the generation of illumination quality white light over the full range of color temperatures required for lighting applications. For the model device demonstrated in this work, an overall electrical to optical power conversion efficiency of 1.3% was achieved at a brightness of 1080 cd/m 2 .

Experimental demonstration of increased organic light emitting device output via volumetric light scattering

A set of experimental measurements of scattering films and organic light emitting devices (OLEDs) is presented. We measure the reflectance, transmission, and emission characteristics of scattering media and OLED devices separately, and use this data as input into a simple radiative transfer model to predict the effect of light scattering on OLED light output. We find quantitative agreement between the radiative transfer model predictions and experimental results. We find that the introduction of volumetric scattering mechanisms increases the output of OLEDs by as much as 40%, which corresponds to over 70% of the light within a typical glass substrate being coupled to air.

Eu2+–Mn2+ phosphor saturation in 5mm light emitting diode lamps

This letter reports on phosphor quenching by saturation, a sublinear phosphor response with excitation intensity, in light emitting diode (LED) packages using 405nm LEDs and Ca5(PO4)3Cl:Eu2+,Mn2+ phosphors. This saturation is due to the high light flux incident on the phosphors in these LED packages and the slow radiative relaxation rate of Mn2+. Apart from known saturation processes of Mn2+ ground state depletion and energy transfer between excited Mn2+ ions, an additional quenching pathway, Eu2+→Mn2+ (excited) energy transfer, is taken into account to quantitatively fit both the efficiency under pulsed operation and the time-resolved luminescence of Ca5(PO4)3Cl:Eu2+,Mn2+.

Application of radiative transport theory to light extraction from organic light emitting diodes

One limitation on organic light emitting diode (OLED) performance is the optical extraction efficiency ηex, which is defined as the ratio of light generated within the device to light emitted into the ambient. Typical estimates for ηex, in OLEDs range between 0.17 and 0.5. We develop a simple radiative transport model that quantifies the effect of volumetric light scattering on light output in OLEDs in terms of a small set of readily measured parameters. The methodology is sufficiently general to parametrize and describe many of the light extraction schemes found in the literature. A set of model calculations is presented using typical OLED parameters; these calculations show that the introduction of light scattering sites within the otherwise transparent substrate can increase light extraction efficiencies to values between 0.55 and 1.

Solution-Processed Organic Light-Emitting Diodes for Lighting

In this paper, the vapor-deposited and solution-processed organic light-emitting diode (OLED) technology development paradigms are described and then compared with respect to their prospects for enabling general lighting applications. Two key development needs are improved device efficiency and lower cost fabrication methods. Progress in these areas for solution-processed OLEDs is illustrated by describing recent methods for attaining high efficiency blue emission and introducing novel low cost process methods for device fabrication which enable high performance devices without the need for any vacuum processing steps

Efficient bottom cathodes for organic light-emitting devices

Bilayers of aluminum and an alkali fluoride are well-known top cathode contacts for organic light-emitting devices but have never been successfully applied as bottom contacts. We describe a bilayer bottom cathode contact for organic electronic devices based on reversing the well-known top cathode structure such that the aluminum, rather than the alkali fluoride, contacts the organic material. Electron-only devices were fabricated showing enhanced electron injection from this bottom contact. Kelvin probe, x-ray photoelectron spectroscopy experiments, and thermodynamic calculations suggest that the enhancement results from n doping of the organic material by dissociated alkali metals.

4.1: Invited Paper: Large Area White OLEDs

A large area white OLED panel that emits 1200 lumens of illumination-quality light is described. The technical approach utilizes down-conversion from an underlying blue OLED made with a fault-tolerant monolithically series-connected architecture. The high device efficiency suggests that the singlet/triplet ratio for polymer OLEDs can be greater than 25%.

Phosphor quenching in LED packages: measurements, mechanisms, and paths forward

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.

Light extraction from OLEDs using volumetric light scattering

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.

Illumination-quality OLEDs for lighting

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.