Anil Raj Duggal

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.

Transparent hybrid inorganic/organic barrier coatings for plastic organic light-emitting diode substrates

We have developed a coating technology to reduce the moisture permeation rate through a polycarbonate plastic film substrate to below 1×10−5g∕m2∕day using plasma-enhanced chemical vapor deposition. Unlike other ultrahigh barrier (UHB) coatings comprised of inorganic and organic multilayers, our UHB coating comprises a graded single hybrid layer of inorganic and organic materials. Hardness and modulus of the inorganic and the organic materials are tailored such that they are similar to those of typical glass-like materials and thermoplastics, respectively. In this barrier structure, the composition is periodically modulated between the inorganic and the organic materials, but instead of having distinctive interfaces between two materials, there are “transitional” zones where the coating composition changes continuously from one material to another. Our UHB coating also has superior visible light transmittance and color neutrality suitable for the use of display and lighting device substrates.

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

A Transparent, High Barrier, and High Heat Substrate for Organic Electronics

The use of plastic film substrates for organic electronic devices promises to enable new applications, such as flexible displays and conformal lighting, and a new low-cost paradigm through high-volume roll-to-roll fabrication. Unfortunately, presently available substrates cannot yet deliver this promise because of the challenge in achieving the required combination of optical transparency, impermeability to water and oxygen, mechanical flexibility, high-temperature capability, and chemical resistance. Here, we describe the development and performance of a plastic substrate comprising a high heat polycarbonate film combined with a unique transparent coating package that is aimed at meeting this challenge.

Fault-tolerant, scalable organic light-emitting device architecture

High performance organic light emitting diodes (OLEDs) become problematic as emitting area increases due to the high resistivity of the transparent electrode and the increasing probability of encountering a catastrophic short-circuit defect during fabrication. In this letter, a monolithic series-connected OLED architecture is demonstrated. It is shown that such devices exhibit the same power efficiency as traditional small area OLEDs but are, in addition, relatively insensitive to electrode resistivity and tolerant to normally catastrophic short-circuit defects. This architecture should enable applications such as lighting where scalability to large emitting area without high fabrication cost or design complexity is required.

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.

High power switching behavior in electrically conductive polymer composite materials

The electrical switching properties of electrically conductive polymer composites are studied at high current densities. It is shown that the switching properties at high current densities are substantially different from those at low current densities which depend on a positive temperature coefficient of resistance effect. This type of switching appears to be a general feature of conductor-filled polymer composite materials and should lead to a new class of fast, current-limiting devices for power circuits.

A NOVEL HIGH CURRENT DENSITY SWITCHING EFFECT IN ELECTRICALLY CONDUCTIVE POLYMER COMPOSITE MATERIALS

The electrical switching properties of electrically conductive polymer composites are studied at high current densities. It is shown that the switching properties at high current densities are substantially different from those at low current densities which depend on a positive temperature coefficient of resistance effect. Experiments are described that show that this type of switching appears to be a general feature in conductor-filled polymer composite materials and a qualitative model for the switching phenomenon is proposed. These results suggest that conductor-filled polymer composite materials can provide a new nonmechanical way of rapidly limiting high power short-circuit currents. This should have broad applications in the circuit protection industry.