M.-H. Lu

Ink-jet printing of doped polymers for organic light emitting devices

Ink-jet printing was used to directly deposit patterned luminescent doped-polymer films. The luminescence of polyvinylcarbazol (PVK) films, with dyes of coumarin 6 (C6), coumarin 47 (C47), and nile red was similar to that of films of the same composition deposited by spin coating. Light emitting diodes with low turn-on voltages were also fabricated in PVK doped with C6 deposited by ink-jet printing.

Improvement of output coupling efficiency of organic light-emitting diodes by backside substrate modification

The emission intensity of an organic light-emitting diode at normal viewing angle and the total external emission efficiency have been increased by factors of 9.6 and 3.0, respectively, by applying spherically shaped patterns to the back of the device substrate. The technique captures light previously lost to waveguiding in the substrate and, with proper choice of substrate, light previously lost to waveguiding in the organic/anode layers. A method of applying the technique using laminated films and an optical model for evaluating coupling efficiency are also presented.

Optimization of external coupling and light emission in organic light-emitting devices: modeling and experiment

The emission of light and external coupling after the appropriate excitons have been formed in the organic light-emitting devices (OLEDs) has been investigated. The internally emitted light can be classified into three modes: externally emitted, substrate waveguided, and indium–tin–oxide (ITO)/organic waveguided. A combined classical and quantum mechanical microcavity model is used to calculate the distribution of light emission into these three modes in an OLED on planar substrates. The ITO/organic modes maybe suppressed due to the thinness of the ITO/organic layers. Consequently, as much as over 50% of the internally generated light is emitted externally in some structures, much greater than the ∼20% figure given by classical ray optics. This model is used to examine how this distribution varies with exciton to cathode distance, the thickness of the ITO layer, and the index of refraction of the substrate. It can also be applied to OLEDs on shaped substrates where an increase in the total external emissi...

External coupling efficiency in planar organic light-emitting devices

The external coupling efficiency in planar organic light-emitting devices is modeled based on a quantum mechanical microvavity theory and measured by examining both the far-field emission pattern and the edge emission of light trapped in the glass substrate. The external coupling efficiency is dependent upon the thickness of the indium–tin–oxide layer and the refractive index of the substrate. The coupling efficiency ranges from ∼24% to ∼52%, but in general it is much larger than the 18.9% expected from classical ray optics.

Thin film transistors for foldable displays

We report the first amorphous silicon thin film transistors (TFTs) on flexible, ultra-thin substrates of 25 /spl mu/m thick stainless steel foil. The transistors remain operational under convex or concave bending down to 2.5 mm radius of curvature. The goal of our work is a transistor backplane circuit that can be folded, for use in active matrices in highly rugged and portable applications such as foldable intelligent maps. Our results suggest that such foldable backplane circuits are feasible.

Integration of organic LEDs and amorphous Si TFTs onto unbreakable metal foil substrates

A long sought goal has been a flat panel display that is unbreakable, lightweight, flexible, and low-cost. Organic LEDs (OLEDs) have the potential for such a technology because of their demonstrated performance, their lack of a need for a crystalline substrate, and potential low cost (e.g, deposition by spin-coating). Amorphous Si (a-Si) TFTs are in widespread production for the switching and storage elements in active matrix liquid crystal displays (AMLCDs). Both of these technologies, however, are conventionally fabricated on breakable glass substrates. In this paper, we report the integration of amorphous Si TFTs and OLEDs onto thin stainless steel foils, and the demonstration of a simple circuit in which the TFT drives the OLED. The finished substrate (40/spl times/40 mm/sup 2/) has been dropped down 30 feet onto a concrete floor with no degradation in device performance. The work shows that a-Si TFTs can provide adequate current levels to drive OLEDs.

Full-Color OLEDs Integrated by Dry Dye Printing

Dry dye printing and solvent-enhanced dye diffusion were used to locally dope a previously spin-coated poly(9-vinylcarbazole) (PVK) polymer film with different dyes to fabricate side-by-side red, green, and blue (RGB) organic light-emitting device pixels. The fabrication details and the resolution and stability of this patterning technique are discussed. The technique was then used to make combined polymer/small-molecule devices, in which the printability of polymer for color integration was combined with the superior transport properties and thin-layer capabilities of small molecules for high efficiency and low leakage current. To reduce reverse leakage current and raise efficiency, a blanket tris-8-hydroxyquinoline aluminum (Alq3) electron transport layer was deposited on top of the polymer layer after the dye diffusion step, along with a 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline hole/exciton blocking layer between the Alq3 and the PVK to ensure that all light emission occurred from the doped polymer and not from the Alq3. Devices with this hybrid doped polymer/small molecule structure have an extremely low reverse leakage current (with a rectification ratio of 106 at plusmn10 V). The electroluminescence efficiency of the devices was optimized by varying the dye concentration of the printing plate. A three-color passive-matrix test array with 300 mumtimes1 mm RGB subpixels was demonstrated with this structure

Printing approaches for large-area color organic LED displays

In this paper the fundamental properties underlying the transfer of organic fluorescent dyes to local areas in polymer thin films by both liquid phase (ink-jet printing) and evaporation/diffusion transport methods are examined, with the goal of achieving full color displays based on organic light emitting diodes made from such polymers. Ink-jet printing offers a simple non-contact method for forming patterns, but a critical issue is the redistribution of dyes and other molecules in the liquid droplet before it dries. Masked large area evaporations allows one to rapidly pattern large areas, but its rate depends on the ability of dyes to diffuse through polymer films.

Improved external coupling efficiency in organic light-emitting devices on high-index substrates

High-index-of-refraction substrates are shown theoretically and experimentally to increase the external coupling efficiency of organic light-emitting devices (OLEDs) by using a quantum mechanical microcavity model. This increase is due to the elimination of those modes waveguided in the ITO/organic layer. Bi-layer OLEDs were fabricated on standard soda lime glass and high-index glass substrates, and their far-field intensity pattern was measured. Among the devices optimized for external efficiency, those on shaped high-index substrates exhibited a 53% improvement in external quantum efficiency over the devices on shaped standard glass substrates, and an increase by a factor of 2-3 times over those on planar glass substrates. This principle is applicable to any backside patterning technique in conjunction with other OLED structural improvements.

Patterning approaches and system power efficiency considerations for organic LED displays

In this paper we will focus on the various issues which reduce the power efficiency of a complete display system vs. that of a single isolated organic LED, and then discuss the impact of these issues on display integration and design. Critical issues are the necessity of an active matrix design for high definition displays, and the desire for a power- efficient approach for full color. Both dry-etching and ink jet printing will be described as options for achieving patterned films.