Bottom Emission Research Articles

Semitransparent passive matrix organic light-emitting displays

There has been an increased interest in developing top emission organic light-emitting diodes (OLEDs) that are able to emit light from both sides of the OLED display. One important application of the top emission device structure is to achieve monolithic integration of a top-emitting OLED on a polycrystalline or amorphous silicon thin film transistors used in active matrix displays. A high performance dual-sided top-emitting polymer OLED developed in this work exhibited a total luminous efficiency of ∼5.0 cd/A at 4.0 V, which is comparable to that observed for a control device having bottom emission structure. A laser ablation technology was developed to define the pixels. The cathode separation was achieved without using the conventional reverse trapezoid type separators that are normally used for pixellated OLED displays. A prototype of semitransparent polymer light-emitting passive matrix display has a matrix of 100 × 32 with a display area of 32.25 mm by 11.15 mm.

Transparent organic light-emitting devices with LiF/Mg:Ag cathode

Transparent organic light-emitting devices (TOLED) based on a stacked transparent cathode of a LiF/Mg:Ag were investigated. The device has a structure of indium-tin-oxide (ITO)/ N, N'-diphenyl-N, N'-(3- methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD) (90 nm)/ tris-(8- hydroxyquinoline) aluminum (Alq3) (80 nm)/LiF (0.5 nm)/Mg:Ag (20 nm)/Alq3 (50 nm), where the transparent capping layer of 50 nm Alq3 acts as refractive index matching layer to optimize optical output. The turn-on voltage of the device is as low as 2.8 V. The device also shows high optical transparency and low reflectivity in the visible region, approximately 40% of light can emit from the top cathode side and 60% of the light from bottom ITO glass. At injection current density of 20 mA/cm2, the current efficiency, for bottom emission from ITO anode side and top emission from metal cathode side, is 3.4 cd/A and 2.2 cd/A, respectively. The lower turnon voltage and higher efficiency of device are due to enhancing electron injection by using LiF/Mg:Ag cathode.

Beam divergence measurements of InGaN∕GaN micro-array light-emitting diodes using confocal microscopy

The recent development of high-density, two-dimensional arrays of micrometer-sized InGaN∕GaN light-emitting diodes (micro-LEDs) with potential applications from scientific instrumentation to microdisplays has created an urgent need for controlled manipulation of the light output from these devices. With directed light output these devices can be used in situations where collimated beams or light focused onto several thousand matrix points is desired. In order to do this effectively, the emission characteristics of the devices must be fully understood and characterized. Here we utilize confocal microscopy to directly determine the emission characteristics and angular beam divergences from the individual micro-LED elements. The technique is applied to both top (into air) and bottom (through substrate) emission in arrays of green (540nm), blue (470nm), and UV (370nm) micro-LED devices, at distances of up to 50μm from the emission plane. The results are consistent with simple optical modeling of the expected beam profiles.

49.4: Two TFT Pixel Circuit with Non-Uniformity Suppress-Function for Voltage Programming Active Matrix OLED Displays

We proposed the world's first two TFT pixel circuit with an inter-pixel non-uniformity suppress-function for the analog voltage programming to assist in the realization of small pixel bottom emission OLED displays. The pixel circuit provides larger OLED fill factor. We fabricated a 2.5 inch 240 (V) × 960 (H) pixel monochrome bottom emission display.

49.2: 2 Inch AMOLED with a-Si:H TFT using PVP Gate Insulator on Plastic Substrate

We developed an 80 DPI, bottom emission AMOLED (2 inch) on transparent plastic (PES). Hydrogenated amorphous silicon (a-Si:H) on plastic using a PVP (Poly-4-vinylphenol) gate insulator was used for switching and driving TFTs. The low temperature process (<150 °C) and the island formation of inorganic layers give a stress-free AMOLED backplane on PES. The a-Si:H TFT has a field-effect mobility of 0.5 cm2/Vs, a threshold voltage of 4.7 V and subthreshold slope 1.0 V/dec.

Growth and design of deep-UV (240–290 nm) light emitting diodes using AlGaN alloys

Solid-state light sources emitting at wavelengths less than 300 nm would enable technological advances in many areas such as fluorescence-based biological agent detection, non-line-of-sight communications, water purification, and industrial processing including ink drying and epoxy curing. In this paper, we present our recent progress in the development of LEDs with emission between 237 and 297 nm.We will discuss growth and design issues of deep-UV LEDs, including transport in Si-doped AlGaN layers. The LEDs are designed for bottom emission so that improved heat sinking and light extraction can be achieved by flip chipping. To date, we have demonstrated 2.25 mW of output power at 295 nm from 1 mm×1 mm LEDs operated at 500 mA. Shorter wavelength LEDs emitting at 276 nm have achieved an output power of 1.3 mW at 400 mA. The heterostructure designs that we have employed have suppressed deep level emission to intensities that are up to 330×lower than the primary quantum well emission.

P‐82: Top Emission Organic Light Emitting Diode with Ni Anode

AbstractThe hole injection and emissive properties of top emission organic light emitting diodes with Nickel anode were investigated. The effect of oxygen plasma treatment on the Ni anodes was also investigated for enhancing the electrical properties of the devices. The device with Ni anode having an high work function shows better hole injection and emission characteristics than that with ITO. Oxygen plasma treatment of Ni anode induced further improvement of electrical properties and surface toughness. CIE coordinates of top emission device were different from bottom emission device. Ni anode induced the internal quantum efficiency of the device to be improved.

Vertical-cavity surface-emitting lasers with spatially adjustable DBR reflectivity to enable free-space photonic repeaters

We have created bidirectional high-performance top- and bottom-emitting vertical-cavity surface-emitting lasers (VCSELs) from a single epitaxial wafer. Using a spatially selective oxidation of AlAs layers in hybrid distributed Bragg reflectors (DBRs) of a VCSEL structure, devices processed for top emission had threshold currents of 100-110 /spl mu/A with slope efficiencies of 18%-20%. Devices from the same wafer processed for bottom substrate emission had thresholds of 110-120 /spl mu/A and slope efficiencies of 17% and 18%. This technology will greatly simplify the use of VCSEL's in large three-dimensional (3-D) parallel interconnects, photonic repeaters, and smart pixels.

Analysis of heavy metal emission data from municipal waste combustion

Heavy metals contained in municipal solid waste (MSW), after combustion in modern waste-to-energy facilities, are collected in bottom and fly ash, only a small quantity being discharged from the stack as particulate or vapor. These metals are found to be broadly distributed throughout the constituents, limiting the potential for reducing them by targeting specific components. The many factors which determine metals partitioning to bottom ash, boiler hopper and emission control flyash, and stack emissions, shows that the complex relationship between feed composition and emissions makes it difficult if not impossible to assign cause and effect on their quantities and concentrations in these discharges. Data showing the relationship between particulate matter, emission controls, and emission factors for the heavy metals is examined. A finding that substantial spiking of lead and cadmium in the feed resulted in only marginal changes in stack emissions indicates that efforts to remove these metals from the waste would not produce a significant change in stack emissions. The range of trace metal emissions from a single waste-to-energy (WTE) facility over a period of three to four years is compared with the range reported from individual tests of about twenty facilities also having acid gas controls and fabric filters, indicating that the waste composition and the combustion and emission control technology employed all contribute to the variability of metals and particulate emissions. The relationship between annual averages and probable maximum values which may be anticipated from periodic testing is examined. Special attention is given to mercury, its various species, chemical reactions, and the effectiveness of various carbon-based reagents used for emission control. Emissions from WTE facilities are compared with those from oil and coal-fired utility boilers on a mass per kWh generated.