Get it white: color-tunable AC/DC OLEDs

Abstract

Organic light-emitting diodes (OLEDs) have gained considerable attention because of their use of inherently flexible materials and their compatibility with facile roll-to-roll and printing processes. In addition to high efficiency, flexibility and transparency, reliable color tunability of solid state light sources is a desirable feature in the lighting and display industry. Here, we demonstrate a device concept for highly efficient organic light-emitting devices whose emission color can be easily adjusted from deep-blue through cold-white and warm-white to saturated yellow. Our approach exploits the different polarities of the positive and negative half-cycles of an alternating current (AC) driving signal to independently address a fluorescent blue emission unit and a phosphorescent yellow emission unit which are vertically stacked on top of each other. The electrode design is optimized for simple fabrication and driving and allows for two-terminal operation by a single source. The presented concept for color-tunable OLEDs is compatible with application requirements and versatile in terms of emitter combinations. INTRODUCTIONIn recent years, organic light-emitting diodes (OLEDs) have evolved into a mature technology and OLEDs are now used in various display applications. OLEDs provide an internal charge-to-photon conversion efficiency of nearly 100% and deliver homogeneous emission over large areas, making them promising candidates for new and innovative lighting applications. 1 White OLEDs, in particular, offer great potential for energy-efficient general illumination: luminous efficacies of more than 90 lm W 21 , comparable to the best fluorescent tubes, have already been reported. 2,3 Furthermore, OLED-based light sources can be made mechanically flexible and transparent, offering new opportunities for architecture, visual art and decoration. 4 The reliable realtime tunability of the OLED emission color would impart further momentum to OLED technology on its way to becoming a widespread source of general illumination. Thus far, two different color-tuning concepts have prevailed in the literature. One exploits voltagedependent changes in emission color and was demonstrated as early as 1994 for OLEDs fabricated from polymer blends. 5 Voltage-dependent color shifts are the result of a variety of mechanisms, e.g., voltagedependent charge trapping, a spatial shift of the recombination zone, a modified exciton distribution, or exciton quenching at high current densities. [5][6][7] However, this approach has several drawbacks: not only are the mechanisms that lead to voltage-dependent color-shifts difficult to control, but adjusting the driving voltage also unavoidably results in a dramatic and undesired change in device brightness. The second concept overcomes the disadvantages of the voltage-controlled approach by using a stacked tandem OLED structure with two (or more) independently addressable units emitting light of different colors. 8,9 In comparison with the first method, this approach provides

Recent advances in light outcoupling from white organic light-emitting diodes

“…At this thickness, silver films have significant reflectance and absorption (transmittance only 30 to 75%, depending on wavelength). 73 Alternatively, as reported by Freitag et al, CNT meshes can be used as highly transparent, conductive top electrodes for white OLEDs. 71,72 Transmission data of these ultrathin, composite electrodes are compared to ITO and thicker films of pristine silver in Fig.…”

Efficient Deep‐Blue Electroluminescence Based on Phenanthroimidazole‐Dibenzothiophene Derivatives with Different Oxidation States of the Sulfur Atom

“…1 After evaporating the solvent, the crude product was purified by column chromatography by using dichloromethane as eluent to give al ight-green solid.…”

“…Organicl ight-emitting diodes( OLEDs) are expected to be the next-generation lighting and displayt echnology because of their unique advantages such as low weight, flexibility,e nergy conservation, ands oo n. [1] Although great progress has been made since the pioneering work of Tang and co-workers, [2] some issues are still not well resolved, one of which is the realization of efficient deep-blue electroluminescence (EL). [3] Deepblue emission can not only provide one of the RGB primary colors and enhance the color gamutf or full color display,b ut also lower powerc onsumptionf or white lightings.…”

Controlling the Behavior of Single Live Cells with High Density Arrays of Microscopic OLEDs

“…By using stacked devices with two separately controlled OLEDs of different color, co-located target molecules with different activity spectra may be addressed individually. [29,30] Our work thus paves the way for accurate light-activated control of a variety of biological systems.…”