Multiple OLEDs

Liao et al(2004) and Kido et al (2003) have demonstrated a variant of the SOLED that allows the contacts between intermediate OLED in the stack to electrically "float" and performs as a series of independent OLEDs, with a single electron exciting the multiple OLEDs as it passes through the circuit. 

These results also have certain implications on tandem OLEDs, which have multiple OLED units stacked vertically in series [24-26], In the simplest viewpoint, a proportional increase in the luminance efficiency could be expected (i.e., cd A) with the number of emitting units. However, the results here suggest that the enhancement in cd/A efficiency could significantly exceed such proportional increase (extra efficiency gain from the tandem structure), since the emitting unit further away from the reflective metal electrode could contribute a larger cdA efficiency or total outcoupling. 

Since multiple electrical and optical functionality must be combined in the fabrication of an OLED, many workers have turned to the techniques of molecular self-assembly in order to optimize the microstructure of the materials used. In turn, such approaches necessitate the incorporation of additional chemical functionality into the molecules. For example, the successive dipping of a substrate into solutions of polyanion and polycation leads to the deposition of poly-ionic bilayers [59, 60]. Since the precursor form of PPV is cationic, this is a very appealing way to tailor its properties. Anionic polymers that have been studied include sulfonatcd polystyrene [59] and sulfonatcd polyanilinc 159, 60]. Thermal conversion of the precursor PPV then results in an electroluminescent blended polymer film. 

The redispersion microreactor is applied for the liquid-liquid polycondensation to yield an OLED material by multiple Suzuki coupling. As the initial test reaction, the following single Suzuki coupling is currently being explored in the liquid-liquid system made from water/ dioxane/toluene. 

The organic deposition sources are made of a variety of materials including ceramics (e.g., boron nitride, aluminum oxide, and quartz) or metallic boats (e.g., tantalum or molybdenum). Deposition is carried out in high vacuum at a base pressure of around 10-7 torr. The vacuum conditions under which OLEDs are fabricated are extremely important [41] and evaporation rates, monitored using quartz oscillators, are typically in the range 0.01 0.5 nm/s in research and development tools. In manufacturing, higher rates or multiple sources may be used to reduce tact times. 

FIG. 13. Transmission electron micrograph of a freeze fractured w/o cream. The aqueous phase is dispersed as droplets within the continuous lipophilic phase the interface consists of multiple bilayers of hydrated surfactant molecules, bar 500 mn. From Miiller-Goymann, C., Halbfeste emulsionsahnliche Zustande, Seifert, Ole, Fette, Wachse, 110 395-400 (1984). 

Fig. 9.2. Close-coupled showerhead technology for industrial-scale production of multiple layers and OVPD. OLED example deposited in a single OVPD module.

Gohla, S.H. and Nielsen, J. (1995) Partial phase solu-inversion technology (PPSIT). A novel process to manufacture long term stable multiple emulsions by an in situ one step procedure. Seife Ole Fette Wachse, 121(10), 707-10. 

As mention above, Eq. 3 is similar to the equation describing the transmittance of a Fabiy-Perot, except for the cosine at the numerator. As in Fabry-Perot filters, the multiple internal reflections in OLEDs induce, at some specific wavelengths, a resonance of the light electric-field intensity (or more accurately, the irradiance) distribution inside the OLED. 

Fig. 9. Schematic diagram of a multiple quantum well white OLED...

Multilayer Devices. The OLED structure consists in several emissive layers (or a single-layer doped with multiple emitters) with different emission colors. By controlling the recombination zone(s) (e.g., through the use of several electron and/or hole blocking layers), it is then possible to balance the different colors... 

Another potential application for LEDs is in illumination. The requirements for devices that serve as illumination sources are somewhat different than the monochromatic OLEDs described above. OLEDs targeted for RGB displays have to give electroluminescent spectra with a relatively narrow line shape centered on the peak wavelength. On the other hand, an illumination source is meant to approximate the blackbody solar spectrum and needs to have a broad line shape with roughly equal intensity across the entire visible spectrum. Therefore, in order to attain complete coverage across the visible spectrum, an OLED used for illumination purposes typically employs multiple emitters are that are either co-deposited into a single emissive layer or distributed into different layers or regions of the device. A number of the different device architectures have been reported to achieve efficient white EL and are discussed below. 

The OLED structure, as an array of small-size pixels, presents an opportunity to fabricate multiple sensors in this compact design. Any number of pixels is individually addressable and can be associated with a different sensor film. Single- or multi-color OLEDs can be used for generating such sensors. 

OLED-Based Multiple Analyte Sensing Platform [91]... 

This section reviews some recent developments on a multiple analyte sensor that is based on identical Alpg-based OLED pixels and monitors the serum analytes glucose, lactate, ethanol, and oxygen, which are of clinical, health, industrial, and environmental importance. These analytes were monitored sequentially or simultaneously comparable results were obtained using both approaches. 

In the second approach, in which an array of PDs compatible with the OLED pixels was used to simultaneously monitor the analytes in a mixture, the results obtained were, as expected, comparable to those obtained using the sequential monitoring. Importantly, the results show that use of the compact OLED-based sensor array is a viable approach for simultaneous monitoring of these multiple analytes. 

---