OLED Display Architectures

The aim of this chapter is to give the reader a broad overview of the field of vapor-deposited small-molecule OLEDs. It is beyond the scope of this chapter to cover every aspect of these devices, however key references are given throughout the text for those readers who are interested in delving more deeply into this topic. Section 7.2 describes the key elements of a typical OLED. Alternative device architectures are also briefly described. Section 7.3 describes the typical fabrication methods and materials used in the construction of vapor-deposited OLEDs. Section 7.4 describes the physics of an OLED in addition to the improvement of the performance over time made through advances in device architectures and materials. Section 7.5 discusses OLED displays and Section 7.6 looks at the future exciting possibilities for the field of vapor-deposited organic devices. 

Other device architectures include inverted OLEDs. Here the cathode is in intimate contact with the substrate. The organic layers are then deposited onto the cathode in reverse order, i.e., starting with the electron transport material and ending with the HIL. The device is completed with an anode contact. In this case, as above, one of the electrodes is transparent, and light exits from the device through that contact. For example, Bulovic et al. [38], fabricated a device in which Mg/Ag was the bottom contact and ITO the top electrode. The advantage of this type of architecture is that it allows for easier integration with n-type TFTs (see Section 7.5 for a discussion of active-matrix drive OLED displays). 

Kodak is commercialising its low molecular weight OLEDs for use in both passive and active matrix display architectures. It has also licensed its technology to Pioneer Corp who have commercialised passive matrix displays for car radios and cellular phone displays. TDK has displays for cellular phones, personal digital assistants and car instrumentation clusters. Perhaps the most significant collaboration to date has been with Sanyo. Sanyo s capabilities in low-temperature polycrystalline silicon have been married with Kodak s low MW materials to produce a full colour, 5 inch active matrix display, commercialisation of which was expected in 2001. 

Fig. 4. (Left) Schematic representation of an active matrix LCD display, showing single transistors driving capacitive pixel elements. (Right) OLED displays, on the other hand, require current-based driving, and therefore, multi-transistor pixel architectures are more common.

In addition to the potential cost advantage due to easier processing via printing or evaporation, OFETs potentially offer reduced bias stress in current drive applications over a-Si transistors fabricated at less than 200°C. At these temperatures, transistors can be fabricated on a range of transparent flexible substrates and are particularly applicable to flexible OLED displays. There are also circuit and architecture advantages to using PFETS for bottom emission OLED displays [136]. 

Hatwar, T. K., Spindler, J. R, Vargas, J. R. et al. 2007. Advances in white OLED tandem architectures for next generation AMOLED displays. IMID 2007 Digest. 

The minimum target set by many manufacturers for test pixel architectures prior to adoption in a commercial display is a lifetime of 10,000 h at display brightness. However, the lifetime of a similar pixel obtained in a display is often less than this value due to additional complications such as pixel yield and added heat load [26] to the pixel from the display. However, great strides in lifetime have been made within the OLED community, with several manufacturers claiming lifetimes at display brightness of >100,000 h. 

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. 

Fig. 7.1. Two architectures for display driving, (a) shows a single transistor architecture appropriate for latching and holding charge on the pixel for field driven display elements such as liquid crystal displays or e-Ink. (b) schematically shows the matrixed version of this element, (c) shows a two transistor voltage-programmed current driver appropriate for OLEDs. More advanced voltage and current programmed circuits are also possible, but require more transistors.

OLED Materials and Device Architectures for Full-Color Displays and Solid-State Lighting... 

The tandem architecture has been found to be very useful for tuning the color of the white OLEDs for both display and lighting applications, where tandems of the same or different white-emitting units can be connected through a semiconducting p-n junction. In the following section, we will describe the white tandems for display applications developed at Eastman Kodak Company. 

Ir complexes were also used to develop white OLEDs (WOLEDs) for large-scale production of solid-state light sources and backlights in liquid-crystal displays. Several device architectures have been introduced to achieve high brightness and efficiency in WOLEDs. By controlling the recombination current within individual... 

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