Pixels, OLED

Early displays [8] based on vapor-deposited OLEDs were simple alphanumeric devices. More recently, there have been rapid increases in the complexity of these devices. In 1996, Pioneer Corporation demonstrated a monochrome 64 x 256 pixel OLED display [9] that was subsequently developed into a product and was incorporated into automobile stereos (see Figure 7.2). Today full-color, high-resolution vapor-deposited OLED displays as large as 24" have been developed [10]. 

An additional limit to the size of a passive array relates to the current which flows in an OLED when it is under reverse bias [189]. When a given pixel is turned on in the array, there are many possible parallel paths for the current, each involving two diodes in reverse bias and one forward. Hence, as the number of rows and columns increases, there is a higher level of background emission from non-selected pixels that limits the contrast ratio of the array. As a result, the contrast degrades as N increases. 

The minimum size of the monochrome pixels (we consider color in Section 13.7.3) that can be fabricated using OLEDs is dictated primarily by the ability to pattern the electrode which is deposited on top. OLEDs are not sufficiently robust to withstand the normal processes of photolithography. Among the schemes which have been suggested for high resolution patterning is one in which the substrate is pne-pattemed to provide its own shadow mask [1911. By this means, pixel sizes down to 300 p have been demonstrated, and a lower limit of about 100 p is estimated. 

The fluorescence color converter technique [32] can, in principle, overcome much of this power loss by replacing the white light emitter with a blue-emitting organic stack, and the absorbing filters with green and red fluorescent dyes. Thus when a green pixel is desired, the OLED underneath is turned on and the blui... 

Figure 13-18. Diagram of a simple pixel circuit for active matrix addressing of an OLED array. For a color display of N lows and M columns, this circuit must be reproduced Ny.My.7t limes.

The fifth of the color methods places the three emitting structures in a stack one on top of the other, rather than side by side ]20l ]. Clearly there is a requirement here that the two electrodes in the middle of the structure must be transparent. The advantages are that the display can be made much brighter with up to three times the luminance from each pixel, and the requirements for high resolution patterning are relaxed by a factor of three. The disadvantages are that three times as many layers must be coated (without defects) over the area of the display and electrical driving circuitry must make contact with four sets of elec- trades. It will be extremely difficult to incorporate a stacked OLED into a active matrix array. 

Power Consumption of AMOLEDs for Three Sets of OLED Emitters (Portion on Pixel Driver Is Excluded)... 

The issue of power consumption of an OLED display panel changing with the information content has not been well addressed in the OLED field. For AMLCD display panel, the power consumption is almost independent of the information content. For an AMOLED or an AMPLED panel, the power consumption is directly proportional to the number of pixels lighting up. For each display pixel, the power consumption is nearly proportional to the level of brightness (gray level). Thus, AMOLED display only consumes the power necessary, without any waste. This effect is similar to the concept of Pay-Per-View developed in cable and satellite TV industries. Two direct consequences of the Pay-Per-View effect are ... 

Figure 7.3 shows a simple schematic example of the basic steps required to fabricate a bottom emitting vapor-deposited OLED test pixel similar to the device shown in Figure 7.1. 

FIGURE 7.3 Schematic representation of the basic steps required in fabricating a vapor-deposited OLED test pixel, (a) anode patterning via lithography, (b) deposition of the organic, and (c) metal cathode layers through shadow masks. 

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. 

Displays based on OLEDs may be addressed either passively or actively [124], and the drive requirements are quite different in each case. In passive-matrix addressing, the display is addressed one line at a time, so if a display has 480 lines then a pixel can only be emitting for... 

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