Pixels, Patterning, and Drivers

Indicator lights, fixed pattern, and segmented displays are applications which have been suggested for OLED deployment. Manufacturers of automobile components have shown interest in OLED indicators for the dashboard, where the primary considerations are those of cost, form factor, brightness, and stability over a wide range of ambient conditions. Power consumption is not particularly critical. The possibility of molding a thin light into a curved dashboard is attractive. 

13 The Chemistry, Physics and Engineering of Organic Light-Emitting Diodes 

An additional limit to the size of a passive array relates to the current which flows in an OLED when it is imder 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 upper limit for passive matrix OLEDs thus depends on many factors, including the reverse current of the diodes, the current delivery of the row drivers, the roll-off in OLED power efficiency, and the brightness and contrast ratio requirements of the application. The practical limit is probably less than VGA resolution (480 rows by 640x3 columns, where the factor of 3 reflects the RGB subpixels for color) for power-sensitive portable applications. However, such considerations are less important where a wall-plug is available and one company in Japan (Idemitsu Kosan [190]) has demonstrated full color television using passive OLED arrays. 

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 pre-patterned to provide its own shadow mask [191]. By this means, pixel sizes down to 300 p have been demonstrated, and a lower limit of about 100 p is estimated. 

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