Color in Electronic Display Systems by Michael E. Miller

Author:Michael E. Miller
Language: eng
Format: epub
ISBN: 9783030028343
Publisher: Springer International Publishing

6.3.1 Other Power Considerations

We have discussed the structure of OLEDs without much discussion of the active matrix display itself. Similar to LCD, modern OLED displays are formed on active matrix substrates where data is provided to each light emitting element through a thin film transistor to control the light provided by each light emitting element. The requirements for backplanes in OLED displays are, however, significantly different than the backplanes for LCDs. As we discussed earlier, the backplane in an LCD must deliver voltage to the electrode of an LCD to establish an electric field within the liquid crystal cell. As such, current does not flow through the liquid crystal, but the liquid crystal reacts to the presence of the electric field. However, as we have discussed, light output of an OLED is modulated by modifying the current which flows through an OLED. Therefore, in an OLED display, the backplane must provide current to the OLED and the complimentary electrode must carry current away from the OLED. Further, the Thin Film Transistors (TFTs) must be capable of reliably regulating current uniformly across the substrate without degrading significantly with time.

It is important that the need to provide current places additional requirements on the backplane and the materials from which TFTs are made. LCDs often employed amorphous silicon (a-Si) as a semiconductor within the TFTs, unfortunately these transistors have low mobility, restricting the rate of current flow through the TFT . Further, transistors made from a-Si degrade relatively rapidly with time when exposed to high currents, reducing the amount of current to the OLED, which enhances the appearance of burn in . High end LCDs often employed Low Temperature Polysilicon (LTPS) . LTPS is formed by depositing amorphous silicon and then heating the silicon with a laser to cause it to crystalize. Unfortunately, this process does not result in a uniform layer of semiconductor and therefore the mobility of the semiconductor is not uniform, typically resulting in a substrate where some rows of the TFTs within the display have a higher mobility than other rows. As a result, more current flows through some rows of TFTs than others and the brightness of the OLEDs vary across rows of the display, often resulting in a very apparent striped pattern. Image processing has been applied to improve the quality of OLED displays formed with either of these two types of backplanes [3, 7]. To avoid this complication, the industry has sought improved semiconductor materials for use in OLED display manufacturing, including the use of new materials such as doped zinc oxides [10] and new ways of crystalizing silicon to form materials that are more like single crystal silicon than LTPS [4]. However, the mass production of stable, highly uniform backplanes provides a continuing challenge to the industry.

The fact that OLED displays must provide current to each light emitting element creates additional constraints for the OLED substrate. Figure 6.11 shows an illustration of a backplane. Similar to the portion on the LCD backplane shown in Fig. 5.

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