Pixel AMOLED

Servati, R Nathan, A. 2005. Functional pixel circuits for elastic AMOLED displays. Proc. IEEE 93 1257-1264. 

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 7.12 A 24" AMOLED display fabricated by Sony using top-emission pixel architectures. 

J.L. Sanford and F.R. Libsch, Vt compensation performance of voltage data AMOLED pixel circuits, Proc. IDRC, 38-41, 2003. 

Y. Hong, R. Hattori, and J. Kanicki, Novel poly-Si TFT pixel electrode circuits and current programmed active-matrix driving methods for AMOLEDs, SID Tech. Dig., 33, 618-621, 2002. 

A. Yumoto, M. Asano, H. Jasegawa, M. Sekiya, Pixel-driving methods for large-sized poly-Si AMOLED displays, ... 

Fig. 14. Two-transistor and one-capacitor pixel circuits for AMOLED (a) the operating circuit and (b) the recovery circuit during idle time (Vsub<0 V, Vdd=0 V)...

Fig. 21. Structure and circuit implementation of normal top-emission AMOLED (TOLED) pixel (a) anode-contact with a-Si H TFT (ACTOLED) and (b) cathode-contact with a-Si H TFT (CCTOLED)...

Cathode-contact structure pixel structure employing normal TOLED was proposed for a-Si H TFT backplane. The new top-emission AMOLED pixel structure employing the TOLED as well as the cathode-drain contact structure was proposed and fabricated. The structure of TOLED had a cathode at bottom and an anode on top. The negative photo-resist separator wall successfully patterned the pixel cathode layers. As the electrical performances of CCTOLED and ACTOLED were compared, the CCTOLED was verified more suitable for better display performance having a high luminance and a high contrast ratio. 

Ashtiani, S.,J. Servati, P. Striakhilev, D. Nathan, A. (2005). A 3-TFT Current-Programmed Pixel Circuit for AMOLEDs. IEEE Trans. Electron Devices, Vol. 52, 0uly, 2005) 1514-1518, ISSN 0018-9383... 

The first demonstrations of using OTFTs to drive OLEDs came from independent groups at Bell Laboratories and Cambridge University nearly simultaneously in 1998 [41,42]. These were single smart pixels, each incorporating one polymer-based OTFT and one OLED. Since AMOLED displays require at least two TFTs per pixel, these demonstrations were not examples of complete AMOLED pixels. 

In 2004 Pioneer reported the first OTFT AMOLED display, containing an 8 x 8 array of two TFT pixels using pentacene OTFTs on a glass substrate [43]. In 2005 Penn State University demonstrated a 48 x 48 OTFT AMOLED display on a PET substrate, also using two TFT pixels [44]. Table 6.4.2 lists these and other significant OTFT AMOLED display results. 

Figure 6.4.17 illustrates PMOLED and AMOLED display architectures, with only one pixel of the AMOLED display shown for simplicity. The pixel in Figure 6.4.17(b) is the simplest AMOLED pixel, a two TFT pixel. It was proposed in 1975 for use with any electro-optic material that requires the flow of current [45]. Later, we describe AMOLED pixels that are more complicated than the two TFT pixel. 

The advantage of AMOLED over PMOLED displays arises because emission in a passive matrix occurs one line at a time, so each OLED element operates at high peak currents and low duty-cycle. The duty-cycle in a PMOLED display is approximately equal to the inverse of the number of rows. For example, in an SXGA (1280 X 1024) display, the duty cycle is approximately 0.1%. The peak current of an OLED pixel may be 1 mA or more. High OLED currents lead to reduced power efficiency and operational lifetime and also place greater demands on the current capacity of the row driver circuits, which may have to handle currents of hundreds of miUiamperes on each output (although not simultaneously). 

The demands that an AMOLED display places on TFT performance are more stringent than those placed by liquid-crystal or electrophoretic displays. Table 6.4.3 compares the TFT requirements of high-resolution displays of these types. The different TFT requirements arise from the fact that LCD and electrophoretic display pixels have a single TFT that is used as a switch, while an AMOLED pixel has one or more TFTs used as a switch, plus one or more used to drive current. A transistor used as a switch typically has stringent requirements for off-current because it must not leak significant charge when it is off, but the requirements for mobility, uniformity, and stability are modest. A transistor that is used to drive current has stringent requirements for mobility, uniformity, and stability, but off-current requirements are lenient. Because the AMOLED pixel has both types of... 

FIGURE 6.4.19 Cross-section of an OTFT AMOLED display pixel showing the drive transistor and OLED (not to scale). 

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