Active-matrix display

For high information-content displays, active-matrix (AM) pixel addressing provides improved display performance and reduced power consumption. In active matrix addressing each individual pixel is controlled by one or more thin-film transistors (TFTs). To date, most AM OLED displays have used polysilicon TFTs as the active elements, because they can provide sufficient current at low voltages and acceptable device dimensions, and they are capable of integrated drive electronics... 

Compared to STN displays, active matrix addressing in TFTs allows enhanced sharpness and greater multiplexing, each liquid crystal pixel is addressed by a transistor, which thus primarily governs the response of the device. TFT i greater number of pixels (higher resolution) and number of colour levels than STN devices. They are widely used ii computers, although they are more expensive than STN displays. Further details can be found elsewhere [115]. 

Kaneko E Active matrix addressed displays Handbook of Liquid Crystals Vol 2A. Low Molecular Weight Liquid Crystals led D Demus, J Goodby, G W Gray, H-W Speiss and V Vill (New York Wiley-VCH)... 

Liquid Crystals. Based on worldwide patent activity, numerous compounds containing fluoroaromatic moieties have been synthesized for incorporation into liquid crystals. For example, fluoroaromatics are incorporated in ZLI-4792 and ZLI-4801-000/-100 for active matrix displays (AMD) containing super fluorinated materials (SFM) (186,187). Representative stmctures are as follows. 

FET is ihe on-off current ratio, which indicates its ability to shut down the current, and is particularly relevant in applications such as active matrix displays and logical circuits. Because of the presence of p-n junctions at both the source and drain electrode, the on-off ratio of MOSFETs is in the I0y range [12J, while that of a-Si H TFT is limited to 106 [13], High mobility ensures high on-current and, hence, also contributes to a high on-off ratio. 

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. 

Another disadvantageous phenomenon in TFTs is the photoconductivity of a-Si H [626]. Electrons and holes are photogenerated and recombine at the back surface (gate insulator). The photocurrent reduces the on/off ratio of the TFT. Illumination, however, cannot always be avoided, e.g., in active matrix displays. A way of circumventing this is to make the a-Si H as thin as possible. 

Gelinck, G. H. et al. 2004. Flexible active-matrix displays and shift registers based on solution-processed organic transistors. Nature Mater. 3 106-110. 

G Yu, G Srdanov, B Zhang, M Stevenson, J Wang, P Chen, E Baggao, J Macias, R Sun, C McPherson, P Sant, J Innocenzo, M Stainer, and M O Regan, Active-Matrix Polymer Displays Made with Electroluminescent Polymers, Cockpit Displays X, Orlando, Florida, 2003, pp. 192-199. 

NCvd Vaart, EA Meulenkamp, ND Young, and M Fleuster, Next-generation active-matrix polymer OLED displays. Asia display/IMID 04, Digest, 337-342, 2004. 

J Wang and G Yu, Performance Simulation of Active-Matrix OLED Displays, Photonics Asia 2004 Light-Emitting Diode Materials and Devices, Beijing, China, 2004, pp. 32-44. 

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). 

The metal cathode is deposited onto the organic layers through a shadow mask (see Figure 7.3c). For active-matrix OLED (AMOLED) displays, a single unbroken cathode is often used over the entire display area. 

Many LCDs are based on active-matrix addressing, in which an active device circuit containing one or more TFTs is connected to each pixel. The TFT circuit at each pixel effectively acts as an individual electrical switch that provides the means to store display information on a storage capacitor for the entire frame time, such that the pixel can remain emitting during this entire time rather than for a small fraction of time, as is the case in passive addressing. 

Y He, R Hattori, and J Kanicki, Four-thin film transistor pixel electrode circuits for active-matrix organic light-emitting displays, Jpn. J. Appl. Phys., 40 (Part 1) 1199—1208, 2001. 

JJ Lih, CF Sung, MS Weaver, M Hack, and JJ Brown, A Phosphorescent Active-Matrix OLED Display Driven by Amorphous Silicon Backplane, Proceedings of the Society for Information Displays International Symposium Digest of Technical Papers, Vol. 34(Book 1), Baltimore, 2003, pp. 14-17. 

J Blochwitz, J Brandt, M Hofmann, J Birnstock, M Pfeiffer, G He, P Wellmann, and K Leo, Full Color Active Matrix OLED Displays with High Aperture Ratio, Proceedings of the Society for Information Display, Digest of Technical Papers, Vol. 35, Seattle, 2004, pp. 1000-1003. 

Amorphous Silicon Thin-Film Transistor Active-Matrix Organic Light-Emitting Displays... 

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