Flexible OLED Display

Although there have been several examples of flexible OLED displays on plastics, including passive-matrix OLED displays on poly(ethylene terephthalate) (PET) substrates from Pioneer and Universal Display Corporation [26, 27], and a-Si H TFT-driven monochrome active-matrix OLED displays on poly(ethylene naphthalate) (PEN) from Honeywell [28], there have been no demonstrations of organic TFT- 

An important part of this study was to recognize and address difficulties resulting from processing on polymeric substrates. Problems encountered included substrate handling, stability of the substrate, surface smoothness of the substrates, and effects of chemical exposure. Effective encapsulation of an OLED on a flexible polymeric substrate is a challenge but not one we will address in detail here. 

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AC Chwang, MA Rothman, SY Mao, RH Hewitt, MS Weaver, JA Silvernail, K Rajan, M Hack, JJ Brown, X Chu, M Lorenza, T Krajewski, and N Rutherford, Thin film encapsulated flexible OLED displays, Appl. Phys. Lett., 83 413-415, 2003. 

Fig. 15.23. The flexible OLED display brightness under different data voltages.

Fig. 15.24. The flexible OLED display working while bent.

In addition to the potential cost advantage due to easier processing via printing or evaporation, OFETs potentially offer reduced bias stress in current drive applications over a-Si transistors fabricated at less than 200°C. At these temperatures, transistors can be fabricated on a range of transparent flexible substrates and are particularly applicable to flexible OLED displays. There are also circuit and architecture advantages to using PFETS for bottom emission OLED displays [136]. 

L. Zhao, Z.-L. Zhou, Z. Guo, G. Gibson, J. A. Brug, S. Lam, J. Pei, S. S. Mao, Development of Semi-interpenetrating Polymer Networks and Quantum Dots-Polymer Ncuiocomposites for Low-Cost, Flexible OLED Display Application. J. Mater. Res. 2012,27,639-652. 

Figure 21.43 OLED on bacterial cellulose nanocomposite. Reprinted from Ummartyotin, S., Juntaro, Sain, M., Manusp, H., Development of transparent bacterial nanocellulose nanocomposite film as a substrate for flexible OLED display, Ind. Crops Prod., 35,92-97. Copyright (2012) with permission from Elsevier] [84].

OLED displays can also be fabricated on flexible substrates [159-161] such as metal foils or plastic. This enables entirely new display features such as conformability, ruggedness, flexibility, and reduced weight. 

OLEDs grown and encapsulated using these techniques are beginning to show significant promise. Recently, Chwang et al. demonstrated the effects of flexing a 64x64 (180 dpi) passive-matrix flexible OLED (FOLED) display fabricated on a PET substrate with thin film encapsulation [166], In addition, lifetimes of thin-film-encapsulated OLED test pixels on flexible substrates have now been demonstrated to be thousands of hours [162,167],... 

In addition to the thin-film solar cells, this system is suitable for use in other flexible electronics, such as a flexible, transparent, and light-weight encapsulation of organic light-emitting diode (OLED) displays and lighting. 

These properties have been discussed in the text and elsewhere [5, 6], This table shows that both heat-stabilized PET (e.g. Melinex ST504) and heat stabilized PEN (TeonexQ65A) have an excellent balance of the key properties required for flexible electronics. TeonexQ65A has a higher-temperature performance than Melinex (Fig. 7.9) and as a result of this set of properties TeonexQ65A is emerging as a leading material for the base substrate of OLED displays and active matrix backplanes. 

As discussed in the introduction requirements for the different applications envisaged for printable electronics are very different and will require substrates with different sets of properties. This is summarized in Fig. 7.13 and for this classification covers simple organic circuitry, e.g. RFID, organic based active matrix backplanes, OLED displays, but also includes the requirements of inorganic TFTs on flexible substrates. 

A 48 x 48 active-matrix OLED display was completed on flexible PET substrates. The completed display is shown bent, after wire bonding, in Fig. 15.22. Figure 15.23 shows the display brightness under different data voltages, and Fig. 15.24 demonstrates its flexible functionality during operation. Despite the many noticeable defects on the display, which are expected for displays fabricated outside a dean room, the display was characterized by good drive performance and uniformity. 

OLED display manufacturers have so far used circular polarizers borrowed from the LCD technology to improve contrast (Trapani et al, 2003). This approach does not require introduction of new layers in the OLED structure and results in reflectance similar to that of glass. However, polarizers are expensive, generally not flexible, and absorb a substantial amount of the light (up to 40%) (Wu, 2005). 

Wu, C.C. et al.. Integration of organic LEDs and amorphous Si. TFTs onto flexible and kghtweight metal foil substrates, IEEE Electron Dev. Lett., 18, 609, 1997. Tsujimura, T. et al., A 20-inch OLED display driven by super-amorphous-silicon technology, SID Int. Symp. Dig. Tech. Papers, 34, 6, 2003. 

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