Polymers, OLED

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

FIGURE 6.22 Schematic diagrams of top-emitting polymer OLED with a configuration of (a) glass/ metallic mirror/ITO/PEDOT/Ph-PPV/semitransparent cathode, (b) Al-PET/acrylic layer/metallic mirror/ITO/PEDOT/Ph-PPV/semitransparent cathode, and (c) Al-PET/acrylic layer/metallic mir-ror/anode/Ph-PPY/semitransparent cathode. 

FIGURE 10.7 Power consumption simulation for a 2.2-in. full-color OLED display using Universal Display s phosphorescent OLEDs, small-molecule fluorescent devices, and polymer OLEDs along with a comparison of the power consumed by an active-matrix liquid crystal display backlight. R G B= 3 6 1, 50% polarizer efficiency, and 30% of pixels lit. (From Mahon, J.K., Adv. Imaging, June, 28, 2003. With permission.)... 

Trilayer OLEDs using LEPs have been fabricated with external quantum efficiencies (rjeff 4-10%) comparable to those obtained for analogous OLEDs using small molecules or LEDs. The low threshold and operating voltages ( 5 V) of these multilayer polymer OLEDs means that they can be used as displays in battery operated devices. 

Fig. 10.19 Pictiva polymer OLED displays developed by OSRAM Opto Semiconductors. Main picture a batch of 128 by 64 pixel monochrome displays. Insert an operating display. Reproduced with permission of OSRAM Opto Semiconductors, Inc.

The semi-conducting properties of conjugated polymers originate from the delocalized n orbitals formed in these carbon-containing compounds such as poly(phenylene vinylene), polythiophene and polyfparaphenylene). Like the OLED s described above, the polymer-OLED consists of a linninescent film, sandwiched between an anode and a cathode. 

Several typical properties of PEDT PSS polymers that depend on the PEDT PSS ratio are summarized in Table 10.1. To meet requirements for conductivity, antistatic grades of PEDT PSS have relatively low PSS-contents, and therefore higher conductivity values. In contrast, PEDT PSS grades designed for hole-injection in polymer OLEDs have larger PSS contents, smaller particles (Figure 10.6), and lower conductivities. Specifically, PEDT PSS grades useful for passive matrix OLED displays have the lowest... 

Polysilanes can be employed as hole transporting materials or emitters in OLED devices, more specifically in polymer OLEDs. Polymer OLEDs are prepared by spin coating techniques on transparent substrates like indium-tin-oxide (ITO) coated glass serving as anode. The basic design of a polymer OLED is shown in Eig. 23. 

J. Halls. 2007. Recent Advances in Polymer OLED-Technology, SID Seminar May 2007. 

Although one ean produee polymer OLEDs without an FTTL, the performance of the device improves dramatieally if a material specialized for hole injection from the anode is used. The FTTL reduces the device operating voltage, inereases lifetime, and... 

Figure 9.24 The chemical formula for the two components poly(3,4-ethylene dioxythiophene) (PEDOT) and poly(styrene sulphonic acid) (PSS) making up the PEDOT PSS polymer blend used in contact with the anode and for hole transport in polymer OLEDS. Note that the two molecules bind together strongly as a double bond is broken in the PEDOT units and an electron is transferred to the PSS, resulting in release of a hydrogen atom (which may bond with the reaetive radical site of the double bond. The molecules are held together, in part by the attraction of the ionic charges.

Comparisons of polymer OLEDs generally show an order of magnitude improvement in light emission efficiency for PPV-based devices when the surface of the ITO anode is treated with oxygen plasmas or acid cleaning methods (which modify the effective work function of the TTO surface). However, a thousand fold improvement results from the use of hole injeetion layers such as PEDOT PSS. 

Many other materials have been considered in polymer OLEDs. The results are reflected in the dramatic gains in performance of the devices as shown in Figure 9.10. All of these materials combine multiple functions (as compared to molecular materials where each separate constituent molecule has a specific purpose). The molecules are designed by quantum chemical methods supplemented with a considerable experience base among synthetic chemists. The final materials represent a compromise between performance and manufacturabihty. The most successful materials in manufactured devices are based on soluble polymers. 

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