OLED and PLEDs

We saw in Chapter 20 that some organic molecules and polymers can be doped to make them either p-type or n-type semiconductors. Therefore, it should be possible to make a p-n junction and, if there is a direct bandgap, it should also be possible to make a small molecule organic light-emitting diode (OLED) and a polymer light-emitting diode (PLED). 

Later a Cambridge group made a single-layer OLED from polyphenylene vinylene with a Ca cathode that emits yellow light with a brightness of 500 cd/m with an 8 V bias. 

An important breakthrough came with the discovery that with the addition of phosphorescent dopant emitters such as Ft, Ir, Os, etc., the energy could be transferred from the triplet states to the dopant molecules which could decay radiatively. As a result, the internal quantum efficiency (the number of photons generated inside the device per electron-hole pair injected) of these phosphorescent organic light-emitting diodes (PHO-LEDs) (PHOLED is a trademark of the Universal Display Corporation, Ewing, New Jersey) can approach a quantum efficiency 100% at a luminescence of 100 cd/sr. 

Despite the gains in internal quantum efficiency, the overall efficiency is still less than 19%. As much as 80% of the photons produced cannot get out because of the high index of refraction of the polymer material. Approaches such as texturing the surface of the glass, index matching or using low index substrate materials, and the use of arrays of microlenses have made increases in the extraction efficiency. 

OLED flat and flexible panel display technology is advancing rapidly and full color displays are currently being used in cell phones. Sony recently annoimced a 2.5 in. flexible screen TV that is only 0.3 mm thick and now has an 11 in. OLED TV in production. Samsimg also annoimced a prototype 17 in. high definition (1600 x 1200 pixels) active matrix OLED display panel. 

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As a class of n-type organic semiconductors, PBI derivatives have received considerable attention for a variety of applications [312, 313], for example, for organic or polymer light-emitting diodes (OLEDs and PLEDs) [314, 315], thin-film organic field-effect transistors (OFETs) [316, 317], solar cells [318, 319], and liquid crystals [320]. They are also interesting candidates for single-molecule device applications, such as sensors [321], molecular wires [322], or transistors [141]. 

B Werner, J Posdorfer, B Webling, H Becker, S Heun, H Vestweber, and T Hassenkam, Polyaniline as hole injection layer for OLEDs and PLEDs, SID Digest Tech. Pap., 33 603-605, 2002. 

The ground state of most of the luminescent molecules and polymers which are used as the emitters in OLEDs and PLEDs is the symmetric singlet 11 Ag state.22 Figure 1.4 shows thebasic processes which may occur following photoexcitation of the molecule or conjugated segment of the polymer. Since the material is assumed to be luminescent, the antisymmetric 11 Bu state must lie below the symmetric 2-photon 21 Ag state. Otherwise, photoexcitation will still populate the 11 Bu state, but that state will quickly decay to the 21 Ag, and the latter will decay nonradiatively to the ground state, with lifetimes as short as 2 ps.23... 

This Section 1.5 has provided a brief survey of the small molecules and polymers currently in use in various academic and industrial laboratories developing novel OLEDs and PLEDs. It is obviously incomplete, and only highlights some of the major molecules and polymers utilized to date. It is also obvious that an enormous variety of existing and novel compounds, yet to be synthesized, could be utilized for novel future devices. 

After this brief overview of the physical concepts at play in OLEDs and PLEDs, the details of the chemicals of PLEDs will be described further, with a classification based on their color emission. 

Uses Conductive polymer for antistatic and electrostatic coatings, capacitor electrodes, through-hole plating of printed circuit boards interfacial hole injection layer in OLED and PLED devices to lower operating voltages, increase luminescence efficiency, enhance display lifetimes... 

PAni as Hole Injection Layer for OLEDs and PLEDs [111]... 

P30T is a conducting LEP that finds use in OLED and PLED materials, FETs, and, probably most widely studied, polymer-based solar cells [116, 117]. Other alkyl substituted derivatives (particularly the regioregular 3-hexyl P3HT) are also widely used and are important materials for bulk heterojuncticm PV systems. 

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