Electroluminescence in Conjugated Polymers Polymer LED

The most important issues in the performance of polymer LEDs are quantum efficiency and device lifetime [1169]. Although the exciton-based model sets some limits to the efficiency of operation for polymer LEDs (up to 25% of PL efficiency), reported efficiencies are relatively high (above 20 Im/W for green emission), and compare favorably to those reported for sublimed molecular-film devices. 

Photoinduced Charge IVansfer in Coiyugated Polymer/Fullerene Composites 

As a new form of carbon, fullerene ( 50) is an excellent electron acceptor capable of taking on as many as sfac electrons. Therefore, fullerene can easily form charge-transfer salts with a variety of alkaline metal donors [1257]. Sariciftci et al. [1170] found photoinduced electron transfer from semiconducting polymers onto fullerene, with interesting and unique photophysical properties in those composites. In such cases, conjugated polymers act as electron donors upon photoexcitation (electrons promoted to the antibonding TT band), and then fullerene absorbs photoexcited electrons in the LUMO level. As a result, new absorption occurs in the photoexcitation spectrum of those composites, assigning to the allowed HOMO (Tig)-LUMO (Tju) transitions of [1258]. Once 

On the other hand, electron injection into Cgo and electron removal from the semiconducting polymer is energetically favorable, resulting in relatively high current densities under forward bias. 

As explained previously for polymer LEDs, a semiconducting polymer with asymmetric contacts (a low-work-function metal on one side and a high-work-function metal on the opposite side) functions as a tunneling injection diode [1262]. In forward bias, tunneling injection diodes exhibit relatively high efficiency electroluminescence. In reverse bias, Yu et al. (1265,1266] reported that the devices exhibit a strong photoresponse with a quantum yield larger than 20% (electron/photon at 10-V reverse bias), which comparable to UV-sensitized Si photodiodes. 

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