Polymer electrically pumped

EL), conjugated polymers are also of interest as materials for optically or electrically pumped stimulated emission. For effects of this type, the ratio of stimulated emission to photoinduced absorption (PA) is of particular interest for conjugated polymers. In this context, the orign of the PA is controversial the PA can be a result of the formation of either charge-separated polaron pair -states or excimers. Initial experiments support the conjecture that LPPP 26 is significantly superior [49], as the stimulated emission of LPPP 26 is markedly more intense than that of PPV under comparable conditions. 

All systems presented in this section show lasing only in the optical pumping mode. There is much interest in electrically pumped devices, but for molecular glasses the difficulties in achieving high excitation densities and low absorption due to charge carriers and electrodes have yet to be overcome. This problem and the related semiconducting polymer lasers that are based on the same principles will not be covered here, but are treated in recent reviews [214-216]. 

The absorption spectrum of m-LPPP (see Figs. 8.5 and 8.14) is characterized by a steep onset of the absorption above the energy gap (ow 1.1 x 105 cm-1 at 453 nm) and a very low subgap absorption ( ,], < 12 cm-1).4,5,77 The minimum value of a is more determined by scattering effects than by true absorption, so that this value can be drastically decreased if the film quality of the polymer film is further improved, which is essential for the application of this polymer in waveguide structures for optical23,78 79 and electrically pumped lasers. 

Interestingly, Vg-controlled electroluminescence and ambipolar characteristics have been recently observed in conjugated polymer OFETs [67,68], which indicates a balanced electron and hole injection. However, low hole and electron mobilities ( 10 cmWs), typical for polymer semiconductors, limit the channel current and therefore may present a serious problem for realizatiou of electrically pumped polymer lasers. For this reasou, ordered smaU-molecule orgauic semicouductors with higher mobilities are very promisiug for research in this direction. 

Other possible applications of gradient junctions are in polymer-nanocom-posite LEDs, where a voltage-dependent color is expected, and even for the manufacture of lasers with electrical pumping, since the injection current densities in the case of gradient junctions can be made quite high. However, in the case of lasers, the nanopartides must be fairly uniform in size, a condition which is barely achievable by the chemical synthetic methods usually used for the preparation of nanocomposites. 

The fast kinetics of the photoluminescence emission, picosecond and femtosecond relaxation, in ladder-type polyparaphenylenes is notable [142,143,186], since such measurements reveal stimulated emission and therefore promising aspects for LPPPs as a suitable material for optically pumped solid-state homopolymer lasers as well as for the realization of electrically pumped solid-state polymer lasers [144]. To illustrate the situation we depict the photoluminescence intensity as a function of the excitation power (excitation wavelength A = 295 nm) for... 

PPO forms one of a group of rigid, heat-resistant, more-or-less selfextinguishing polymers with a good electrical and chemical resistance, low water absorption and very good dimensional stability. This has led to a number of applications in television such as tuner strips, microwave insulation components and transformer housings. The excellent hydrolytic stability has also led to applications in water distribution and water treatment applications such as in pumps, water meters, sprinkler systems and hot water tanks. It is also used in valves of drink vending machines. 

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