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Light-induced softening of azobenzene dye-doped polymer films probed with quartz crystal resonators
(2000)
Surface light emitting diodes SLEDs , in which previously microfabricated electrodes were coated with a conjugated polymer, were made with greatly different electrode spacings 250 nm and 10 or 20 mm and with different electrode material combinations. The fabrication process allowed us to compare several electrode materials. The SLED structures also enabled imaging of the light emission zone with fluorescence video microscopy. Conventional sandwich structures were also made for comparison electrode separation 50 nm. In this study, the emitting layer was poly[3- (2',5'-bis(1'',4'',7''trioxaoctyl)phenyl)-2,2'-bithiophene] (EO-PT), a conjugated polymer based on polythiophene with oligo ethyleneoxide side chains. The current-voltage (I(V)) and light-voltage (L(V)) characteristics of the SLEDs were largely insensitive to electrode separation except at high voltages, at which the current in the devices with the largest separations was limited. Sandwich structures had the same light output at a given current. Light could be obtained in forward and reverse bias from indium tin oxide ITO -aluminum, gold silicide-aluminum, and gold silicide-gold SLEDs, but the turn-on voltages were lowest with the ITO-aluminum devices, and these were also the brightest and most reliable. Adding salt to the EO-PT increased the current and brightness, decreased the turn-on voltages, and made the I(V) characteristics symmetric; thus, a device with an electrode separation of 10 mm had the extraordinarily low turn-on voltage of 6 V. The location of the light emission was at the electron-injecting contact.
Dielectric loss spectroscopy (DLS) was performed at compact samples and lamellary organized Langmuir-Blodgett (LB) films from various fatty acid salts. Previous thermoanalytical measurements at compact samples revealed the appearance of two different phase transition temperatures; the lower one is related to the acid the second one to the acid salt molecules. In spite of ill defined electrical contacts with the film the characteristic DLS frequencies obtained from about 100nm thick multilayer films are similar to those recorded from bulk samples. No significant variations of frequencies were found changing the counter ions. Besides conductivity influence at low frequncies we found two relaxations related to the mobility of the dipolar carboxylat-metal group at about 100 and 10000Hz. One of these frequencies is related to the rotation around the chain axis. The strength of this relaxation increases significantly with increasing the sample temperature above 105°C. This temperature is connected with a structural phase transition observed by X-ray reflectometry. In case of Pb-stearate the results of the dielectric measurements help to interprete this structural change as a transition from an orthorhombic into a free-rotator phase. The uncorrelated rotation of molecules around their molecular axes initiates a much increased relaxation strength at the carboxylat-metal sites.