Dynamics of space charge distributions in side-chain PPV LEDs

A single wavelength electro-optical reflection technique was used to study the creation of space charge distributions (SCDs) in an AN PPV/ITO LED by monitoring the third order NLO response of the electro-luminescent layer, at a wavelength far from the main absorption band (lambda = 632.8 nm). The polymer is a PPV derivative with oxidiazole groups grafted as side chains. The measurements show the presence of SCD given by the sum of a static contribution close to the ITO, and of a slowly changing and bias dependent one due to mobile charges injection. Taking into account Debye-Huckel screening in the polymer, we estimate the density of the fixed charges to be rho (0) = 1.5 x 10(24) m(-3), for a uniform distribution in a 110 nm thick layer. The dynamics of the mobile charges was investigated by time-dependent measurements. The samples were biased at a voltage Vb for a time interval and the time dependence of the signal was studied after switching off V-b. The measurements were performed for several temperatures between 60 and 110 degreesC, indicating complex dynamics of the signal. Measurements performed at low temperature (88 K) show the features of dispersive transport. The dynamic measurements were investigated by the electro-ab sorption technique also, with wavelengths laying in the main absorption band of PPV (C) 2001 Elsevier Science B.V. All rights reserved.