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 m onitoring the third order NLO response of the electro-luminescent layer, at a wavelength far from the main absorption band (lambda = 632.8 nm). The po lymer is a PPV derivative with oxidiazole groups grafted as side chains. Th e measurements show the presence of SCD given by the sum of a static contri bution close to the ITO, and of a slowly changing and bias dependent one du e to mobile charges injection. Taking into account Debye-Huckel screening i n 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. Th e dynamics of the mobile charges was investigated by time-dependent measure ments. 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 meas urements were performed for several temperatures between 60 and 110 degrees C, 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, w ith wavelengths laying in the main absorption band of PPV (C) 2001 Elsevier Science B.V. All rights reserved.