A single wavelength electro-optical reflection technique was used to study the creation of space charge distributions (SCDs) in an Al/PPV/ITO LED by monitoring the third order NLO response of the electro-luminescent layer, at a wavelength far from the main absorption band (λ=632.8nm). 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-Hückel screening in the polymer, we estimate the density of the fixed charges to be ρ0 = 1.5 × 1024m-3, for a uniform distribution in a 110nm 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 Vb. The measurements were performed for several temperatures between 60 and 110°C, indicating complex dynamics of the signal. Measurements performed at low temperature (88K) show the features of dispersive transport. The dynamic measurements were investigated by the electro-absorption technique also, with wavelengths laying in the main absorption band of PPV. © 2001 Elsevier Science B.V. All rights reserved.
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|Published - Oct 22 2001