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 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 =632.8 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 ρ 0 =1.5×10 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 V b 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°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-absorption technique also, with wavelengths laying in the main absorption band of PPV.