Comparative study of space-charge effects in polymer light emitting diodes by means of reflection electro-optic and electroabsorption techniques

We used two nonlinear optical techniques, single wavelength ellipsometry and electroabsorption spectroscopy, to study the creation of space charge distributions in an Al/polymer/ITO organic light emitting diode. The polymer is OXA1-PPV, a derivative of $\mathrm{poly}(p-\mathrm{p}\mathrm{h}\mathrm{e}\mathrm{n}\mathrm{y}\mathrm{l}\mathrm{e}\mathrm{n}\mathrm{e}-\mathrm{v}\mathrm{i}\mathrm{n}\mathrm{y}\mathrm{l}\mathrm{e}\mathrm{n}\mathrm{e})$ in which oxidiazole groups are grafted as side chains to improve electroluminescence efficiency. Both the techniques are sensitive to the spatial profile of electric field via the nonlinear effect in the bulk of the polymer. Results indicate the creation of an asymmetric charge distribution with depletion region close to the ITO electrode. The comparison between ellipsometry and electroabsorption measurements is an original feature of this work. Such an integrated analysis shows as a further result that the mostly used electroabsorption setup, working with wavelengths in the spectral region of maximum absorption, modifies the spatial charge distribution optically reactivating trapped carriers.