Abstract
AbstractPoly(p‐phenylenevinylene) (PPV) and its derivatives exhibit strong luminescence, being serious candidates to be used as active layers in organic light‐emitting diodes. However, the structural degradation caused by photo‐oxidation is an obstacle for commercial applications of such materials. Here, we show that spectroscopy ellipsometry is a useful technique to investigate the photo‐oxidation of poly[(2‐methoxy‐5‐hexyloxy)‐p‐phenylenevinylene] (MH‐PPV), a PPV derivative, which emits a red color light. Spectroscopy ellipsometry enables determination of the complex dielectric function—ϵ*(E)—of MH‐PPV thin‐layer films exposed to air, in the 2.1–4.2 eV energy range, as a function of the light exposure time (te). By using the Lorentz model to fit the experimental ϵ*(E) curves, it was inferred that the interactions among polymeric chains increase with te. From ϵ*(E), it is also possible to obtain the complex refractive index, N*(E) = n + ik. At higher energies (where k ≪ n), n increases from 1.32 to 1.40 with the photo‐oxidation progress. The behavior of n was investigated by using the Lorenz–Lorentz equation, taking into account the contribution for n by the chromophores of MH‐PPV. The effect of photo‐oxidation, mainly due to the replacement of vinyl CC by the ketone CO bonds, is confirmed by Fourier transform infrared measurements, an effect that reduces the average effective polymer conjugation length. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1033–1041, 2004
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