Propriedades Ópticas de Semicondutores Orgânicos à Base de Polímeros Emissores de Luz / Optical proprieties of organic semiconductors based on light emitting polymers.




In this work, we studied the optical proprieties of absorption and emission of luminescent conjugated polymers based on poly(p-phenylene vinylene) (PPV). This material was processed in thin films by casting, spin-coating, self-assembled (SA) e Langmuir-Blodgett (LB) techniques, available in the Grupo de Polímeros Bernhard Gross, where the samples were characterized optically and chemically. The optical proprieties of PPV were investigated by the following techniques: photoluminescence (PL), photoluminescence excitation spectroscopy and optical absorption. The measurements were carried out in function of sample temperature and polarization of the excitation and the emission light. The chemical and structural characterization of the material was performed by infrared spectroscopy and elemental analysis. The molecular anisotropy in plane of LB-PPV films were studied by circular dichroism and birefringence experiments. A new methodology in the material and film processing was developed in this work. Here we have adopted an alternative approach consisting in substituting the chloride counter ion of a water-soluble precursor, poly(xylylidene tetrahydrothiophenium chloride) (PTHT), by a long chain sulfonic counter ion (DBS) using a sodium salt of dodecylbenzenesulfonic acid. The advantage of this precursor polymer lies in the possibility of converting PPV films with a high conjugation length at 115 °C within only 3 min. Using DBS allowed PPV films to be converted under atmospheric pressure at temperatures as low as 80 oC, with conjugation length and optical properties better than for standard films converted at temperatures above 200 oC under controlled atmospheres. Stable Langmuir PTHT-DBS monolayers were transferred onto quartz substrates in the form of LB films. These LB-PPV films are highly anisotropic as demonstrated by linear dichroism experiments using linearly polarized optical absorption and emission and by birefringence measurements. Furthermore, SA-PPV films were produced by a different methodology. The adsorption on alternate PTHT and DBS layers result in PPV films with high conjugation degree and well-resolved spectral structure. These results are not similar in the literature. A strong PL enhancement was observed in PPV films caused by light excitation in the presence of air. This effect is accompanied by a blue-shift in the absorption spectrum resulting in shortened effective conjugation length and by a formation of defects such as carbonyl groups. The PL enhancement can be explained by an efficient incoherent diffusion of excited carriers to non-degraded PPV segments by Förster transfer, which is activated by the formation of an energy profile in the film due to distribution conjugation lengths generated by photodegradation. A theoretical model based on experimental data and considering the geometric parameters is proposed. Finally, the spectral line shape of absorbance and emission of PPV with different conjugation degrees was analyzed with success in the region of p-p* non-localized electronic transitions by Franck-Condon analysis.


optical proprieties organic semiconductors propriedades ópticas semicondutores orgânicos

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