Estudo das propriedades ópticas e da condutividade do PEDOT:PSS modificado com nanoestruturas plasmônicas

Abstract

PEDOT: PSS is a conductive polymer of the polythiophene class widely used as the hole injection layer in photovoltaic devices. In this work, resonance Raman, SERS and UVVIS-NIR spectroscopies were used to study the optical properties of pure PEDOT: PSS and modified with AuNPs. PEDOT: PSS was synthesized via oxidative polymerization with sodium persulfate and with persulfate mixture with ferric sulfate hydrate, followed by purification with ion exchange resin or dialysis in deionized water. The evolution of the syntheses was monitored via UV-VIS-NIR spectroscopy, in which the existence of two distinct electronic regimes for the synthesis process was evidenced, with a faster kinetics for the syntheses performed with the mixture of salts. The polymers formed exhibited a high level of oxidation, characterized by a strong absorption of electromagnetic radiation by the species that contain polarons and bipolarons. The efficiency of purification methods with ion exchange resin compared to dialysis in deionized water was also evaluated. Purification by dialysis proved to be distinct from stirring with ion exchange resins, yielding less doped polymers than those purified with resins, which was attributed to the rearrangement of the polymer chains by migration of small ions and the small PSS chains for the feed solution. The evolution of the vibrational profile was monitored by resonant Raman spectroscopy with excitation wavelength of 633, 785 and 1064 nm. The variation for the region of high wavenumber of the bands attributed to the thiophene group, responsible for the high delocalization of charge in the conducting species, indicated that the polymers formed have a high degree of oxidation, with the existence of two regimes for the evolution of the vibrational profile. Doping that occurs during the synthesis process favored the formation of more oxidized species in the polymeric matrix, and the purification process of PEDOT: PSS changed the structure of the polymer. Thin and opaque films were obtained, and showed sheet resistance comparable to that obtained with commercially available polymers. It was also observed, through SERS spectroscopy of small chains of PEDOT:PSS adsorbed on gold nanoparticles, using incident radiation of 1064 and 785 nm, the intensification of vibrational modes characteristic of the oxidized species, probably near the metallic surface, although there was no evidence of specific interaction between the polymer and the nanostructures.