Nanocompósitos de óxido de grafeno reduzido contendo óxidos metálicos e moléculas orgânicas para aplicações em fotovoltaicos

Abstract

In this work, aiming potential materials to act as electrodes in optoelectronics, graphene oxide (GO) and reduced graphene oxide (rGO) containing oxides (TiO2 or SnO2) or molecule (perylene tetracarboxylic dianhydride, PTCDA) were prepared by a modified Hummers method, where the precursors (oxide or molecule) are added at the GO synthesis, potassium permanganate (KMnO4) acts as oxidizing agent and the reduction was performed by thermal annealing in air. Structural and morphology features were investigated by powder x-ray diffraction (XRD), small angle electron diffraction (SAED), scanning and transmission electronic microscopy (SEM and TEM) and atomic force microscopy (AFM). Additional data about optical properties, as well as the presence of functional groups, were recorded by ultraviolet-visible (UV-Vis), infrared (FTIR), Raman, X-ray photoelectron (XPS) and ultraviolet photoelectron (UPS) spectroscopy. Both XRD and RAMAN results confirmed the formation of GO and further reduction to rGO at the nanocomposites, as indicated by the peaks at 2θ= ~10.2º (GO) and ~26.2º (rGO), besides changes at the relative intensity between the D and G bands at the Raman spectra corresponding to graphene. Additionally, FTIR spectra pointed out the presence of C꞊O, C-H, COOH and C-O-C functional groups, which are characteristics of rGO. Presence of TiO2 and SnO2 at the composites was confirmed by SEM and TEM images, which pointed out nanoparticles attached at the rGO surface and, by the presence of the characteristic oxide bands at the FTIR spectra around 500-600 cm-1. In the rGO_PTCDA, SEM and TEM pointed out the absence of phase separation between PTCDA and rGO, the SAED pattern indicate high cristallinity and regular distribution of PTCDA molecules between the rGO sheets. FTIR spectra point out that in the rGO_PTCDA the perylene can be covalently linked to rGO structure. By adding perylene to the reaction medium, the acid would promote hydrolysis of the anhydride, forming carboxylic acid. Part of these groups could react with OH groups of rGO, generating an ester bond between rGO and perylene. This assumption is based on the presence of peaks at 1730, 1175 and 1069 cm -1 attributed to the stretching of the ester type C=O and C-C-O bonds, respectively. Thin films were produced from aqueous suspension using spin coating method, the resulting films are transparent to the visible light, presenting low roughness and electrical resistivity and thus, they are suitable for use as electrodes. The potential of rGO, rGO_SnO2 and rGO_PTCDA films as buffer layer was evaluated in organic photovoltaics produced with the commercial PBDTTT-CT copolymer and the PC71BM molecule as electron donor and acceptor materials, respectively, at the active layer.