Propriedades ópticas de blendas e bicamadas de polímeros semicondutores e aplicações em dispositivos emissores de luz

Abstract

In this work, we studied optical and morphological properties for binary blends of poly[2-(2,5-bis(2ethyl-hexyloxy)phenyl)-1,4-phenylenevinylene] (BEHP-PPV) and poly[3-hexylthiophene](P3HT) semiconducting polymers, with different P3HT relative concentrations. The optical characterization of the blends was made using photoluminescence andabsorption techniques, at room temperature and at low temperature (~ 60 K). From the obtained results, we investigated the possible presence of energy transfer effects from the BEHP-PPV molecules (donor) to P3HT molecules (acceptor). Morphological studies were also made usingAtomic Force Microscopy (AFM) and Electrical Force Microscopy (EFM), which confirmed the immiscibility character of this polymer system. We also studied self-absorption effects for both polymers and their influence on the final shape of emission spectra for blended films. As a technological application, we fabricated organic light emitting diodes (OLEDs) using the blends with 25% and 50% relative P3HT concentrations as active layers. A comparison between the photoluminescence (PL) and electroluminescence (EL) spectra at room temperaure, for both devices, shows a red shift for the EL curves. The spectra shift toward the more pure red region as the applied bias is increased. This phenomenon was interpreted as being caused byaccumulation of charge carriers into P3HT domains and the interfaces. We confirmed this explanation by making two OLEDs using bilayers of these polymers as active layers, simulating the two possible carriers injection configuration into the blends. We also observed an EL blueshift, caused by one barrier of potential, introduced between the polymer layers. This barrier consists of a lithium fluoride layer, wich is used to keep the polymer films separated.