Optical and electrical properties of PPV/SiO2 and PPV/TiO2 composite materials

Composites made by incorporation of SiO2 or TiO2 nanoparticles into poly(p-phenylene vinylene) (PPV) have been fabricated and their optical and electrical properties have been investigated. The UV-vis absorption band of the composite films showed a large blue-shift with SiO2 nanoparticles, but only little difference with TiO2 nanoparticles. Photoluminescence (PL) spectra showed the same blue-shift trend for SiO2 composites, and in addition, an increase in intensity of the high energy shoulder (515 nm) when the concentration of nanoparticles increased. Raman spectra showed a reduction of the 1547/1625 cm−1 band ratio in SiO2 composites but not in TiO2 ones. These results suggest that SiO2 nanoparticles reduced the PPV conjugation length, while TiO2 nanoparticles did not. For SiO2 particles, the reduction of conjugation lengths is more pronounced on increasing the oxide concentration or on decreasing the particle size. Fourier-transform Infrared (FT-IR) spectra showed that both types of nanoparticles reduced the formation of carbonyl groups in PPV main chains. Current-voltage characteristics measured in ITO-composite-MgAg diodes exhibit different electrical behavior of the composites depending on the particle size and the nature of the oxide. The composite-electrode contact morphology, the polymer-dielectric particle contact and the change in the polymer chain length are the possible explanations for these changes in behavior of the diodes.