Study of Thickness Effect on Absorption, Photo and Electroluminescence Spectra in ROPPV Polymer Films

The study of EL in thin films is of vital interest both for fundamental and technological reasons. Research in case of organic polymers as the active semiconductors in light-emission has advanced rapidly, and prototype devices now meet realistic specifications for applications. The ROPPV films of various thicknesses have been prepared using solution casting technique by dissolving different amounts of ROPPV granules in DMF. Their absorption, photo and electroluminescence spectra have been studied. The films are found to be transparent in the visible region and absorption peaks are obtained only in the ultraviolet range. The thickness of polymer layer was measured by Fizeau fringes technique. The ROPPV films thicknesses were varied between 200 to 400 nm. Absorption spectra were taken by Perkin - Elmer - Lambda -12 spectrometer. Photo luminescence has been studied by monocromator. The electroluminescence (EL) of ROPPV films have been studied by sandwiching them between SnO₂ coated conducting glass plate and aluminum electrodes. Result of absorption spectra shows that the peaks are obtained only in the ultraviolet region. Thick films show only one peak at 310 nm, whereas decreasing the thickness it is divided in two or three peaks. Thin films show only one peak at 310 nm indicating the HOMO level, whereas increasing the thickness, other higher energy orbital is also revealed The PL spectrum of ROPPV films was excited through violet filter. It has been observed that peaks are obtained at wavelength 380 nm, 450 nm, and 510 nm by violet filter. For lower thickness the PL intensity is less. The EL brightness is found to increase with increasing thickness of ROPPV emissive layer. It is observed that higher threshold voltage is required for the thicker films. The EL spectrum shows two main peaks one at 450 and the other at 510 nm. The intensity of peak at 510 nm increases linearly with the emissive layer thickness; whereas no systematic variation has been observed in the intensity of the peak at 450 nm. The results are analyzed on the basis of energy levels and recombination profile of ROPPV.