Preparation and characterization of oxadiazole based electron transporting thin films

Abstract

To study the effect of aggregation of the 2-(4-biphenylyl)-5-phenyl-1,3,4-oxadiazole (PBD) molecule in solid state, thin films of PBD have been prepared by the thermal evaporation technique onto glass and quartz substrates under different experimental conditions. These films have been studied for their structural, optical and electrical properties. AFM investigations of the films revealed that the films were smooth, dense and crack free with RMS roughness of 11–14 nm. XRD measurements indicate that films deposited on quartz are more crystalline than films deposited on glass substrate. Both absorption and reflectance spectra over the wavelength range 200–800 nm have been recorded to find optical parameters, namely, absorption, extinction coefficient, refractive index and dielectric constants. The inter-band transition energies are found to lie within the range 3.45–3.49 eV. Optical studies of the films indicate that PBD molecules preferred J-aggregation. A prominent single emission peak in the range of 370–390 nm has been observed which confirms that the fluorescent property of this molecule is not quenched in the thin film state. The electrical conductivity results for the evaporated films exhibited semiconductor behaviour within the investigated field and temperature range. The nature of the substrate is found to be a useful tool to modify the film morphology and for enhancing the charge transport within the films.