Investigation of the optical and electrical properties of plasma polymerized pyrrole - N , N ,3 ,5 tetramethylaniline composite thin films

Abstract

A capacitively coupled parallel plate glow discharge reactor has been used to deposit plasma polymerized pyrrole (PPPy), plasma polymerized N,N,3,5 tetramethylaniline (PPTMA) and plasma polymerized pyrrole-N,N,3,5 tetramethylaniline (PPPy-PPTMA) bilayer composite thin films on to glass substrates at room temperature. To deposit single-layer PPPy and PPTMA thin films, the deposition parameters such as flow-rate, power, vacuum order, etc. were kept almost same for all samples, but deposition time was varied to grow thin films with different thicknesses, so that the comparison of the results could be made for various plasma polymerized thin films. On the other hand, to deposit the bilayer composite films, pyrrole-monomer has been used as the parent-material and N,N,3,5 tetramethylaniline monomer has been deposited in different deposition time ratios after the pyrrole films were formed. The structural analyses by Fourier transform infrared (FTIR) spectroscopy have indicated that the monomer has undergone re-organization and the ring structure is retained during the plasma polymerization of PPPy and PPTMA thin films, and the bilayer composite thin films contain the characteristics of both of its components. From DTA and TGA traces it is observed that the weight loss in PPPy-PPTMA bilayer composite thin films is much higher than those of its component thin films even at relatively lower temperature. This behavior suggests PPPy-PPTMA bilayer composite thin films are less thermally stable and less breakdown thermal energy is needed for bilayer structure to dissociate the bonds than those of its components. From the UV-Visible absorption spectra, it is observed that both the allowed direct transition (Eqd) and allowed indirect transition (Eqi) energy gaps are decreased with the decrease of the thickness for all types of thin films, which is an indication of decreasing the resistivity and increasing the conductivity of the thin film of lower thicknesses. Moreover, it is also observed that the energy gaps of the PPPy-PPTMA bilayer composite thin films are higher compared to those of the PPPy and PPTMA thin films which indicates a higher electrical resistivity of the bilayer thin films than those of its component films. The change in the resistivity suggests a probable change in physical properties during the formation of the plasma polymerized thin films and the result is interpreted in terms of oxidation, inhomogeneties and irregularities in the complex polymer-polymer interface of the bilayer composite structure. In the study of the direct current conduction mechanism in PPPy-PPTMA bilayer composite thin films of different composition and different thicknesses the current density – voltage (J-V) characteristics indicate an increase in conductivity of bilayer thin films as the proportion of PPTMA is increased in the films. It is also observed that the conductivity of the bilayer composite thin film is reduced compared to its component thin films. This result is consistent with the previous result that the higher values of the optical band gaps were observed for bilayer composite thin films as compared to its component thin films. It is seen that in the low voltage region, the current conduction obeys Ohm’s law while the charge transport phenomenon appears to be the space charge limited in the higher voltage regions. The free charge carrier density, the mobility and the trap density has been calculated from the J-V characteristics and it is found that the SCLC is trap-mediated. From the study of ac electrical properties the variation of the dielectric constant ε ' and dielectric loss factor ε "with the frequency shows electrical relaxation properties which is related with multi-component contribution of polarizability of the polar materials; e.g. interfacial or space charge polarization, orientational polarization of molecular chain, α-relaxation process, etc. The ac conductivity, σac , is observed to be increased with the frequency which is attributed to the relaxations caused by the motion of electrons or atoms, which could involve hopping between equilibrium sites; and it is therefore concluded that, the ac conduction mechanism is due to the hopping of carriers.