Photoconductivity of poly-p-xylylene + CdS nanocomposite films over a wide temperature range

Abstract

For poly-p-xylylene + CdS (PPX + CdS) nanocomposite films, the dependences of the photo-conductivity σph(T) on the concentration C of CdS nanoparticles, intensity and wavelength of exciting light, and temperature T within 15–300 K are examined. An appreciable photocurrent appears at C ≥ 10 vol %, when a large percolation cluster of CdS nanoparticles is formed. The photocurrent spectrum is compared to the absorption spectrum of the film. The photocurrent Iph(P) increases with the intensity of light flux P in a wavelength range near 435 nm according to the Iph(P) ∼ Pn power law, where n 11.5 vol %, the σph(T) dependence at low T exhibits a metal-like character (σph(T) decreases with increasing temperature). Atomic force microscopy is used to examine the surface topography of PPX + CdS films, which is found to be strongly dependent on the concentration of nanoparticles. The dark conductivity and photoconductivity of nanocomposite films arise due to the thermo- and photoexcitation transfer of electrons from the CdS nanoparticles to the PPX matrix with the formation of an electronic double layer at the PPX matrix-large percolation CdS cluster interface, a process that populates the phenyl rings of the adjacent PPX layer with excess electrons. As a result, various mechanisms of electron transfer in the polymer matrix can be realized: Mott’s hopping conduction mechanism with variable-range hopping in the matrix between CdS clusters and the metal-like behavior of the conductivity in the polymer shell of the large cluster at low temperatures. The polymer shell contains excess electrons on the phenyl rings -C6H4- in the composition of anion-resonances -C6H4−-.