Photoconductivity of polymer films of vanadium composites with copper and cobalt

Abstract

This is a study of the photoconductive properties of polymer composite fi lms based on non-photoconducting polyvinylbutyral doped with different metallic complexes V/M (M = Cu, Co). The activation energy of the photocurrent is found to rise with an increasing external electric fi eld. The possible infl uence of an external electric fi eld on recombination and the separation of nonequilibrium charge pairs is considered as a possible explanation for the temperature dependence of the photoconductivity. A phenomenological model for the effect of an electric fi eld on these processes is proposed. Introduction. Different complexes of transition metals in polymer composite fi lms (PCFs) can facilitate the development or stimulation of the internal photoeffect and lead to photoconductivity in these fi lms (1, 2). In the visible range corresponding to optical d-d transitions of the metals, the photoconductivity in these PCFs is caused by photogeneration of charge carriers and their transport, both inside the particles of the complexes separated by the polymer binder, and among these particles. This latter point suggests the use of these compounds as nanostructures for information media (3) and photoelectric converters of solar energy (4). Vanadium complexes are of ongoing research interest because of their biological and catalytic properties (5, 6). In the meantime, the question of creating PCFs with added vanadium-containing compounds has not been discussed in the literature. As an extension of research on the internal photoeffect in PCFs with complexes of different metals, here we study the photoconductive properties of PCFs based on polyvinylbutyral (PVB) doped with complexes of different metals, (H3O)2(H2en)(Cu(en)2(H2O)2)(V10O28)·3H2O (C1), (NH4)2(Co(H2O)6)2(V10O28)·4H2O (C2), and (H3O)2(Co(H2O)5(β-HAla))2(V10O28)·H2O (C3) (where en denotes ethylenediamine and β-HAla, 2-aminopropanoic acid), obtained by direct synthesis, as in (7). Samples and Experimental Technique. Samples with PCF based on nonphotoconducting PVB were prepared with a free surface of the PCF (glass substrate-PCF) or in the form of sandwich structures (glass substrate-electrically conducting SiO2:In2O3 layer-PCF-Ag). The amount of C1-C3 in the PVB was 33 mass%. The method for fabricating the samples is described elsewhere (8). The PCF thickness, L = 2-3 μm, was measured using an MII-4 (LOMO, Russia) interference microscope. The following characteristics of the prepared samples were measured: optical thickness D of the PCF for wavelengths λ = 400-800 nm; photocurrent density jPH during and after illumination at λirr ~ 540 nm as a function of the voltage U applied to the electrical contacts; the intensity I0 of the light; the duration t of the illumination and the time after the light was turned off; and, the temperature T. The light source was an incandescent lamp with a glass fi lter. U was varied over 1-300 V. The intensity of the light was varied over a range of 5-50 W/m 2 using glass fi lters. A thermostat with an optical window was used for the temperature measurements; the temperature in the thermostat was varied over a range of 293-370 K. The time variation in the photocurrent was recorded using a Tektronix TDS1001B memory scope. Results and Discussion. The PVB fi lms are transparent in the visible, but with added particles of complexes C1-C3 they have a yellow-brown tint (Fig. 1) that is independent of the second metal, since the absorption of visible light in these PCFs is caused by photoexcitation of metallic centers of decavanadate anions (V10O28) 6- with the following structure: