JOURNAL OF MATERIALS SCIENCE LETTERS 8 (1989) 1009-1010 Effect of iodine doping on electrical conduction in PVB films S. CHAND National Physical Laboratory, New Delhi 110012, India N. KUMAR Department of Physics, Vardhman College, Bijnor, India In recent years [1-7] investigations on the iodine doping of polymers has found considerable interest in polymer physics primarily due to an important role played by iodine in governing the electrical and dielec- tric properties of the polymers. For example the doping of iodine in polymers has shown [1-7] many effects such as enhancement in their electrical conduc- tivity, increase/decrease in their charge storing capacity, anomalous discharge currents, reversal of thermally stimulated currents, etc. Various of the above effects of iodine have been explained by these authors in terms of (1) formation of charge transfer com- plexes (CTCs), (2) diffusion of iodine at crystalline/ amorphous phases of the polymer and introducing conducting paths, (3) formation of molecular aggre- gates, (4) iodination of electrodes and providing blocking at the electrode polymer interface, etc. Recently in iodine doped polyvinylbutyral (PVB) films we have observed some interesting results while study- ing their I-V characteristics especially at high fields, i.e. iodine doping not only enhances its conductivity and reduces the activation energy of the charge carriers but it also changes the nature of the I-V curves especi- ally at high fields and high iodine concentrations where the curves tend to become ohmic rather than being non-ohmic. The results of these investigations are discussed in this letter. PVB in white powder form, was obtained from M/s Polysciences Inc., USA. Thick films (~ 50 Ftm) of both pure and iodine-doped PVB were prepared by the solution evaporation technique [8]. The iodine con- centration in these films was varied from 0 to 1.0%, by weight. The Ag/PVB/Ag sandwich configuration of area ~ 1 cm 2 was obtained by vacuum depositing the silver electrodes on two sides of the PVB films. The method of conductivity measurement was the same as reported earlier [2]. Figure 1 shows the current voltage (I-V) curves on a log-log scale of both pure and iodine-doped PVB films (thickness ~50#m) at a fixed temperature 303 K, but for different iodine concentrations; curves 1 to 5 correspond to iodine concentrations of 0, 0.1, 0.2, 0.5 and 1.0 wt %, respectively. It is seen from these curves that general behaviour of iodine-doped PVB films is almost similar to that of pure PVB films (curve 1), i.e. in both cases two regions of conduction namely ohmic conduction at low fields with slope of I-V curves ~ 1 and non-ohmic conduction at high fields with slope of I-V curves ~ 2, are observed. It is also seen that with the increase in iodine concentration the overall conductivity in the PVB films goes on increas- 0261-8028/89 $03.00 + .12 © 1989 Chapman and Hall Ltd. ing. The actual variation of current density with iodine concentration is shown in Fig. 2. Further it is observed from Fig. 1 that the non-ohmicity in the curves goes on decreasing with the increase in iodine concentration and the curves become practically ohmic at large con- centrations of iodine ~ 1.0wt % (curve 5). Also the transition voltage Vt .... separating the two regions of conduction is seen to shift towards higher voltages with the increase in iodine concentration. In order to evaluate the activation energy (U) of the charge carriers, the conductivity of pure and iodine- doped PVB films (~ 50 pro) has been studied at a fixed bias (~ 100 V) but at different temperatures and log I against lIT plots have been made. Such plots for different concentrations of iodine are shown in Fig. 3 where curves 1 to 5 correspond to iodine concentrations of 0, 0.1, 0.2, 0.5 and 1.0 wt % respectively. The acti- vation energy evaluated from the slope of the above plots has been found to decrease with the increase in iodine concentration as seen in Fig. 2. It has already been suggested [9] that high field conduction in PVB films is predominantly governed by space charge limited currents (SCLC) where the charge carriers are injected from the electrodes into the bulk of the material at a particular level of traps (trap depth ~ 0.60 eV) and gets transported as a space charge at this level through the material; the trap 10-4 -- 10-5 I iO_6 IO -8 I io I 10 4 5 /' I I io~ io 3 V (V)---~ Figure 1 Log/-log V plots of pure and iodine doped PVB films. Curves ! to 5 correspond to iodine concentrations of 0, 0.1, 0.2, 0.5 and 1.0wt %, respectively. 1009