The electrical transport properties of organic semiconductor polypropylene doped with various concentrations of bromine
Electrical transport properties of undoped and bromine-doped polyprophlene were studied by investigating the temperature dependence of dark electrical conductivity and thermoelectric power. The doping process involved letting the polypropylene sample absorb liquid bromine molecules and, by varying the immersion time, it was possible to obtain samples of different bromine concentrations. The samples of different concentrations, by weight uptake, of 0.00%, 0.7%, 1.6%, 2.2%, 3.5%, 4.2%, 5.9%, 6.8% and 7.2% were prepared. The isotacticity of these samples was confirmed by infrared spectroscopy. Conductivity (s) values were calculated in the temperature range 208K to 373K. The conductivity is found to increase sharply up to 5% concentration, beyond which it tends to a constant value. Below room temperature the 1ns versus, inverse tempeture curves are straight lines each of which has two gradients, which is in conformity with the expected contribution to the resultant conductivity from extrinsic and intrinsic conduction. The activation energy (Eg), for both regions, was calculated with the aid of the equation s = so exp (-Eg/2kBT), where so is the pre-exponential constant. The E.g. values for the extrinsic (lower temperature) regime lie between 0.22 ± 0.04 eV and 0.92 ± 0.02 eV, while in the intrinsic conduction regime, values varied from 0.91± 0.03 eV to 3.45 ± 0.04 eV. The values calculated for thermo power (S) were found to be positive which is an indication of a P-type conduction process with holes as the majority carriers. The equation S = (kB/e)[Es/2kBT = B], where B is a constant which depends upon the scattering mechanism, was used to explain the linear temperature dependent thermo power and for the determination of the thermal activation energy of carrier generation (Es). E|s values ranged from 0.20 ± 0.04 eV to 0.97 ± 0.08 eV.