On the first stage of the project, I focused the work on monitoring the hybridization process in situ, using a Quartz Crystal Microbalance (QCM). QCM was integrated into the reactor dedicated for vapor phase infiltration of thermoelectric (TE) materials. The in situ monitoring allowed studying the saturation behaviour of the VPI process and quantitatively controlling the loading of inorganic precursors into a polymer. The combination of my experience in in situ QCM analysis of vapor-phase processes and the host´s expertise in polymer infiltration allowed investigations of infiltration. When the chemical and physical mechanistic behind these processes was understood, I moved to second stage of the project.
In the second stage of HYTEM, VPI was applied to the conducting polymers to grow hierarchical inorganic nanostructures and defect sites in the polymer bulk to increase the phonon scattering. I developed VPI of conductive polymers with n- and p-type semiconducting materials like Bi2S3, Sb2S3, Sb2Se3, Sb2Te3, Bi2Se3. Using VPI, I obtained a great control over the size distribution and gradient of the inorganic fraction and the overall composition of the designed hybrid materials.
In the third stage, during the secondment at Leibniz Institute for Solid State and Materials Research, I was doing full thermoelectric characterization of the obtained samples. Due to the better performance, later the work had been focused on p-type polymers and therefore on p-type inorganic materials respectively: Sb2S3, Sb2Se3, Sb2Te3. Sufficient TE activity at from room temperature considering their relatively small bandgap was achieved. The strategy of rational engineering of semiconductor interfaces allowed to enhance the ZT performance in TE materials with heterostructures.
Interesting results were obtained by doping p-type polymer PEDOT:PSS with n-type Bi2S3 to investigate how much we can impact on the nature of carriers and they mobility and whether we can measure that. As a result of the experiment- both, electrical and thermal conductivities decreased, that showed that the introduced n-type carriers compensated intrinsic p-type carriers, impacting carrier transport in PEDOT:PSS -Bi2S3 film. The measurement results for the Seebeck coefficient indicated transformation from p to n-type conductivity. That showed that we can impart properties, opposite to the intrinsic transport properties of the polymer using VPI infiltration with inorganic materials of different nature.
Unique results are obtained for PEDOT:PSS-Sb2Te3 hybrid film. The thermal conductivity of the film increased during first two temperature cycling stages, but within the third run when semiconducting behaviour transformed to metallic, thermal conductivity went down to a value of 0.3 W/mK. It could be that with the cooling, the degree of disorder increases by out of the plane orientated Sb2Te3 crystals formation, reducing thermal conductivity. For a typical material, when electrical conductivity is going up, thermal conductivity is also going up. Here I observed the unique case, that was the goal of my project- to decouple electrical from thermal conductivity. The maximum obtained ZT thermoelectric performance was 0.16 at 150 ˚C.
