Thermoelectric and Charge Transport in Doped Semi-Crystalline Polymers
In recent years, organic semiconductors (OSCs) have generated many research interests from both academia and industry due to the exotic properties and potential applications in flexible displays, light-emitting diodes, solar cells as well as thermoelectrics. Compared with their inorganic counterparts, OSCs have several advantageous features including low cost, lightweight, mechanically robustness, ease of synthesis and tailoring material properties, environmental friendless, flexibility. For many of the mentioned applications, OSCs are doped to improve the conductivity. This is particularly important to improving the thermoelectric performance of OSCs and the figure of merit ZT. However, another equally important parameter, the Seebeck coefficient, tends to decrease when carrier concentration and conductivity increase. Therefore, it is crucial to find a balanced point where ZT maximizes. To achieve this, it is paramount to understand charge and entropy transport as well as the related key parameters affecting the electrical and thermoelectric properties in doped OSCs. Herein, doped poly(2,5-bis(3-tetradecylthiophen2-yl)thieno-[3,2-b]thiophene), PBTTT and its derivates are selected in this thesis for the investigation of thermoelectric and charge transport in highly conducting polymers. An unprecedented doping regime can be achieved via ion-exchange doping method, which is already approaching the metal-insulator (M-I) transition. The enhanced charge delocalization has been evidenced by the systematic spectroscopic as well as transport studies for samples with a broad range of conductivities. The clear Hall effect has been detected by a high-resolution ac Hall set-up, with the heavily doped sample showing a temperature-independent Hall coefficient, which indicates nearly "ideal" band-like transport. The temperature dependences of conductivity and Seebeck coefficient for samples at different doping levels witness the onset of metallic states evolved from pure hopping transport. The Seebeck coefficient over a wide range of conductivity can be analyzed by a model that accounts for the energy dependence of the transport function and show a transition from an exponent s=3 to s=1 or 2. Such a transition also correlates to the point where power factor maximizes. We correlate these observed transport phenomena for ion-exchange doped PBTTT with microstructural ordering as function of increasing doping levels, as supported by the diffraction data. Moreover, side-chain modification and chalcogen substitution have been established to affect the properties of semiconducting organic materials. In this thesis, we also investigate their effects on doping as well as electrical and thermoelectric properties. The adopted polar side chain modification with alkoxy and glycol groups was verified to give more efficient doping but show an adverse effect on thermoelectric and electrical performance. Intriguingly, the introduction of single chalcogen atom into the polymer backbone, which is believed to enhance the interchain interaction, gives rise to decent electrical and thermoelectric properties for doped samples. We attribute such improvement to the peculiar microstructures induced by the heteroatom. The presented fundamental studies of both charge and thermoelectric transport properties on doped semi-crystalline polymers aim to contribute more insights into the structure-property relationship in conducting polymers and thus provide guidelines to improve electric and thermoelectric properties of organic materials for future applications.
Organic thermoelectrics, Doping, Charge transport
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