Thermoelectric and charge transport properties in doped and aligned conjugated polymers

Abstract

This thesis presents an investigation aimed at enhancing the thermoelectric power factor for semicrystalline conjugated polymers, particularly PBTTT, through methodologies such as ion-exchange doping, high-temperature rubbing and electrochemical doping. Central to this investigation is a deepened comprehension of the transport physics and the modelling of temperature dependence in doped conjugated polymers, pivotal for unlocking their full potential in thermoelectric applications. Building on the emerging ion-exchange doping technique, a high-throughput screening system was developed to evaluate the thermoelectric power factor across a range of doped conjugated polymers. This system, harnessing the capabilities of a semi-automatic probe station and Peltier elements, ensures the expeditious generation of high-quality data, crucial for the precise measurement of conductivity and Seebeck coefficient. Through the systematic screening of various conjugated polymer candidates, PBTTT has emerged as a frontrunner in terms of thermoelectric performance. However, it was intriguing to note that P8T2Z-C12, despite its high crystallinity, lagged behind PBTTT in terms of thermoelectric properties, a phenomenon potentially attributable to reduced long-range order. The device design also facilitated the concurrent measurement of carrier density, enabling the plotting of its correlation with conductivity, Seebeck coefficient, mobility, and thermopower. Upon acknowledging PBTTT as a superior thermoelectric conjugated polymer, a critical progress was achieved through the optimization of high-temperature rubbing process parameters for PBTTT. The design-of-experiments approach enabled a systematic probe into the pivotal factors affecting the high-temperature rubbing process, identifying the optimal parameters with minimized experimental effort and time. The alignment of polymer chains through this process markedly enhanced conductivity by facilitating improved charge mobility, as charge transport is more efficient along the polymer backbone. This optimization led to a significant 3-4 times increase in the conductivity of aligned PBTTT films compared to their unaligned counterparts, accompanied by a remarkably high dichroic ratio. Notably, the power factor for aligned PBTTT films was observed to be quadruple that of unaligned films, marking a critical milestone in the field of thermoelectric devices. In a systemic exploration utilizing organic electrochemical transistors (OECTs), the charge transport characteristics of highly aligned PBTTT were studied in detail, unveiling distinct carrier density levels and a characteristic temperature-dependence of the conductivity and Seebeck coefficient. The charge transport mechanism evolved from Arrhenius behaviour to various variable-range hopping regimes, which can be described by Kaiser's heterogeneous model at ultrahigh doping levels. This systematic study yielded a summary of the interrelations between doping level, doping domain, carrier density, and transport behaviours in aligned PBTTT. A sign reversal in the Seebeck coefficient for PBTTT at low temperatures was also observed, an unprecedented finding for p-type conjugated polymers. The insights gained from this investigation are anticipated to catalyse advancements in the field of energy harvesting and conversion. The findings pave the way for more efficient thermoelectric devices, contributing to sustainable energy solutions for a rapidly changing world.