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Charge and Thermoelectric Transport in Semicrystalline Conjugated Polymers and Single-Walled Carbon Nanotube Networks



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Due to their flexibility, solution-processability and continuously improving electronic performance, conjugated polymer semiconductors and single-walled carbon nanotube (SWCNT) networks are promising candidates for wearable electronics, flexible optoelectronic devices and thermoelectric generators. In the past two decades the development of high-mobility donor-acceptor copolymers outperforming amorphous silicon, employed in commercial display technologies, and the ability to tune the diameter distribution in SWCNT networks via selective dispersion with conjugated polymers and other sorting methods have been major breakthroughs for these material systems. This thesis provides an improved understanding of their charge and thermoelectric transport. In particular, the charge density and temperature dependence of their field-effect mobility and gated Seebeck coefficient are investigated. When a temperature difference is applied to a conducting system, a thermal voltage builds up as a response. The Seebeck coefficient is the ratio of the thermal voltage to the temperature difference and characterizes the entropy transported by a carrier divided by its charge. Consequently, it offers insights into the transport energetics and the density of states (DoS). It can be used to identify the prevailing transport mechanisms, such as phonon-assisted hopping between localized states or scattering-limited transport through delocalized states, scattering mechanisms and carrier-carrier interactions as well as the extent of charge carrier trapping. Firstly, it is demonstrated that charge transport in semicrystalline high-mobility copolymers is incompatible with disorder-based transport models that were developed for preceding, more disordered polymers. Instead the charge density and temperature dependence of the field-effect mobility and gated Seebeck coefficient of the semicrystalline n-type polymer P(NDI2OD-T2) with varying degrees of crystallinity provides direct evidence for low-disorder, narrow-band conduction. The inclusion of short-range electron-electron interactions and the consideration of a spatially inhomogeneous DoS allow to explain both the measured mobility and Seebeck coefficient. These findings outline the extension of crystalline domains as a mean for improved thermoelectric conversion efficiencies. Subsequently, the charge density and temperature-dependent field-effect mobility and gated Seebeck coefficient of polymer-sorted monochiral small diameter (6,5) (0.76 nm) and mixed large diameter SWCNT (1.17-1.55 nm) networks with different network densities and length distributions are reported. It is shown that charge and thermoelectric transport in SWCNT networks can be modelled by the Boltzmann transport formalism incorporating transport in heterogeneous media and fluctuation-induced tunneling. The charge density and temperature dependence of the Seebeck coefficient can be simulated via the consideration of the diameter-dependent one-dimensional DoS of the SWCNTs composing the network. Due to the carrier relaxation time being anti-proportional to energy, the simulations further point towards a more two-dimensional character of scattering, as opposed to one-dimensional acoustic and optical phonon scattering in single SWCNTs, as well as the potential necessity to consider scattering at SWCNT junctions. Trap-free, narrow DoS distribution, large diameter SWCNT networks, allowing low tunnel barriers and a large thermally accessible DoS, are proposed for both electronic and thermoelectric applications. Finally, the thermoelectric performance of the molecularly doped semicrystalline polymer PBTTT is presented in the high charge density limit, which is relevant for applications in thermoelectric generators. Using the recently reported ion-exchange doping routine charge densities on the order of one carrier per monomer repeat unit can be obtained, allowing to attain a highly conductive system. Ongoing investigations of the impact of polymer alignment on charge and thermoelectric transport in this regime are presented.





Sirringhaus, Henning


charge transport, thermoelectric transport, organic semiconductors, single-walled carbon nanotube networks, Seebeck coefficient, Boltzmann transport formalism, density of states


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
EPSRC (1805383)
EPSRC studentship