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

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.EPSRC studentshi

Interaction-driven (quasi-) insulating ground states of gapped electron-doped bilayer graphene

Bernal bilayer graphene has recently been discovered to exhibit a wide range of unique ordered phases resulting from interaction-driven effects and encompassing spin and valley magnetism, correlated insulators, correlated metals, and superconductivity. This letter reports on a novel family of correlated phases characterized by spin and valley ordering, observed in electron-doped bilayer graphene. The novel correlated phases demonstrate an intriguing non-linear current-bias behavior at ultralow currents that is sensitive to the onset of the phases and is accompanied by an insulating temperature dependence, providing strong evidence for the presence of unconventional charge carrying degrees of freedom originating from ordering. These characteristics cannot be solely attributed to any of the previously reported phases, and are qualitatively different from the behavior seen previously on the hole-doped side. Instead, our observations align with the presence of charge- or spin-density-waves state that open a gap on a portion of the Fermi surface or fully gapped Wigner crystals. The resulting new phases, quasi-insulators in which part of the Fermi surface remains intact or valley-polarized and valley-unpolarized Wigner crystals, coexist with previously known Stoner phases, resulting in an exceptionally intricate phase diagram

Probing the tunable multi-cone bandstructure in Bernal bilayer graphene

Controlling the bandstructure of Dirac materials is of wide interest in current research but has remained an outstanding challenge for systems such as monolayer graphene. In contrast, Bernal bilayer graphene (BLG) offers a highly flexible platform for tuning the bandstructure, featuring two distinct regimes. In one regime, which is well established and widely used, a tunable bandgap is induced by a large enough transverse displacement field. Another is a gapless metallic band occurring near charge neutrality and at not too strong fields, featuring rich 'fine structure' consisting of four linearly-dispersing Dirac cones with opposite chiralities in each valley and van Hove singularities. Even though BLG was extensively studied experimentally in the last two decades, the evidence of this exotic bandstructure is still elusive, likely due to insufficient energy resolution. Here, rather than probing the bandstructure by direct spectroscopy, we use Landau levels as markers of the energy dispersion and carefully analyze the Landau level spectrum in a regime where the cyclotron orbits of electrons or holes in momentum space are small enough to resolve the distinct mini Dirac cones. We identify the presence of four distinct Dirac cones and map out complex topological transitions induced by electric displacement field. These findings introduce a valuable addition to the toolkit for graphene electronics

Structural and dynamic disorder, not ionic trapping, controls charge transport in highly doped conducting polymers

Doped organic semiconductors are critical to emerging device applications, including thermoelectrics, bioelectronics, and neuromorphic computing devices. It is commonly assumed that low conductivities in these materials result primarily from charge trapping by the Coulomb potentials of the dopant counter-ions. Here, we present a combined experimental and theoretical study rebutting this belief. Using a newly developed doping technique, we find the conductivity of several classes of high-mobility conjugated polymers to be strongly correlated with paracrystalline disorder but poorly correlated with ionic size, suggesting that Coulomb traps do not limit transport. A general model for interacting electrons in highly doped polymers is proposed and carefully parameterized against atomistic calculations, enabling the calculation of electrical conductivity within the framework of transient localisation theory. Theoretical calculations are in excellent agreement with experimental data, providing insights into the disordered-limited nature of charge transport and suggesting new strategies to further improve conductivities

Structural and dynamic disorder, not ionic trapping, controls charge transport in highly doped conducting polymers

Doped organic semiconductors are critical to emerging device applications, including thermoelectrics, bioelectronics, and neuromorphic computing devices. It is commonly assumed that low conductivities in these materials result primarily from charge trapping by the Coulomb potentials of the dopant counter-ions. Here, we present a combined experimental and theoretical study rebutting this belief. Using a newly developed doping technique, we find the conductivity of several classes of high-mobility conjugated polymers to be strongly correlated with paracrystalline disorder but poorly correlated with ionic size, suggesting that Coulomb traps do not limit transport. A general model for interacting electrons in highly doped polymers is proposed and carefully parameterized against atomistic calculations, enabling the calculation of electrical conductivity within the framework of transient localisation theory. Theoretical calculations are in excellent agreement with experimental data, providing insights into the disordered-limited nature of charge transport and suggesting new strategies to further improve conductivities

High‐Efficiency Ion‐Exchange Doping of Conducting Polymers

Abstract: Molecular doping—the use of redox‐active small molecules as dopants for organic semiconductors—has seen a surge in research interest driven by emerging applications in sensing, bioelectronics, and thermoelectrics. However, molecular doping carries with it several intrinsic problems stemming directly from the redox‐active character of these materials. A recent breakthrough was a doping technique based on ion‐exchange, which separates the redox and charge compensation steps of the doping process. Here, the equilibrium and kinetics of ion exchange doping in a model system, poly(2,5‐bis(3‐alkylthiophen‐2‐yl)thieno(3,2‐b)thiophene) (PBTTT) doped with FeCl3 and an ionic liquid, is studied, reaching conductivities in excess of 1000 S cm−1 and ion exchange efficiencies above 99%. Several factors that enable such high performance, including the choice of acetonitrile as the doping solvent, which largely eliminates electrolyte association effects and dramatically increases the doping strength of FeCl3, are demonstrated. In this high ion exchange efficiency regime, a simple connection between electrochemical doping and ion exchange is illustrated, and it is shown that the performance and stability of highly doped PBTTT is ultimately limited by intrinsically poor stability at high redox potential

The high-resolution map of Oxia Planum, Mars; the landing site of the ExoMars Rosalind Franklin rover mission

This 1:30,000 scale geological map describes Oxia Planum, Mars, the landing site for the ExoMars Rosalind Franklin rover mission. The map represents our current understanding of bedrock units and their relationships prior to Rosalind Franklin’s exploration of this location. The map details 15 bedrock units organised into 6 groups and 7 textural and surficial units. The bedrock units were identified using visible and near-infrared remote sensing datasets. The objectives of this map are (i) to identify where the most astrobiologically relevant rocks are likely to be found, (ii) to show where hypotheses about their geological context (within Oxia Planum and in the wider geological history of Mars) can be tested, (iii) to inform both the long-term (hundreds of metres to ∼1 km) and the short-term (tens of metres) activity planning for rover exploration, and (iv) to allow the samples analysed by the rover to be interpreted within their regional geological context

The Effects of Biofeedback and Relaxation Training on Resting-Level EMG, Stress-Recovery and Tension Awareness

Conflictual findings have been reported as to the effects of biofeedback and other forms of relaxation training on reduction of frontalis muscle activity as measured by an electromyograph (EMG). Also related to the utility of relaxation procedures is the question regarding how to effectively discriminate the effects of one method over another. The present study investigated the effects of EMG biofeedback, progressive muscle relaxation and passive self-control on resting level EMG and on a stress-recovery and tension awareness task, the latter having been noted in prior research as potential discriminating measures in relaxation training. Subjects were 2b (8 per group) undergraduate females with approximately average levels of self-reported trait anxiety across groups. Results showed no statistically significant difference among groups in reducing resting-level and stress-recovery level EMG although a significant difference was shown across the short duration of time from pre- to post-training with the means for groups showing the greatest change occuring in the biofeedback and passive self-control groups while the progressive muscle relaxation group showed the least change. Also, no significant difference was shown in the effects of training on the tension awareness task. However, better than chance estimates of changes in tension awareness were noted across groups prior to training. Implications for future research were discussed

Charge transport in single polymer fiber transistors in the sub-100 nm regime: temperature dependence and Coulomb blockade

Even though charge transport in semiconducting polymers is of relevance for a number of potential applications in (opto-)electronic devices, the fundamental mechanism of how charges are transported through organic polymers that are typically characterized by a complex nanostructure is still open. One of the challenges which we address here, is how to gain controllable experimental access to charge transport at the sub-100 nm lengthscale. To this end charge transport in single poly(diketopyrrolopyrrole-terthiophene) fiber transistors, employing two different solid gate dielectrics, a hybrid Al _2 O _3 /self-assembled monolayer and hexagonal boron nitride, is investigated in the sub-50 nm regime using electron-beam contact patterning. The electrical characteristics exhibit near ideal behavior at room temperature which demonstrates the general feasibility of the nanoscale contacting approach, even though the channels are only a few nanometers in width. At low temperatures, we observe nonlinear behavior in the current–voltage characteristics in the form of Coulomb diamonds which can be explained by the formation of an array of multiple quantum dots at cryogenic temperatures