Seebeck coefficient in organic semiconductors

When a temperature differential is applied across a semiconductor, a thermal voltage develops across it in response. The ratio of this thermal voltage to the applied temperature differential is the Seebeck coefficient, a transport coefficient that complements measurements of electrical and thermal conductivity. The physical interpretation of the Seebeck coefficient is the entropy per charge carrier divided by its charge and is hence a direct measurement of the carrier entropy in the solid state. This PhD thesis has three major outcomes. The first major outcome is a demonstration of how the Seebeck coefficient can be used as a tool to quantify the role of energetic disorder in organic semiconductors. To this end, a microfabricated chip was designed to perform accurate measurements of the Seebeck coefficient within the channel of the active layer in a field-effect transistor (FET). When measured within an FET, the Seebeck coefficient can be modulated using the gate electrode. The extent to which the Seebeck coefficient is modulated gives a clear idea of charge carrier trapping and the distribution of the density of states within the organic semiconductor. The second major outcome of this work is the observation that organic semiconducting polymers show Seebeck coefficients that are temperature independent and strongly gate voltage modulated. The extent to which the Seebeck coefficient is modulated in the polymer PBTTT is found to be larger than that in the polymer IDTBT. Taken together with conventional charge transport measurements on IDTBT, the voltage modulated Seebeck coefficient confirms the existence of a vanishingly small energetic disorder in this material. In the third and final outcome of this thesis, the magnitude of the Seebeck coefficient is shown to be larger for organic small molecules as compared to organic polymers. The basis for this is not yet clear. There are reports that such an observation is substantiated through a larger contribution from vibrational entropy that adds to the so called entropy-of-mixing contribution so as to boost the magnitude of the Seebeck coefficient in organic small molecules. As of now, this remains an open question and is a potential starting point for future work. The practical implications of this PhD thesis lie in building cost-effective and environmentally friendly waste-heat to useful energy converters based on organic polymers. The efficiency of heat to energy conversion by organic polymers tends to be higher than that for conventional semiconductors owing to the presence of narrow bands in organic polymer semiconductors

The effect of the dielectric end groups on the positive bias stress stability of N2200 organic field effect transistors

Bias stress degradation in conjugated polymer field-effect transistors is a fundamental problem in these disordered materials and can be traced back to interactions of the material with environmental species,[1,2,3] as well as fabrication-induced defects.[4,5] However, the effect of the end groups of the polymer gate dielectric and the associated dipole-induced disorder on bias stress stability has not been studied so far in high-performing n-type materials, such as N2200.[6,7] In this work, the performance metrics of N2200 transistors are examined with respect to dielectrics with different end groups (Cytop-M and Cytop-S).[8] We hypothesize that the polar end groups would lead to increased dipole-induced disorder, and worse performance.[1,9,10] The long-time annealing scheme at lower temperatures used in the paper is assumed to lead to better crystallization by allowing the crystalline domains to reorganize in the presence of the solvent.[11] It is hypothesized that the higher crystallinity could narrow down the range at which energy carriers are induced and thus decrease the gate dependence of the mobility. The results show that the dielectric end groups do not influence the bias stress stability of N2200 transistors. However, long annealing times result in a dramatic improvement in bias stress stability, with the most stable devices having a mobility that is only weakly dependent on or independent of gate voltage

Thermoelectric nanospectroscopy for the imaging of molecular fingerprints

We present a nanospectroscopic device platform allowing simple and spatially resolved thermoelectric detection of molecular fingerprints of soft materials. Our technique makes use of a locally generated thermal gradient converted into a thermoelectric photocurrent that is read out in the underlying device. The thermal gradient is generated by an illuminated atomic force microscope tip that localizes power absorption onto the sample surface. The detection principle is illustrated using a concept device that contains a nanostructured strip of polymethyl methacrylate (PMMA) defined by electron beam lithography. The platform's capabilities are demonstrated through a comparison between the spectrum obtained by on-chip thermoelectric nanospectroscopy with a nano-FTIR spectrum recorded by scattering-type scanning near-field optical microscopy at the same position. The subwavelength spatial resolution is demonstrated by a spectral line scan across the edge of the PMMA layer

Epitaxial Zn(x)Fe(3-x)O(4) Thin Films: A Spintronic Material with Tunable Electrical and Magnetic Properties

The ferrimagnetic spinel oxide Zn(x)Fe(3-x)O(4) combines high Curie temperature and spin polarization with tunable electrical and magnetic properties, making it a promising functional material for spintronic devices. We have grown epitaxial thin films with 0<=x<=0.9 on MgO(001) substrates with excellent structural properties both in pure Ar atmosphere and an Ar/O2 mixture by laser molecular beam epitaxy. We find that the electrical conductivity and the saturation magnetization can be tuned over a wide range during growth. Our extensive characterization of the films provides a clear picture of the underlying physics of this spinel ferrimagnet with antiparallel Fe moments on the A and B sublattice: (i) Zn substitution removes both Fe3+ moments from the A sublattice and itinerant charge carriers from the B sublattice, (ii) growth in finite oxygen partial pressure generates Fe vacancies on the B sublattice also removing itinerant charge carriers, and (iii) application of both Zn substitution and excess oxygen results in a compensation effect as Zn substitution partially removes the Fe vacancies. A decrease (increase) of charge carrier density results in a weakening (strengthening) of double exchange and thereby a decrease (increase) of conductivity and the saturation magnetization. This scenario is confirmed by the observation that the saturation magnetization scales with the longitudinal conductivity. The combination of tailored films with semiconductor materials such as ZnO in multi-functional heterostructures seems to be particularly appealing.Comment: 13 pages, 8 figures, Hall effect data removed, anti-phase boundary discussion added, accepted for publication in PRB79 (2009

High-mobility, trap-free charge transport in conjugated polymer diodes

Charge transport in conjugated polymer semiconductors has traditionally been thought to be limited to a low mobility regime by pronounced energetic disorder. Much progress has recently been made in advancing carrier mobilities in field-effect transistors through developing low-disorder conjugated polymers. However, in diodes these polymers have to date not shown much improved mobilities, presumably reflecting the fact that in diodes lower carrier concentrations are available to fill up residual tail states in the density of states. Here, we show that the bulk charge transport in low-disorder polymers is limited by water-induced trap states and that their concentration can be dramatically reduced through incorporating small molecular additives into the polymer film. Upon incorporation of the additives we achieve space-charge limited current characteristics that resemble molecular single crystals such as rubrene with high, trap-free SCLC mobilities up to 0.2 cm2/Vs and a width of the residual tail state distribution comparable to kBT.We gratefully acknowledge financial support the Engineering and Physical Sciences Research Council (EPSRC) through a Programme Grant (EP/M005141/1). M.N. acknowledges financial support from the European Commission through a Marie-Curie Individual Fellowship (EC Grant Agreement Number: 747461)

Chasing the ‘Killer’ Phonon Mode for the Rational Design of Low Disorder, High Mobility Molecular Semiconductors

Molecular vibrations play a critical role in the charge transport properties of weakly van der Waals bonded organic semiconductors. To understand which specific phonon modes contribute most strongly to the electron – phonon coupling and ensuing thermal energetic disorder in some of the most widely studied high mobility molecular semiconductors, we have combined state-of-the-art quantum mechanical simulations of the vibrational modes and the ensuing electron phonon coupling constants with experimental measurements of the low-frequency vibrations using inelastic neutron scattering and terahertz time-domain spectroscopy. In this way we have been able to identify the long-axis sliding motion as a ‘killer’ phonon mode, which in some molecules contributes more than 80% to the total thermal disorder. Based on this insight, we propose a way to rationalize mobility trends between different materials and derive important molecular design guidelines for new high mobility molecular semiconductors.Royal Society German Research Foundation European Research Council Engineering and Physical Sciences Research Council ARCHER UK National Supercomputing Service Belgian National Fund for Scientific Research Leverhulme Trust Wiener-Anspach Foundation Belgian Walloon Region GENCI-CINES/IDRI

Investigation of Electrode Electrochemical Reactions in CH3 NH3 PbBr3 Perovskite Single-Crystal Field-Effect Transistors.

Optoelectronic devices based on metal halide perovskites, including solar cells and light-emitting diodes, have attracted tremendous research attention globally in the last decade. Due to their potential to achieve high carrier mobilities, organic-inorganic hybrid perovskite materials can enable high-performance, solution-processed field-effect transistors (FETs) for next-generation, low-cost, flexible electronic circuits and displays. However, the performance of perovskite FETs is hampered predominantly by device instabilities, whose origin remains poorly understood. Here, perovskite single-crystal FETs based on methylammonium lead bromide are studied and device instabilities due to electrochemical reactions at the interface between the perovskite and gold source-drain top contacts are investigated. Despite forming the contacts by a gentle, soft lamination method, evidence is found that even at such "ideal" interfaces, a defective, intermixed layer is formed at the interface upon biasing of the device. Using a bottom-contact, bottom-gate architecture, it is shown that it is possible to minimize such a reaction through a chemical modification of the electrodes, and this enables fabrication of perovskite single-crystal FETs with high mobility of up to ≈15 cm2 V-1 s-1 at 80 K. This work addresses one of the key challenges toward the realization of high-performance solution-processed perovskite FETs

Roadmap on energy harvesting materials

Ambient energy harvesting has great potential to contribute to sustainable development and address growing environmental challenges. Converting waste energy from energy-intensive processes and systems (e.g. combustion engines and furnaces) is crucial to reducing their environmental impact and achieving net-zero emissions. Compact energy harvesters will also be key to powering the exponentially growing smart devices ecosystem that is part of the Internet of Things, thus enabling futuristic applications that can improve our quality of life (e.g. smart homes, smart cities, smart manufacturing, and smart healthcare). To achieve these goals, innovative materials are needed to efficiently convert ambient energy into electricity through various physical mechanisms, such as the photovoltaic effect, thermoelectricity, piezoelectricity, triboelectricity, and radiofrequency wireless power transfer. By bringing together the perspectives of experts in various types of energy harvesting materials, this Roadmap provides extensive insights into recent advances and present challenges in the field. Additionally, the Roadmap analyses the key performance metrics of these technologies in relation to their ultimate energy conversion limits. Building on these insights, the Roadmap outlines promising directions for future research to fully harness the potential of energy harvesting materials for green energy anytime, anywhere

Research Data supporting "Dynamic self-stabilisation in the electronic and nanomechanical properties of an organic polymer semiconductor"

The data set contains higher eigen mode images of C16-IDTBT polymer films together with their nanomechanical properties. The measurements were taken on a Park Systems NX20 AFM

Research data supporting The effect of the dielectric end groups on the positive bias stress stability of N2200 organic field effect transistors

Data to accompany "The effect of the dielectric end groups on the positive bias stress stability of N2200 organic field effect transistors" by D. Simatos et al., APL Materials 9, 041113 (2021); https://doi.org/10.1063/5.0044785 also available on Apollo (https://www.repository.cam.ac.uk/handle/1810/319841) and arXiv (https://arxiv.org/abs/2104.07089). The zip folder contains raw datasets (.txt and .csv files) of transfer and output curves as well as of positive bias stress (PBS) of OFETs (Organic Field Effect Transistors) fabricated with the semiconducting organic polymers indacenodithiophene-co-benzothiadiazole (C16-IDTBT) and poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (C14-PBTTT) as well as Cytop-M (CTL-809M) and Cytop-S (CTX809-SP2) gate dielectrics (AGC Inc.). The analysis (.opju file) was carried out in Origin (the data analysis and graphing software from OriginLab). Dielectric thicknesses measured with a profilometer are also included. See the main manuscript for more details. Folder names include information relating to the batch & date of device fabrication, contact geometry (TGBC - top gate bottom contact), type of polymer (IDTBT, N2200), type of dielectric (Cytop-M, Cytop-S), fabrication and testing conditions (nitrogen atmosphere, air exposure or vacuum) and the number of days elapsed since fabrication. In addition, Cryo denotes measurements as a function of temperature and PBS - Positive Bias Stress results. Filenames include information relating to the fabrication batch, contact geometry (TGBC - top gate bottom contact), type of polymer (IDTBT, N2200), type of dielectric (Cytop-M, Cytop-S), fabrication and testing conditions (nitrogen atmosphere, air exposure and vacuum) as well as the number of days elapsed since fabrication, T- transfer curve, O- output curve. PBS filenames also include the time elapsed from the beginning of an experiment and Cryo filenames include the temperature in Kelvin