부산대학교 도서관

Understanding how Seebeck effect of organic thermoelectric devices is associated with the chemical structure of active molecules within the devices is not only a significant challenge in chemistry but also a key goal in the research of organic and molecular thermoelectrics. Chapter 1 describes advances in the use of molecular junctions for studies of molecular thermoelectrics based on physical-organic approaches. Through a stepwise approach, this chapter provides (i) a summary of the types of thermoelectric molecular junctions and thermopower measurement techniques employed in the field of molecular thermoelectrics, (ii) an extensive discussion on how they affect the thermoelectric data in terms of absolute value and distribution, and (iii) a comprehensive summary and analysis of structure–thermopower relationships established in the field. Chapter 2 shows a new, efficient approach for thermoelectric characterization of molecular monolayers using liquid eutectic gallium-indium (EGaIn). Cone-shaped EGaIn microelectrode permits access to non-invasive, reversible top-contact formation onto organic surfaces in ambient conditions, high yields of working devices (up to 97%), and thus statistically sufficient thermoelectric data sets (~6000 data per sample in a few hours). We here successfully validated our platform with widely studied molecules, oligophenylenethiolates.Chapter 3 describes the length dependence of thermopower in self-assembled monolayers (SAMs) comprising structurally simple wide-bandgap molecules, n-alkanethiolates (SCn; n = 2, 4, 6, 8, 10, 12, 14, 16, 18) chemisorbed on gold. A plot of the Seebeck coefficient (S, μV/K) versus the length of the n-alkane chain reveals the presence of two different length-dependence regimes. The rate of decrease of the Seebeck coefficient as the molecular length increases changes at SC10 from −0.54 to −0.10 μV(K·nC)-1. The theoretically proposed presence of metal-induced gap states (MIGS) in the short but not in the long n-alkanethiolates accounts for the two observed length-dependence regimes. Owing to the length dependence of the transmission function coefficient of MIGS in the short n-alkanethiolates, the Seebeck coefficient decreases linearly as the length increases. The nearly zero rate of decrease in the long n-alkanethiolates mirrors the insignificant MIGS in the long n-alkanethiolates. We envisage that the new energy states at the SAM−metal interface can be chemically tuned and harnessed for new applications in organic and molecular thermoelectrics and electronics.Chapter 4 describes a series of physical-organic studies that investigate structure-thermopower relationships in self-assembled monolayers (SAMs) through measurements of the Seebeck coefficient using the EGaIn-based junction technique. Several hypotheses were derived from a transmission function-based Mott formula. These hypotheses were tested by comparing S values for simple alkyl and aryl molecules with different structures in terms of backbone, length, spacer, anchor, and substituent, and for different electrodes (Au vs. Ag). Experimentally obtained S values were further reconciled with values simulated by the Mott formula and by interfacial electronic structure and molecule-electrode coupling strength, independently measured by ultraviolet photoelectron spectroscopy (UPS) and transition voltage spectroscopy (TVS).In Chapter 5, the study systematically investigates the thermal properties of self-assembled monolayer (SAM)-based molecular junctions and relates them to the thermoelectric performance of the junctions. The electrode temperatures for the bare AuTS, AuTS/EGaIn, and AuTS/TPT SAM//Ga2O3/EGaIn samples placed on a hot chuck were measured under different conditions, such as air vs vacuum and the presence and absence of thermal grease, which generates a heat conduction channel from a hot chuck to gold. It was revealed that the SAM was the most efficient thermal resistor, which was responsible for the creation of a temperature differential (ΔT) across the junction; ΔT in an air atmosphere is overestimated to some extent, and air mainly contributes to large dispersions of thermovoltage (ΔV) data. While junction measurements in the air were possible at low ΔT, the new optimal condition, under a vacuum and with thermal grease, allowed us to examine a wide temperature range up to ΔT = 40 K and obtain a more reliable Seebeck coefficient. The value of S under the new condition was ∼1.4 times higher than that measured in the air without thermal grease. Chapter 6 describes that introducing a noncovalent interface in a molecular junction leads to a remarkable enhancement of thermopower as compared to the analogous junction with a covalent interface. Thermoelectric junction measurements exhibit that the value of the Seebeck coefficient in large-area junctions based on n-alkylamine monolayer on graphene is increased up to five-fold compared to the analogous junction based on n-alkanethiolate monolayer on gold. Our work demonstrates that control of interfacial bonding nature in molecular junctions improves the Seebeck effect.Chapter 7 shows that N-heterocyclic carbene (NHC) can be a robust anchor and leads to molecular junctions with consistent Seebeck coefficient under harsh thermal environments (heating temperatures up to 573 K), generating thermovoltage up to ca. |1900 μV|. Structural analysis indicates that the NHC anchor maintains without appreciable structural change under the thermal environments, whereas thiol degrades into unbound species and leads to deteriorated thermoelectric performance. Our work demonstrates that NHC-based anchor chemistry can contribute to resolving the stability problem in energy conversion devices.Chapter 8 describes the fabrication of molecular epitaxy films via imine condensation between phenylamine and aldehyde derivatives and traces the response of the Seebeck coefficient as a function of epitaxy cycles, using a liquid-metal technique. A linearly increasing trend of the Seebeck coefficient as the thickness of the molecular layer increases was observed. Interestingly, the increasing rate transitioned from 0.99 μV∙K−1Å−1 to 0.38 μV∙K−1Å−1 at d = 3.4 nm. This finding was attributed to the transition of the transport regime from tunneling to hopping. Our work demonstrates the benefit of molecular epitaxy in the research of thermoelectrics, particularly for bridging a gap between bulk (oligomer/polymer) and small-molecule (monomer) systems.Chapter 9 shows a study that experimentally determines the power factor (PF) of one molecule-thick organic films, SAMs. The SAMs composed of n-alkanethiolates, n-cycloalkanethiolates, and oligophenylenethiolates of different lengths are focused. These SAMs are electrically and thermoelectrically characterized on an identical junction platform using a liquid metal top-electrode, allowing the straightforward estimation of the PF of monolayers. A parametric semi-empirical model describing the length dependence of PF is further developed. Our simulation using this equation proposes that ultrathin organic films (e.g., less than 10 nm) can yield PF values that rival conventional high-performance organic materials. Furthermore, how the transition of transport regime from tunneling to hopping as molecules become long affects power factors is examined. We envisage that this work offers an unprecedented opportunity to harness SAM-based junctions as a nanoscale platform for establishing structure-PF relationships at the molecular level.