Open Access

Show Abstract ▼

Abstract: Conducting polymers are mixed ionic–electronic conductors that are emerging candidates for neuromorphic computing, bioelectronics and thermoelectrics. However, fundamental aspects of their many-body correlated electron–ion transport physics remain poorly understood. Here we show that in p-type organic electrochemical transistors it is possible to remove all of the electrons from the valence band and even access deeper bands without degradation. By adding a second, field-effect gate electrode, additional electrons or holes can be injected at set doping states. Under conditions where the counterions are unable to equilibrate in response to field-induced changes in the electronic carrier density, we observe surprising, non-equilibrium transport signatures that provide unique insights into the interaction-driven formation of a frozen, soft Coulomb gap in the density of states. Our work identifies new strategies for substantially enhancing the transport properties of conducting polymers by exploiting non-equilibrium states in the coupled system of electronic charges and counterions.

Open Access

Show Abstract ▼

Abstract: Two-dimensional conjugated coordination polymers (cCPs) based on square-planar transition metal-complexes (such as MO4, M(NH)4, and MS4, M = metal) are an emerging class of (semi)conducting materials that are of great interest for applications in supercapacitors, catalysis, and thermoelectrics. Finding synthetic approaches to high-performance nickel and nitrogen (Ni-N) based cCP films has been a long-standing challenge. Here, we develop a general, dynamically controlled on-surface synthesis method that produces highly conductive Ni-N-based cCP thin films and study the thermoelectric properties as a function of the molecular structure and their dependence on interactions with the ambient atmosphere. Among the four studied cCPs with different ligand sizes hexaminobenzene- and hexaaminotriphenylene-based films exhibit record electrical conductivity (100 – 200 S cm–1) in this Ni-N based cCP family, which is one order of magnitude higher than previous reports, and the highest thermoelectric power factors up to 10 μW m–1 K–2 among reported 2D cCPs. We study the transport physics of these films and show that depending on the host-guest interaction with oxygen/water the majority carrier type and the value of the Seebeck coefficient can be largely regulated. The high conductivity is likely reflecting good interconnectivity between (small) ordered domains with grain boundaries supporting disordered metallic transport.

Show Abstract ▼

Abstract: Coinage-metal chalcogenides and chalcogenide halides are mixed ionic and electrical semiconductors with low thermal conductivities. They show a high potential for thermoelectric applications. Despite these general aspects, a small number of those materials is known to show a reversible switching from p-type to n-type semiconduction and back during a temperature induced phase transition. This switching enables a widespread potential for applications in one-compound diodes and transistors, as well as solar cells or electrocatalysis, due to the generation of p-type and n-type regions in one single material. In this work, Ag18Cu3Te11Cl3, AgCuS and Cu1.5Se1-yTey are synthesized in standard solid-state approaches in a muffle furnace. The chemical composition is assessed by powder and single crystal X-ray diffraction and substantiated by energy-dispersive X-ray spectroscopy. To monitor the temperature dependent behavior, differential scanning calorimetry is used, and the thermoelectric properties are examined by Seebeck coefficient and electrical conductivity measurements as well as laser flash analysis. For the pnp-switching materials AgCuS and Ag18Cu3Te11Cl3, the change in conduction mechanism is reassessed during those measurements. They are therefore identified as possible candidates for the construction of one-compound diode devices, where a pn-junction is created by a simple temperature gradient within a uniform material without any additional doping. A new measurement setup is created, allowing conductivity measurements on single crystalline samples with specifically applied temperature gradients on them. The existence of the rectifying diode behavior is proven by UI- curve measurements, and Schottky-like behavior is ruled out by the assessment of switching times and the electronic response of the system under isothermal conditions. Both materials are successfully utilized as one-compound diode devices. Ag18Cu3Te11Cl3 shows a phase transition accompanied by a pnp-switch at around 295 K, allowing its use in one compound diodes at room temperature (RT) with switching times of ~9 s. AgCuS can be used at slightly elevated temperatures. A heat gradient of 333/368 K enables the creation of a one-compound diode with a faster switching time of 2.7 s due to the accelerated carrier mobility. The proof of the generated pn-junction in a single material enables a wide new field of electrical applications, from one-compound diodes and transistors to solar cells or electrocatalysis.

Open Access

Show Abstract ▼

Abstract: Poly(Ni-btt), an organometallic coordination polymer (OMCP) characterized by the coordination between benzene-1,2,4,5-tetrakis(thiolate) (btt) and Ni2+ ions, has been recognized as a promising p-type thermoelectric material. In this study, we employed a constitutional isomer based on benzene-1,2,3,4-tetrakis(thiolate) (ibtt) to generate the corresponding isomeric polymer, poly(Ni-ibtt). Comparative analysis of poly(Ni-ibtt) and poly(Ni-btt) reveals several common infrared (IR) and Raman features attributed to their similar square-planar nickel-sulfur (Ni-S) coordination. Nevertheless, these two polymer isomers exhibit substantially different backbone geometries. Poly(Ni-btt) possesses a linear backbone, whereas poly(Ni-ibtt) exhibits a more undulating, zigzag-like structure. Consequently, poly(Ni-ibtt) demonstrates slightly higher solubility and an increased bandgap in comparison to poly(Ni-btt). The most noteworthy dissimilarity, however, manifests in their thermoelectric properties. While poly(Ni-btt) exhibits p-type behaviour, poly(Ni-ibtt) demonstrates n-type carrier characteristics. This intriguing divergence prompted further investigation into the influence of OMCP backbone geometry on the electronic structure and, particularly, the thermoelectric properties of these materials.

Open Access

Show Abstract ▼

Abstract: Understanding the structure-property relationships of materials in order to supress thermal conductivity is crucial for developing efficient thermoelectric generators and thermal barrier coatings. We synthesise two mixed-anion materials, Bi8CsO8SeX7 (X = Cl and Br), with low thermal conductivities of 0.27(2) and 0.22(2) W m-1 K-1 respectively, associated with their c-axes at room temperature. These materials possess a combination of bond strength hierarchies and low frequency Cs+ rattling, which significantly inhibits phonon transport along different crystallographic directions. Due to sharp bond strength contrast between the van der Waals gaps and [Bi2O2]2+ layers, Bi8CsO8SeX7 materials exhibit thermal conductivities <50% of the theoretical minimum when measured along the stacking direction. Conversely, the thermal conductivity associated with the ab-plane is reduced by Cs+ rattling when compared to the structurally and compositionally related BiOCl. This highlights how combining different structural features into one material can aid in the design and identification of new materials with low thermal conductivities.