Periodic Reporting for period 2 - NANO-DECTET (Nanoscale characterisation and decoupling of charge and heat transport across interfaces in novel, molecular thermoelectric materials)
Reporting period: 2023-08-01 to 2025-01-31
Across the global energy economy only about 1/3 of primary energy is converted into useful energy services, the other 2/3 are wasted as heat in the various industrial, transportation, residential energy conversion and electricity generation processes. Given the urgent need for transitioning to a zero-carbon energy supply in order to mitigate dangerous consequences of climate change a waste of energy on this scale needs to be addressed. Thermoelectric waste-heat-to-electricity conversion could offer a potential solution but the performance of thermoelectric materials is currently insufficient, particularly for the majority of heat that is generated at low heat source temperatures below 300-400°C. Based on recent scientific breakthroughs the NANO-DECTET project explores the potential for organic semiconductors (OSCs) with their unique molecular characteristics to be used as low-temperature thermoelectric materials.
The thermoelectric physics of this class of soft materials with complex microstructures and strong electron-phonon coupling remains as yet largely unexplored and poorly understood. The aim of NANODECTET is (a) to develop novel experimental methods for the nanoscale characterisation of the thermoelectric properties of a very promising new generation of molecular thermoelectric materials, (b) to achieve a clear fundamental understanding of their thermoelectric physics and how thermoelectric transport coefficients can be decoupled and enhanced independently in these materials and (c) to translate this into the design of high performance molecular thermoelectric materials with unprecedented performance as measured by the figure of merit ZT for operation at temperatures < 300-400°C. This is a fundamental and interdisciplinary, high-risk research programme stretching across theory, condensed matter physics, chemistry and materials science as well device engineering. If successful the project could make a significant contribution to improved energy efficiency and a successful transition to a zero-carbon energy economy.
The thermoelectric physics of this class of soft materials with complex microstructures and strong electron-phonon coupling remains as yet largely unexplored and poorly understood. The aim of NANODECTET is (a) to develop novel experimental methods for the nanoscale characterisation of the thermoelectric properties of a very promising new generation of molecular thermoelectric materials, (b) to achieve a clear fundamental understanding of their thermoelectric physics and how thermoelectric transport coefficients can be decoupled and enhanced independently in these materials and (c) to translate this into the design of high performance molecular thermoelectric materials with unprecedented performance as measured by the figure of merit ZT for operation at temperatures < 300-400°C. This is a fundamental and interdisciplinary, high-risk research programme stretching across theory, condensed matter physics, chemistry and materials science as well device engineering. If successful the project could make a significant contribution to improved energy efficiency and a successful transition to a zero-carbon energy economy.
In the first part of the project we have focussed on studying the thermoelectric physics of model systems with the aim of identifying fundamental mechanisms that can be exploited generally to improve the thermoelectric properties of OSCs.
As a first step early on in the project we developed an ion exchange doping method for conjugated polymers that allows incorporating charge compensating counterions of different shape and size (Jacobs, et al., Advanced Materials 34, 2102988 (2022)). This then allowed a systematic study of how the electrical conductivity of conducting polymers depends on the size of the counterions. We had hypothesized that the conductivity would increase with the size of the counterions because a larger counterion remains further away from the conducting polymer backbone and therefore attracts the electronic charge carriers on the backbone less strongly. Surprisingly, we found the conductivity to be independent of the size of the counterion, which showed that the electronic charge carriers remain sufficiently delocalized, so that they can average effectively over the Coulombic energy landscape created by the counterions. The ion exchange method has also allowed us to minimize dopant-induced disorder in conjugated polymers (Wang et al., Advanced Materials 2314062 (2024)).
One of the polymer model systems to which we applied this doping method are so-called ribbon phases. These ribbon phases comprise a regular array of polymer crystallites. In each crystallite the polymer chains are tightly packed and aligned parallel to each other from the beginning of the chain to its end. In between two such crystallites is a domain boundary, in which the chain ends are located. Such domain boundaries constitute bottlenecks for charge transport. We were able to demonstrate a method for incorporating so-called tie chains into these polymer crystallites, which are longer than the average polymer length and can form conducting bridges between adjacent crystalline domains. The incorporation of such tie chains enhances the electrical conductivity and thermoelectric performance of these ribbon-phase polymers significantly (Zhu et al., Advanced Materials, 2310480 (2024)) and provides an example how the study of such controlled model systems allows investigating how targeted modifications to the polymer microstructure and electronic structure affect the thermoelectric properties.
As a first step early on in the project we developed an ion exchange doping method for conjugated polymers that allows incorporating charge compensating counterions of different shape and size (Jacobs, et al., Advanced Materials 34, 2102988 (2022)). This then allowed a systematic study of how the electrical conductivity of conducting polymers depends on the size of the counterions. We had hypothesized that the conductivity would increase with the size of the counterions because a larger counterion remains further away from the conducting polymer backbone and therefore attracts the electronic charge carriers on the backbone less strongly. Surprisingly, we found the conductivity to be independent of the size of the counterion, which showed that the electronic charge carriers remain sufficiently delocalized, so that they can average effectively over the Coulombic energy landscape created by the counterions. The ion exchange method has also allowed us to minimize dopant-induced disorder in conjugated polymers (Wang et al., Advanced Materials 2314062 (2024)).
One of the polymer model systems to which we applied this doping method are so-called ribbon phases. These ribbon phases comprise a regular array of polymer crystallites. In each crystallite the polymer chains are tightly packed and aligned parallel to each other from the beginning of the chain to its end. In between two such crystallites is a domain boundary, in which the chain ends are located. Such domain boundaries constitute bottlenecks for charge transport. We were able to demonstrate a method for incorporating so-called tie chains into these polymer crystallites, which are longer than the average polymer length and can form conducting bridges between adjacent crystalline domains. The incorporation of such tie chains enhances the electrical conductivity and thermoelectric performance of these ribbon-phase polymers significantly (Zhu et al., Advanced Materials, 2310480 (2024)) and provides an example how the study of such controlled model systems allows investigating how targeted modifications to the polymer microstructure and electronic structure affect the thermoelectric properties.
In the first phase of the project we have deepened our understanding of the thermoelectric physics of conjugated polymers as well as other soft functional materials and identified several new fundamental mechanism that can be used to enhance their thermoelectric performance. We have also developed novel experimental methods that allow nanoscale characterisation of the relevant charge and heat transport processes and developed theoretical models for the Seebeck coefficient in organic semiconductors and the relevant charge transport processes in a complicated transport regime, in which both electron-electron interactions and electron-ion interactions play an important role.
To study the dependence of the thermoelectric properties on the doping concentration we developed a device architecture that allows measurements of the thermoelectric Seebeck coefficient and the electrical conductivity of polymer films within an organic electrochemical transistor (OECT). This conveniently allows electrical tuning of the doping level, while providing accurate measurements of the Seebeck coefficient and conductivity as a function of temperature at each doping level. We used this architecture to demonstrate that in some conducting polymers extremely high doping levels can be achieved. In these systems all electrons can be removed from the valence band and even conduction in deeper lying valence band states can be accessed. Such high doping levels could not be achieved in covalently bonded inorganic semiconductors without inducing structural collapse. In a second experiment using the OECT architecture we added a second field-effect gate to the device and demonstrated that, when a few extra charges are induced in the doped polymer film by applying a voltage to the field-effect gate electrode under conditions where the dopant counterions are immobile, these few extra charges can be highly delocalised and mobile and give rise to surprisingly large increases in conductivity (Tjhe, et al., Nature Materials (2024), https://doi.org/10.1038/s41563-024-01953-6)(opens in new window)). This was a very unexpected, serendipitous discovery, which provided fundamental insights into the charge transport physics of these systems which is now allowing us to identify new pathways for increasing the thermoelectric properties of conjugated polymers.
In the remaining time of the project we hope to be translating these advances in fundamental understanding into novel materials with higher thermoelectric performance that can be used in practical applications for efficient waste-heat-to-electricity conversion.
To study the dependence of the thermoelectric properties on the doping concentration we developed a device architecture that allows measurements of the thermoelectric Seebeck coefficient and the electrical conductivity of polymer films within an organic electrochemical transistor (OECT). This conveniently allows electrical tuning of the doping level, while providing accurate measurements of the Seebeck coefficient and conductivity as a function of temperature at each doping level. We used this architecture to demonstrate that in some conducting polymers extremely high doping levels can be achieved. In these systems all electrons can be removed from the valence band and even conduction in deeper lying valence band states can be accessed. Such high doping levels could not be achieved in covalently bonded inorganic semiconductors without inducing structural collapse. In a second experiment using the OECT architecture we added a second field-effect gate to the device and demonstrated that, when a few extra charges are induced in the doped polymer film by applying a voltage to the field-effect gate electrode under conditions where the dopant counterions are immobile, these few extra charges can be highly delocalised and mobile and give rise to surprisingly large increases in conductivity (Tjhe, et al., Nature Materials (2024), https://doi.org/10.1038/s41563-024-01953-6)(opens in new window)). This was a very unexpected, serendipitous discovery, which provided fundamental insights into the charge transport physics of these systems which is now allowing us to identify new pathways for increasing the thermoelectric properties of conjugated polymers.
In the remaining time of the project we hope to be translating these advances in fundamental understanding into novel materials with higher thermoelectric performance that can be used in practical applications for efficient waste-heat-to-electricity conversion.