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Advanced Simulation Design of Nanostructured Thermoelectric Materials with Enhanced Power Factors

Periodic Reporting for period 4 - NANOthermMA (Advanced Simulation Design of Nanostructured Thermoelectric Materials with Enhanced Power Factors)

Reporting period: 2021-01-01 to 2022-12-31

Roughly one-third of all energy consumption ends up as low-grade heat. Thermoelectric (TE) materials could potentially convert vast amounts of this waste heat into electricity and reduce the dependence on fossil fuels. State-of-the-art nanostructured materials with record-low thermal conductivities (κ~1-2W/mK) have recently demonstrated large improvements in conversion efficiencies, but not high enough to enable large scale implementation. Central to this low efficiency problem lies the fact that the Seebeck coefficient (S) and the electrical conductivity (σ), the parameters that determine the TE power factor (σS2), are inversely related. Relaxing this inverse interdependence has never been achieved, and TE efficiency remains low. The project is an integrated project, consisting of large scale simulations and experiments, and utilizes nanostructured designs to relax the adverse interdependence of the σ and S, and achieve unprecedentedly large power factors, thus, enabling the thermoelectric technology to achieve high conversion efficiencies. High thermoelectric efficiencies will allow waste heat energy harvesting, and contribute to the reduction of the use of fossil fuels and promote energy sustainability.

The project focuses around four ambitious objectives: i) Theoretically establishes and generalizes the strategies that relax the adverse interdependence of σ and S in nanostructures and achieve power factors (PF) >5× compared to the state-of-the-art; ii) Experimentally validate the theoretical propositions through well-controlled material design examples; iii) Develop a state-of-the-art, high-performance, electro-thermal simulator to generalize the concept and guide the design of the entirely new nanostructured TE materials proposed; iv) Optimize the design, targeting maximum performance.

Conclusions of the action:
The project has reached a theoretical design consisting of several ‘ingredients’, which when all put together, a power factor larger than 10x can be achieved. The advanced software that was developed was crucial is providing confidence in the device operation and optimization. An experimental fabrication setup is realized, which indicates that the developed concept has merit and can indeed allow for very high power factors.
The work performed and the main results achieved revolve around three main categories: a) high power factor (PF) and high thermoelectric (TE) efficiency strategies, b) simulation tools development, c) experimental validation of high PF concepts.

High power factor (PF) and high thermoelectric efficiency strategies
1. The project has theoretically demonstrated the full design guidelines on how to nanostructure a material to provide very high power factors (PF). The design involves guidelines for the geometrical features of the nanostructure, the impurity distribution in the material, and potential energy barriers that are introduced in the presence of nanocrystallinity. We have laid out how to optimize and generalize the design for different type of materials. The PI has given 11 invited talks in top conferences and workshops on this topic. The project showed that it is possible to improve the TE PF of a material by at least 10x using this particular design.
2. With regards to understanding thermal conductivity in highly disordered nanostructured materials, we developed novel compact models which correct existing models for the effect of disorder. We further demonstrated through large-scale atomistic simulations novel effects that the heat flow experiences, which allow for reduction in thermal conductivity with a minimum degree of nanostructuring, something beneficial for the overall material performance.
3. With regards to understanding thermoelectricity in materials with arbitrary crystal and electronic structure complexity, we have also developed advanced methods that would allow accurate and robust examination of their transport properties, which relax many approximations currently performed, and offer a trusted degree of accuracy. These studies enabled us to form appropriate descriptor parameters that indicate the performance of a material without the need of large-scale simulations, which can then be used in machine learning studies.
4. With regards to exploring thermoelectricity in novel nanomaterials, we have developed a formalism to evaluate the thermoelectric properties of 2D materials, and indicated that very large responses are possible in 2D stacks of monolayer materials.

Major simulation tools development. A series of simulator packages have been developed, with the most important as follows:
1. A fully quantum mechanical electronic transport simulator has been developed specifically for nanostructured thermoelectric materials. Parts of the complete strategy for the factors that affect the PF in nanostructured materials have been identified and tested with this code. Importantly, the code considers the full quantum nature of electrons, electron-phonon interactions, and large-scale disordered domains, which makes it unique to tackle nanostructured TE simulations.
2. A fully functional Monte Carlo simulator for phonon transport has been developed. It was used to investigate phonon transport in hierarchically nanostructured Si-based materials, with results receiving the best poster award in two conference occasions.
3. A fully functional Boltzmann Transport simulator is developed, which can extract the TE properties of complex electronic structure materials. The simulator was used to provide design and optimization strategies for many materials, and examine the role that different parameters play.
4. A Monte Carlo simulator for electron transport is developed, which was used to examine the effect of doping variability in the channel, an important component of the strategy for large PFs.

Experimental validation:
In order to test the basic high PF design ‘ingredients’ identified in the nanostructured material, we developed an experimental process, fabricated a device, and carried out a first batch of measurements to test those assumptions. At the end of the project we have preliminary evidence of devices with very high PFs compared to the maximum PF that a non-nanostructured material can provide.
Progress beyond the state-of-the-art
The most significant scientific achievement of the project, with unexpected results far beyond the state of the art, is the theoretical realization of a novel nanostructured material design which allows for more than 10-fold increase in the TE PF of a material. The partial experimental demonstration of this prediction is also an achievement which goes beyond the state of the art, since nanostructured materials have not achieved high PFs to date. The improvement was speculated in the beginning of the project, but the magnitude of this improvements was unexpected.
The software developed in the project, and especially the quantum simulator, provides a significant advancement in the field. The project provided novel theoretical formalisms to evaluate transport in nanostructures and accurate and efficient parameter extraction, which are used within these codes. The later was unplanned, but it provides the possibility for accurate and efficient computational studies for TE materials, something which is not possible at the moment.
Illustration of a hierarchically nanostructured thermoelectric material