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.
