Interfacial Patterning to Create High ZT Thermoelectric Materials

With ~60% of all generated energy wasted as heat[1], energy harvesting is an important avenue of study to help provide for our worlds increased power demand in a sustainable way. Thermoelectrics (TEs) offer the unique opportunity to recover this previously wasted energy, and improve performance, via a direct conversion of heat to electricity in a solid state device. The key limitation of TEs is their poor power conversion efficiency, characterised by the dimensionless figure of merit, ZT. This value depends on both the electron and phonon transport characteristics, which themselves are heavily interdependent. For optimal TE behavior the electronic conductivity should be high whilst the thermal conductivity is kept to a minimum. However, in the majority of cases an increase in electrical conductivity is accompanied by an increase in thermal conductivity due to the Wiedemann-Franz Law[2], which links the thermal conductivity of electrons to the charge carrier density. It is this interdependence which has effectively limited the maximum ZT achievable to its current value of ~3[3]. Therefore, the ideal technique for raising ZT further would break this interdependence and optimise these characteristics independently. With this in mind, we investigate interfacial patterning as a method for controlling the transport properties of phonons. This method utilises the fundamental difference in electron and phonon wavelengths to selectively scatter phonons and hence reduce thermal conductivity, whilst having a minimal effect on electronic transport. Using density functional theory, we demonstrate the effectiveness of interfacial patterning on Si/Ge structures. We investigate three differently patterned interfaces, noting how subtle changes in the patterning can have dramatic effects on properties such as the effective mass and the lattice thermal conductivity. This patterning provides a technique to enhance ZT and a framework for producing more viable devices from any heterostructure-based TE materials, including materials not traditionally considered for the application.<br/>1. Perspectives on thermoelectrics: from fundamentals to device applications, Energy & Environmental Science, 2012, 5, 5147<br/>2. Ueber die Wärme-Leitungsfähigkeit der Metalle, Annalen der Physik, 1853, 165, 8, 497<br/>3. Advances in thermoelectric materials research: Looking back and moving forward, Science, 2017, 357, 6358, 9997