Low Effective Mass Leading to High Thermoelectric Performance

Abstract: High Seebeck coefficient by creating large density of state (DOS) around the Fermi level through either electronic structure modification or manipulating nanostructures, is commonly considered as a route to advanced thermoelectrics. However, large density of state due to flat bands leads to large effective mass, which results in a simultaneous decrease of mobility. In fact, the net effect of high effective mass is a lower thermoelectric figure of merit when the carriers are predominantly scattered by acoustic phonons according to the deformation potential theory of Bardeen-Shockley. We demonstrate the beneficial effect of light effective mass leading to high power factor in n-type thermoelectric PbTe, where doping and temperature can be used to tune the effective mass. This clear demonstration of the deformation potential theory to thermoelectrics shows that the guiding principle for band structure engineering should be low effective mass along the transport direction.

• The paper presents a novel finding that reducing effective mass via iodine doping significantly improves the thermoelectric power factor in n-type PbTe.
• It employs rigorous numerical models and experimental analyses, including a single Kane band approach and acoustic phonon scattering evaluation.
• The study suggests that targeting low effective mass in thermoelectric materials can enhance energy conversion efficiency across a broad temperature range.

Analysis of Effect of Effective Mass on Thermoelectric Performance in PbTe

This paper explores the intricate relationship between effective mass and thermoelectric performance, challenging established notions by investigating n-type PbTe (lead telluride). The authors present robust numerical studies and experimental analyses to elucidate how light effective mass can elevate the thermoelectric power factor, a critical parameter in optimizing thermoelectric materials.

In the field of thermoelectrics, the Seebeck coefficient, electrical resistivity, and thermal conductivity intricately influence the thermoelectric figure of merit (zT). Enhancement of zT is imperative for efficient thermoelectric energy conversion. Traditionally, strategies have involved augmenting the density of states (DOS) near the Fermi level to increase the Seebeck coefficient, often leading to high effective mass and flat band structures. However, these conditions tend to reduce carrier mobility, adversely affecting the power factor. The work draws upon the Bardeen-Shockley deformation potential theory to explore scattering mechanisms predominated by acoustic phonons, revealing that a high effective mass generally diminishes thermoelectric performance.

The authors systematically demonstrate the beneficial impact of a reduced effective mass through doping and thermal treatment in n-type PbTe. Specifically, iodine (I) doping, compared to lanthanum (La) doping, resulted in a ~20% reduction in effective mass and consequently, a ~20% enhanced power factor. This enhancement is attributed to the higher carrier mobility maintained within PbTe due to the lower effective mass along the conduction direction.

The results are substantiated by a single Kane band (SKB) model, which accurately describes the electronic transport properties using carrier concentration-dependent Seebeck coefficients and mobilities. The model integrates plausible assumptions about band non-parabolicity, acoustic phonon scattering prevalence, and temperature-dependent variations in effective mass to match empirical measurements across different doping levels and temperatures.

From an experimental perspective, varying Hall carrier concentrations were measured, confirming consistent n-type conduction across samples. Theoretical calculations coupled with detailed experimental data showcase that I-doped PbTe retains a superior power factor across a broad temperature range when compared to La-doped variants. This directly leads to a higher zT over the temperature spectrum, despite La-doped samples exhibiting larger Seebeck coefficients due to heavier effective mass.

Theoretical implications of these findings suggest that thermoelectric material engineering should target low effective mass configurations to enhance power factors, a strategy that contrasts with conventional methods focusing on high DOS-induced high effective mass. Practically, this insight could pivot thermoelectric material synthesis towards doping strategies and structural optimizations that favor light effective mass without compromising the Seebeck coefficient.

Looking forward, this work lays the groundwork for future exploration into materials with similar band characteristics as PbTe, potentially guiding the engineering of new thermoelectric systems with enhanced energy conversion efficiencies. It also poses intriguing questions about how other scattering mechanisms might be leveraged to further optimize thermoelectric performance across various material systems.