7.4 Advanced semiconductor materials for thermoelectrics

Advanced semiconductor materials are pushing the boundaries of thermoelectric performance. Nanostructured materials like quantum dots and superlattices exploit quantum confinement effects to enhance electrical properties while reducing thermal conductivity. Two-dimensional materials offer unique opportunities for tuning thermoelectric properties in planar structures.

Novel electronic structures are revolutionizing thermoelectric design. Topological insulators and phonon glass electron crystals combine low thermal conductivity with high electrical conductivity. Strategies like band convergence and energy filtering optimize carrier transport, while defect engineering fine-tunes both electronic and thermal properties.

Nanostructured Materials

Quantum Confinement Effects in Nanostructures

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• Nanostructured materials reduce thermal conductivity while maintaining electrical conductivity
• Quantum dots consist of semiconductor nanocrystals with size-dependent electronic properties
  • Exhibit discrete energy levels due to quantum confinement
  • Tunable bandgap enables optimization for specific thermoelectric applications
• Superlattices comprise alternating layers of different materials with nanoscale thickness
  • Create periodic potential wells for charge carriers
  • Enhance electron mobility and reduce phonon transport
• Nanowires offer one-dimensional confinement of charge carriers and phonons
  • Increased surface scattering of phonons reduces thermal conductivity
  • Quantum confinement effects can enhance the Seebeck coefficient

Two-Dimensional Materials for Thermoelectrics

• 2D materials possess unique electronic and thermal properties due to their planar structure
• Graphene serves as a prototype 2D material with high electrical conductivity
  • Limited thermoelectric performance due to high thermal conductivity
• Transition metal dichalcogenides (TMDs) offer tunable electronic properties
  • MoS2 and WSe2 show promising thermoelectric performance
• Phosphorene exhibits anisotropic thermal and electrical transport
  • Potential for direction-dependent thermoelectric optimization

Advanced Thermoelectric Concepts

Novel Electronic Structures for Enhanced Performance

• Topological insulators feature insulating bulk with conductive surface states
  • Bi2Te3 and Sb2Te3 demonstrate improved thermoelectric properties
  • Surface states contribute to enhanced electrical conductivity
• Phonon glass electron crystal (PGEC) materials combine low thermal conductivity with high electrical conductivity
  • Skutterudites and clathrates exemplify PGEC behavior
  • Complex crystal structures scatter phonons while preserving electron transport
• Band convergence involves aligning multiple electronic bands near the Fermi level
  • Increases the density of states and enhances the Seebeck coefficient
  • PbTe1-xSex alloys demonstrate successful band convergence

Carrier and Phonon Engineering Strategies

• Energy filtering selectively blocks low-energy carriers to increase the Seebeck coefficient
  • Nanocomposites and heterostructures create energy barriers for carrier filtering
  • Superlattices with carefully designed band offsets enable effective energy filtering
• Defect engineering introduces controlled imperfections to optimize thermoelectric properties
  • Point defects (vacancies, interstitials) scatter phonons and reduce thermal conductivity
  • Extended defects (dislocations, grain boundaries) can enhance electrical transport
  • Modulation doping creates spatially separated dopants and charge carriers
    • Reduces ionized impurity scattering and improves carrier mobility