Innovative Thermoelectric Materials

The following sections are included:

• Contents
• List of Contributors
• Preface

The following sections are included:

• Thermoelectric Effect Mechanisms
• Thermoelectric Materials
• Summary
• References

The following sections are included:

• Introduction
• Electronic Structure and Morphology of Organic Semiconductors
• Transport Models for Organic Semiconductors
• Thermoelectric Performance of Organic Semiconductors
• Thermal Conductivity of Organic Semiconductors
• Conclusion
• References

The introduction of nanostructures in thermoelectric materials has led to significant improvements of the thermoelectric figure of merit, ZT, over the past two decades. Initial efforts have focused on using low-dimensional materials to engineer electron band structures and reduce phonon mean free paths. Subsequent efforts have been made to develop bulk thermoelectric materials maintaining the features of the low-dimensional materials, and the effort has led to the high ZT of bulk nanostructured materials recently reported. In this chapter, we summarize the theoretical background and the experimental demonstrations that resulted in these advancements. We also address several unanswered issues regarding electron/phonon transport in thermoelectric materials, which may hold the key to further advances in ZT.

The following sections are included:

• Introduction
• Transport Background for Thermoelectrics
• Carrier Filtering and Energy Level Alignment: Engineering μ(E), and τ(E)
• Engineering Power Factor Enhancement through “Trap” States: D(E), n(E)
• Heterojunction Charge Transfer: n, μ, and
• Reducing Kappa: κ = κ_e + κ_ph
• Structural Changes in the Organic Material at the Organic–Inorganic Interface: S(S₁, S₂, S_interface), σ(σ₁, σ₂, σ_interface)
• Summary, Outlook, and Conclusion
• References

Dopants affect fundamental thermoelectric transport quantities including carrier concentration (n), carrier mobility (μ), and thermal conductivity (κ). While optimizing n by chemical doping is a well-known method to maximize the thermoelectric power factor (S²σ, where S and σ are the Seebeck coefficient and electrical conductivity, respectively), the effect of finite dopant volume can greatly impact μ and hence S²σ in organic semiconductors (OSCs), since the hopping probability exponentially decreases with hopping distance. In this chapter, the role of dopants in tuning OSC thermoelectric transport parameters is discussed.

Polymer–inorganic composites have become widely anticipated and explored for thermoelectric (TE) applications, particularly with the parallel development of conducting organic polymers and compound semiconductor nanostructures. Physical mixing, solution processing, and in situ synthesis of polymers and/or inorganic crystals have been used to prepare polymer–inorganic composites which show TE improvements over the individual constituents, leading to useful power output and high practical efficiencies. The incorporation of conducting inorganic compounds in conjugated polymers is expected to increase composite electrical conductivity and can also enhance the Seebeck coefficient, while the polymer matrix retains a very low thermal conductivity. Composites are expected to have improved TE performance due to enhanced current pathways, donation or extraction of carriers, morphological confinement, passivation or functionalization, energy alignment and filtering, and interface scattering effects.

The following sections are included:

• Introduction
• Continuum Considerations
• Noncontinuum Methods
• Wave versus Particle
• First-Principles Calculations
• Electronic Structure Methods
• References

The quality of a thermoelectric material is judged by the size of its temperature-dependent thermoelectric-figure of merit (zT). The ordered phase of Cu₂Se shows a zT peak in its ordered phase, doubling to 0.7 at 406K over 30K. The enhancements are due to anomalous increase in their Seebeck coefficients, beyond that predicted by carrier concentration measurements, and band structure modeling. As the Seebeck coefficient is the entropy transported per carrier, this suggests that there is an additional quantity of entropy co-transported with charge carriers. Such co-transport has been previously observed via co-transport of vibrational entropy in bipolaron conductors and spin-state entropy in Na_xCo₂O₄. The correlation of the temperature profile of the increases in each material with the nature of their phase transitions indicates that the entropy is associated with the thermodynamcis of ion ordering. This suggests a new mechanism by which high thermoelectric performance may be understood and engineered, and the phenomenological basis of that mechanism is explored.

The following sections is included:

• Index