Enhancement of Energy Conversion Efficiency via Interfacial Carrier Diffusion in Microscale Segmented Thermoelectric Materials

Abstract

Explicit carrier diffusion may play a significant role in energy conversion of nano-and microscale segmented thermoelectric (TE) devices because of the combined effect of the diffusion induced and Seebeck effect induced currents near the segment interfaces. This work presents a computational thermoelectricity model with diffusion current to investigate carrier diffusion across the interface in a microscale segmented n-type TE material and its influence on the TE energy conversion efficiency. The carrier concentration distribution in the segmented TE leg is obtained and the energy efficiency equation is developed based on the theoretical model. The energy efficiency model shows that the efficiency is dependent on the carrier concentration at the interface between the two segments. The numerical examples for a segmented ntype Bi 2 Te 3 leg show that when the hot side segment has a higher donor impurity concentration than the cold side segment, the interfacial carrier diffusion leads to a significantly higher energy conversion efficiency than that based on the thermoelectricity model without explicitly considering carrier diffusion. The efficiency enhancement via the interfacial carrier diffusion may become less significant for longer TE legs subjected to larger temperature differentials between the hot and cold sides wherein the Seebeck effect induced current becomes more dominant.

A Linear Thermoelectricity Theory for Thermoelectric Materials

“…Carrier transport plays a significant role in determining TE properties and energy conversion efficiency. [23][24][25][26] Lu et al [24,25] developed a transport model for the Seebeck coefficient of organic TEs and reviewed the progress in carrier transport theory for organic semiconductors. Jin and Yang [26] showed that the explicit carrier diffusion across the segment interface in a segmented TE material may significantly increase the TE energy conversion efficiency.…”

“…[23][24][25][26] Lu et al [24,25] developed a transport model for the Seebeck coefficient of organic TEs and reviewed the progress in carrier transport theory for organic semiconductors. Jin and Yang [26] showed that the explicit carrier diffusion across the segment interface in a segmented TE material may significantly increase the TE energy conversion efficiency. A first-order linear theory will be developed in a future study to examine if a linear theory will also be accurate when explicit carrier diffusion is included.…”

“…Equation ( 24) to (26) indicate that the peak efficiency in the linear theory is also characterized by the figure of merit parameter Z.…”

“…Consider an exponentially graded material with the following properties [12] S ¼ S 0 expðβx=LÞ k ¼ k 0 expðβx=LÞ ρ ¼ ρ 0 expðγx=LÞ (33) where S 0 , k 0 , and ρ 0 are the properties at the cold side x ¼ 0, and β and γ are the gradation parameters. Note that γ in Equation ( 33) is different from that in Equations ( 24) to (26) for the homogeneous TE material. The energy efficiency is now given by…”