Seeking new, highly effective thermoelectrics

Thermoelectric technology can directly and reversibly convert heat to electrical energy. Although thermoelectric energy conversion will never be as efficient as a steam engine (), improving thermoelectric performance can potentially make a technology commercially competitive. Thermoelectric conversion efficiency is estimated by the so-called dimensionless figure of merit, ZT = S²σ T/κ, where S, σ, T, and κ denote the Seebeck coefficient, electrical conductivity, working temperature, and thermal conductivity, respectfully . These parameters are strongly coupled, and improving the final is challenging as a result. Strategies for boosting thermoelectric performance include nanostructuring, band engineering, nanomagnetic compositing, high-throughput screening, and others (). Many of these strategies create a high ZT in a narrow range of temperatures, limiting the overall energy conversion. Finding materials with wider operating temperature ranges may require rethinking development strategies.

A thermoelectric device is assembled with many cascading n-type and p-type couples. The device efficiency is closely related to the performance of thermoelectric materials. Thermoelectric materials are categorized into three temperature ranges depending on their working temperatures. Bismuth tellurides are typical thermoelectric materials that operate under 400 K. Lead chalcogenides are typical for the 600 to 900 K range. Silicon-germanium and Zintl phases exhibit the best performance above 1000 K. The best thermoelectric performance at the optimal working temperature is restricted by the bandgap ( E_g) owing to intrinsic excitation. This ba ⁿdg ^ap is given by a rule E_g = 2 eS_maxT, where e is unit charge, S_max is the maximum Seebeck coefficient, and T is the temperature that corresponds to S_max. The Seebeck coefficient is also called the thermopower and is a measurement of the voltage produced within a temperature gradient ( S = Δ V/Δ T, where V is voltage). The bandgap rule means that the most well-known thermoelectric materials are narrow-bandgap semiconductors, such as (Bi,Sb) ₂Te ₃ ( E_g ∼ 0.13 eV) (), PbTe ( E_g ∼ 0.28 eV) (), and GeTe-AgSbTe ₂ ( E_g ∼ 0.39 eV) (). The maximum ZT ( ZT_max) values for these thermoelectric materials shift to higher temperatures as the bandgap increases (see the figure). To fully realize their potential, thermoelectric materials must work over the entire, several-hundred-kelvin operating range. One method for doing this is with a segmented leg (see the figure), but interfacial resistance and mismatched compatibility factors deteriorate the long-term performance under high temperature ().

Wide-bandgap semiconductors could solve this temperature range issue but often have poor electrical properties, but the wide-bandgap SnSe (∼0.86 eV) has proved to be an excellent thermoelectric material. Its ZT curve for SnSe covers several narrow-bandgap thermoelectrics (-). SnSe possesses attractive ZT values at low temperatures, which continuously increase without saturation up to 800 K. Several special features of SnSe provide some general selection rules for new thermoelectric materials that may work over a wide temperature range. First, the wide bandgap avoids the intrinsic excitation and the ZT values are not saturated at high temperatures. Second, layered structures can have high in-plane transport properties that circumvent the normally low carrier density that plagues wide-bandgap semiconductors. Wide-bandgap semiconductors were neglected as promising thermoelectrics because of their intrinsically low carrier density. This deviates from the optimal carrier density ( n) owing to the ZT parameter interrelations. To achieve high electrical transport properties in materials with low carrier density, high carrier mobility (µ) can be found along an in-plane direction in layered structures, and thus layered materials can reach high electrical conductivity σ = neµ. Furthermore, the low carrier density allows for a high Seebeck coefficient, and, consequently, an ultrahigh power factor ( PF = S²σ) (, ). Third, a low-symmetry structure is connected to low lattice thermal conductivity (κ _lat), which is a lower electronic themal conductivity (κ _ele) owing to low carrier density, and contributes to the total thermal conductivity (κ = κ _lat + κ _ele). Asymmetric crystal structures have strong anharmonic lattice vibrations useful for lowering thermal conductivity, and their more complex electronic band structure is also attractive for thermoelectric materials. The selection rules will not work for all materials because of the complex interplay between the ZT parameters but should provide at least a rough guide for candidate materials. The selection rules for identifying potentially highly effective thermoelectrics are appropriate for both n-type and p-type materials because of their similar transport principles.

Within these selection rules, some promising thermoelectrics can be identified, such as BiCuSeO (), BiSbSe ₃ (), K ₂Bi ₈Se ₁₃ (), and Sb ₂Si ₂Te ₆ (). The anisotropic transport properties should lead to improved performance in crystalline forms of these materials where we expect, as for SnSe and SnS crystals, higher carrier mobility (, ). Moreover, much more highly effective thermoelectric performance from these anisotropic thermoelectric materials could be expected through integrating present selection rules with the approach to reveal the intrinsically low thermal conductivity (). Finally, it must also be mentioned that not every material with high-range ZT values, ZT_ave, is going to immediately make for a device-ready material. High ZT_ave thermoelectric materials may be challenging to ultimately turn into commercial devices, especially at higher temperatures. Interfacial resistivity and diffusion between the high-performance thermoelectrics and contact electrode that can degrade thermoelectric performance over time are exacerbated at high temperatures. Some of these issues may be solved by device engineering, but the present selection rules also provide a rough guide for finding different types of thermoelectric materials.

Acknowledgments: We acknowledge support from the National Key Research and Development Program of China (2018YFA0702100; 2018YFB0703600), National Natural Science Foundation of China (51772012; 51671015), Beijing Natural Science Foundation (JQ18004), Shenzhen Peacock Plan team (KQTD2016022619565991), National Postdoctoral Program for Innovative Talents (BX20190028), 111 Project (B17002), and Postdoctoral Science Foundation of China (2019M660399). L.-D. Z. has support from the National Science Foundation for Distinguished Young Scholars (51925101).