Supported by the National Natural Science Foundation of China (Grant Nos. 51925101 and 52250090) and other fundings, Professor Li-dong Zhao and his team from Beihang University have made significant progress in the field of thermoelectric energy materials. The relevant research results entitled (Lattice plainification advances highly effective SnSe crystalline thermoelectrics) were published in the Science journal on May 26, 2023 (link: https://www.science.org/doi/10.1126/science.adg7196).

Thermoelectric cooling is a green cooling technology that directly converts electrical energy into thermal energy using the Peltier effect. It can achieve continuous high-precision control of refrigeration capacity and temperature simply by adjusting the working voltage and current. Due to its precise temperature control, flexible size, diverse structure, and local cooling advantages, thermoelectric cooling technology has stronger competitive advantages than traditional mechanical compression refrigeration technology in key areas such as precision guidance, sensors, and 5G optical modules. Therefore, researching and developing high-performance cooling materials and enhancing the cooling device efficiency is of great significance for precise temperature control in various scientific and technological fields that require self-reliance.

The cooling efficiency of the device is mainly determined by the dimensionless figure of merit (ZT value) of the material. The ZT value, as defined by ZT=(S²σ/κ) T, indicates that high-performance materials should exhibit high thermoelectromotive force S (generating high voltage), high electrical conductivity σ (decreasing joule heat losses), and low thermal conductivity κ (producing a high temperature difference) at a specific temperature T. However, the intricate interconnections among multiple physical parameters give rise to a tight phonon-electron coupling relationship, rendering it extremely challenging to optimize the thermoelectric properties of materials. Consequently, regulating these strongly coupled complex thermoelectric parameters is the essential approach to improving the ZT value of materials and enhancing the cooling efficiency.

At present, Bismuth telluride (Bi₂Te₃) based materials are still the only applicable thermoelectric cooling materials. However, the earth-abundant level of Te is on par with platinum (and 50% of the photovoltaic material CdTe holds market share), and Bi₂Te₃ materials and Bi₂Te₃-based thermoelectric cooling devices suffer from inadequate processability, insufficient cooling performance, and high operating power consumption. Therefore, it is imperative to explore and develop novel thermoelectric cooling materials and devices. After long-term screening research, SnSe crystal was found to have excellent application potential and could become a new generation of green cooling materials. In 2021, the multi-bands coordination effect in the momentum space and energy space (named Synglisis effect) was discovered and utilized in P-type SnSe crystals, achieving a significant improvement in room temperature thermoelectric performance. The thermoelectric device based on P-type SnSe crystals can achieve a maximum cooling temperature difference of ~ 45.7K, which can reach 70% of the commercial Bi₂Te₃-based cooling device. In 2022, a "grid-design" strategy based on component and process control was proposed to optimize thermoelectric cooling materials and devices. By controlling the intrinsic defects of the materials, higher mobility and thermoelectric cooling performance at near-room temperature could be achieved.

Inspired by the “compositional plainification” strategy, this study proposes the “lattice plainification” strategy to boost carrier mobility by repairing lattice defects. By introducing trace amounts of Cu into SnSe, Cu atoms can fill Sn vacancies, weakening the scattering of Sn vacancies on carriers (Figure). The additional Cu can substitute Sn sites in the lattice and act as hole dopant, and effectively enhances the Synglisis effect of multi-valent bands, thereby significantly increasing the carrier mobility and effective mass. At room temperature, ultra-high electrical transport performance of ~ 100μW cm^-1 K^-2 and room temperature ZT values of ~ 1.5 and average ZT values of ~ 2.2 (300-773K) were achieved, indicating that SnSe crystals have great potential in both "power generation" and "cooling" fields.

The thermoelectric device prepared using the obtained high-performance SnSe crystal exhibits excellent performance in both power generation and cooling. The thermoelectric device can achieve ~ 12.2% power generation efficiency under a 300K temperature difference, and the thermoelectric cooling device can achieve a cooling temperature difference of ~ 61.2K at a room temperature hot-end temperature (T_h). When T_h > 323K, the SnSe-based cooling device shows superior cooling advantages to Bi₂Te₃. At T_h ~ 343 K, a cooling temperature difference of up to ~ 90.6K can be obtained. Moreover, SnSe crystals possess numerous advantages over commercial Bi₂Te₃, including low cost, abundant availability, environmental friendliness, excellent processability, and low operating power consumption.

This work first proposes the "lattice plainification" strategy, which significantly improves carrier mobility by reducing the defect concentration in the crystal, thereby achieving excellent electrical transport performance and room temperature ZT values. This strategy provides a paradigm shift for the development of new thermoelectric cooling materials and the optimization of devices and has landmark significance.

Figure. The significant increase in carrier mobility was achieved through the lattice plainification strategy.