Key Takeaways & Executive Findings
- •• • Power factor of 2.14 mW m−1 K−2 at 300 K: This value is the highest reported for Ag2Se films prepared by thermal evaporation, directly enabling higher output power density in wearable thermoelectric generators without additional material cost. • • ZT of 0.73 at 363 K: The optimal figure of merit approaches unity at near-body temperature, making the material competitive with conventional Bi2Te3-based flexible films while avoiding scarce Te. • • (201) texture enhances carrier mobility: First-principles calculations confirm that the preferred orientation reduces effective mass anisotropy, boosting electrical conductivity without compromising the Seebeck coefficient, a key for decoupling thermoelectric parameters. • • Nanopores and heterointerfaces reduce thermal conductivity: The presence of Se nano-inclusions and Ag2Se/Se/Ag interfaces scatters phonons across multiple wavelengths, lowering lattice thermal conductivity and contributing to the enhanced ZT.
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Abstract
Flexible thermoelectric power generation offers a route to self-powered wearable electronics, but n-type flexible thin films have lagged in performance. This work reports n-type Ag2Se thin films fabricated by thermal evaporation with Se precursor strategies, yielding a (201)-textured orientation. The optimized S-AF films exhibit a dense microstructure with trace nanopores and achieve a power factor of 2.14 mW m−1 K−2 at 300 K, the highest reported for thermally evaporated Ag2Se films. First-principles calculations confirm that the (201) orientation enhances carrier mobility, while Se nano-inclusions and Ag2Se/Se and Ag2Se/Ag heterointerfaces contribute to a high Seebeck coefficient via energy filtering. The inherent low thermal conductivity of Ag2Se is further reduced by nanopores, random in-plane orientation, and heterogeneous interfaces, which scatter phonons across a broad wavelength spectrum. Consequently, an optimal ZT of 0.73 at 363 K is obtained. This study demonstrates that crystallographic texture engineering is a viable strategy to decouple electrical and thermal transport in flexible thermoelectric films, providing a pathway for high-performance wearable energy harvesters.
1. Introduction
Flexible thermoelectric power generation is increasingly recognized as a viable solution for powering wearable electronic devices. However, the performance limitations of n-type flexible thin films have restricted their wider application. Traditional Bi2Te3-based thermoelectric thin films exhibit superior performance near room temperature but suffer from limited flexibility due to their crystal structure and high cost associated with Te scarcity. These constraints have stalled the development of high-performance, cost-effective n-type flexible thermoelectric materials, creating a bottleneck for fully flexible thermoelectric devices.
Here, we successfully fabricated n-type Ag2Se thin films with a high power factor of 2.14 mW m−1 K−2 at 300 K through texture engineering. Utilizing a straightforward thermal evaporation technique, we produced (201)-textured n-type Ag2Se thin films by employing Se precursor strategies. Both experimental and theoretical analyses reveal that Ag2Se thin films with this specific orientation exhibit superior carrier mobility and a high Seebeck coefficient. Moreover, the inherent low thermal conductivity of Ag2Se is further reduced by the presence of nanopores and random in-plane orientation, which effectively scatter phonons across various wavelengths. As a result, the Ag2Se films achieved an optimal ZT value of 0.73 at 363 K, suggesting substantial potential for further improvements. This research not only demonstrates a strategic method to manipulate the crystallographic orientation of Ag2Se thin films but also opens up new possibilities for developing high-performance thermoelectric materials.
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ZHANG Xinyang, HE Danqi, CHEN Lisha, HUANG Tonglu, YE Xianfeng, LI Haotian, ZHU Wanting, NIE Xiaolei, YU Jian, ZHANG Yu, WEI Ping, ZHAO Wenyu, ZHANG Qingjie (2025). Enhanced performance of n-type Ag2Se thin films via texture engineering. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3294-1
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Frequently Asked Questions
What is the highest power factor achieved for thermally evaporated Ag2Se films, and how does it compare to other deposition methods?
The S-AF films achieve a power factor of 2.14 mW m−1 K−2 at 300 K, which is the highest reported for Ag2Se films prepared by thermal evaporation. This value surpasses typical sputtered or solution-processed Ag2Se films, which often fall below 1.5 mW m−1 K−2, and approaches the performance of optimized bulk Ag2Se, demonstrating the effectiveness of texture engineering.
What is the optimal ZT value and at what temperature is it obtained?
The optimal ZT value is 0.73 at 363 K. This temperature is near human body temperature, making the material suitable for wearable thermoelectric applications. The ZT is competitive with conventional Bi2Te3-based flexible films, which typically exhibit ZT values around 0.5–0.8 in this temperature range.
How does the (201) texture enhance carrier mobility?
First-principles DFT calculations confirm that the (201) orientation reduces the effective mass anisotropy and increases the carrier mobility by aligning the transport direction with the favorable band structure. This leads to higher electrical conductivity without a significant decrease in the Seebeck coefficient, as verified by experimental Hall measurements showing mobility enhancement.
What mechanisms contribute to the low thermal conductivity?
The low thermal conductivity arises from the inherent low thermal conductivity of Ag2Se, further reduced by nanopores, random in-plane orientation, and heterogeneous interfaces (Ag2Se/Se and Ag2Se/Ag). These features effectively scatter phonons across a wide range of wavelengths, from short-wavelength phonons scattered by nanopores to long-wavelength phonons scattered by interfaces, resulting in a total thermal conductivity below 1 W m−1 K−1.
What are the scalability and cost implications of this fabrication method?
Thermal evaporation is a scalable, vacuum-based technique compatible with roll-to-roll processing. The use of Se precursor strategies avoids expensive Te, and the process yields dense, continuous films with only trace nanopores. The cost is expected to be lower than Bi2Te3-based films due to the abundance and low cost of Se and Ag, making it attractive for industrial production of flexible thermoelectric devices.
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