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High carrier mobility is critical to improving thermoelectric performance over a broad temperature range. However, traditional doping inevitably deteriorates carrier mobility. Herein, we develop a strategy for fine tuning of defects to improve carrier mobility. To begin, n-type PbTe is created by compensating for the intrinsic Pb vacancy in bare PbTe. Excess Pb 2+ reduces vacancy scattering, resulting in a high carrier mobility of ∼3400 cm 2 V −1 s −1 . Then, excess Ag is introduced to compensate for the remaining intrinsic Pb vacancies. We find that excess Ag exhibits a dynamic doping process with increasing temperatures, increasing both the carrier concentration and carrier mobility throughout a wide temperature range; specifically, an ultrahigh carrier mobility ∼7300 cm 2 V −1 s −1 is obtained for Pb 1.01 Te + 0.002Ag at 300 K. Moreover, the dynamic doping-induced high carrier concentration suppresses the bipolar thermal conductivity at high temperatures. The final step is using iodine to optimize the carrier concentration to ∼10 19 cm −3 . Ultimately, a maximum ZT value of ∼1.5 and a large average ZT ave value of ∼1.0 at 300−773 K are obtained for Pb 1.01 Te 0.998 I 0.002 + 0.002Ag. These findings demonstrate that fine tuning of defects with <0.5% impurities can remarkably enhance carrier mobility and improve thermoelectric performance.

Rational engineering of nanostructured anode materials is important to develop lithium-ion batteries (LIBs). In this study, hierarchical composites of fluoridated carbonaceous GeO 2 (F-GeO 2 @C) with rich oxygen vacancies were prepared by a simple annealing method. It is found that F − ions not only exist in the carbon matrix but also replace O 2− of metallic oxides. The abundant introduced oxygen vacancies can provide more active sites and contribute to better electronic conductivity. Moreover, density functional theory (DFT) calculations confirm that Fdoping greatly changes the electronic structure of the GeO 2 composite, exhibiting interesting metallic behavior. Consequently, the F-GeO 2 @C anode shows an enhanced initial Coulombic efficiency (ICE) value of 71.6% and delivers excellent rate capability, much higher than most reported GeO 2 -based anodes. The enhancement of the electrochemical performance for F-GeO 2 @C is attributed to the hierarchical nanostructure and F-doping by increased reaction kinetics, reversibility, and cycling stability. Thus, such rational fabrication of the composite can motivate other high-performance germanium-based materials in LIBs.

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Heterogeneous composites consisting of Bi6Cu2Se3.6Cl0.4O6 and Bi2O2Se are prepared according to the concept of modulation doping. With prominently increased carrier mobility and almost unchanged effective mass, the electrical transport properties are considerably optimized resulting in a peak power factor ≈1.8 µW cm−1 K−2 at 873 K, although the carrier concentration is slightly deteriorated. Meanwhile, the lattice thermal conductivity is lowered to ≈0.62 W m−1 K−1 due to the introduction of the second phase. The modified Self‐consistent Effective Medium Theory is utilized to explain the deeper mechanism of modulation doping. The enhancement of apparent carrier mobility is derived from the highly active phase interfaces as fast carrier transport channels, while the reduced apparent thermal conductivity is ascribed to the existence of thermal resistance at the phase interfaces. Ultimately, an optimized ZT ≈0.23 is obtained at 873 K in Bi6Cu2Se3.6Cl0.4O6 + 13% Bi2O2Se. This research demonstrates the effectiveness of modulation doping for optimizing thermoelectric properties once again, and provides the direct microstructure observation and consistent theoretical model calculation to emphasize the role of interface effects in modulation doping, which should be probably applicable to other thermoelectrics.

Compared with traditional thermoelectric materials, layered oxyselenide thermoelectric materials consist of nontoxic and lower-cost elements and have better chemical and thermal stability. Recently, several studies on n-type layered oxyselenide thermoelectric materials, including BiCuSeO, Bi2O2Se and Bi6Cu2Se4O6, were reported, which stimulates us to comprehensively summarize these researches. In this short review, we begin with various attempts to realize an n-type BiCuSeO system. Then, we summarize several methods to optimize the thermoelectric performance of Bi2O2Se, including carrier engineering, band engineering, microstructure design, et al. Next, we introduce a new type of layered oxyselenide Bi6Cu2Se4O6, and n-type transport properties can be obtained through halogen doping. At last, we propose some possible research directions for n-type layered oxyselenide thermoelectric materials.

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