Thermoelectric (TE) materials are functional materials that can realize the direct and reversible conversion between heat and electricity. Their conversion efficiency is determined by their average figure of merit (ZTave). Generally, high ZTave requires TE materials to possess both excellent electrical transport properties and low thermal conductivity, called “electron crystal-phonon glass.” To date, although commonly used band manipulation and defect designing strategies can optimize the carrier effective mass and lattice thermal conductivity, they reduce the carrier mobility and thus limit the improvement of ZTave. Therefore, maintaining high carrier mobility is essential for improving ZTave over a wide temperature range. In this review, the methods to optimize carrier mobility, including crystal defect manipulations and multiple coupling parameter manipulations, were summarized. Specifically, crystal defect manipulations include strategies of crystal growth, crystal symmetry manipulation, and point defect manipulation, and the multiple coupling parameter manipulations include band alignment strategies, modulation doping, and band sharpening. Further, the applications of these strategies in multiple TE material systems were discussed, such as in SnSe/S, PbTe/Se/S, BiCuSeO, and BiAgSeS compounds. It was proven that the above strategies can well optimize the TE performance over the entire working temperature by effectively balancing carrier and phonon scattering and synergistically manipulating the coupling relationships between carrier mobility, effective mass, and carrier density. The importance of carrier mobility optimization in TE materials and a new research idea for developing high-efficiency TE materials were presented.
Fund: National Key Research and Development Program of China(2018YFA0702100);National Natural Science Foundation of China(51772012);National Science Fund for Distinguished Young Scholars(51925101);National Postdoctoral Program for Innovative Talents(BX20190028);Postdoctoral Science Foundation of China(2019M660399);Programme of Introducing Talents of Discipline to Universities(B17002);Beijing Natural Science Foundation(JQ18004);Shenzhen Peacock Plan team(KQTD2016022619565991)
Fig.3 Strategy of crystal symmetry manipulation (SnSe-(SnTe[29]/Na[64,66]/Ag[66]/SnS[67]))
Fig.4 Strategy of point defect manipulation (PbTe-(Cu[72]/Cu2Te[73]/Ag2Te[76]/CdTe[77]/InSb[78]), PbSe-(Cu[74]/Zn[75]/CdSe[79]/SrSe[80]))
Fig.5 Strategy of band alignment in energy (p-type: PbS-(CdS[8]/Ag2S[83]/CaS[8]/SrS[8]), n-type: PbS-(PbTe[30]/Sb2S3[84]/Pb(Pb, Sb)S2[85])(a) schematic of the relationship between band alignment and carrier mobility (C—conduction band, V—valence band, —band gap of second phase, Eg—band gap of matrix, ΔE—energy offset of conduction band or valence band between matrix and second phase)
Fig.6 Strategy of modulation doping
Fig.7 Strategy of band sharpening ((Pb1-xSnx)(Te1-xSex)[98], PbTe-(In-I[99]/Pb-Sb[100]/Ag2Te[101]/Pb vacancy[102]), Pb1-xSnxS[30], PbS-(Sb-Cu[103]/Bi2S3-PbCl2[84]/Sb2S3-PbCl2[84]/Sb[85]))
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