Tippireddy Sahil, Azough Feridoon, Tompkins Frances Towers, Bhui Animesh, Freer Robert, Grau-Crespo Ricardo, Biswas Kanishka, Vaqueiro Paz, Powell Anthony V
Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6DX, United Kingdom.
Department of Materials, University of Manchester, Manchester, M13 9PL, United Kingdom.
Chem Mater. 2022 Jul 12;34(13):5860-5873. doi: 10.1021/acs.chemmater.2c00637. Epub 2022 Jun 25.
The dearth of -type sulfides with thermoelectric performance comparable to that of their type analogues presents a problem in the fabrication of all-sulfide devices. Chalcopyrite (CuFeS) offers a rare example of an -type sulfide. Chemical substitution has been used to enhance the thermoelectric performance of chalcopyrite through preparation of Cu Sn FeS (0 ≤ ≤ 0.1). Substitution induces a high level of mass and strain field fluctuation, leading to lattice softening and enhanced point-defect scattering. Together with dislocations and twinning identified by transmission electron microscopy, this provides a mechanism for scattering phonons with a wide range of mean free paths. Substituted materials retain a large density-of-states effective mass and, hence, a high Seebeck coefficient. Combined with a high charge-carrier mobility and, thus, high electrical conductivity, a 3-fold improvement in power factor is achieved. Density functional theory (DFT) calculations reveal that substitution leads to the creation of small polarons, involving localized Fe states, as confirmed by X-ray photoelectron spectroscopy. Small polaron formation limits the increase in carrier concentration to values that are lower than expected on electron-counting grounds. An improved power factor, coupled with substantial reductions (up to 40%) in lattice thermal conductivity, increases the maximum figure-of-merit by 300%, to ≈ 0.3 at 673 K for CuSnFeS.
缺乏热电性能与其同类类似物相当的β型硫化物给全硫化物器件的制造带来了问题。黄铜矿（CuFeS₂）是β型硫化物的一个罕见例子。通过制备Cu₁₋ₓSnₓFeS₂（0≤x≤0.1），化学取代已被用于提高黄铜矿的热电性能。取代会引起高水平的质量和应变场波动，导致晶格软化和点缺陷散射增强。与透射电子显微镜鉴定出的位错和孪晶一起，这提供了一种散射具有广泛平均自由程的声子的机制。取代材料保留了较大的态密度有效质量，因此具有较高的塞贝克系数。结合高载流子迁移率以及由此产生的高电导率，功率因数提高了3倍。密度泛函理论（DFT）计算表明，取代导致了小极化子的产生，涉及局域化的Fe态，这一点已通过X射线光电子能谱得到证实。小极化子的形成将载流子浓度的增加限制在低于基于电子计数预期的值。功率因数的提高，再加上晶格热导率大幅降低（高达40%），使得CuSnFeS₂在673 K时的最大优值提高了300%，达到≈0.3。
