Application of zinc-blende-related chalcogenide absorbers such as CdTe and Cu(In,Ga)Se2 (CIGSe) has enabled remarkable advancement in laboratory- and commercial-scale thin-film photovoltaic performance; however concerns remain regarding the toxicity (CdTe) and scarcity (CIGSe/CdTe) of the constituent elements. Recently, kesterite-based Cu2ZnSn(S,Se)4 (CZTSSe) materials have emerged as attractive non-toxic and earth-abundant absorber candidates. Despite the similarities between CZTSSe and CIGSe/CdTe, the record power conversion efficiency of CZTSSe solar cells (12.6%) remains significantly lower than that of CIGSe (22.6%) and CdTe (22.1%) devices, with the performance gap primarily being attributed to cationic disordering and associated band tailing. To capture the promise of kesterite-like materials as prospective “drop-in” earth-abundant replacements for closely-related CIGSe, current research has focused on several key directions to control disorder, including: (i) examination of the interaction between processing conditions and atomic site disorder, (ii) isoelectronic cation substitution to introduce ionic size mismatch, and (iii) structural diversification beyond the zinc-blende-type coordination environment. In this review, recent efforts targeting accurate identification and engineering of anti-site disorder in kesterite-based CZTSSe are considered. Lessons learned from CZTSSe are applied to other complex chalcogenide semiconductors, in an effort to develop promising pathways to avoid anti-site disordering and associated band tailing in future high-performance earth-abundant photovoltaic technologies.

Defect Engineering in Multinary Earth-Abundant Chalcogenide Photovoltaic Materials

Cu-based thermoelectric materials

SnSe, a "simple" and "old" binary compound composed of earth-abundant elements, has been reported to exhibit a high thermoelectric performance in single crystals, which stimulated recent interest in its polycrystalline counterparts. This work investigated the electrical and thermal transport properties of pristine and Na-doped SnSe1-xTex polycrystals prepared by mechanical alloying and spark plasma sintering. It is revealed that SnSe1-xTex solid solutions are formed when x ranges from 0 to 0.2. An energy barrier scattering mechanism is suitable for understanding the electrical conducting behaviour observed in the present SnSe polycrystalline materials, which may be associated with abundant defects at grain boundaries. The thermal conductivity was greatly reduced upon Te substitution due to alloy scattering of phonons as well explained by the Debye model. Due to the increased carrier concentration by Na-doping, thermoelectric figure of merit (ZT) was enhanced in the whole temperature range with a maximum value of 0.72 obtained at a relatively low temperature (773 K) for Sn0.99Na0.01Se0.84Te0.16.

Thermoelectric transport properties of pristine and Na-doped SnSe1−xTex polycrystals

Cu_3SbSe_4-based compounds composed of earth-abundant elements have been found to exhibit good thermoelectric performance at medium temperatures. High zT values were achieved in previous studies, but further insight into the transport mechanism as well as some key material parameters is still needed. In this work, we studied the electrical and thermal transport properties of Sn-doped Cu_3SbSe_4 between 300 K and 673 K. It was found that the single parabolic band model explains the electrical transport very well. Experimentally, we determined the band gap to be around 0.29 eV. The density-of-state effective mass was found to be about 1.5 me for the doped samples. The transport properties suggested degeneracy splitting near the valence band maximum that was not captured by previous band structure calculations. The maximum zT ~0.70 was obtained at 673 K, and the optimized carrier density was ~1.8 × 10^20 cm^(−3), and the potential for further improvement of zT via material engineering is briefly discussed.

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Thermoelectric properties of Sn-doped p-type Cu3SbSe4: a compound with large effective mass and small band gap

Cu-based chalcogenides have received increasing attention as promising thermoelectric materials due to their high efficiency, tunable transport properties, high elemental abundance and low toxicity. In this review, we summarize the recent research progress on this large family compounds covering diamond-like chalcogenides and liquid-like Cu2X (X=S, Se, Te) binary compounds as well as their multinary derivatives. These materials have the general features of two sublattices to decouple electron and phonon transport properties. On the one hand, the complex crystal structure and the disordered or even liquid-like sublattice bring about an intrinsically low lattice thermal conductivity. On the other hand, the rigid sublattice constitutes the charge-transport network, maintaining a decent electrical performance. For specific material systems, we demonstrate their unique structural features and outline the structure-performance correlation. Various design strategies including doping, alloying, band engineering and nanostructure architecture, covering nearly all the material scale, are also presented. Finally, the potential of the application of Cu-based chalcogenides as high-performance thermoelectric materials is briefly discussed from material design to device development.

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Copper chalcogenide thermoelectric materials

The compositional dependence of the crystal structure and lattice thermal conductivity in the Cu3SbSe4-Cu3SbS4 system has been studied. The lattice parameters of the Cu3SbSe4-xSx compounds decrease linearly with x, and the tetragonal structure (space group 14−2m no. 121) of the end compounds is maintained at all compositions. The lattice thermal conductivity is much lower than that predicted by a simple rule of mixtures, which is typical for a solid solution. The Debye model produces a very reasonable fit to the experimental lattice thermal conductivity data when phonon scattering due to atomic mass and size differences between Se and S is taken into account. Compounds in this series are likely to improve upon the thermoelectric performance of Cu3SbSe4, which has shown ZT = 0.72 when optimized.

Lattice thermal conductivity of the Cu3SbSe4-Cu3SbS4 solid solution

We present the first systematic doping study on the ternary semiconductor Cu3SbSe 4 . We have developed a novel synthesis procedure that produces high-quality polycrystalline samples with hole concentrations an order of magnitude lower than have been reported for the undoped compound. The hole concentration can be increased by adding small amounts of either Ge or Sn on the Sb site. The power factor increases with increasing doping, reaching a maximum value of 16 W/cmK^2 . The thermoelectric properties are optimized for the 2% Sn doped compound which has ZT=0.72 at 630K, rivaling that of state-of-the-art thermoelectric materials in this temperature range.

Doping Effects on the Thermoelectric Properties of Cu3SbSe4

We report the results of an investigation on the structural evolution of a potential new thermoelectric material, Cu3SbSe3, as a function of temperature from 25 to 390 °C. From high-temperature x-ray diffraction data, the refined lattice parameters were seen to change nonlinearly, but continuously, with temperature, with an increased rate of thermal expansion in the a and b lattice parameters from around 125 °C to 175 °C and negative thermal expansion in the c axis from around 100 °C to 175 °C. Crystallographic charge flipping analysis indicated an increase in the disorder of the copper cations with temperature. This reversible order/disorder phase transition in Cu3SbSe3 affects the transport properties, as evidenced by thermal conductivity measurements, which change from negative to positive slope at the transition temperature. This structural change in Cu3SbSe3 has implications for its potential use in thermoelectric generators.

High-temperature order/disorder transition in the thermoelectric Cu3SbSe3

High-temperature order/disorder transition in the thermoelectric

We report on the effects of thermal processing on the microstructure and composition of a system with overall stoichiometry of 3Cu:1Sb:3Se with the aim of producing single-phase Cu3SbSe3. It was found that slow cooling from the melt produced a multiphase material consisting of Cu2Se and CuSbSe2, but devoid of Cu3SbSe3. Cooling rapidly from the melt resulted in three-phase microstructures consisting of Cu2Se, CuSbSe2, and Cu3SbSe3. Subsequent annealing of the three-phase material between 325 and 400 °C shifted the composition toward nearly pure Cu3SbSe3—the target compound of this work. The kinetics of the transformation into Cu3SbSe3 was successfully described using a modified Avrami model which suggests that diffusion is the rate-controlling step. Values of Young’s modulus and hardness, obtained by nanoindentation, are reported for Cu2Se, CuSbSe2, and Cu3SbSe3.

Paul Majsztrik

Papers

Lattice thermal conductivity of the Cu3SbSe4-Cu3SbS4 solid solution

Doping Effects on the Thermoelectric Properties of Cu3SbSe4

High-temperature order/disorder transition in the thermoelectric Cu3SbSe3

High-temperature order/disorder transition in the thermoelectric

Effect of thermal processing on the microstructure and composition of Cu–Sb–Se compounds