BiCuSeO as state-of-the-art thermoelectric materials for energy conversion: from thin films to bulks
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BiCuSeO-based thermoelectric material has attracted great attention as state-of-the-art thermoelectric materials since it was first reported in 2010. In this review, we update the studies on the BiCuSeO thin films first. Then, we focus on the most recent progress of multiple approaches that enhance the thermoelectric performance including advanced synthesized technologies, notable mechanisms for higher power factor (optimizing carrier concentration, carrier mobility, Seebeck coefficient) and doping effects predicted by calculation. And finally, aiming at further enhancing the performance of these materials and ultimately commercial application, we give a brief discussion on the urgent issues to which should be paid close attention.
KeywordsBiCuSeO State-of-the-art thermoelectric materials Thin films Bulks
The contradiction between the increasing demand for energy and decreasing supplies of fossil fuels needs to be solved in the worldwide range. Therefore, a wide range of alternative energy technologies such as solar and wind energy have been developed. In particular, thermoelectric (TE) materials, a solution for direct conversion from the low-quality heat energy to versatile electricity, have drown vast attention [1, 2]. The performance of TE material is determined by the dimensionless figure of merit (ZT), which is derived from the combinatorial parameters of Seebeck coefficient (S), electrical conductivity (σ), thermal conductivity (κ) and absolute temperature (T): ZT = σS2T/κ. σS2 is defined as the power factor (PF) of the TE material. And σ = nμe, where n is carrier concentration, μ is carrier mobility, and e is carrier charge. To maximize ZT value, we need a large Seebeck coefficient and electrical conductivity with low thermal conductivity. However, these parameters have a complex relationship with each other, which makes it hard to effectively improve thermoelectric performance.
The reviews by Zhao et al.  and Zhang et al.  summarized some approaches for enhanced TE performance of bulk BiCuSeO. However, great progress has been achieved in the BiCuSeO thin films so far. Here in this review, not only do we mainly focus on the most recent progress on multiple ways for higher power factor of BiCuSeO bulks including optimizing carrier concentration, carrier mobility, Seebeck coefficient, but also update the development of BiCuSeO TE films. Besides, possible issues and perspectives for the future research have been briefly discussed as well.
2 BiCuSeO TE thin films
The researches on BiCuSeO TE films just get started in recent years. We can see the amazing physical phenomena through the films. The development on TE films requires more attention and resources.
3 BiCuSeO polycrystalline bulks
3.1 Advances in synthesis and processing techniques for BiCuSeO
Even though BiCuSeO was found to be promising TE materials only a few years ago, it has been proven that BiCuSeO could be prepared by high-pressure and high-temperature (HPHT) synthesis, sol–gel, solid-state reaction (SSR), mechanical alloying (MA) and self-propagating high-temperature synthesis (SHS).
3.1.1 High-pressure and high-temperature process
Some physical and transport properties (room temperature) of BiCuSeO prepared by SSR and sol–gel, where m0 being free electron mass
Reduced Fermi energy/η
Density of states effective mass/(m*/m0)
3.1.3 Solid-state reaction
Indeed, BiCuSeO bulks were fabricated by SSR in majority of studies [15, 39, 40, 41, 42]. It requires two-step heating. The mixed powders, which were sealed in evacuated ampules, were calcined at 573 K and then 973 K for several hours. SSR is a very simple and useful method to prepare oxides. However, it is hard to control the purity of the product. As SSR is one of the most conventional methods, it is not introduced in detail here. Note that this method is time- and energy-consuming, which motivates people to find other approaches to synthesize BiCuSeO instead of SSR.
3.1.4 Mechanical alloying
In the mechanical alloying process, the initial element powders can form the compound powders by the mechanochemical effect . BiCuSeO compounds were synthesized using MA process by Wu et al. . Bi, Se and CuO powders (not Bi2O3, Cu, Se and Bi) were put in a dry milling form, so that the compound powders could be prepared by the high energy generated by the collision between balls.
A metastable phase of BiSe was obtained first and when the energy was high enough to form the equilibrium compound, the reaction went on. Compared with the two-step SSR, MA is very competitive as a facile method to prepare BiCuSeO powders.
3.1.5 Self-propagating high-temperature synthesis
The required ignition temperature and heating rate for SHS process were determined by differential scanning calorimetry (DSC)  (Fig. 6b). It was found that the ignition temperature should be higher than 540.7 K and heating rate should be 75 K·min−1 for SHS process of BiCuSeO. It should be noted that SHS is the simplest and fasted method for preparation of materials, which needs only several minutes, but it can be only applied to certain materials, for instance, some selenides. Besides, as SHS is a non-equilibrium process, the synthesized samples by SHS may possess rich nanostructures. As an example for comparison, the lattice thermal conductivity (κL) of SSRed  and SHSed  Bi1-yPb y CuSeO samples at 323 and 623 K is shown in Fig. 6c. For SSRed samples, the experimental results agree well with the calculation data (solid line) indicating there are no additional microstructures that can scatter phonons. But the κL of SHSed samples is obviously lower than that of SSRed samples and calculated ones. As seen in Fig. 6d, Pb-doped BiCuSeO has large number of nanodots embedded in the matrix homogeneously. The abundant nanostructures including nanodots and nanophases can induce strong phonon scattering, which is beneficial to enhance ZT value.
3.2 Tuning carrier concentration
3.2.1 Acceptor doping
BiCuSeO has an impressively low thermal conductivity, but electrical conductivity is far from that of good TE performance. At the initial stage, divalent ions M2+ (Sr , Mg , Pb , Ca , Ba ) doping at Bi3+ site were adopted to successfully optimize carrier concentration and thus electrical conductivity. And up to now, many studies still focus on this. Note that the carrier concentration boosts from ~ 1 × 1018 of pristine BiCuSeO to ~ 1 × 1020 after M2+ doping.
3.2.2 Vacancies at Bi and Cu sites
3.2.3 Band gap tuning
3.3 Improving carrier mobility
Another limitation of further improvement of BiCuSeO TE performance is the low carrier mobility. The mobility of pristine BiCuSeO is about 20 cm2·V−1·s−1. After optimizing carrier concentration, it can be reduced to ~ 1–2 cm2·V−1·s−1 because of strong phonon coupling. It is much lower than that of any other TE materials. Therefore, multiple methods are required to further improve the low carrier mobility.
3.3.1 Increasing bond covalency
Carrier concentration and mobility of Bi0.96Pb0.04CuSe1−xTe x O
x = 0
x = 0.025
x = 0.050
x = 0.075
x = 0.100
3.3.2 Tuning band structure
3.3.3 Modulation doping
3.3.4 Textured microstructure by hot-forging
3.4 Enhancing Seebeck coefficient
So far, due to the largely increased carrier concentration, the Seebeck coefficient of doped BiCuSeO drops heavily. Wen et al.  selected Ba and Ni as dopants at Bi site and Cu site of BiCuSeO, respectively. The magnetic ions can induce degeneracy of electronic spin configurations in real space, which can give extra spin entropy, and as a result, Seebeck coefficient can be enhanced.
3.5 Investigation on doping effects by calculation
3.5.1 Si as an unexpected n-type dopant
3.5.2 Inducing resonant states by In and Tl
Band engineering has been proved as an effective way to increase Seebeck coefficient. Method of inducing resonant levels through impurity doping has been used in many TE materials system such as PbTe [62, 63, 64]. The resonant levels can enhance the Seebeck coefficient by the added density of states (DOS) or a strong energy filtering effect . According to Shen et al. , In and Tl doping can induce resonant states at conduction band minimum (CBM) and valence band maximum (VBM), which is based on a band unfolding and density functional theory.
4 Summary and perspectives
In this review, we introduce the development of BiCuSeO TE thin films and the most recent progress in BiCuSeO bulks including notable mechanisms of optimizing carrier concentration, carrier mobility, Seebeck coefficient to enhance power factor and the calculation results. Apart from the mentioned work, the research on the BiCuSeO systems still has space for further exploration, which at least includes the following.
The studies on the BiCuSeO thin films need more attention and resources. A lot of physical phenomena can be shown in the films. And the thin films can offer other possibilities of investigations including heterostructure, modified layers, etc., which are almost impossible to achieve in the bulks. Besides, technologies of measuring temperature-depended in-plane thermal conductivity of thin films need to be further developed.
For BiCuSeO TE bulks, the urgent issue is to increase Seebeck coefficient and overall power factor. The effective method of band engineering can be extended to BiCuSeO systems. In addition, even though the p-type BiCuSeO TE performance can have a ZT value more than 1, the corresponding n-type BiCuSeO-based thermoelectric materials with high TE properties remains to be developed. The Cl-doped BiCuSeO is still p-type material ; however, trials are needed to explore n-type BiCuSeO thermoelectric materials.
Apart from pushing the TE performance of BiCuSeO to a higher level, assembling TE devices is also important. Advanced technologies of assembling different TE materials are needed to weaken the effect of elemental diffusion and contact resistance, which will pave the way for the future commercial application of energy conversion.
This work was financially supported by the National Key Research Programme of China (No. 2016YFA0201003), the National Basic Research Program of China (No. 2013CB632506), the National Natural Science Foundation of China (No. 51772016), the National Natural Science Foundation of China (Nos. 51672155, 51532003) and China Postdoctoral Science Foundation (No. 2016M601020).
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