Enhanced Thermoelectric Properties of Cu3SbSe4 Compounds by Isovalent Bismuth Doping

Cu3SbSe4, featuring its earth-abundant, cheap, nontoxic and environmentally friendly constituent elements, can be considered as a promising intermediate temperature thermoelectric (TE) material. Herein, a series of p-type Bi-doped Cu3Sb1−xBixSe4 (x = 0–0.04) samples were fabricated through melting and hot pressing process, and the effects of isovalent Bi-doping on their TE properties were comparatively investigated by experimental and computational methods. TEM analysis indicates that Bi-doped samples consist of Cu3SbSe4 and Cu2−xSe impurity phases, which is in good agreement with the results of XRD, SEM and XPS. For Bi-doped samples, the reduced electrical resistivity (ρ) caused by the optimized carrier concentrations and enhanced Seebeck coefficient derived from the densities of states near the Fermi level give rise to a high power factor of ~ 1000 µWm−1 K−2 at 673 K for the Cu3Sb0.985Bi0.015Se4 sample. Additionally, the multiscale defects of Cu3SbSe4-based materials involving point defects, nanoprecipitates, amorphous phases and grain boundaries can strongly scatter phonons to depress lattice thermal conductivity (κlat), resulting in a low κlat of ~ 0.53 Wm−1 K−1 and thermal conductivity (κtot) of ~ 0.62 Wm−1 K−1 at 673 K for the Cu3Sb0.98Bi0.02Se4 sample. As a consequence, a maximum ZT value ~ 0.95 at 673 K is obtained for the Cu3Sb0.985Bi0.015Se4 sample, which is ~ 1.9 times higher than that of pristine Cu3SbSe4. This work shows that isovalent heavy element doping is an effective strategy to optimize thermoelectric properties of copper-based chalcogenides.


Introduction
Thermoelectric (TE) materials, dealing with environmental protection and economic development, have gained great interest of research for waste energy recovery, temperature control and heat management in last two decades owing to the direct interconversion between heat and electricity [1][2][3][4][5]. TE materials like chalcogenides [6,7], skutterudites [8], clathrates [9] and half-Heuslers [10], are intensively studied for power generation and thermoelectric cooling [11]. The efficiency of a TE material is determined by the dimensionless thermoelectric figure of merit, ZT = S 2 T/qj, where S, q, T and j presents the Seebeck coefficient, electrical resistivity, absolute temperature, total thermal conductivity (including lattice thermal conductivity j lat and electronic thermal conductivity j ele ), respectively [12,13]. Generally speaking, an important goal of TE research is to achieve as large ZT value as possible because the efficiency increases nonlinearly with ZT, which approaches the Carnot efficiency as ZT approaches infinity [11,14]. As is known, increasing power factor (PF = S 2 /q) or/and depressing j lat can enhance the ZT values. On one hand, one can use methods to decouple the internal relationship between S and q to guarantee a high PF value, such as modulation doping [15], band engineering [16][17][18], carrier concentration optimization [19][20][21], carrier blocking [22], energy filtering and effect [23]; On the other hand, to obtain a low j lat , one can select materials with intrinsically low j lat [24][25][26], or strengthen phonon scattering to suppress phonon transports by nanostructure engineering [27,28], alloying [29], and allscale hierarchical microstructure construction at atomic-, nano-and meso-scale, including point defects [30], dislocations [31,32], precipitates and boundaries [33,34]. However, a large PF is usually along with a high j on account of the interrelated relationship among the S, q and j ele , which makes the optimization of TE properties and development of high-performance TE materials quite challenging [13]. Furthermore, the exploitation of earth-abundant, cheap, nontoxic TE materials (more economical) is urgent in large-scale practical application of TE devices [6,35].
In this regard, copper-based chalcogenides have sprung up as potential TE materials because of their low thermal transport and relatively good electrical transport properties [5,36,37]. In particular, Cu 3-SbSe 4 , featuring its earth-abundant, cheap, nontoxic and environmentally friendly elements, has been extensively investigated as a promising intermediate temperature TE material. Meantime, it shows a narrow bandgap (* 0.29 eV), p-type conductivity with large carrier effective mass (m* *1.1 m e ), and a zincblende-derived stannite structure (tetragonal, I-42 m space group) [14,38]. However, the low carrier concentration limits high ZT values in pristine Cu 3 SbSe 4 , for which how to improve ZT is a research focus. To the best of our knowledge, the Cu-Se bond can act as the carrier conduction framework while Sb orbitals make less contribution to the valence-band maximum (VBM) in Cu 3 SbSe 4 [39]. Consequently, partial substitution for Sb lattice site is an effective approach to optimize carrier concentration, such as Ag [13], In [39], Ti [40], Sn [41], Ge [42], Ga [43], Al [44] and other element substitutions [14]. Recently, Dou et al. reported a new strategy to enhance TE properties of Cu 3 SbSe 4 via alkali-ion (Na and K) doping, and the peak ZT values reached 0.52 and 0.71 at 673 K for Cu 3 Sb 1 -x Na x Se4 (x = 0.02 and 0.03) and Cu 3 Sb 0.99-K 0.01 Se 4 , respectively [45]. In contrast to light elements doping, heavy elements doping (Bi, Te, etc.) have more virtues in lowering j lat because of stronger mass and strain field fluctuations, which can block the propagation of phonons [46,47]. Furthermore, the ZT value can be further increased by a co-doped strategy, such as Ag-Sn [48], Sn-Bi [49] and Ag-Bi [50] in Cu 3 SbSe 4 .
In this article, we report a high-performance Cu 3-SbSe 4 -based material via Bi-doping. Isovalent element dopant Bi can reduce q by the enhanced carrier concentration and the changed Fermi level, and the j lat is depressed through the strengthened phonon scattering caused by multiscale defects including point defects, nanoprecipitates, amorphous phases and grain boundaries, respectively. Consequently, a maximum ZT (ZT max ) of * 0.95 at 673 K is achieved for the Cu 3 Sb 0.985 Bi 0.015 Se 4 compound due to the decoupled internal relationships between electrical and thermal transport properties.

Experimental procedures 2.1 Synthesis
The Cu 3 Sb 1-x Bi x Se 4 (x = 0, 0.005, 0.01, 0.015, 0.02, 0.03 and 0.04) polycrystalline samples were fabricated through vacuum melting and hot pressing (HP) process. Nominal composition of high purity elements (Cu: 99.99%; Sb: 99.99%; Se: 99.999%, Bi: 99.999%) were weighted and placed in quartz tubes, which sealed under high vacuum and then putted into a digitally controlled furnace. The samples were heated at a rate of 20°C /h to 1173 K with a holding time of 12 h, then cooled to 773 K in 40 h. Finally, the quartz tubes were quenched in water for cooling from 773 K down to room temperature. After quenching, to promote Cu 3 SbSe 4 -based compounds homogeneity, the samples were annealed at 573 K for two days. After that, the obtained ingots were milled into powders by handing in an agate mortar for 30 min and sintering in graphite dies with an inner diameter of 10 mm by HP at 673 K for 2 h under 50 MPa.

Characterizations
The phase structure of bulk samples was checked using X-ray diffractometer (XRD, Bruker D8 Advance) in the h-2h scan mode with Cu Ka (k = 1.5418Å ) radiation as an X-ray source. The surface morphologies and chemical elemental composition of the samples were characterized by field emission scanning electron microscopy (FESEM; Nova NanoSEM450) combined with energy-dispersive X-ray spectroscopy (EDS). The valence state of the elements and chemical composition of the Bidoped Cu 3 SbSe 4 sample was identified using X-ray photoelectron spectroscopy (XPS) (Escalab 250Xi, Thermo VG Scientific). Transmission electron microscopy (TEM) was performed on a Talos-F200X operated with an acceleration voltage of 200 kV. The Seebeck coefficient (S) and electrical resistivity (q) were simultaneously measured by the commercial measuring system (LSR-3) under a helium atmosphere from room temperature to 673 K. The thermal diffusivity (D) was measured using the laser flash method (Netzsch, LFA-457). The thermal conductivity was calculated from the relationship of j = DC p d, where D, C p and d are the measured thermal diffusivity, specific heat and density, respectively. The concentration of carrier and transportation in the samples were measured using a Hall apparatus (LAKE SHARE, 7707 A, USA) by the Vander Pauw method under constant magnetic fields of 0.68 T. Densities of sintered samples were conducted by Archimedes' method, which reached over 95% of the theoretical density of Cu 3 SbSe 4 (5.86 g cm -3 ) (Tab. S1).

Computational methods
Theoretical calculations based on DFT were performed using Vienna Ab-initio Simulation Package (VASP) [51,52]. Generalized gradient approximation (GGA) method with Perdew-Burke-Ernzerhof (PBE) function was used to describe electronic exchange and correlation effects [53]. A supercell consisting of 2 9 2 9 2 pristine cell of Cu 3 SbSe 4 (128 atoms, Cu 48 Sb 16 Se 64 ) were built for randomly replacing Sb atoms with Bi atoms. The kinetic energy cutoff for the plane wave basis was set as 500 eV and the a monkhorst-Pack 3 9 3 9 2 k-point mesh is adopted. All structures were relaxed until the forces exerted on each atom were less than 0.01 eV/Å , and the energy convergence criteria for electronic iterations were assumed to be 10 -5 eV. The HSE06 model are adopted to calculate the density of states (DOS) [54,55].

Results and discussion
As seen in Fig. 1a, the crystal structure of pristine Cu 3 SbSe 4 is the tetragonal structure (I-42 m space group) [38]. The XRD patterns of the synthesized Cu 3 Sb 1-x Bi x Se 4 (x = 0-0.04) compounds are shown in Fig. 1b. The major diffraction peaks match well with the standard peaks of Cu 3 SbSe 4 (JCPDS No.85-0003) phase, which belongs to the zinc blende structure with tetragonal system. As the concentration of Bi element increases to 0.03, some weak diffraction peaks at the 2h angle of * 24°, 31°and 65°are observed, indicating the Bi and Cu 2-x Se phases [56]. Previous work also demonstrated that Bi phase was precipitated from the matrix as Bi concentration exceeds 0.02 in Cu 3 SbSe 4 [46]. It can be deduced that the addition of Bi is propitious to the precipitation of Cu 2-x Se phase, which is beneficial to decrease in lattice thermal conductivity.
The morphologies and compositions of Cu 3 SbSe 4based samples were examined by SEM and EDS analyses. As depicted in Fig. 2, SEM images of the fracture surfaces (Fig. 2a-d) indicate that Cu 3 SbSe 4 materials are isotropic and some impure phases are embedded into the Cu 3 SbSe 4 matrix. The EDS result demonstrates that the impurity phase contains Cu and Se elements (Fig. 2e) and the atomic ratio (%) of copper to selenium is 56.20: 28.99, implying that the impure phase may be copper selenide of Cu 2-x Se, which is consistent with XRD results. The back-scattered electron (BSE) image of Cu 3 Sb 0.985 Bi 0.015 Se 4 sample clearly presents the micro or even nanoscale impurity phases (Fig. S1), which are useful to reduce j lat by scattering long-wave phonons [47]. The Cu 2-x Se with extremely low j lat can decrease the j tot of Cu 3 SbSe 4 to some degree [48].
XPS is used to check the valence state of elements and chemical composition of the Cu 3 SbSe 4 -based samples. The experimental data of XPS spectra peaks of Cu 3 Sb 0.985 Bi 0.015 Se 4 samples are presented in Fig. 3. XPS spectra confirm the presence of Cu, Sb, Se and Bi elements in the Cu 3 SbSe 4 -based compounds [57]. As shown in Fig. 3a, Cu 2p core electron peak of specimen splits into two peaks, and the XPS spectra of Cu 2p3/2 (931.2 eV) and the Cu 2p1/2 (952.5 eV) peaks indicate the existence of Cu ? in the sample [58]. The Sb 3d spectra can be deconvoluted into two peaks of Sb 3d 5/2 (528.4 eV) and Sb 3d 3/2 (537.7 eV) (Fig. 2b), which can be indexed to Sb 5? [59]. The Se 3d spectra can also be split into two peaks at 53.1 eV (3d5/2) and 54.0 eV (3d3/2) (Fig. 2c), which are matched with Se 2of oxidation state [60]. The Bi 4f spectra do not display distinguished changes. Consequently, the components of as-sintered Bi-doped samples are speculated to be consisted of Cu 3 SbSe 4 , Cu 2-x Se, which is in agreement with XRD and SEM results.
The presence of secondary phase is clearly visualized by employing a high-resolution TEM. The lowmagnification images presented in Fig. 4a demonstrate that some impurity phases are distributed in the matrix. As shown in Fig. 4b, a crossed fringe, with interplanar spacing values of 0.5164 nm, which can be indexed to (101) planes of Cu 3 SbSe 4 [41,61]. The 0.2986 nm, 0.1041 nm and 0.1157 nm of spacing are in response to (200), (511) and (422) planes of Cu 2 Se, respectively [56]. It can be seen that some amorphous phases exist in the matrix (The marks ''A'' and ''C'' present amorphous phases and crystallized phases, respectively). In addition, the interfaces (blue dot lines) among Cu 3 SbSe 4 , Cu 2 Se and amorphous phases can be clearly observed. All of which are beneficial for reducing lattice thermal conductivity. The SAED pattern, corresponding to (105) and (220) planes of Cu 3 SbSe 4 , as well as (111) and (220) planes of Cu 2 Se, which may further support that the asprepared sample is consists Cu 3 SbSe 4 and Cu 2-x Se [37,48].
To validate our experimental design and investigate the change of electronic properties, the density  [13]. To ensure the optimal doping model is consistent with experiment and guarantee the rationality for theory calculation, the Cu 48 Fig. 5a. After Bi-doping, the Fermi level changes from 3.99 eV to 3.77 eV, thus improving electrical transport characteristics. The bandgap (E g ) of Cu 48 Sb 16 Se 64 is 0.25 eV, which is basically consistent with previous report [60], and increase to 0.27 eV for Cu 48 Sb 15 Bi 1 Se 64 . More importantly, the DOS near the Fermi level (0 eV) are increased, which is composed of the hybridized Cu 3d, Sb 5 s, Se 4p and Bi 6 s orbitals (Fig. 5b), leading to large Seebeck coefficients.
The temperature-dependent electrical transport properties of Cu 3 SbSe 4 -based samples are depicted in Fig. 6. As seen in Fig. 6a, the q of pristine Cu 3 SbSe 4 Fig. 2 The fracture morphology images of a pristine Cu 3 SbSe 4 and b Cu 3 Sb 0.985 Bi 0.015 Se 4 samples; c, d high magnification images of (a, b); e the corresponding EDS spectrum of B zone in (d) sample decreases with elevating temperature, indicating that it is non-degenerate semiconductor. However, Bi-doped samples become partially degenerate semiconductors, especially for the samples with low Bi content (x \ 0.015). Clearly, the q of all Bi-doped Cu 3 SbSe 4 compounds is lower than that of pristine Cu 3 SbSe 4 due to the increased carrier concentration (Tab. S1). In comparison with Sb 5? (0.62 Å ), the bigger radius of Bi 5? (0.74 Å ) can generate Cu and/or Sb vacancies [46]. Meanwhile, the q    of the samples increases with increasing Bi content (x \ 0.03), however, the q sharply decreases as x = 0.03 and 0.04 because of the relatively high carrier mobility and carrier concentration. In addition, the q shows the temperature-independent behavior at high temperature because the intrinsic excitation gradually becomes dominant at 573-673 K, which is similar to the transport behavior of non-degenerate semiconductor. Figure 6b demonstrates the temperature-dependent S of Cu 3 SbSe 4 -based samples. It can be seen that they are p-type conductivity derived from the positive values of S, and the charge carriers in these compounds are holes [62]. After Bi-doping, the S first decreases and then rises with increasing temperature, achieving a minimum S, which should be attributed to the a-b phase transition of Cu 2-x Se phase [37,56]. Although the increased hole concentration (Tab. S1) deteriorates S, Bi-doping can enhance DOS near Fermi level (E f ) and enlarge E g according to the DFT calculations, resulting in the relatively large S [46]. Figure 6c presents the temperature-dependent PF of Cu 3 SbSe 4 -based samples. It can be seen that the PF values of pristine Cu 3 SbSe 4 sample increase with elevating temperature. Besides, PF values of Bidoped compounds decrease slightly and then increases with rising temperature, and are higher than that of pristine Cu 3 SbSe 4 at high temperatures, which is attributed to the enhancement of electrical conductivity after Bi-doping. However, excessive Bi content (x C 0.02) does not further enhance PF values due to the deteriorated Seebeck coefficient. The maximum PF value of * 1000 lWm -1 K -2 is achieved at 673 K for Cu 3 Sb 0.985 Bi 0.015 Se 4 compound.
The temperature-dependent thermal transport properties of the Cu 3 SbSe 4 -based samples are displayed in Fig. 7. The D decreases with the increase of temperature and the room temperature D drops from * 1.7 mm 2 s -1 for pristine Cu 3 SbSe 4 to * 1.45 mm 2 s -1 for Cu 3 Sb 0.97 Bi 0.03 Se 4 sample after Bi-doping (Fig. 7a). As presented in Fig. 7b, the j tot reduces with rising temperature within the range of measurement due to the enhanced lattice vibration scattering at high temperatures [10,63]. Besides, the j tot of all the Cu 3 SbSe 4 -based samples decreases after Bidoping, drops from * 3.11 Wm -1 K -1 at 300 K and * 1.03 Wm -1 K -1 at 673 K for pristine Cu 3 SbSe 4 to * 2.81 Wm -1 K -1 at 300 K and * 0.69 Wm -1 K -1 at 673 K for Cu 3 Sb 0.985 Bi 0.015 Se 4 sample. Generally, the j lat can be calculated by subtracting the electronic part j ele from the j tot . According to the Wiedemann-Franz law Eq. (1): where L is the Lorenz number and can be calculated from Eq. (2) [48,64]: The calculated L values of the Cu 3 SbSe 4 -based samples are presented in Fig. S3 and L ranges from 1.51 to 2.13 W X K -2 . The increased j ele of Bi-doped compounds is shown in Fig. 7c, which can be closely related to electrical conductivity derived from the rising carrier concentration and high carrier mobility. As seen in Fig. 7d, the prominent scattering mechanism of Cu 3 SbSe 4 -based samples is the phonon-phonon Umklapp scattering, which can be inferred from the relationship of j lat µ T -1 . As expected, the Bidoping can decrease j lat in the measured temperature range. On one hand, the isovalent substitutions on Sb can reduce j lat because of stronger mass and strain field fluctuations, which is similar to the case that In Sb point defects in Cu 3 SbSe 4 can scatter high frequency phonons [40]. On the other hand, the heavy element Bi has much larger ionic radius in comparison with Sb, leading to the reduced j lat . The heat-carrying acoustic phonons can be blocked by softening the chemical bonding interaction in crystal due to the expansion of unit cell [47]. Furthermore, the impurity phase Cu 2 Se and amorphous phase with extremely low j lat can also decrease j lat of Bi-doped Cu 3 SbSe 4 [48,56]. Consequently, the reduction of j tot should be attributed to the depressed j lat caused by the strengthened phonon scattering of multiscale defects involving point defects, nanoprecipitates, amorphous phases and grain boundaries [65][66][67]. The lowest j lat * 0.53 Wm -1 K -1 and j tot * 0.62 Wm -1 K -1 is achieved at 673 K for the Cu 3 Sb 0.98 Bi 0.02 Se 4 sample.
The temperature-dependent ZT of Cu 3 SbSe 4 -based samples with various Bi concentration is presented in Fig. 8a. Benefiting from the synergistically optimized electrical and thermal transport properties, the ZT values obtain an enhancement at high temperature for Bi-doped compounds, which can reach the maximum ZT value of * 0.95 at 673 K for the Cu 3-Sb 0.985 Bi 0.015 Se 4 sample. To further analyze our experimental results, the previous ZT max values of other single-doped Cu 3 SbSe 4 samples are listed in Fig. 8b [13,[39][40][41][42][43][44][45][46]. Accordingly, isovalent heavy element Bi-doping in Cu 3 SbSe 4 compounds can simultaneous optimize the electrical and thermal parameters, which is highly effective in improving TE properties.

Conclusions
In summary, the improvement of the overall TE properties in p-type Cu 3 SbSe 4 was realized by Bidoping. The results of XRD, XPS, SEM and TEM analysis indicate that Bi-doped samples consist of Cu 3 SbSe 4 with Cu 2-x Se impurity phases. The q of all the isovalent Bi-doped samples is lower than that of pristine Cu 3 SbSe 4 because the enhanced carrier concentration, and a high power factor of * 1000 lWcm -1 K -2 is achieved at 673 K for the Cu 3 Sb 0.985-Bi 0.015 Se 4 sample. In addition, the multiscale defects of Cu 3 SbSe 4 -based materials involving point defects, nanoprecipitates, amorphous phases and grain boundaries caused by Bi-doping can strongly scatter phonons to depress j lat . Consequently, the thermoelectric properties improve significantly and the maximum ZT value of * 0.95 is achieved at 673 K for the Cu 3 Sb 0.985 Bi 0.015 Se 4 sample, which is * 1.9 times higher than that of pristine Cu 3 SbSe 4 . The present Fig. 7 Temperature-dependent a specific heat capacity C p , b total thermal conductivity j tot , c electronic thermal conductivity j ele , d lattice thermal conductivity j lat of Cu 3 SbSe 4 -based samples work presents a selectable route to enhance the comprehensive TE properties by isovalent heavy element doping for other TE materials, which is also important to TE device application.