High thermoelectric performance of BiCuSeO by optimized carrier concentration and point defect scattering through a Cr-induced compositing effect

Abstract

BiCuSeO has intrinsically low electrical conductivity, a high Seebeck coefficient, and low thermal conductivity. Therefore, the improvement of the thermoelectric performance of BiCuSeO mainly focuses on enhancing its electrical conductivity. In this study, we report the high thermoelectric performance of BiCuSeO achieved by optimizing the carrier concentration and phonon scattering through the composite effect induced by Cr addition. The enhanced electrical conductivity coupled with a moderate Seebeck coefficient leads to an almost one order of magnitude improvement of the power factor from ∼0.07 mW m⁻¹ K⁻² for the pristine BiCuSeO to ∼0.57 mW m⁻¹ K⁻² for the BiCu_0.96Cr_0.04SeO sample at 473 K. The Cr addition reduces the lattice thermal conductivity by ∼23%, as confirmed by both experimental results and Debye–Callaway model calculations. The analysis indicates that the lattice thermal conductivity can be reduced to 0.5 W m⁻¹ K⁻¹ at 773 K due to enhanced point defect scattering. The combination of optimized power factor and intrinsically low thermal conductivity yield a relatively high zT. The maximum zT value of ∼0.7 is obtained at 773 K for the BiCu_0.96Cr_0.04SeO sample. This study demonstrates the potential of utilizing the compositing approach for the development of efficient thermoelectric materials based on BiCuSeO oxyselenides.

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Introduction

The first layered Bi-containing oxyselenides were synthesized by L. N. Kholodkovskaya et al. in 1993.¹ Several years later T. Ohtani et al. revealed that BiCuSeO is a degenerate semiconductor with holes as the dominant charge carrier.² In 2010, L. D. Zhao et al. reported that BiCuSeO can be considered as a promising thermoelectric material for mid-temperature applications and its thermoelectric performance can be improved by heterovalent doping.³ High-performance thermoelectric materials can lead to efficient power generation by their modules.⁴ Pristine BiCuSeO has poor electrical conductivity, a high Seebeck coefficient, and low thermal conductivity.⁵ Considering the high Seebeck coefficient and low thermal conductivity, several studies have been conducted to improve the thermoelectric performance of BiCuSeO through charge carrier optimization. The single-doped materials with monovalent (Ag,⁶ Na,⁷ and Cs⁸) ions or divalent (Pb,⁹ Mg,¹⁰ Ca,¹¹ Ba,¹² Cd,¹³ and Sn¹⁴) ions are established for doping in the Bi site. Doping with monovalent or divalent ions can improve the electrical conductivity by increasing the charge carrier concentrations from 10¹⁹ cm⁻³ to 10²¹ cm⁻³. In addition, they can reduce the thermal conductivity by phonon point defect scattering. The point defect scattering depends on the mass and strain difference between monovalent or divalent ions and the host lattice. The large mass difference between Bi and monovalent or divalent ions yields the predominant perturbation effect at a point defect, contributing to reduced lattice thermal conductivity.

Single doping on the Bi site has limitations due to a dramatic decrease in carrier mobility. Several studies have been done to introduce dual doping on the Bi site by using Pb/In,¹⁵ Pb/Ca,¹⁶ Pb/Na,¹⁷ Pb/Ba,¹⁸ and Pb/In,¹⁹ on the Cu site by using Ba/Co,²⁰ Pb/Ag,²¹ Ba/Ni,²² and Pb/Fe,²³ and on the Se sites by using Ba/Te.²⁴ The introduction of Pb doping on the Bi site increases the carrier concentration by producing more holes in the insulating layers. The substitution of Bi³⁺ by Pb²⁺ introduces an additional peak in the density of state (DOS) near the Fermi level. This peak is caused by the delocalized lone-pair 6s orbital.⁹ The introduction of Pb doping can maintain a moderate Seebeck coefficient by enhancing the density of state effective mass. The excessive usage of heavy metals and toxic elements, such as Pb, has the potential to cause environmental issues. To avoid the usage of heavy and toxic metals, various transition metals have been used as dopants in Cu sites, such as Mn,²⁵ Fe,²³ Co,²⁶ Ni,²² and Zn.²⁷ The incorporation of Zn into the Cu site has been demonstrated to effectively enhance the carrier concentration by a factor of two orders of magnitude. As a result, the zT_max of 0.9 was achieved at 873 K for BiCu_0.9Zn_0.1SeO, which is approximately 39% larger than that of the undoped BiCuSeO.²⁷ Similarly, simultaneous substitution of Bi with Ba and Ni doping at the Cu site was reported to lead to an increase in electrical conductivity by enhancing carrier concentration. Moreover, the spin entropy introduced by magnetic ion incorporation into the lattice has significantly improved the Seebeck coefficient by 173%. The zT_max of 0.97 was achieved for Bi_0.875Ba_0.125Cu_0.85Ni_0.15SeO at 923 K.²² Transition metal doping significantly enhanced the electrical conductivity and increased the Seebeck coefficient, leading to an improved power factor.^28,29 This strategy seems promising due to the intrinsically low thermal conductivity of BiCuSeO.

In this report, we present the synthesis of a high-performance BiCuSeO prepared using an additional transition metal in the system. A transition metal element, such as Cr, was selected as an addition to the Cu site to improve the electronic transport properties. The Cr addition can modify the carrier concentration, which raises the electrical conductivity while keeping a moderate Seebeck coefficient. This results in a higher power factor. The role of Cr addition is not only increasing the power factor but also decreasing lattice thermal conductivity. A decrease in lattice thermal conductivity is dominated by point defect scattering. The maximum zT value has been achieved in the BiCu_0.96Cr_0.04SeO sample around ∼0.7 at 773 K.

Materials and methods

Polycrystalline samples with nominal composition of BiCu_1−xCr_xSeO (x = 0, 0.02, 0.04, 0.06, 0.08) were prepared by solid-state reaction followed by spark plasma sintering (SPS). The raw materials of Bi₂O₃ nanopowder (90–210 nm particle size, 2 N, Sigma Aldrich), Bi (2 N, Sigma Aldrich), Cu (2 N, Sigma Aldrich), Cr (2 N, Sigma Aldrich), and Se shot (2–6 mm, 5 N, Alfa Aesar) were weighed according to the stoichiometric ratio. The raw materials were mixed in an agate pestle and mortar, then pressed into pellets under 20 MPa in a steel die. The pellets were sealed in quartz tubes under a vacuum of ∼10⁻³ Pa. Prior to use, the quartz tube had been baked in a dry box for 1.5 h at 423 K to ensure cleanliness. The ampoules were heated to 673 K at a rate of 2 K min⁻¹, then dwelled for 10 h to ensure that the reaction between chromium and other metals proceeded completely. Afterwards, the temperature slowly raised to 973 K at a rate of 1 K min⁻¹ and held at this temperature for 20 h before cooling to room temperature naturally. The obtained ingots were hand-ground into fine powders and densified using SPS (Dr Sinter-1080 SPS system, Fuji-SPS, Japan) at 973 K for 5 min under a uniaxial pressure of 50 MPa in an argon atmosphere. Finally, dense sintered pellets with a thickness of ∼4 mm and a diameter of 10 mm were obtained.

The X-ray diffraction (XRD) patterns were collected using X-ray diffraction (XRD) on a Rigaku Mini-Flex, Japan, with Cu-K_α radiation (λ = 1.54060 Å). The obtained data were analyzed using GSAS-II software.³⁰ The microstructure and compositional analysis of bulk samples were characterized by field-emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray spectroscopy (EDS) on an ultra-high resolution HRSEM SU8230 (Hitachi, Japan) in conjunction with an X-MaxN Horiba EDS detector (Oxford Instruments, UK). Samples for EDS were embedded in a conductive resin and polished with standard metallography procedures. The electrical resistivity (ρ) and Seebeck coefficient (S) were measured in a low-pressure helium atmosphere on a commercial thermoelectric measurement system, ZEM-2 (Ulvac-Rico, Japan) from 323 K to 773 K. The typical dimensions of samples for the electrical resistivity and Seebeck coefficient measurement are about 2 × 2 × 9 mm³. The thermal diffusivity coefficient (D) was measured using the laser flash method (LFA427, Netzsch, Germany) in the same temperature range. The total thermal conductivity (κ) was calculated from the equation κ = DC_pd, where C_p is the specific heat calculated using the Debye model, and d is the density measured by the Archimedes method. The electronic thermal conductivity (κ_e) is estimated using the Wiedemann–Franz law (κ_e = LσT), where L is derived by calculating the reduced Fermi energy (η) and assuming acoustic phonon scattering (APS) as the main carrier scattering mechanism.^31,32 The pieces of 8.4 mm × 8.4 mm × 0.5 mm were used for Hall coefficient (R_H) measurements under a reversible magnetic field of 0.52 T (ResiTest 8300DC, Tokyo, Japan). The Hall carrier concentration (n_H) and mobility (μ_H) were calculated using n_H = 1/(eR_H) and μ_H = σR_H, respectively. The extended uncertainty measurement estimates for electrical resistivity, Seebeck coefficient, thermal diffusivity, total thermal conductivity, and density are 5%, 5%, 5%, 10%, and 2%, respectively. The combined uncertainty for all measurements involved in zT determination is below 20%.

Results and discussion

Phase composition and microstructure

Fig. 1(a) shows the normalized XRD patterns of the BiCu_1−xCr_xSeO (x = 0, 0.02, 0.04, 0.06, 0.08) samples at room temperature. All the diffraction patterns of the obtained BiCu_1−xCr_xSeO samples correspond to tetragonal BiCuSeO (PDF card # 67-00327) with P4/nmm space group. There are no additional peaks of secondary phases identified within the XRD detection limit for samples with x ≤ 0.06, indicating the high purity of our samples. However, an additional peak appears at 2θ = 29.4° in the samples of x = 0.08, indicating the presence of a minor secondary phase. The main peaks are slightly shifted to the higher angle 2θ with the increase of Cr content, as shown in Fig. 1(b). All the Rietveld refinements can be found in the ESI,† as shown in Fig. S1. The lattice parameters of the BiCu_1−xCr_xSeO samples decrease slightly with the increase of Cr content, as shown in Table 1. The lattice parameter reduction is most likely associated with the formation of copper vacancies (or ‘with Cu deficiency’) in the BiCuSeO matrix, while the majority of Cr appears to form the CrO secondary phase. The decrease in lattice parameters is consistent with Y. Liu et al. when Cu deficiency is introduced into BiCuSeO.³³

Fig. 1 (a) The XRD patterns of the BiCu_1−xCr_xSeO (x = 0, 0.02, 0.04, 0.06, 0.08) samples at room temperature, and (b) the enlarged section of (a) in a 2θ range from 29° to 31°.

Table 1 Lattice parameters of the BiCu_1−xCr_xSeO samples, atomic positions of Bi and Se in the z-axis, and occupancy of Cu in the unit cell obtained from Rietveld analysis using GSAS-II

x	a (Å)	c (Å)	z (Bi)	z (Se)	Occupancy (Cu)	Vol. (Å)^3	GoF	RW	RF
a Standard deviations are given in parentheses and refer to the estimated errors in the least significant units.
0	3.9296(3)^a	8.9296(7)	0.1405	0.6760	1	137.8(9)	1.17	5.68	4.55
0.02	3.9295(2)	8.9299(7)	0.1399	0.6760	0.97	137.8(9)	1.36	6.42	2.49
0.04	3.9293(4)	8.9297(1)	0.1408	0.6767	0.96	137.8(7)	1.58	6.97	4.11
0.06	3.9287(2)	8.9279(7)	0.1405	0.6767	0.93	137.8(3)	1.34	6.38	3.82
0.08	3.9288(6)	8.9299(5)	0.1402	0.6758	0.90	137.8(4)	1.31	6.24	2.55

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Fig. 2(a) and (b) display the SEM image of the fractured surface of BiCu_1−xCr_xSeO with x = 0 and x = 0.04, respectively. The SEM image of pristine BiCuSeO illustrates a variety of grain shapes and sizes, while the BiCu_0.96Cr_0.04SeO sample shows a lath-like microstructure with randomly arranged platelet grains stacked densely. The average grain size of the pristine BiCuSeO and BiCu_0.96Cr_0.04SeO samples is approximately 2.1 μm and 1.8 μm, respectively, as shown in Fig. S2 (ESI†). Fig. 3 shows the SEM and EDS mapping of the polished surface of the BiCu_0.96Cr_0.04SeO sample. The Bi, Cu, and Se elements are homogeneously distributed in the BiCuSeO matrix. The BiCu_0.96Cr_0.04SeO sample shows an uneven distribution of Cr and O elements. The Cr and O elements are segregated to form a chromium oxide. The point mapping and EDX composition analysis on the Cr-rich area of the BiCu_0.96Cr_0.04SeO confirms that the chromium oxide forms a composite in the BiCuSeO matrix even at low Cr concentrations, as shown in Fig. S3 (ESI†).

Fig. 2 The SEM images of the fractured sample of BiCu_1−xCr_xSeO (a) x = 0 and (b) x = 0.04 samples.

Fig. 3 The elemental mapping of the polished surface of the BiCu_0.96Cr_0.04SeO sample.

Thermoelectric properties

The electrical conductivity (σ) for the BiCu_1−xCr_xSeO (x = 0, 0.02, 0.04, 0.06, 0.08) samples as a function of temperature is illustrated in Fig. 4(a). The pristine BiCuSeO has a very low electrical conductivity, as reported in various papers.^7,11,34 It has a continuously increasing tendency as the temperature rises, indicating the semiconductor behavior. The electrical conductivity of the BiCu_1−xCr_xSeO (x = 0.02, 0.04, 0.06, 0.08) samples increases with temperature, reaching a maximum at about 473 K, and subsequently decreases at higher temperatures. When the temperature increases above 473 K, the phonon contribution becomes more significant, resulting in increased atomic vibration, reduced carrier mean free path, and decreased electrical conductivity. Table 2 shows the carrier concentration and mobility at room temperature as a function of nominal composition. The carrier concentration increases at a composition of x = 0.02 and x = 0.04, yielding an enhancement in electrical conductivity. However, at a composition of x = 0.06 and x = 0.08, the decreased carrier concentration results in lower electrical conductivity.

Fig. 4 Thermoelectric properties as a function of temperature of the BiCu_1−xCr_xSeO (x = 0, 0.02, 0.04, 0.06, 0.08) samples, (a) electrical conductivity, (b) Seebeck coefficient, (c) the weighted mobility (the dash-line represent the trend of μ_w ∼ T^−1.5), (d) power factor (PF), (e) total thermal conductivity and (f) lattice thermal conductivity (κ_L = κ − κ_e).

Table 2 Carrier concentration, carrier mobility, and density of the BiCu_1−xCr_xSeO samples at room temperature

x	n (10^19 cm^−3)	μ (cm^2 V^−1 s^−1)	ρ (g cm^−3)
0	0.596	4.979	8.453
0.02	1.298	1.412	8.454
0.04	5.683	0.223	8.459
0.06	2.128	0.293	8.673
0.08	1.846	0.338	8.421

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The increase in carrier concentration is commonly observed during aliovalent doping. The substitution of Cu⁺ by Cr²⁺ is theoretically expected to follow the equation, . It was hypothesized that the substitution of Cu⁺ by Cr²⁺ should produce more electrons than holes, resulting in a decrease in electrical conductivity and an increase in the Seebeck coefficient due to the charge carrier.^22,23 However, the observed mechanism in our system does not align with this hypothesis. Considering data from XRD, EDS and Hall measurements, the explanation for this discrepancy lies in the possibility that unlike Fe and Ni,^22,23 Cr is not incorporated into the lattice at the Cu site; instead, it forms a secondary phase based on Cr and O, which leads to the formation of O and Cu vacancies in the main BiCuSeO matrix. Even at the lowest Cr concentration, the Cr tends to form nano- to microscale precipitates of CrO, indicating very low solubility of Cr in the BiCuSeO matrix (see Fig. S4, ESI†). The CrO precipitates will create additional holes in the oxide layer due to the presence of oxygen vacancies and in the conducting layer due to a copper deficiency. The additional holes increase the carrier concentration in the system. Upon further increase in Cr concentration, the small amount of Cr may occupy Cu sites, while the majority of Cr forms the CrO secondary phase. As a result, the tendency of CrO to provide holes is suppressed by the introduction of electrons, which slightly decreases the carrier concentration. The introduction of Cr in BiCu_1−xCr_xSeO can be illustrated by eqn (1):

where V⁻_Cu is copper vacancies, V²⁺_O is oxygen vacancies, and h⁺ is the produced holes introduced by the compositing effect of Cr addition.

Fig. 4(b) shows the temperature dependence of the Seebeck coefficient (S) of the BiCu_1−xCr_xSeO (x = 0, 0.02, 0.04, 0.06, 0.08) samples. The positive value of the Seebeck coefficient indicates that all samples are p-type semiconductors. The pristine BiCuSeO exhibits a high value of Seebeck coefficient of around 425–500 μV K⁻¹ within a temperature range of 325–773 K. This value is higher than that reported in previous studies on pristine BiCuSeO.^7,11,34 The pristine BiCuSeO is sensitive to the synthesis technique, mainly due to the different number of copper vacancies formed in the system.³⁵ The Seebeck coefficient decreases from ∼484.5 μV K⁻¹ in pristine BiCuSeO to ∼305.4 μV K⁻¹ in BiCu_0.94Cr_0.06SeO at 473 K. This reduction in Seebeck coefficient is consistent with the enhancement in carrier concentration as the Cr content increases. As illustrated in Fig. S5 (ESI†), the electrical and thermal properties of the synthesized samples demonstrate good reproducibility.

The electronic transport properties of the material are well explained by its weighted mobility. The weighted mobility is calculated by eqn (2) using the experimental data on electrical conductivity and Seebeck coefficient proposed by J. Snyder et al., which are expressed as follows:³⁶

where h, e, σ, k_B, m_e, S, and T represent the Planck constant, the charge carrier, the experimental electrical conductivity in Ω⁻¹ cm⁻¹, the Boltzmann constant, the mass of electrons, the experimental Seebeck coefficient, and the absolute temperature, respectively. Fig. 4(c) shows the weighted mobility of the BiCu_1−xCr_xSeO (x = 0, 0.02, 0.04, 0.06, 0.08) as a function of temperature. The μ_w of the Cr addition samples follows the ∼T^−1.5 tendency, indicating that the acoustic–phonon scattering dominates across the entire temperature range.

Fig. 4(d) shows the power factor (PF) of the BiCu_1−xCr_xSeO (x = 0, 0.02, 0.04, 0.06, 0.08) samples. The BiCu_0.96Cr_0.04SeO sample shows high electrical conductivity, while maintaining its Seebeck coefficient of around 350–425 μV K⁻¹ in the whole temperature range. The power factor of the BiCu_0.96Cr_0.04SeO sample is the highest compared to all samples over the entire temperature range. The maximum power factor of the BiCu_0.96Cr_0.04SeO is around ∼0.60 mW m⁻¹ K⁻² at 473 K. This value is higher than that of Mg,³⁴ Ca,¹¹ Fe,²⁵ Sb,³⁷ La,³⁸ and Pr³⁹ doped BiCuSeO at the same temperature.

The total thermal conductivity of the pristine BiCuSeO sample decreases with increasing temperature, from ∼1.09 W m⁻¹ K⁻¹ at 300 K to ∼0.55 W m⁻¹ K⁻¹ at 773 K, as shown in Fig. 4(e). The heat capacity and thermal diffusivity data as a function of temperature are shown in Fig. S6(a) and (b) (ESI†). The total thermal conductivity decreases with increased Cr content caused by multiple scattering factors. The maximum reduction in the total thermal conductivity is observed at room temperature for x = 0.06 & 0.08, with a decrease of about 18% compared to the pristine BiCuSeO. In order to investigate the electronic and phonon contributions, we calculate the electronic thermal conductivity using the Wiedemann–Franz relation, κ_e = LσT, where L is Lorenz number. The Lorenz number was derived by eqn (3) and expressed as:^31,32

where F_n(η), η, k_B, e, and r are the n-th order Fermi integral, the reduced Fermi energy, the Boltzmann constant, the electron charge, and the scattering parameter. The calculation details for the Lorenz number can be found in the ESI.† Fig. S7(a) and (b) (ESI†) show the Lorenz number and electronic thermal conductivity as a function of temperature. Based on the calculation, the electronic thermal conductivity of the BiCu_1−xCr_xSeO samples maintains a very low value over the temperature range. The maximum contribution of the electronic part to the total thermal conductivity is less than 2% for all samples. The lattice thermal conductivity decreases as the temperature increases, as shown in Fig. 4(f). In this system, the addition of Cr tends to form a secondary phase of CrO, which introduces a vacancy in the Cu site. The vacancy has a significant effect in reducing the lattice thermal conductivity by point defect scattering and broken chemical bonds.⁴⁰ To understand the role of point defects in reducing the lattice thermal conductivity, theoretical calculations are conducted using the Klemens model, which is defined as the ratio of the lattice thermal conductivity of a material containing a defect to that of the parent material (see ESI†). Here κ_L and u can be calculated using eqn (4) and (5):^41–43where κ_L and κ_L,p are the lattice thermal conductivities of the defected and parent materials, respectively, and the parameter u is defined by:where h, Ω, v_a and θ_D stand for the Planck constant, average volume per atom, lattice sound velocity (v_a = 2107 m s⁻¹),¹¹ and the Debye temperature (θ_D = 243 K).⁴⁴ The disorder parameter (Γ) is understood to be a combination of both mass fluctuation (Γ_M) and strain field fluctuation (Γ_SF).^45,46 However, in some cases, the mass difference will be the dominant effect at a point defect.^11,12,34,47 In this study, we estimated the effect of Cr addition in the lattice thermal conductivity at 300 K by considering the influence of copper vacancies with the virial-theorem treatment for broken bonds as proposed by Gurunathan.⁴⁰ It should be noted that the effect of oxygen vacancies was not included in this calculation. It is assumed that all samples were fabricated by SSR in vacuum conditions and that the oxygen vacancy is present in all samples with insignificant variations.^48,49 The disorder parameter (Γ) is evaluated using mass difference scattering alone based on the fractional occupation of Cu vacancies. The calculation details can be found in the ESI.† This assumption is validated by the good agreement between the calculated and the measured value of the lattice thermal conductivity, as shown in Fig. 5(a). The disorder parameter and the difference between the calculated and measured values of the lattice thermal conductivity at 300 K are listed in Table S2 (ESI†), respectively.

Fig. 5 (a) A comparison of the experimental and the calculated lattice thermal conductivity at 300 K, (b) the dashed line shows the prediction of κ_L using the Debye–Callaway model with the Umklapp process (U), grain boundary (GB) and point defect (PD) scattering mechanisms, (c) the contribution of grain boundary (GB) and point defect scattering (PD) to the reduction of the lattice thermal conductivity for the BiCu_1−xCr_xSeO (x = 0.08) sample is 1.6% and 23%, respectively.

The reduction in lattice thermal conductivity as a function of temperature remains to be investigated. Several factors may contribute to a decrease in lattice thermal conductivity, including the Umklapp process, grain boundary, and point defect scattering. To elucidate the contribution of each scattering mechanism as a function of temperature, the Debye–Callaway model was employed to evaluate the lattice thermal conductivity. Here, the lattice thermal conductivity can be calculated from the following eqn (6) and (7):⁵⁰

τ_tot⁻¹ = τ_U⁻¹ + τ_GB⁻¹ + τ_PD⁻¹ (7)

where τ_tot, τ_U, τ_GB, and τ_PD are the relaxation times for the total phonon, Umklapp scattering, grain boundary scattering and point defect scattering, respectively. The detailed calculations and fitting parameters for this model are provided in the ESI.† As depicted in Fig. 5(b), the calculation of the lattice thermal conductivity using the Debye–Callaway model shows good agreement with the measured values. This model highlights the significant role of various scattering mechanisms on the enhancement of phonon scattering. Fig. 5(c) illustrates the contributions of each scattering mechanism to the reduction of lattice thermal conductivity. The dashed green, red, and blue lines represent the contributions from the Umklapp process (U), the Umklapp process and grain boundary scattering (U + GB), and the Umklapp process, grain boundary, and point defect scattering (U + GB + PD), respectively.

Fig. 5(c) provides a further comparison between the model that excludes point defect scattering (U + GB) and the model that includes it (U + GB + PD). The analysis shows that the Umklapp process and grain boundary scattering contribute about ∼1.6% to the reduction of the lattice thermal conductivity. As expected from the oxide materials, including BiCuSeO, the grain boundary does not significantly influence the phonon transport due to it exhibiting relatively short phonon mean free path.⁵¹ When point defect scattering is included, the calculated lattice thermal conductivity decreases by approximately 23%, which shows good agreement between theoretical and experimental data. The calculation of lattice thermal conductivity shows a discrepancy of more than 10% between the experimental data and the model calculation for T > 600 K. This highlights the necessity for additional scattering mechanisms to be taken into account for model optimization.

The figure of merit (zT = S²σT/κ) of the BiCu_1−xCr_xSeO (x = 0, 0.02, 0.04, 0.06, 0.08) samples as a function of temperature is shown in Fig. 6(a). The zT shows an increasing trend with temperature. The maximum zT value of ∼0.7 is obtained at 773 K for the BiCu_0.96Cr_0.04SeO sample. This value is almost 2.3 times higher than that of pristine BiCuSeO. As seen in Fig. 6(b), this value surpasses that of BiCuSeO doped with transition metals,^22,25,27 Cu deficiency,³³ and composites^52,53 at 773 K when compared to Pb-free doping. This outcome shows that the thermoelectric performance of BiCuSeO oxyselenides is successfully improved through the composite effect induced by Cr addition. Furthermore, our method replaces Pb with a more environmentally friendly element.

Fig. 6 (a) The zT as a function of temperature, (b) the zT of this work compared to Pb-free BiCuSeO with transition metal doping, such as Zn, Ni, and Mn, Cu deficiency, and composite materials using La_0.8Sr_0.2CoO₃ and Cu₂Se at 773 K, respectively.^{22,25,27,33,52,53}

Conclusions

In summary, polycrystalline samples with nominal compositions of BiCu_1−xCr_xSeO (x = 0, 0.02, 0.04, 0.06, 0.08) were synthesized by combining SSR with SPS method. The thermoelectric properties were analyzed in the temperature range from 300 K to 773 K. The results demonstrate that the Cr addition increases the charge carrier concentration yield to improve the electrical conductivity from ∼3.44 Ω⁻¹ cm⁻¹ (x = 0) to ∼52.46 Ω⁻¹ cm⁻¹ (x = 0.04) at 473 K while maintaining a moderate Seebeck coefficient (300–400) μV K⁻¹. By combining electrical conductivity and Seebeck coefficient, the maximum power factor of ∼0.57 mW m⁻¹ K⁻² can be obtained from the BiCu_0.96Cr_0.04SeO sample at 473 K. The formation of a secondary phase simultaneously with Cu vacancies in the BiCuSeO matrix leads to reduce thermal conductivity. Consequently, the BiCu_0.96Cr_0.04SeO sample exhibits a relatively high zT value of about 0.7 at 773 K. Our results demonstrate an improvement in the zT. Moreover, this result can serve as a starting point for further optimization of these compositions, for example, through carrier concentration optimization and additional reduction in thermal conductivity via heterovalent substitution of Bi.

Author contributions

Asep Ridwan Nugraha: conceptualization, data curation, formal analysis, investigation, visualization, writing – original draft, writing – review & editing; Shamim Sk: project administration, writing – review & editing; Andrei Novitskii: data curation, investigation, writing – review & editing; Dedi: resources, supervision, funding acquisition; Fainan Failamani: conceptualization, formal analysis, investigation, supervision, writing – review & editing; Bambang Prijamboedi: conceptualization, investigation, supervision, writing – review & editing; Takao Mori: resources, funding acquisition, supervision, writing – review & editing; Agustinus Agung Nugroho: conceptualization, formal analysis, investigation, supervision, writing – review & editing.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by The Indonesia Endowment Fund for Education (LPDP). T. M., S. S., and A. N. acknowledge JST Mirai JPMJMI19A1. The authors are very grateful to B. D. Napitu from Institut Teknologi Bandung for helpful discussion.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc05369a

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