Silicon As an Unexpected n-Type Dopant in BiCuSeO Thermoelectrics

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Silicon as an unexpected n-type dopant in BiCuSeO thermoelectrics Jiahong Shen, and Yue Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06872 • Publication Date (Web): 03 Aug 2017 Downloaded from http://pubs.acs.org on August 6, 2017

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ACS Applied Materials & Interfaces

Silicon as an unexpected n-type dopant in BiCuSeO thermoelectrics Jiahong Shen1,2 and Yue Chen1,* 1. Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China 2. Department of Materials Science, Fudan University, 220 Handan Road, Shanghai 200433, China

Supporting Information Placeholder ABSTRACT: As a promising thermoelectric material,

BiCuSeO is of great interest for energy conversion. A higher figure of merit in n-type BiCuSeO than that in the p-type was predicted from theory, suggesting a need of in-depth investigations on the doping effects. In this work, the influences of group IV elements (Si, Ge, Sn, and Pb) on the electronic structures of BiCuSeO are studied from first principles. Despite the similar electronegativities of the group IV elements, Si is found to be an n-type dopant, being distinctly different from Ge, Sn, and Pb, which exhibit typical p-type behaviors. Detailed analysis on the doping effects is performed based on a recently developed band unfolding technique. Furthermore, Si-doped BiCuSeO is shown to have a higher power factor than p-type BiCuSeO from the Boltzmann transport theory.

sistance. Nonetheless, the ZT values of many widely studied TE oxides, including SrTiO36, In2O37, and Ca3Co4O98 remain relatively low.

KEYWORDS: thermoelectrics, defect formation energy, electronic structure, charge density, Boltzmann transport theory

Thermoelectric (TE) materials play an important role in energy conservation. Their applications including refrigeration and thermal power generation1 provide a prospective solution to environmental issues related to energy. The TE performance is determined by the dimensionless figure of merit (ZT), defined as ZT = (S2σT)/κ, where S, σ, κ, and T are Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively2. Several classes of bulk materials have been demonstrated to exhibit high ZT values, including PbTe3, BiSbTe4, and Bi2Te35; however, the applications of these materials are restrained due to the limitations in chemical and thermal stabilities at high temperatures. Thermoelectric oxides display high thermal and chemical stabilities, and good oxidation re-

Figure 1. (a) Primitive unit cell of BiCuSeO with a layered crystal structure. (b) Theoretical power factor of pristine BiCuSeO normalized by the relaxation time as a function of carrier concentration. (c) Formation energies of different doped BiCuSeO systems calculated from density functional theory.

Recently, the layered quaternary oxyselenide BiCuSeO with its naturally layered structure (see Fig. 1a), was found to be a good TE material in high temperature range due to its intrinsically low lattice thermal conductivity (~0.45 Wm-1K-1 at 923 K9). Different approaches have been conducted to further improve the energy conversion efficiency of BiCuSeO, including the introduction of Cu vacancies10, carrier concentration modification11 and band gap tuning12. A high ZT value of 1.4 was reported by Sui et al13 in Ba-doped BiCuSeO at 923 K. Most studies availbale in the literatures focus on p-type

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BiCuSeO; e.g., the doping of alkali metals (Na11 and K14), alkaline earth metals (Mg15, Ca9, Sr16, and Ba13), Pb17, La12, and Ni18 on the Bi sublattice site, the doping of Zn19, Cd19, and Ag20 on the Cu sublattice site, and the doping of Te21 on the Se sublattice site. N-type BiCuSeO is relatively less studied, but Yang et al22 predicted that its ZT value can be higher than p-type BiCuSeO by 32% because of the increase of power factor. BiCuSeO adopts a crystal structure with the tetragonal P4/nmm space group. The fluorite-like (Bi2O2)2+ layers and the (Cu2Se2)2- layers are alternatively stacked along the c axis of the unit cell23. As the (001) plane of BiCuSeO has the lowest surface energy, c-axis-oriented growth of BiCuSeO has been achieved in experiments17. We have calculated the electrical transport properties along the caxis from Boltzmann transport theory. It is seen from Fig. 1b that the power factor of n-type BiCuSeO is much higher than that of p-type BiCuSeO. Here, we investigate the doping effects of group IV elements (Si, Ge, Sn, and Pb) on the electrical transport properties and the electronic structures of BiCuSeO. It was found in experiments24 that Pb exhibits a +2 charge state and Bi shows a +3 charge state in BiCuSeO; thus, the substitution of Pb for the Bi atom leads to a p-type doping behavior. Given that the electronegativities of group IV elements are similar (1.916, 1.994, 1.824, and 1.854 in Pauling units25 for Si, Ge, Sn, and Pb, respectively), it is expected that they all have p-type doping behaviors when they occupy the same sublattice site in BiCuSeO. Interestingly, Si is found to be a promising ntype dopant whereas other elements in this group lead to a p-type semiconductor as expected. Effective band structures and charge density distributions are calculated to provide insight into the different doping effects. Electrical transport properties of the doped systems are also calculated and compared to available experimental measurements. The lattice constants of BiCuSeO obtained from our calculations are a = 3.955 Å and c = 9.083 Å, and the experimental values are a = 3.928 Å and c = 8.929 Å26. It is well-known that the generalized gradient approximation (GGA) usually overestimates the lattice constants of solids27; thus, the small differences between our theoretical lattice constants and the experimental values are reasonable. To investigate the doping effects, one atom in a 3×3×1 supercell is substituted, corresponding to a doping concentration of 5.56%. The formation energy is calculated according to the following equation28 to determine the preferable substitutional lattice site: ∆ ∆

where R represents the doping elements (Si, Ge, Sn, or Pb) and X represents the different sublattice sites (Bi, Cu, Se, or O). and are the total energies of the R-doped supercell and the pristine supercell, respectively. and are the energies of one R atom and one X

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atom in their respective bulk phases. represents the charge states. refers to the energy at the top of the valence band of the pristine system and ∆ is the Fermi energy relative to . ∆ is the correction term to aligh the reference potentials of pristine and doped BiCuSeO. The results corresponding to neutral charge states (q=0) are shown in Fig. 1c; it is seen that all doping elements tend to occupy the Bi site because of the lowest formation energies. With a similar atomic radius to Bi, Pb has a relatively low formation energy comparing to Sn and Ge. Interestingly, Si-doped system has the lowest formation energy among the four elements. The valence states of Bi, Cu, Se and O atoms in BiCuSeO are +3, +1, -2 and -2, respectively. The common valence state of both Si and Ge is +4, while the common valence states of both Sn and Pb are +4 and +2. Thus, we have considered the charge states of +1, 0 and -1 when the dopants occupy the Bi site and the charge states of +3, +1 and 0 when the dopants occupy the Cu site. The Se and O sites are not considered for charged dopants because these sites are unlikely to be occupied due to their negative charges, which is also evidenced in the neutral defect formation energy calculations. It is seen from Fig. 2 that the defect formation energies corresponding to the Bi site are always lower than those corresponding to the Cu site for all doping systems; therefore, the Bi site is more likely to be occupied by the dopants regardless of the charge states and chemical potentials. In the following discussion, we focus on the systems in which the doping atoms occupy their most preferable lattice site.

Figure 2. Formation energies of (a) Si-doped, (b) Ge-doped, (c) Sn-doped and (d) Pb-doped BiCuSeO corresponding to different charge states of dopants. The solid lines represent the occupations of the Bi site, and the dashed lines represent the occupations of the Cu site.

Electronic structures are calculated to analyze the doping effects of group IV elements. As can be seen in Fig. 3a, Ge-, Sn-, and Pb-doped BiCuSeO have a similar density of states (DOS) with Fermi level moving slightly into the valence band, indicating that these elements are p-type dopants. This agrees with the expectation that Ge and Sn have similar doping effects with Pb because they


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have comparable electronegativities and they all tend to occupy the Bi sublattice site. We choose Pb-doped BiCuSeO as the representative p-type system since it has already been synthesized in experiments24. Most interestingly, the DOS of Si-doped BiCuSeO is distinctly different from other systems (see Fig. 3b); Fermi level moves into the conduction band, leading to an n-type semiconductor. The n-type behavior of Si-doped BiCuSeO indicates that Si may exhibit a +4 charge state, in comparison with the Pb-doped system where Pb exhibits a +2 charge state24. Because of the similar electronegativities of the group IV elements, the distinctly different doping effects of Si are unexpected and require further investigations. To unveil the detailed modifications to the band dispersions, we have calculated the unfolded effective band structures of Pb and Si-doped BiCuSeO, as shown in Fig. 3c and 3d. The conduction band minimum (CBM) of both systems is between the Γ and Ζ points, while the valence band maximum (VBM) is in the Μ-Γ and Z-R directions. In Pb-doped system, the energy of VBM exceeds Fermi level, corresponding to the p-type behavior observed in DOS. The effective band structures of Sn- and Ge-doped BiCuSeO are similar with that of the Pb-doped system (see Supporting Information). From the effective band structure of the Si-doped system, it is seen that Fermi level is higher than the CBM, which is in consistent with the n-type behavior observed in DOS. In addition, the weights of electronic bands near Fermi level are significantly modified due to the introduction of Si impurity in BiCuSeO, indicating changes in the electrical transport properties.

Bader charge analysis29 is performed to study the charge states of different atoms. As can be seen from Table 1, Ge, Sn, and Pb donate fewer electrons than Bi, rationalizing the p-type behavior of the doped systems where holes dominate the electrical transport. On the other hand, Si donates more electrons than Bi, indicating that electrons are the majority carriers and the system becomes an n-type semiconductor. The Bader charges of Bi, Cu, Se, and O atoms are less affected by the doping of Ge, Sn, or Pb comparing to Si; particularly, Si induces a large change in the Bader charge of O, which suggests that Si may have a strong interaction with the O atom.

Figure 3. Density of states of (a) Ge-, Sn-, and Pb-doped BiCuSeO and (b) Si-doped BiCuSeO. Effective band structures of (c) Pb-doped and (d) Si-doped BiCuSeO. Fermi level is located at 0 eV

Figure 4. Charge density distributions in Pb-doped BiCuSeO; (a) top view, (b) side view, and (c) the (001) plane crossing the oxygen atomic layer. Charge density distributions in Si-doped BiCuSeO; (d) top view, (e) side view, and (f) the (001) plane crossing the oxygen atomic layer. The isosurface level is 0.12 e/Å3.

Table 1. Bader charge of different dopants and of the Bi, Cu, Se, and O atoms nearest to the doping atoms.

pristine Si-doped Ge-doped Sn-doped Pb-doped

Bi 1.59 1.54 1.60 1.60 1.60

Cu 0.30 0.31 0.32 0.31 0.31

Se -0.79 -0.85 -0.74 -0.76 -0.78

O -1.10 -1.31 -1.11 -1.12 -1.10

R(dopants) 2.80 1.19 1.27 1.15

The total charge density distributions of Pb- and Sidoped BiCuSeO are shown in Fig. 4. These two systems have similar charge density distributions around the Bi, Cu, and Se atoms, in agreement with the Bader charge analysis. Nonetheless, the charge densities around the O atoms are significantly different in Pb- and Si-doped BiCuSeO; a stronger chemical bond is found to exist between the Si and O atoms while a weaker interaction is observed between the Pb and O atoms. The Si and O interaction is so strong that the Si atom is attracted to the oxygen layer, rationalizing the lowest formation energy of Si-doped BiCuSeO. It can be clearly seen from Fig. 4f that these strong Si-O bonds result in a transfer of electrons from Si to O, corresponding to the increase of Bader charge of the O atom.



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For 5.56% Pb-doped BiCuSeO, we obtain a carrier concentration of 7.82 10 cm-3, which agrees reasonably with the experimental result24 of 8.39 10 cm-3 for a 5% doping concentration. Based on the relation " #$ %& '%&/) , we know that relaxation time " decreases as temperature T increases and " is proportional to '%&/) , in which ' represents the carrier concentration30. The experimental electrical conductivity24 of 5% Pb doping concentration is 1.11 10+ Ωm%& at 700 K. Comparing this value to the electrical transport properties calculated from Boltzmann transport theory, we obtain " 7.392 10%. $ %& '%&/) ; inserting this value to the above equation, we determine the value of C, which is 7.392 10%. Kscm%& . Based on the expression of relaxation time, it is seen from Fig. 5a that our theoretical electrical conductivity and Seebeck coefficient are in good agreements with experiments.

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trations. The predicted increase of power factor is promising for achieving high performance n-type BiCuSeO. In summary, the doping effects of group IV elements (Si, Ge, Sn, and Pb) on the electronic structures and electrical transport properties of BiCuSeO are investigated from first-principles calculations. The site preferences of dopants are determined using the defect formation energies and it is found that all of these dopants tend to occupy the Bi sublattice site regardless of the charge states and chemical potentials. The doping effects of Si are found to be distinctly different from other elements from group IV; Si is found to be an n-type dopant in BiCuSeO while other elements (Ge, Sn, and Pb) are p-type dopants. A recently developed band unfolding technique is utilized to obtain the effective band structures of the doped systems; significant band modifications are observed in Si-doped BiCuSeO near Fermi level. From Bader charge analysis and charge density distributions, we rationalize the different doping effects of Si. Despite the similar electronegativities of the group IV elements, they exhibit different charge states in BiCuSeO. Electrical transport properties are calculated using the Boltzmann transport theory and Si-doped BiCuSeO is found to be a promising n-type thermoelectric material with an enhanced power factor. ASSOCIATED CONTENT

Figure 5. (a) Seebeck coefficient and electrical conductivity of Pb-doped polycrystalline BiCuSeO at 700 K together with the experimental results24 (b) Absolute values of the Seebeck coefficients as functions of carrier concentration at 700 K (dashed lines) and 900 K (solid lines). (c) Electrical conductivities normalized by the relaxation time at different carrier concentrations. (d) Power factors normalized by the relaxation time.

The electrical transport properties along the c-axis are shown in Fig. 5b-5d. It is seen that the absolute value of Seebeck coefficient of Si-doped n-type BiCuSeO is lower than those of the pristine and Pb-doped p-type BiCuSeO. On the other hand, the electrical conductivity normalized by relaxation time of Si-doped BiCuSeO is much higher than those of the Pb-doped and pristine BiCuSeO (see Fig. 5c). As can be seen from Fig. 3c and 3d, the effective mass of the top of the valence band is larger than that of the bottom of the conduction band; therefore, the carrier mobility in Si-doped BiCuSeO is higher. Because conductivity is proportional to carrier mobility, the Si-doped BiCuSeO has a higher conductivity. The power factors of these systems are compared in Fig. 5d; it is seen that the Si-doped BiCuSeO has a larger power factor over a broad range of carrier concen-

Supporting Information. Computational details, formation energies, DOS and band structures from full relaxations (Fig. S1); effective band structures and charge density distributions of pristine, Sn-, and Ge-doped BiCuSeO (Fig. S2); effective band structures of Si-doped BiCuSeO at different doping concentrations (Fig. S3). The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author

[email protected] Funding Sources

Early Career Scheme of RGC

ACKNOWLEDGMENT We are grateful for the financial support from RGC under project numbers 27202516 and 17200017, and the research computing facilities offered by ITS, HKU.

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(a) Primitive unit cell of BiCuSeO with a layered crystal structure. (b) Theoretical power factor of pristine BiCuSeO normalized by the relaxation time as a function of carrier concentration. (c) Formation energies of different doped BiCuSeO systems calculated from density functional theory. 79x78mm (600 x 600 DPI)



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Formation energies of (a) Si-doped, (b) Ge-doped, (c) Sn-doped and (d) Pb-doped BiCuSeO corresponding to different charge states of dopants. The solid lines represent the occupations of the Bi site, and the dashed lines represent the occupations of the Cu site. 469x303mm (72 x 72 DPI)


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Density of states of (a) Ge-, Sn-, and Pb-doped BiCuSeO and (b) Si-doped BiCuSeO. Effective band structures of (c) Pb-doped and (d) Si-doped BiCuSeO. Fermi level is located at 0 eV. 74x76mm (600 x 600 DPI)



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Charge density distributions in Pb-doped BiCuSeO; (a) top view, (b) side view, and (c) the (001) plane crossing the oxygen atomic layer. Charge density distributions in Si-doped BiCuSeO; (d) top view, (e) side view, and (f) the (001) plane crossing the oxygen atomic layer. The isosurface level is 0.12 e/Å^3. 59x40mm (300 x 300 DPI)


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(a) Seebeck coefficient and electrical conductivity of Pb-doped polycrystalline BiCuSeO at 700 K together with the experimental results21 (b) Absolute values of the Seebeck coefficients as functions of carrier concentration at 700 K (dashed lines) and 900 K (solid lines). (c) Electrical conductivities normalized by the relaxation time at different carrier concentrations. (d) Power factors normalized by the relaxation time. 61x47mm (600 x 600 DPI)



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Silicon As an Unexpected n-Type Dopant in BiCuSeO Thermoelectrics

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