Enhanced thermoelectric properties of n-type Bi2Te2.7Se0.3 semiconductor by manipulating its parent liquid state

Bin Zhu, Yuan Yu, Xiao-yu Wang, Zhong-yue Huang*, Fang-qiu Zu*

Liquid/Solid Metal Processing Institute, School of Materials Science & Engineering, Hefei University of Technology, Hefei 230009, China

Keywords: Bismuth-Telluride; Thermoelectric-materials; Casting; Liquid state; Energy filtering effect;

1. The effect of KI doping

The temperature dependence of electrical conductivity (σ)for Bi2Te2.7Se0.3 bulk samples with various KI contents (x=0, 0.20, 0.25, 0.30 and 0.35) was measured and plotted in fig1 (a).σ−T curve showedsemiconducting behavior for x=0, i.e.dσ/dT>0, and metallic-like behavior for 0.20≤x≤0.30 in the whole measurement temperature range, i.e. dσ/dT<0; while for x=0.35, thesample showed metallic-like behavior at low temperatureand then semiconducting behavior at high temperature, which may be attributed to the excessive I- doping. The electrical conductivity increased significantly with increasing the TeI4 content, e.g. from about 0.2×105 (x=0) to 0.8×105Sm−1(x=0.30) at 300 K.

The corresponding Seebeck coefficient (S) is shown in fig1 (b). Anti-site defects can generate more readily with holes due to the low formation energy of antisite defects, so the un-doped specimen is p-type semiconductors at room temperature. The decrease of S at higher temperatures is associated with the onset of intrinsic conduction[1-3]. Finally,Schanges from positiveto negative with temperature rising for the Bi2Te2.7Se0.3 alloy without KI. And the effect of intrinsic conduction decreased when the doping amount of KI was increased, thusthe conduction translated to absolute n-type for samples with KI content more than 0.20 wt.%, and |S|at 300K decreases from 174to 144μV/K with the KI content varyingfrom 0.20 to 0.35wt.%. For Bi2Te2.7Se0.3 alloy, σ and S can be expressed as follows [4, 5]:

FigS1. The temperature dependence of a) electrical conductivity; b)Seebeck coefficient; c) Power Factor for Bi2Te2.7Se0.3 doped with various contents of KI (0, 0.20, 0.25, 0.30 and 0.35).

(S1)

(S2)

Where n, , and are carrier concentration, mobility, the effective mass and Boltzmann’s constant, respectively. Perrin et al[6] proposed that the halogen atoms may substitute for tellurium, generating electrons (carriers). According to equation (1) and (2), we can note that is proportional to, while and have an inverse relationship. As a result, the power factor (PF) at 300K was 0.5, 9.8, 13.1, 15.2 and 10.3μW/(cm·K2) for x=0, 0.20, 0.25, 0.30 and 0.35, respectively as shown in fig1 (c), which means only optimum KI doping (x=0.3 in this work) can get good electrical properties.

2. Calculation of thermoelectric parameters

Table S1. Sample density, reduced Femi levels, Lorenz number and effective mass at room temperature for sample A and B.

Samples / ρ(g/cm3) / / m*/m0 / L(V2K-2)
A / 7.701 / 1.35 / 1.2 / 1.77
B / 7.703 / 1.37 / 1.3 / 1.77
ZM[7] / 7.70 / 0.1 / 1.5 / 1.6

On the basis of single parabolic band (SPB) model with acoustic phonon scattering dominates, the Lorenz number, L can be derived from the followingequations: [8]

(S3)

(S4)

(S5)

Where is the Boltzmann constant, e is the electron charge, r is the scattering parameter which equals -0.5 for acoustic phonon scattering, is the n-th order Fermi integral and is the reduced Fermi energy, which can be calculated from the measured Seebeck coefficient according to equation (5). The calculating process was automatic run by the software Matlab R2012a with pre-compiled program.

The electron mfp is calculated according to the model in the reference[9]

(S6)

(S7)

Where EFis the Fermi energy, m*is the single band effective mass, µis the mobility,kBis the Boltzmann constant, S is the Seebeck coefficient.

According to the Debye-Callaway model, κph can be calculated by

(S8)

In the above equation, x=ћω/kBT is the reduced phonon frequency, is the Boltzmann constant, is the average sound velocity, which could be calculated by (with and respectively denoting the longitudinal and transverse sound velocities), is reduced Plank’s constant, is the Debye temperature, and is the frequency of phonons. is the total relaxation time and calculated according to the Matthiessen’s rule:

(S9)

The detailed calculation of the relaxation time and the parameters can be found in thereference. The phonon mfp is calculated according to the formula mfp(ω)=vτ(ω).

FigS1 Phonon and carrier mean free path of Bi2Te2.7Se0.3 ingot experienced TI-LLST.

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