Enhancing thermoelectric performance by Fermi level tuning and thermal conductivity degradation in (Ge1\u2212xBix)Te crystals

Pai-Chun Wei,Mitch M C Chou,Cheng-Xun Cai,Yang-Yuan Chen,Ching-Ming Wei,Hsin-Jay Wu,Da-Hua Wei,Duc-Long Nguyen,Cheng-Rong Hsing,Shih-Hsun Yu,Cheng-Lung Chen

doi:10.1038/s41598-019-45071-9

Abstract

In this work, a high thermoelectric figure of merit, zT of 1.9 at 740 K is achieved in Ge1−xBixTe crystals through the concurrent of Seebeck coefficient enhancement and thermal conductivity reduction with Bi dopants. The substitution of Bi for Ge not only compensates the superfluous hole carriers in pristine GeTe but also shifts the Fermi level (EF) to an eligible region. Experimentally, with moderate 6–10% Bi dopants, the carrier concentration is drastically decreased from 8.7 × 1020 cm−3 to 3–5 × 1020 cm−3 and the Seebeck coefficient is boosted three times to 75 μVK−1. In the meantime, based on the density functional theory (DFT) calculation, the Fermi level EF starts to intersect with the pudding mold band at L point, where the band effective mass is enhanced. The enhanced Seebeck coefficient effectively compensates the decrease of electrical conductivity and thus successfully maintain the power factor as large as or even superior than that of the pristine GeTe. In addition, the Bi doping significantly reduces both thermal conductivities of carriers and lattices to an extremely low limit of 1.57 W m−1K−1 at 740 K with 10% Bi dopants, which is an about 63% reduction as compared with that of pristine GeTe. The elevated figure of merit observed in Ge1−xBixTe specimens is therefore realized by synergistically optimizing the power factor and downgrading the thermal conductivity of alloying effect and lattice anharmonicity caused by Bi doping.

Highlights

IntroductionThe conversion efficiency is mainly determined by the dimensionless figure of merit zT = σ S2T/κ, in which σ, S, T, κ and σ S2 are the electrical conductivity, Seebeck coefficient, absolute temperature, thermal conductivity, and power factor (PF), respectively
The elevated figure of merit observed in Ge1−xBixTe specimens is realized by synergistically optimizing the power factor and downgrading the thermal conductivity of alloying effect and lattice anharmonicity caused by Bi doping
These samples are ~40 g in weight with 13 mm in diameter. They have a rhombohedral lattice at room temperature (the inset in Fig. 1(d)) and are free from secondary phases, as confirmed by the powder X-ray diffraction (XRD) patterns (Fig. 1(b))

Summary

Introduction

The conversion efficiency is mainly determined by the dimensionless figure of merit zT = σ S2T/κ, in which σ, S, T, κ and σ S2 are the electrical conductivity, Seebeck coefficient, absolute temperature, thermal conductivity, and power factor (PF), respectively. These parameters are strongly coupled with each other, leading to the difficulty in manipulation of zT enhancement[1]. GeTe is a heavily p-type semiconductor with an inherent high carrier concentration of ~1021 cm−3 It stabilizes in a non-centrosymmetric rhombohedral structure with an space group R3m (No 160) at room temperature, which undergoes a second-order ferroelectric phase www.nature.com/scientificreports/

Methods

Results

Conclusion