Boosting the thermoelectric performance of p-type heavily Cu-doped polycrystalline SnSe via inducing intensive crystal imperfections and defect phonon scattering.

Xiaolei Shi,Yuan Wang,Weidi Liu,Jin Zou,Raza Moshwan,Kun Zheng,Zhi-Gang Chen,Min Hong,Xianlin Qu

doi:10.1039/c8sc02397b

Abstract

In this study, we, for the first time, report a high Cu solubility of 11.8% in single crystal SnSe microbelts synthesized via a facile solvothermal route. The pellets sintered from these heavily Cu-doped microbelts show a high power factor of 5.57 μW cm-1 K-2 and low thermal conductivity of 0.32 W m-1 K-1 at 823 K, contributing to a high peak ZT of ∼1.41. Through a combination of detailed structural and chemical characterizations, we found that with increasing the Cu doping level, the morphology of the synthesized Sn1-x Cu x Se (x is from 0 to 0.118) transfers from rectangular microplate to microbelt. The high electrical transport performance comes from the obtained Cu+ doped state, and the intensive crystal imperfections such as dislocations, lattice distortions, and strains, play key roles in keeping low thermal conductivity. This study fills in the gaps of the existing knowledge concerning the doping mechanisms of Cu in SnSe systems, and provides a new strategy to achieve high thermoelectric performance in SnSe-based thermoelectric materials.

Highlights

With the capability of directly converting between heat and electricity, thermoelectric materials provide a promising alternative energy supplement in applications by collecting the waste-heat and assisting in nding new energy solutions.[1,2] To evaluate the converting efficiency, the unitless gure of merit ZT is de ned as ZT 1⁄4 S2sT/k and k 1⁄4 ke + kl, where s, S, k, kl, ke, and T are the electrical conductivity, the Seebeck coefficient, the thermal conductivity, the lattice thermal conductivity, the electrical thermal conductivity, and the absolute temperature,[3,4,5] respectively
Through detailed electron probe micro-analysis (EPMA) studies, the Cu doping level from different r values was found as 0%, 1%, 2%, 5%, 7.5%, 10%, 11.8%, and 11.8%, respectively, indicating that the solubility of Cu in the SnSe system is 11.8%
The strong bipolar effect,[51] arising between 500 and 600 K, can produce additional holes, leading to a rapid n increase, and in turn increasing s.18. These results indicate that the doped Cu can signi cantly improve the s of pure SnSe at high temperature by strengthening the thermal excitation of the carriers, even though it results in a slight reduction of s at medium temperature, which is why the pure SnSe sample outperforms most of the Cu-doped samples in this temperature range