High thermoelectric power factor of p-type amorphous silicon thin films dispersed with ultrafine silicon nanocrystals

Shi-Li Zhang,Junji Guo,Örjan Vallin,Ngan Hoang Pham,Jun Luo,M Venkata Kamalakar,Zhi-Bin Zhang,Chenyu Wen,J Panda

doi:10.1063/5.0004318

Abstract

Silicon, a candidate as an abundant-element thermoelectric material for low-temperature thermal energy scavenging applications, generally suffers from rather low thermoelectric efficiency. One viable solution to enhancing the efficiency is to boost the power factor (PF) of amorphous silicon (a-Si) while keeping the thermal conductivity sufficiently low. In this work, we report that PF &gt;1 m Wm−1 K−2 is achievable for boron-implanted p-type a-Si films dispersed with ultrafine crystals realized by annealing with temperatures ≤600 °C. Annealing at 550 °C initiates crystallization with sub-5-nm nanocrystals embedded in the a-Si matrix. The resultant thin films remain highly resistive and thus yield a low PF. Annealing at 600 °C approximately doubles the density of the sub-5-nm nanocrystals with a bimodal size distribution characteristic and accordingly reduces the fraction of the amorphous phase in the films. Consequently, a dramatically enhanced electrical conductivity up to 104 S/m and hence PF &gt; 1 m Wm−1 K−2 measured at room temperature are achieved. The results show the great potential of silicon in large-scale thermoelectric applications and establish a route toward high-performance energy harvesting and cooling based on silicon thermoelectrics.

Highlights

A thermoelectric generator (TEG) represents a unique device to directly convert waste heat into electricity
We report that power factor (PF) >1 m Wm−1 K−2 is achievable for boron-implanted p-type amorphous silicon (a-Si) films dispersed with ultrafine crystals realized by annealing with temperatures ≤600 °C
As thin films consisting of ultrafine nanocrystals possess very low κ due to efficient phonon scattering at grain boundaries,[11] our study provides a solution to achieve high performance p-type thermoelectric silicon thin films, in combination of the n-type counterpart, for the applications of inexpensive and distributed thermal energy harvesting and cooling operating in a low temperature range

Summary

Introduction

A thermoelectric generator (TEG) represents a unique device to directly convert waste heat into electricity. One is the persistently poor conversion efficiency, which is primarily characterized by the dimensionless figure of merit ZT = (σS2/κ)T, where σ, S, κ, and T represent the electrical conductivity, the Seebeck coefficient, the thermal conductivity, and the absolute operating temperature, respectively. The other issue is associated with the use of rare and toxic tellurides, e.g., Bi2Te3 and PbTe, for their unprecedented TE performance in the low temperature regime below 400 °C; in bulk form, they typically deliver a ZT value close to 1 at RT. Single crystalline silicon (sc-Si) suffers from a rather low ZT around 0.01 due to its large bulk thermal conductivity (∼150 Wm−1 K−1).[7,8] An efficient strategy to enhance ZT of sc-Si is suppressing its thermal conductivity by nanostructuring in the form of, e.g., superlattice, nanowires, and nano-meshes of sc-Si, which generally involves demanding and complicated processing.[9,10]

Methods

Results

Conclusion