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

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 >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 > 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.