ZnO/aSi interface charge carriers transport in Li-ion secondary cell anodes

Abstract

Electron and Li-ion transport at the n-ZnO/p-aSi (amorphous Si) heterojunction interface is analyzed for the initial charging conditions of a secondary battery anode. The ohmic and diode-type current-voltage characteristics of the junction are investigated for varying doping levels of aSi and ZnO layers. The interface potential barrier impacts the electrons supply to control the Li + ZnO -> Li2O + LixZn reaction. The interface electric field could exceed similar to 10(5) V cm(-1) and draws in Li ions from zinc oxide into the silicon layer. Relatively low-level doping (similar to 10(18) cm(-3)) of the semiconductors is preferred for the optimum draw-in effect. During the initial charging, when the Li content in ZnO (as substitution Li-Zn acceptors) does not exceed the solubility level (similar to 10(19) cm(-3)), the overall doping maintains the n-type, and the interface electric field continues to draw in Li ions towards silicon. Under further increase of Li content at interstitials, the layer conductivity is converted, and the heterojunction becomes n(-)-n-p (or even p-n-p) type. During the subsequent transport of Li ions, the interface potential barrier diminishes and vanishes, and the current-voltage characteristics become ohmic. The importance of doping level control for both the materials is emphasized. The results are applicable for interface engineering in LIB anodes. (C) 2014 Elsevier B.V. All rights reserved.