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

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 ∼105 V cm−1 and draws in Li ions from zinc oxide into the silicon layer. Relatively low-level doping (∼1018 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 LiZn acceptors) does not exceed the solubility level (∼1019 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.