Low-Temperature Atomic Layer Deposited Magnesium Oxide as a Passivating Electron Contact for c-Si-Based Solar Cells

In this article, we explore magnesium oxide (MgO) as electron-selective contact layer in silicon heterojunction solar cells. We report on the successful deposition of MgO layers by atomic layer deposition at low temperatures ≤200 °C using bis(ethylcyclopentadienyl)magnesium (Mg(CpEt) 2) and H 2O as precursors. Depositions were carried out on bare crystalline silicon (c-Si) wafers and c-Si wafers with an intrinsic amorphous hydrogenated silicon (i-aSi:H) passivation layer. The resulting interfacial properties, surface passivation quality, and contact resistivity were investigated. Upon initial deposition of MgO on an i-aSi:H/c-Si stack, the c-Si surface passivation degrades drastically. However, with an additional annealing step of 5 min at 200-250 °C, it is possible to reverse the degradation and even to achieve charge carrier lifetimes in excess of those achieved with an i-aSi:H alone. Furthermore, we show that MgO forms an ohmic contact with both MgO/i-aSi:H/c-Si and MgO/c-Si stacks, and we demonstrate solar cells using both types of stacks as electron contact layers.