Atomic-layer-deposited Al-doped zinc oxide as a passivating conductive contacting layer for n+-doped surfaces in silicon solar cells

Stacks consisting of an ultrathin SiO2 coated with atomic-layer deposited (ALD) zinc oxide (ZnO) and aluminum oxide (Al2O3) have been shown to yield state-of-the-art passivation of n-type crystalline silicon surfaces. The distinguishing aspect of this novel passivation stack is the very conductive nature of the passivating ZnO layer. In this work, it is demonstrated that such a stack can provide additional functionalities relevant for silicon solar cells. Specifically, it is shown that the conductive and transparent stacks can passivate textured and n+-diffused silicon surfaces and that they can form an Ohmic contact to n+ -diffused surfaces with a low contact resistivity, provided the ZnO is Al-doped. The Al2O3 capping layer has previously been shown to be crucial in the passivation mechanism by preventing the effusion of hydrogen during annealing. Here, it is demonstrated to enable a significant improvement in both the transparency and lateral conductivity of the ZnO upon annealing as well, up to a level typically only attainable by In-based transparent conductive oxides. It is furthermore shown that the passivation of the stacks is thermally stable up to 500–600 oC, depending on the preparation method for the interfacial SiO2. Together, these properties make the presented stack an interesting building block for crystalline silicon solar cells, with possibilities for integration as passivating front contact in Passivated Emitter and Rear Cell (PERC)-like solar cells, e.g. as bottom cell top contact in silicon-perovskite tandem cells, as well as a conductive hydrogenation source for poly-Si passivating contacts.