Aluminum Oxide-aluminum Stacks for Contact Passivation in Silicon Solar Cells

We investigate passivation of the aluminum-silicon interface by thin aluminum oxide (Al2O3) layers grown by thermal atomic layer deposition (ALD) on HF-last silicon surfaces. We first report effective lifetimes of Al2O3-passivated n- and p-type silicon wafers as a function of the number of ALD cycles. Then, we present saturation current density and contact resistance measurements of aluminum contacts on n/n+ and n/p+ junctions, passivated with a selection of the investigated layers. Our results show that aluminum contacts on n+ silicon can be successfully passivated with thin Al2O3 layers without compromising contact resistance. However, we did not observe significant contact passivation for acceptable contact resistance in the case of Al2O3 passivated aluminum contacts on p+ silicon. We explain our experimental results from the asymmetry between conductance and valence band offsets of Al2O3 on silicon.