Anodic Aluminum Oxide Passivation For Silicon Solar Cells

The requirement to form localized rear metal contact regions for higher silicon solar cell efficiencies places demand on patterning techniques in terms of the small size of the openings and the ability to perform the patterning at commercial wafer processing rates. We suggest here the possibility of using a self-patterning approach which offers the potential of enhanced surface passivation and nanoscale patterning achieved using a single electrochemical anodization process. It is shown that when nanoporous anodic aluminum oxide (AAO) layers are formed by anodizing an aluminum layer over an intervening SiO2 or SiNx dielectric layer, the implied open-circuit voltages of p-type silicon test structures can be increased by an average of 40 and 47 mV, respectively. Capacitance-voltage measurements show that these passivating AAO dielectric stack layers store positive charges, which differs from what is observed for Al2O3 layers deposited by plasma-enhanced chemical vapor deposition or atomic layer deposition. Furthermore, we show that the magnitude of the stored charge in the dielectric stacks depends on the anodization conditions, highlighting the possibility of controlling the charge storage properties of these layers for specific cell requirements. Although the passivating properties of the anodized aluminum layer appear to be strongly influenced by charge effects, it is also possible that hydrogenation effects may play a role as has been previously observed for other electrochemical processes, such as metal plating.