The Effect of Aluminum Thickness on Solar Cell Performance

For crystalline silicon solar cells having an alloyed aluminum back contact, open-circuit voltage increases with aluminum thickness. The increase is ascribed to the formation of a p+/p "high-low " junction by liquid-phase epitaxy, resulting in increased efficiency for the cell. With thick aluminum layers, the characteristics of the p+ regrown region are in reasonable agreement withpredictions based on the A1-Si phase diagram. With thin aluminum layers the junction is nonideal, and the improvement in Voo is much smaller. Pile-up of metallic impurities, at the aluminum/p + interface is attributed to rejection of these impurities during the silicon regrowth process. The formation of back contacts to silicon solar cells by a luminum alloying is well known. Typically, an a luminum layer is fo rmed on the back surface of a p-type silicon wafer, typically by evaporation or sputtering I'2 or by print-ing of an a luminum paste. 3-s The contact is subsequently " formed " by an appropriate heat-treatment. Thicker layers of a luminum typically lead to improved cell performance, particularly with respect to open-circuit voltage (Voo) and