Influence of atomic layer deposition Al_2O_3 nano-layer on the surface passivation of silicon solar cells

摘要

A stack of Al_2O_3/SiNx dual layer was applied for the back side surface passivation of p-type multicrystalline silicon solar cells, with laser-opened line metal contacts, forming a local aluminum back surface field (local Al-BSF) structure. A slight amount of Al_2O_3, wrapping around to the front side of the wafer during the thermal atomic layer deposition process, was found to have a negative influence on cell performance. The different process flow was found to lead to a different cell performance, because of the Al_2O_3 wrapping around the front surface. The best cell performance, with an absolute efficiency gain of about 0.6% compared with the normal full Al-BSF structure solar cell, was achieved when the Al_2O_3 layer was deposited after the front surface of the wafer had been covered by a SiNx layer. We discuss the possible reasons for this phenomenon, and propose three explanations as the Ag paste, being hindered from firing through the front passivation layer, degraded the SiNx passivation effect and the Al_2O_3 induced an inversion effect on the front surface. Characterization methods like internal quantum efficiency and contact resistance scanning were used to assist our understanding of the underlying mechanisms.