Al₂O₃ rear surface passivation for silicon ribbon solar cells

Abstract

In this work the application of an Al2O3 surface passivation layer to low-cost multicrystalline silicon ribbon material is investigated. Symmetrical lifetime samples are prepared from adjacent p-doped EFG (Edge-defined Film-fed Growth) wafers and spatially resolved μPCD (Microwave detected PhotoConductance Decay) measurements are carried out to determine the minority charge carrier lifetime. It is shown that even very thin layers (5 nm) of Al2O3 can provide an excellent surface passivation of the mentioned silicon ribbon material. Instead of annealing in N2 ambience a MIRHP (Microwave Induced Remote Hydrogen Plasma) treatment is applied. This process step allows hydrogen atoms to pass the thin Al2O3 layer and to increase lifetime by passivating recombinative defects in the bulk material. In addition to the Al2O3 layer, hydrogen-rich silicon nitride (SiNx:H) is deposited by PECVD (Plasma Enhanced Chemical Vapor Deposition) on top of selected Al2O3 samples to increase the mechanical and chemical stability during a subsequent solar cell process. This deposition can replace the necessary Al2O3 annealing step leading to a comparable passivation quality. Finally, 2 x 2 cm2 solar cells are processed including the Al2O3 rear side passivation in a photolithography based highefficiency process reaching efficiencies above 18% on multicrystalline EFG material [1].