“Zero-charge” SiO2/Al2O3 stacks for the simultaneous passivation of n+ and p+ doped silicon surfaces by atomic layer deposition

To achieve high conversion efficiencies, advanced silicon solar cell architectures such as interdigitated back contact solar cells demand that defects at both the n+ and p+ doped Si surfaces are passivated simultaneously by a single passivation scheme. In this work, corona charging experiments show that the fixed charge density Qf is a key parameter governing the passivation of both surface types. Alternatively, Qf can be controlled from strongly negative to even positive values by carefully tuning the SiO2 interlayer thickness in SiO2/Al2O3 stacks prepared by atomic layer deposition (ALD). This control in Qf allows for a superior passivation of n+ Si surfaces by SiO2/Al2O3 stacks compared to a single layer Al2O3. For instance, for SiO2 interlayer thicknesses of ~3-14 nm, the recombination parameter of an n+ Si surface having a high surface doping concentration Ns of 2×1020 cm−3 was reduced from J0n+=81 fA/cm2 to J0n+=50 fA/cm2. Simulations predict that the SiO2/Al2O3 stacks outperform Al2O3 passivation layers particularly on n+ Si surfaces having a moderate Ns in the range of 1018-1020 cm−3. On p+ Si surfaces, J0p+≤54 fA/cm2 was achieved for all ALD SiO2 interlayer thicknesses investigated (i.e., 1-14 nm). The SiO2/Al2O3 stacks presented in this work are compatible with SiNx capping and subsequent high-temperature firing steps, which are typically used in solar cell processing. Furthermore, the results were successfully reproduced in an industrial ALD batch reactor using a low-temperature process. This makes ALD SiO2/Al2O3 stacks a promising candidate for the simultaneous passivation of n+ and p+ Si surfaces in solar cells.