Oxide-nitride nanolayer stacks for enhanced passivation of p-type surfaces in silicon solar cells

Abstract

In the quest for ultra-high-efficiency silicon solar cells, optimising surface passivation has emerged as a critical pathway to minimise losses and enhance device performance. Recent breakthroughs in aluminium oxide (AlO_x) passivation show an interface to Si with low interface defect density and high negative charge density after activation annealing at 400–450 °C, enabling low surface recombination velocities. The formation of an interfacial SiO_x layer has been recognised as a key factor. In this study, we present an in-depth investigation of a SiO_x/AlO_x/SiN_x nanolayer stack interface with Si, where the SiO_x is wet chemically grown. By varying the AlO_x deposition from 5 to 40 ALD cycles, we observed a reduction in interface defect density, indicating the presence of negatively charged hydrogen in the AlO_x layer. We reveal a distinctly different interface between Si and nanolayer stacks with or without AlO_x. Activation annealing significantly reduced recombination losses for stacks with AlO_x, attributed to increased charge density and decreased carrier capture velocity at the valence band-tail. We find lower electron capture rates in nanolayer stacks containing AlO_x, suggesting effective passivation of donor states by negatively charged hydrogen. Additionally, the formation of new acceptor states was detected by an increase in hole capture velocity at the interface after annealing. Electron energy loss spectroscopy (EELS) identified an Al:SiO_xN_y layer of ∼2.5 nm thick with excess oxygen content and a mixture of tetrahedral and octahedral coordinated Al, likely contributing to the formation of acceptor defects and suggest an intrinsic link between the chemical and field-effect passivation.