Solar cells: oxide-nitride nanolayer stacks

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^oC, enabling low surface recombination velocities.  The formation of an interfacial SiO_x layer has been recognised as a key factor.

The paper 'Oxide-nitride nanolayer stacks for enhanced passivation of p-type surfaces in silicon solar cells', published in Solar Energy Materials and Solar Cells, presents the authors'* 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, they observed a reduction in interface defect density, which indicated the presence of negatively charged hydrogen in the AlO_x layer.  The authors 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, which was attributed to increased charge density and decreased carrier capture velocity at the valence band-tail.

The authors explain that they found lower electron capture rates in nanolayer stacks containing AlO_x, which suggested effective passivation of donor states by negatively charged hydrogen.  In addition, 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:Si_xNy layer ~2.5nm thick with excess oxygen content and a mixture of tetrahedral and octahedral coordinated Al, which they concluded likely contributed to the formation of acceptor defects, and which suggests an intrinsic link between the chemical and field-effect passivation.

*Led by this department, the collaborating authors were from the School of Photovoltaic and Renewable Energy Engineering (UNSW, Australia) and the Shanghai Nuclear Engineering Research and Design Institute Ltd Co (China).