Stoichiometrically graded SiN_x for improved surface passivation in high performance solar cells

The effects of an interface gradient in nitrogen concentration on a number of important properties of amorphous hydrogenated silicon nitride/crystalline silicon (⁠$a-SiNx:H/c-Si$⁠) interfaces in the context of solar cell devices are investigated using molecular dynamics simulations. We simulate interfaces with a gradient of nitrogen which goes from $SiN1.2$ to Si over widths from 2 to 9 nm, in the presence of 10 at. % hydrogen, to recreate the conditions present when $SiNx$ layers are deposited onto c-Si by plasma enhanced vapour deposition. We examine how changing the width of the nitrogen gradient can affect a number of atomic level structural properties, which influence the optical and electrical performances of solar cells. We examine the trajectories of our simulations to search for certain geometries, which have previously been identified as being important at this interface. The number of silicon-silicon and silicon hydrogen bonds, which helps to determine the refractive index of the interface, is shown to increase with increasing N gradient width. The fixed charge in the interface is also shown to increase with the width of the gradient. The results demonstrate how altering the width of the N layer can affect the efficiency of $a-SiNx:H$ as both an anti-reflective coating and a passivation layer, and we suggest an optimal gradient width in the region of 2 nm.