Stoichiometrically graded SiNx 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 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 performance 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. Silicon- silicon and silicon hydrogen bonds, which are help to determine the refractive index of the interface are 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 of, in the region of, 2 nm