Computational Generation of Hydrogenated Amorphous Silicon for Tandem Solar Cells: Strain, Voids, and Interfaces

We present computational generation of hydrogenated amorphous silicon (a-Si:H) for use in studying the electronic properties of "heterojunction with intrinsic thin-layer" (HIT) tandem solar cells, in which a-Si:H passivates c-Si at interfaces and enables efficiencies up to 27%. We generate structures using the Wooten-Winer-Weaire classical-potential Monte Carlo method using our CHASSM (Computational Hydrogenated Amorphous Semiconductor Structure Maker) code. We create hydrogenated structures of varying densities and study the effect on the bond-angle and bond-length distributions. At low densities, voids form which are likely to constitute defects limiting carrier mobilities and may be implicated in the light-induced degradation of the Staebler-Wronski effect. We mathematically characterize the size and position of voids using Voronoi polyhedra and persistent homology. We generate interfaces with c-Si in different crystal orientations and investigate the effect of the interface on local amorphous structure. To ensure physical structures, we monitor total energy and bond-angle deviation during the Monte Carlo process to avoid ending in crystalline structures or a class of excessively coordinated artifact structures.