Gas bubbles and atomic diffusion mechanisms in CdTe solar cells

This thesis examines various atomistic mechanisms associated with the production of cadmium telluride (CdTe) thin film solar cells and gives a fundamental understanding of processes that are used in their production. Thin film CdTe is the most important thin film photovoltaic technology with annual module production increasing to 8 GW. Currently, the champion conversion efficiency achieved from a CdTe solar cell in a research environment is 22.1%. Cells are produced by a deposition process followed by a CdCl2 treatment at 400 C which improves cell efficiency and cell structure. The thesis uses a combination of theoretical modelling tools, Molecular Dynamics, binary collision algorithms, Ab-initio calculations, Parallel trajectory splicing and Kinetic Monte Carlo for the investigation. The viability of magnetron sputtering as an alternative to the closed space sublimation method of thin film deposition is examined since experiment had shown the formation of inert gas bubbles in the structures after the elevated temperature CdCl2 treatment in sputtered cells. Using Molecular Dynamics, the penetration threshold energies are determined for both Ar and Xe, typical working gases in a magnetron, with CdTe in both zinc-blende and wurtzite phases. These calculations show that more Ar than Xe can penetrate into the growing film with most penetration across the (111) surface. The mechanisms and energy barriers for interstitial Ar and Xe diffusion in zinc-blende CdTe were determined. Energy barriers are reduced near existing clusters, increasing the probability of capture based cluster growth. Barriers in wurtzite are higher with non-Arrhenius behaviour observed. This provides an explanation for the increase in the size of voids observed experimentally after the CdCl2 treatment removes these wurtzite structures through its stacking fault removal process. Blister exfoliation was also modelled, showing the formation of shallow craters with a raised rim. Using ab-initio methods, we find that a saturation of Cl at the grain ...