Abstract
Cadmium telluride (CdTe) solar cells are deposited in current production using evaporation-based tech- niques. Fabricating CdTe solar cells using magnetron sputtering would have the advantage of being more cost-efficient. Here, we show that such deposition results in the incorporation of the magnetron working gas Ar, within the films. Post deposition processing with CdCl2 improves cell efficiency and during which stacking faults are removed. The Ar then accumulates into clusters leading to the creation of voids and blisters on the surface. Using molecular dynamics, the penetration threshold energies are determined for both Ar and Xe, 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 are determined. 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 after stacking fault removal. Blister exfoliation was also modelled, showing the formation of shallow craters with a raised rim.
Highlights
Thin film cadmium telluride (CdTe) is the most important thin-film photovoltaic technology with annual module2020 The Authors
After the high-temperature CdCl2 activation treatment, the stacking faults are removed [6] as shown in figure 3b and the remaining film is almost completely zinc-blende structured with a few twin boundaries remaining
The cluster aggregation rate for Xe is slower, the results indicate that Xe inert gas clusters would form in the CdTe film over experimental time scales if sufficient Xe penetrates into the surface layers
Summary
Thin film cadmium telluride (CdTe) is the most important thin-film photovoltaic technology with annual module. The absorber layer is typically only 3 μm in thickness compared with 150 μm for crystalline silicon This provides thin-film CdTe with a natural cost advantage. The champion conversion efficiency achieved from a CdTe solar cell in a research environment is 22.1% [2] This has been achieved by adding selenium to the front of the cell and replacing the cadmium sulfide buffer layer with a transparent metal oxide. The latest cells incorporate Se in the CdTe absorber to reduce the band gap and the CdS is replaced with magnesium-doped zinc oxide or some other transparent oxide layer. Both of these changes have improved the short circuit current [7,8].
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