Abstract
The conversion efficiency of as-deposited, CdTe solar cells is poor and typically less than 5%. A CdCl2 activation treatment increases this to up to 22%. Studies have shown that stacking faults (SFs) are removed and the grain boundaries (GBs) are decorated with chlorine. Thus, SF removal and device efficiency are strongly correlated but whether this is direct or indirect has not been established. Here we explain the passivation responsible for the increase in efficiency but also crucially elucidate the associated SF removal mechanism. The effect of chlorine on a model system containing a SF and two GBs is investigated using density functional theory. The proposed SF removal mechanisms are feasible at the 400 ∘C treatment temperature. It is concluded that the efficiency increase is due to electronic effects in the GBs while SF removal is a by-product of the saturation of the GB with chlorine but is a key signal that sufficient chlorine is present for passivation to occur.
Highlights
The conversion efficiency of as-deposited, cadmium telluride (CdTe) solar cells is poor and typically less than 5%
Modules based on silicon absorbers dominate the current market, second-generation modules based on cadmium telluride (CdTe) are the most commercially successful thin film technology with over 25 GW already installed
Recrystallisation is minimal and little grain growth is observed following CdCl2 activation on CdTe deposited with high-temperature deposition processes such as close spaced sublimation (CSS), which is typically conducted at >500 °C compared to the lower temperature CdCl2 process of 400–450 °C9
Summary
The conversion efficiency of as-deposited, CdTe solar cells is poor and typically less than 5%. This implies that this build up of chlorine at an adjacent grain boundary is key to the removal of stacking faults in each individual grain where the resultant atomic structure can be seen in Fig. 3b after the high-temperature chlorine treatment.
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