Chlorine passivation of grain boundaries in cadmium telluride solar cells

Cadmium Telluride is the most commercially important second generation thin film photovoltaic, with a record solar cell conversion efficiency of 22.1%. However as-deposited cells are <5% efficient and require a cell activation treatment with CdCl2 at about 400 ◦C to reach commercially viable efficiencies. Such a treatment is a routine process during CdTe module manufacturing. However, the precise mechanisms at work for this remarkable efficiency enhancement are not well understood. In this paper, atomistic modelling techniques are used to improve the fundamental understanding of the structural and electronic properties of CdTe by modelling the effects of chlorine and other elements with their interaction with extended defects and grain boundaries. Studies at high spatial resolution with NanoSIMS, TEM and Energy Dispersive X-ray analysis shows that chlorine atoms are concentrated at grain boundaries in CdTe after the CdCl2 treatment. DFT calculations show that both ClTe and for the first time Cli are stabilised at the grain boundaries compared to bulk CdTe. Similar defect formation energies of these defects suggests both will be present at the grain boundaries. As expected, four single particle levels are present in the Σ3 (112) GB band gap which explains the low efficiencies prior to treatment. ClTe substitutions passivate one of these levels and partially passivate another two. Remarkably further addition of Cli fully passivates the remaining single particle levels. This passivation of single particle levels is most likely to be the primary cause of the efficiency enhancement on chlorine treatment. Further to this, alternative halogens were then trialled as activation treatments. All halogens show similar electronic effects and their defect formation energies follow ionic radii trends.