Optical Characterization of Ternary Element Loss during Co-Chloride Passivation of Polycrystalline II-VI Wide-Bandgap Alloys

Alloys of the II-VI semiconductor CdTe with Mg or Zn have tunable bandgaps in the range modelled to be optimal for integration as a top cell in a tandem photovoltaic device with a Si bottom cell. Leveraging the low-cost production techniques developed for CdTe, this pairing has the potential to produce low-cost and highly efficient tandem PV cells. However, the CdCl 2 post-deposition bulk passivation process widely used for CdTe reduces the concentration of the ternary elements in the absorber, reducing the bandgap from the modelled values, and results in poorer activation than the same process applied to pure CdTe material. We study the mitigation of this loss during CdCl 2 treatment by the development of an overpressure of MgCl 2 and ZnCl 2 , thought to be products of the loss reaction, for Cd 1-x Mg x Te and Cd 1-x Zn x Te, respectively. Little effect is seen until a vapor pressure far in excess of CdCl 2 is achieved, reducing the loss—as measured by transmittance—by half.