Measurement of Chlorine Concentrations at CdTe Grain Boundaries

We measure the chlorine concentrations at grain boundaries in CdTe thin films using Time of Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) and find that over 90% of the total Cl in the film is concentrated on the grain boundary. We describe a method for quantifying the grain boundary and intragrain chlorine concentrations. Using this method, the average saturation chlorine concentration in the grain boundary for vapor deposited CdTe with a mixed (1 1 1) and (0 0 1) texture is 2.4 × 10¹⁴ atoms/cm². When the chlorine concentration at the grain boundary is well below the saturation level, solar photovoltaic device performance degrades. We also find that twin boundaries in CdTe with (1 1 1)/〈1 1 1〉 orientations (CSL sigma 3) do not have Cl decorating the boundary above the ToF-SIMS detection limit of 10¹² atoms/cm². However, we find multiple instances, where the (1 0 1)/〈1 1 1〉 twins (CSL sigma 9) are often strongly decorated with Cl at levels comparable with average grain boundaries. Total Cl concentration in the films is strongly correlated with the grain boundary density in the analysis area.