Correlative impurity distribution analysis in cadmium telluride (CdTe) thin-film solar cells by ToF-SIMS 2D imaging

Polycrystalline cadmium telluride (CdTe) thin-films are commonly annealed in an atmosphere containing oxygen and chlorine and this passivation is essential in order to establish high photovoltaic conversion efficiencies. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is employed to acquire two-dimensional (2D) chemical distributions of impurities in CdTe samples with a lateral resolution of ~150 nm. A statistical methodology is developed to examine the ToF-SIMS data via a two-sample Kolmogorov-Smirnov test. This test is used to determine the degree of chemical segregation at the grain boundaries for chlorine, sulfur, oxygen and copper. Chlorine and oxygen are highly segregated at grain boundaries. Sulfur is found to slowly diffuse into the grain interior from the grain boundary and this process is accelerated with an increasing temperature during the chlorine heat treatment. The distribution of copper is found to be uniform in bi-crystalline and polycrystalline CdTe samples and, hence, it is concluded that within the sensitivities of the ToF-SIMS measurement, no segregation of copper at the grain boundary is detected.