Three-dimensional imaging of selenium and chlorine distributions in highly efficient selenium-graded cadmium telluride solar cells

Thin-film solar modules based on cadmium telluride (CdTe) technology currently produce the world's lowest cost solar electricity. However, the best CdTe modules now contain a cadmium selenium telluride (CST) alloy at the front of the absorber layer. Despite this, research characterizing the behavior of selenium in alloyed CdTe devices is currently very limited. Here we employ advanced secondary ion mass spectrometry measurements to map the three-dimensional distribution of selenium in a graded CST/CdTe device for the first time. We find significant interdiffusion of selenium between the CST and CdTe layers in the cell, primarily out of the CST grain boundaries and up into the CdTe grain boundaries and grain fringes above. This results in significant lateral variations in selenium concentrations across grains and hence also lateral fields, which we estimate using the measured selenium concentrations.