Optical Properties and Dielectric Functions of Grain Boundaries and Interfaces in CdTe Thin-Film Solar Cells

Abstract

CdTe thin-film solar cells have complex microstructures, such as grain boundaries within the absorber layer, as well as CdS window, and Au back contact interfaces, where the local structure and chemistry undergo significant changes. The optical properties at these nano-scale defects are unknown, but their accurate measurement is required in order to identify potential losses in device efficiency. Here monochromated electron energy loss spectroscopy (EELS) in an aberration corrected scanning transmission electron microscope (STEM) is used to measure the complex dielectric function for the CdTe1-xSx inter-diffusion layer at the CdS-CdTe interface, high angle CdTe grain boundaries and Au-CdTe interface. CdTe1-xSx is shown to have a lower absorption coefficient than CdTe, but its refractive index is more closely matched to CdS. Grain boundaries have a negligible effect on the light absorption profile within CdTe, despite significant changes in the local structure and chemistry (i.e. Te-depletion) at the grain boundary. Delocalisation in inelastic scattering is the dominant systematic error in the above measurements. Finally a light backscattering mechanism via surface plasmon polaritons at the Au-CdTe interface is uncovered, which could potentially increase the photocurrent extracted from incident light at energies just above the CdTe band gap.