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Copper is a traditional dopant for many types of polycrystalline thin-film CdTe photovoltaic devices. However, Cu can easily distribute through the depth and breadth of the device, segregating at interfaces or grain boundaries and leading to metastability of the device. Directly correlating Cu-related defect species to the local (i.e. nanoscale) charge transport in CdTe devices remains challenging due to relatively low Cu concentrations in the CdTe layer. Using nanoscale X-ray microscopy, we simultaneously probe both the elemental copper distribution and electrical performance of the device in cross-section. Complementary charge transport modelling delineates the possible defect distributions that can exist under low and high Cu loading, and how these defects interact with charge carriers at different depths of the device.

A fully encapsulated c-Si solar cell was evaluated using focused X-ray and laser beams to probe the microscopic electrical performance and composition. Particular emphasis was placed on the influence of the silver fingers on the laser (LBIC) and X-ray beam induced current (XBIC). Therefore, an uncommonly high X-ray energy of 28keV was utilized for maximum sensitivity to the Ag distribution measured by X-ray fluorescence through the back sheet. The direct comparison of LBIC and XBIC measurements yields a comprehensive picture of these techniques, illustrating the advantages and challenges of both approaches. Specifically, the effect of heavy elements acting as a secondary photon source that increase the XBIC signal is discussed and supported by Monte-Carlo simulations.

X-ray absorption spectroscopy (XAS) has been shown to be a powerful tool to unravel the chemical environment of a given atom within a matrix. When used in correlative X-ray microscopy approaches, XAS allows one to probe with nanoscale precision regions of particular interest in an absorber. Herein, we use X-ray absorption near edge structure (XANES) to evaluate the chemical environment of Cu atoms within a CdTe solar cell. The reconstruction of XANES spectra from XRF maps have unfolded 2D maps of Cu chemical structures. In this work, we found that most Cu atoms exist in Cu 2 Te and Cu 1.4 Te phase. Moreover, we found traces of CuTe, Cu 2 O, CuO, Cu 2 S, CuS, and metallic Cu phase. Investigating Cu chemical structures at different performing areas, we found no observable correlation between Cu chemical structures and electrical performance. This approach allows tracking of Cu chemical structures along with electrical performance and elemental distribution simultaneously, with high spatial resolution in a statistically practical way.