Elucidating Materials Paradigm of CIGS by Structure--Composition-- Performance Correlations

Recent developments in focusing hard X-rays to nanoscale beams have enabled scanning X-ray microscopy modalities and their simultaneous exploitation in multi-modal measurement campaigns. Specifically, X-ray beam induced current and X-ray fluorescence measurements have been established for the correlation of the electrical performance with the distribution of absorber and trace elements for thin-film solar cells with absorbers from CIGS to CdTe and perovskites. For CIGS, the composition is in an especially complex interplay with the synthesis conditions and the crystallographic structure due to the tetragonal lattice distortions, steep vertical In/Ga gradients, and lateral inhomogeneities that introduce lattice strain and structural defects. For this contribution, we have added scanning X-ray nano-diffraction to the multi-modal envelope of scanning X-ray microscopy to assess crystallographic properties of a solar-cell series with a varying In/Ga ratio. For the first time, this combination has been used to characterize a statistically significant number of CIGS grains embedded in as-deposited solar cells: mapping out the real and reciprocal space, we have isolated nearly 500 individual grains. This enabled us to elucidate Materials Paradigm of CIGS, by (1) correlating the lateral Cd and In/Ga distribution with the local performance and lattice spacing with unprecedented sensitivity, (2) differentiating voids in the absorber layer that appear (not) to be filled with CdS, and (3) evaluating the crystallographic properties including the grain orientation and grain-boundary classification with sub-grain resolution and powerful statistics in fully assembled devices. In the full presentation, we will elaborate on our methodological advances and unveil performance-relevant findings from the CdS coverage to the strain distribution at small- and large-angle grain boundaries. Beyond applications to CIGS, our work highlights the latest developments in the field of X-ray imaging and paves the way for advanced correlative nanoscopy at diffraction-limited storage rings that will become operational within the next few years.