Correlation of Nanoscale Structure, Composition, and Performance: A Study of the CIGS Materials Paradigm

Multimodal imaging of thin-film solar cells has been demonstrated at hard X-ray nanoprobes: simultaneously assessing X-ray beam induced current and X-ray fluorescence, lateral variations in the electrical performance and the distribution of absorber and trace elements can be correlated. Here, we complement the suite of modalities with scanning X-ray diffraction and map the crystallographic structure of Cu(In,Ga)Se 2 (CIGS) at the nanoscale: in the quaternary compound semiconductor, lattice strain and structural defects induced by tetragonal lattice distortions, steep vertical In/Ga gradients, and lateral inhomogeneities pose a great challenge. Investigating a series of solar cells with varying In/Ga ratio, we probed for the first time a statistically significant number of nearly 500 CIGS grains in the bulk layer of operational cells. Overall, we assessed the entirety of the Cu(In,Ga)Se 2  Materials Science Tetrahedron—thanks to, first, extraordinary sensitivity with K-edge excitation allowing to correlate the lateral Cd and In/Ga distribution, local performance, and lattice spacing, second, detection of voids, some filled with CdS, in the CIGS layer, and third, performance-relevant findings from a crystallographic analysis of grain orientation and boundaries. Beyond further optimization of Cu(In,Ga)Se 2  photovoltaic cells toward the detailed balance limit of solar-cell conversion efficiency, the developed methodology paves the way to extract a maximum of information from correlative hard X-ray nanoscopy at diffraction-limited storage rings.