X-ray Phase Contrast Nano-Tomography of Third Generation Solar Cells

Solar cells are likely to play an increasing role in the future energy scenario and a new generation of solar cells is being developed for this purpose, with a strong focus on sustainability. The solar cells we focus on have organic or kesterite absorber layers. The organic solar cells suffer from degradation and instability, for which their nanomorphology plays a key role whereas the efficiency of kesterite cells is still hampered by presence of secondary phases, which is below detection limits for standard techniques.

Unlike silicon, both technologies are thin-film, with absorber layers that can be as thin as a few microns. Such size makes them suitable for a nanoscale study with synchrotron X-ray imaging techniques, which is the goal of this project. The techniques we dealt with are threedimensional, non-destructive, quantitative, allow for sample sizes of tens of microns, and achieve resolutions in the 10-20 nm range.

Importantly, these techniques are based on coherent diffractive imaging methods, which we have demonstrated in an alternative simulation framework solely based on ray tracing. This approach operates in real space and enables in principle to explore configurations for which an analytical solution is not available.

This framework was developed to simulate an experiment with organic solar cells. The criticality of this sample with respect to resolution, contrast, and radiation damage, is outlined in this work. Experimentally, we demonstrated resonant ptychographic tomography of kesterite solar cells. The measurements achieve an unprecedented combination of high resolution, quantitativeness, and elemental sensitivity to three different elements.

These remarkable imaging capabilities enable quantification and localization of defective features that were overlooked by standard characterization techniques.