Multi-modal X-ray microscopy reveals defect-poor (Ag,Cu)(In,Ga)Se2 supergrains with outstanding charge-carrier lifetime

CIGS solar cells are among the front-runners for thin-film alternatives to mono-Si. Ga-poor (Ag,Cu)(In,Ga)Se2 configurations with a narrow bandgap, grown at low temperature, are particularly promising but require further optimization toward their ideal use in tandem devices. Here, we investigate the lateral inhomogeneity of an Ag-alloyed CIGS solar cell with multi-modal X-ray microscopy. Time-resolved X-ray excited optical luminescence (XEOL, the X-ray equivalent of laser-based photoluminescence) reveals the presence of defect-poor &"upergrains" with a prolonged charge-carrier lifetime and increased luminescence yield. A positive correlation of the spectrally-resolved redshift and lifetime within the supergrains indicates the presence of either photon recycling or band filling. However, the lock-in amplified and simultaneously measured X-ray beam induced voltage (XBIV) registers lower voltage. We elucidate this apparent contradiction using a k-means-based segmentation, thus identifying well-performing, blueshifted areas and redshifted areas of decreased performance, plus a third group that comprises the supergrains. Here, X-ray fluorescence (XRF) suggests copper-deficiency possibly due to partial substitution with silver, providing a potential explanation for their optically superior characteristics. Overall, these measurements are a prime example of lateral performance inhomogeneity in CIGS, where multi-modal X-ray microscopy is key to uncovering further optimization potential.