Defect activation and annihilation in CIGS solar cells: an operando x-ray microscopy study

Abstract

The efficiency of thin-film solar cells with a Cu( -In x 1 Ga x )Se 2 absorber is limited by nanoscopic inhomogeneities and defects. Traditional characterization methods are challenged by the multi-scale evaluation of the performance at defects that are buried in the device structures. Multi-modal x-ray microscopy offers a unique tool-set to probe the performance in fully assembled solar cells, and to correlate the performance with composition down to the micro-and nanoscale. We applied this approach to the mapping of temperature-dependent recombination for Cu( -In x 1 Ga x )Se 2 solar cells with different absorber grain sizes, evaluating the same areas from room temperature to  100 C. It was found that poor performing areas in the large-grain sample are correlated with a Cu-deficient phase, whereas defects in the small-grain sample are not correlated with the distribution of Cu. In both samples, classes of recombination sites were identified, where defects were activated or annihilated by temperature. More generally, the methodology of combined operando and in situ x-ray microscopy was established at the physical limit of spatial resolution given by the device itself. As proof-ofprinciple, the measurement of nanoscopic current generation in a solar cell is demonstrated with applied bias voltage and bias light.

Effects of X-rays on Perovskite Solar Cells

“…Enabled by the great penetration depth in low-Z materials and by the long working distance, hard X-rays are intrinsically suited to serve as probe beams for in situ measurements of complete solar cell stacks under various conditions. 6,7 In the framework of multimodal scanning X-ray microscopy, 8 the combination of XBIC/XBIV with X-ray fluorescence (XRF) and nanodiffraction measurements has turned out to be particularly powerful for the point-by-point correlation of the electrical performance with the elemental composition and strain in thin-film solar cells with compound polycrystalline absorber layers such as CuIn 1−x Ga x Se 2 , 6,9,10 CdTe, 11,12 or methylammonium lead iodide (MAPI, CH 3 NH 3 PbI 3 ). 13−17 The record efficiency of perovskite solar cells (PSCs) with an organometal halide absorber has been increasing at an unprecedented rate, being after a few years of development already beyond the record efficiency of other more established polycrystalline thin-film solar cells.…”

Four-Fold Multi-Modal X-ray Microscopy Measurements of a Cu(In,Ga)Se2 Solar Cell

“…While the statistical significance of the present proof-of-principle study is limited, the multi-modal approach allows the direct correlation of nanoscale properties, leading to the identification of device-relevant defect mechanisms as demonstrated in two-fold multi-modal measurements already [ 12 , 59 ]. Four-fold multi-modal measurements, as we showcased here, will significantly reduce the ambiguity in interpretation and may unveil correlations that have not been detectable so far, thanks to the complementarity of XRF, XBIC, XEOL, and ptychography measurements.…”

Role of Cation Ordering on Device Performance in (Ag,Cu)InSe2 Solar Cells with KF Post-Deposition Treatment

“…In the measurements, a full solar cell device is connected electrically so that the equivalent of nanoscale J sc and V oc can be measured from the carrier generation by the high-energy X-rays . Other examples of how correlative XRF and XBIC/XBIV have been shown on CIGS solar cells as well as perovskites. − …”