Abstract text (incl. figure legends and references)

The performance of Cu(In,Ga)Se2 (CIGS) thin-film solar cells can be improved by partial substitution of Cu with Ag, forming (Ag,Cu)(In,Ga)Se2 (ACIGS) [1]. However, the photovoltaic properties of ACIGS solar cells are significantly influenced by the Ag concentration and its spatial distribution. In order to determine an optimum Ag content and to identify experimental conditions for the fabrication of high-efficient ACIGS devices, ACIGS thin films in this work were prepared by supplying the Ag by two different methods. Providing Ag in both experimental campaigns, the power conversion efficiency (PCE) of the cells decreased around 2% absolute. The chemical and structural properties of the ACIGS layers were studied by (scanning) transmission electron microscopy ((S)TEM) combined with energy-dispersive X-ray spectroscopy (EDXS).

In the first experimental campaign two ACIGS layers were fabricated by in-line co-evaporation of CIGS on Mo coated glass substrates, which were pre-plated with a 40 and 80 nm Ag layer, respectively, acting as precursor. In the second experimental campaign four ACIGS films with integral Ag/(Ag+Cu) ratios (AAC) of 0, 0.05, 0.11, and 0.20, as determined by X-ray fluorescence, were fabricated by co-evaporation of all matrix elements (including Ag) on Mo substrates without a Ag precursor. Cross-sectional TEM lamellae were prepared from these samples by focused-ion-beam milling using an FEI dual beam Helios G4 FX microscope. For TEM and STEM/EDXS investigations, a 200 kV FEI Tecnai Osiris and a 300 kV Thermo Fisher Themis 300 transmission electron microscope was used, where the latter has a Cs-probe corrector.

Fig. 1 shows the depth-dependent line profiles of the Ag concentrations (Figs. 1a,c) together with the resulting Ga/(Ga+In) ratios (GGI) (Figs. 1b,d) of the ACIGS layers deposited with the Ag precursors or by Ag co-evaporation. The average Ag contents are also given in Figs. 1a,c. In general, the GGI profiles determined by STEM/EDXS are in good agreement with corresponding measurements by glow discharge optical emission spectroscopy (not shown here). Evidently, the GGI gradients are flattening with increasing Ag content (Figs. 1b,d). It is noted that the GGI gradient of the Ag-free CIGS with AAC = 0 (red line in Fig. 1d) resembles that of ACIGS with AAC = 0.05 (green line), which is likely due to the small difference in Ag content.

For the precursor-deposited ACIGS films, Ag segregation was observed in some grain-boundary regions, which is more pronounced in the sample with the higher Ag content. Fig. 2 shows elemental maps for a representative Ag-enriched grain boundary in the ACIGS sample with 80 nm Ag precursor. An In and Ag enrichment combined with Se deficiency was detected at the grain boundary with a length of several hundred nanometers. In comparison, Ag segregation was not detected at grain boundaries of the co-evaporated ACIGS films, even for the high AAC ratios.

Since the PCEs of all ACIGS devices in this work decrease with increasing Ag contents, the Ag enrichment at grain boundaries and/or the flattening of the GGI gradients are likely to contribute to the deterioration of the device performance.

[1] M. Edoff et al., IEEE J. Photovolt. 7 (2017) 1789.

[2] The support of the German Federal Ministry for Economic Affairs and Climate Action (BMWK) within the EFFCIS-II project under contract No. 03EE1059A (ZSW) and contract No. 03EE1059E (KIT) is acknowledged.