## 2 Materials and Methods  
Polycrystalline CIGS absorber layers with a thickness of 2.3 \(\upmu\)m, a Ga/(Ga+In) ratio of 0.3 and a Cu/(Ga+In) of 0.8 were grown in an industrially relevant in-line multi-stage co-evaporation process on Mo-coated soda-lime glass [9]. No alkali post-deposition treatment was applied prior to the buffer deposition, which can significantly improve the device performance mainly due to an increased \(V_{\rm OC}\). In the next step, In\({}_{\rm x}\)S\({}_{\rm y}\) as the buffer layer was rf-magnetron sputtered in a VON ARDENNE (Dresden, Germany) high-vacuum sputtering system of type CS 730S from a commercially available In\({}_{2}\)S\({}_{3}\) target with a diameter of 20 cm in a pure argon atmosphere (for further details see [10]). The substrate temperature during sputtering of the In\({}_{\rm x}\)S\({}_{\rm y}\) layers was increased from room temperature (RT) up to 240 \({}^{\circ}\)C at low power densities of approximately 1 W/cm\({}^{2}\) compared to high power densities around 10 W/cm\({}^{2}\) typically used for ZnO:Al sputtering [11]. Thicknesses of In\({}_{\rm x}\)S\({}_{\rm y}\) buffer layers for solar cells were in the range of 30-60 nm for deposition times of 40-80 s, resulting in a sputter deposition rate of approximately 0.8 nm/s. In addition, around 500 nm thick In\({}_{\rm x}\)S\({}_{\rm y}\) layers were deposited on highly transparent suprasil quartz glass substrates for transmittance and gracing incident X-ray diffraction (GIXRD) measurements. The thicknesses of the thin In\({}_{\rm x}\)S\({}_{\rm y}\) layers were determined on CIGS/In\({}_{\rm x}\)S\({}_{\rm y}\) cross-sections with a scanning electron microscope and for the thick layers by evaluation of the transmittance data. The commonly used CdS buffer with approximately 50 nm thickness was grown by CBD with a thiourea-based process as reference material. Subsequently, rf-sputtered undoped ZnO (i-ZnO) was applied as high-resistive layer and dc-sputtered ZnO:Al served as front contact with Ni/Al/Ni grid fingers deposited by e-beam evaporation on top. The total cell area was 0.5 cm\({}^{2}\). Figure 1a illustrates the stacking sequence as used in this work with a sputtered In\({}_{\rm x}\)S\({}_{\rm y}\) buffer in combination with i-ZnO. Alternatively we tried different stacking sequences with rf-sputtered Zn\({}_{0.75}\)Mg\({}_{0.25}\)O as high-resistive layer instead of i-ZnO and without any high-resistive layer with ZnO:Al deposited directly on top of In\({}_{\rm x}\)S\({}_{\rm y}\). Both approaches turned out fine but could not significantly improve the cell efficiencies compared to our standard stacking sequence with i-ZnO, which we used for the present work. Current-voltage (_I-V_) curves were measured with a WACOM (Saitama, Japan) AM1.5G solar simulator with four-point geometry at standard testing conditions with a silicon reference solar cell for calibration. External quantum efficiency (EQE) measurements were performed with a BENTHAM (Berkshire, UK) PVE 300 setup with an additional light bias. For _I-V_ and EQE measurements the CIGS cells were in the as-grown state (no further light-soaking or post-annealing procedures were applied). The transmittance of 500 nm thick In\({}_{\rm x}\)S\({}_{\rm y}\) layers on suprasil quartz glass substrates was measured with a Perkin Elmer (Waltham, MA, USA) UV/VIS/NIR Lambda 900 spectrometer and GIXRD patterns for structural characterization were obtained with a PANalytical (Almelo, The Netherlands) Empyrian diffractometer with Cu K\({}_{\alpha}\) radiation with an incident angle of 0.5\({}^{\circ}\). We performed time-of-flight secondary ion mass spectroscopy (ToF-SIMS) depth profiles with a ToF\({}^{5}\)-SIMS instrument from IONTOF (Munster, Germany). A Bi+ liquid metal gun running at 30 keV generates the secondary ions, which are recorded by a time-of-flight detector. We choose an oxygen gas gun as sputtering source to detect sodium in a good sensibility. To acquire quantified depth profiles of sulfur, oxygen, and CIGS matrix elements, we selected a cesium metal gun.