## 2 Materials and Methods  
CZTSe and CZTGSe absorbers were prepared by the one-step selenization of stacked elemental and alloyed precursor layers (Zn/Cu-Sn/Zn). The samples were produced on 3 mm soda-time glass (SLG) coated with two 250 nm Mo layers and treated with Ar plasma (in a von Ardenne cluster tool) for 3 min before being cut into the desired size of 25 mm by 25 mm, and one more minute of direct treatment (and without leaving the vacuum) before sputtering the metallic-precursor layer (von Ardenne cluster tool). The Ar-plasma-etching steps were performed in order to remove oxides and organic residues from the surface. The substrates were provided by ZSW (Zentrum fur Sonnenenergie und Wasserstofforschung Baden-Wurttemberg) as well as Manz AG. The Zn/Cu-Sn/Zn precursor stack was prepared by using the sputter durations of 324 s, 1625 s, and 44 s, respectively. The sputtering rates were estimated from a sputtering process of long duration over a large area and were approximately 23.7 \(\mathrm{\,\,\mathrm{\,\mathrm{\,\mathrm{\,\mathrm{\,\mathrm{\,\mathrm{\,\,\, \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\  
coating or metallic contact grid was applied to any of the samples presented here. Sample contacting was performed by exposing the Mo layer and placing two Indium dots for the back contacting. The front contacting was established by carefully placing probe heads directly on the ZnO. Each sample was divided into nine smaller cells by mechanical scribing, resulting in laboratory-scale cells of approximately 5 mm by 5 mm.  
The final absorber compositions were determined by cross-calibrated (except for Ge content) energy-dispersive X-ray spectroscopy (EDX) measurements (FEI Helios 600i). An averaging of three independent and randomly positioned measurements was performed per sample. EDX surface mappings and EDX cross-section mappings of cleaved samples were performed for analysis of the elemental distribution. Shallow-angle-focused ion-beam milling was performed on some samples to investigate the depth-resolved element distributions (FEI Helios 600i), which is described in more detail in [11]. Raman measurements were performed with the laser wavelengths 532 nm and 457 nm (LabRAM Aramis Horiba). IV measurements under standard test conditions on each solar cell were performed using an AAA-class sun simulator (Photo Emission Tech Inc., USA). External-quantum-efficiency (EQE) measurements were taken at room temperature with a spectral range of 300 nm to 1400 nm (Bentham TMc300 Monochromator, lamps from Bentham instruments Ltd., ILD 75E QH chop, Xenon and Quartz-Halogen, Stanford research systems SR 830 DSP lock-in amplifier). The band gap \(E_{G}\) of the absorber was extracted by performing a linear extrapolation of EQE\({}^{2}\) over the energy \(E\). Urbach energies \(E_{U}\) were obtained by linearly fitting ln(EQE) as a function of the \(E\) of the exponential tail below the band gap edge [20,21]. The presented values for the \(V_{OC}\) deficit are given relative to the Shockley-Queisser limit according to the empiric Equation (1) [2,22,23]  
\[V_{OCdefSQ}\approx 0.928\times\frac{E_{G}}{q}-0.176\mathrm{V}-V_{OC}, \tag{1}\]  
where \(E_{G}\) is the band gap extracted from EQE, \(q\) is the elementary charge and \(V_{OC}\) is the open-circuit voltage obtained from IV measurements. Accelerated-aging tests were performed in a climate chamber (Weiss Umweltechnik GmbH, type SC 600/70) at a temperature of 85 \({}^{\circ}\)C and relative humidity of 20%. The samples were placed in petri dishes covered with lids to protect the samples from any possible water-droplet condensation in the chamber.