## 3 Results and Discussion  
The effective incorporation of Ge into the kesterite absorber is shown in Figure 2 by the EDX results along with the EQE spectra and the \(E_{G}\) values extracted from them. By varying the added Ge amount, CZTGSe-absorber layers with Ge/(Ge + Sn) ratios in the range of 0.0 to 0.86 were synthesized. For better viewing, only a few selected EQE spectra  
Figure 1: Sample preparation steps: (**a**) Sputtering of Zn and Cu-Sn, (**b**) precursor stack annealed under Se atmosphere with Sn wire and Ge pieces, (**c**) performed temperature profile, (**d**) (simplified) reaction scheme, and (**e**) the solar-cell finishing steps. covering the discussed range are shown in Figure 2a; band gap values extracted from the full set of available EQE spectra are shown in Figure 2b as a function of the Ge content. We find a clear shift in the absorber band gap towards higher energies with increasing Ge content, showing the expected linear behavior for the samples with Ge/(Ge + Sn) \(<\) 0.67. The slope of this relation amounts to (0.600 \(\pm\) 0.014) eV per Ge/(Ge + Sn), which is close to the value of 0.52 reported in the literature for other growth techniques [1,2]. For higher Ge contents, i.e., \(\geq\) 0.67, further increased Ge content in the layer does not lead to the expected band gap rise, indicating that the incorporation of Ge into the kesterite crystal may have reached a saturation point, above which the further addition of Ge may rather lead to condensation, secondary phases, or crystal defects. In this region, major losses, which are predominant in the high wavelength part of the presented EQE spectra, are visible, indicating the presence of transport issues, increased recombination, and reduced collection lengths [2]. The morphology of the samples was influenced by the added Ge content as shown in Figure 3. The morphology of the reference sample shows sharp grains with sizes of approximately 2 \(\upmu\)m. Samples with added Ge show a clear trend towards larger grains and less sharp edges. The most extreme sample with Ge/(Ge + Sn) = 0.86 shows grain sizes of approximately 5-6 \(\upmu\)m. With increased grain size the density of the grain boundaries decreases, which could influence the recombination behavior. Overall, the possible effects of grain size are assumed to be overlapped by other effects (e.g., secondary phases, strain, etc. ); therefore, within this study no conclusions were drawn about the possible influence of grain size on the device performance. Figure 2: (**a**) EQE spectra for selected samples and (**b**) extracted band gaps over Ge/(Ge + Sn) ratio determined from EDX measurements. The estimated uncertainty in values is \(\pm\) 0.01 for Ge/(Ge + Sn) ratio for samples up to Ge/(Ge + Sn) = 0.43 and \(\pm\) 0.04 for the masked data (excluded samples with Ge/(Ge + Sn) \(\geq\) 0.67). The band gap values have an uncertainty around 1 meV, estimated based on the fitting function. The SEM top-view images in Figure 3 show the formation of rod-shaped particles on top of the samples with Ge/(Ge + Sn) \(\geq\) 0.67. Using Raman measurements on the surface, these particles were identified as SnSe. For the performance of a depth-dependent phase analysis, a shallow-angle cross section was created using focused ion-beam milling [24]. The Raman and EDX measurements of the wedge-shaped cross section revealed that the SnSe secondary phase was occurring throughout the full depth of the absorber layer (see Figure S2). Interestingly, the sample with Ge/(Ge + Sn) ratio of 0.86 did not show clear SnSe secondary phases; however, large voids between the grains were observed, possibly indicating an evaporation of volatile secondary phases formed in the process (e.g., SnSe\({}_{2-\mathrm{x}}\) and GeSe\({}_{2-\mathrm{x}}\)).