## 3 Results and Discussion
The overall composition shifted from a Cu-rich start to a Cu-poor (& Zn-rich) absorber during the process due to GeSe incorporation (see Figure S4). The ex-situ Raman and EDX measurements suggest that the Ge incorporation into kesterite does not significantly influence the reported formation path of the kesterite phase [11; 12]. In the case of Ge incorporation, the Cu-rich to Cu-poor composition shift occurs at a slightly lower temperature (480 \({}^{\circ}\)C to 530 \({}^{\circ}\)C) due to the higher vapor pressure of GeSe\({}_{2-\mathrm{x}}\) as compared to SnSe\({}_{2-\mathrm{x}}\) (see Figure S1). This means that up to this temperature the process is unaltered and all reaction steps and pre-steps of the kesterite formation are not influenced. Figure 4: (**a**) Raman spectra and (**b**) atomic percentages of Sn and Ge for the samples prepared from interrupted-annealing experiments, quenched at temperatures of 300 \({}^{\circ}\)C, 350 \({}^{\circ}\)C, 400 \({}^{\circ}\)C, 450 \({}^{\circ}\)C, 480 \({}^{\circ}\)C, 500 \({}^{\circ}\)C, 530 \({}^{\circ}\)C and for samples with \(T_{\mathrm{max}}\) of 530 \({}^{\circ}\)C including a dwelling step of 10 min, 20 min and 30 min duration. (**c**) The cross-section EDX mappings show the distribution of Sn and Ge for samples without and with a dwelling step. We note that XRD measurements of selected samples further confirms that no significant Ge incorporation is observed until 450 \({}^{\circ}\)C. XRD measurements show no signature peaks corresponding to the SnSe\({}_{2-\mathrm{x}}\) and GeSe\({}_{2-\mathrm{x}}\) phases (Figure S5 and description). To measure the PV device characteristics, all the samples were coated with CdS and TCO layers to fabricate the solar cells. A summary of IV parameters and EQE analysis for samples with Ge/(Ge + Sn) \(\leq\) 0.43 is given in Figure 5.  
As expected, the open-circuit voltage \(V_{OC}\) is increased, and the short-circuit current \(J_{SC}\) drops due to the increased band gap (see Figure 2). However, the \(V_{OC}\) deficit follows the trend of the _Voc_, showing that the increased band gap of the absorber is only partially converted into an open-circuit voltage gain. The magnitude of the \(V_{OC}\) deficit, commonly attributed to Sn-related deep defects, remains unaffected by the approach in our study. The Urbach energy \(E_{U}\), extracted from the EQE spectra, describes the magnitude of the band tailing and is considered as an indicator of crystal quality [29; 30]. A clear difference between \(E_{U}\) of the reference and Ge-containing samples is visible. This can be explained by an increase in the general crystal disorder by adding an extra element to the crystal structure in the case of Ge-containing kesterite. The fill factor \(FF\) of all CZTGSe devices was found to be lower than the reference CZTSe. A possible reason for this could be the non-adapted solar-cell-device-processing steps. These were only optimized for the Ge-free kesterite and simplified for a facile procedure that contains no treatments besides the main steps (i.e., no air annealing, no etching, etc.). In the case of Ge-containing kesterite, additional steps such as etching or pre-treatment for the buffer-layer deposition might be necessary to improve the CdS homogeneity and quality of the interface in order to overcome the loss in the observed fill factors. The noticeable scattering of the data shown in Figure 5 is due to process variations  
Figure 5: Device properties for CZTGSe-based solar cells with varying Ge content: \(V_{OC}\) and \(V_{OC}\) deficit, calculated relative to the Shockley-Queisser limit on the left axis; \(\mathrm{n}_{\mathrm{v}}\)\(E_{U}\), \(J_{SC}\) and \(FF\) on the right axis. Trend lines provided as a guide for the eyes. over a longer time range (months), different substrate batches and different CdS deposition batches. The overall PCE \(\eta\) for CZTGSe cells is lower than the Ge-free-CZTSe reference, yet still shows one of the highest reported efficiencies of 10.4%, with a Ge/(Ge + Sn) ratio of 0.32 (\(E_{G}\) = 1.22 eV).