## 3 Results and Discussion
Considering sample homogeneity, morphology and EQE characteristics, the developed process therefore seems suitable for synthesizing CZTGSe-absorber layers with low to intermediate amounts of Ge (i.e., Ge/(Ge + Sn) \(\leq\) 0.43), which appears to be more than sufficient to cover the composition range for highest expected device performance reported in the literature [2,8,25]. The discussion in the following section is therefore focused on the samples with Ge/(Ge + Sn) \(\leq\) 0.43. According to our previous work on Ge-free-kesterite devices [11,12], the CZTSe phase forms below a temperature of 400 \({}^{\circ}\)C in the presence of secondary phases (e.g., ZnSe and Cu\({}_{2}\)Se). At higher temperatures, the SnSe\({}_{2-\mathrm{x}}\) vapor from the supplied Sn wire in the graphite susceptor incorporates into the layer, leading to further growth of the CZTSe phase by reacting with ZnSe and Cu\({}_{2}\)Se, leading to a composition shift (Cu-rich to Cu-poor). To understand the formation path and the composition shift in the Ge-containing process, a series of interrupted-annealing experiments was performed. The selenization process was interrupted at different temperatures by quenching with nitrogen gas. Raman and EDX analyses were carried out on the samples and the results are summarized in Figure 4a. Up to 400 \({}^{\circ}\)C, the process is expected to behave identical to our reference Ge-free kesterite process. No significant amount of Ge was found in the layer (Ge/(Ge + Sn) \(<\) 0.02) and the A1 mode is centered at wavenumbers below 196 cm\({}^{-1}\), indicating no presence of Ge in the CZTSe phase [26]. After reaching the annealing temperature of 480 \({}^{\circ}\)C, the first indications of Ge incorporation were observed. Raman shows a shift of the A1 mode from 194.6 cm\({}^{-1}\) at 430 \({}^{\circ}\)C to 196.3 cm\({}^{-1}\) at 480 \({}^{\circ}\)C and the EDX measurements show a ratio of Ge/(Ge + Sn) \(\approx\) 0.06. Once the temperature reaches 500 \({}^{\circ}\)C, Raman peaks related to secondary phases disappear (as described above for the reference process [11,12], here shown Cu\({}_{2-\mathrm{x}}\)Se at 263 cm\({}^{-1}\)[27,28]), the A1 mode shifts further to 196.8 cm\({}^{-1}\) and the Ge content (from EDX) further increases (Ge/(Ge + Sn) \(\approx\) 0.07). At 530 \({}^{\circ}\)C, the largest A1 mode shift of 201.7 cm\({}^{-1}\) is observed, which would correspond to a Ge content of Ge/(Ge + Sn) \(\approx\) 0.56 (see Figure S3). However, the corresponding Ge content observed from EDX measurements only amounted to Ge/(Ge + Sn) \(\approx\) 0.24. The possible reason for this significant discrepancy could be a higher Ge concentration at the surface region of the layer  
Figure 3: SEM pictures showing the morphology of samples with varied Ge/(Ge + Sn) ratios. detected by Raman, which is more surface sensitive than EDX [8]. For the samples with a dwell time of 10 min and longer, the A1 mode shifts back towards lower wavenumbers, i.e., 199.4 \(\pm\) 0.3 cm\({}^{-1}\), and the peak position matches again with the EDX measured Ge concentration (Ge/(Ge + Sn) \(\approx\) 0.30) (see Figure S3). To investigate the possibility of a composition gradient in the sample from the interruption experiment, we performed cross-sectional EDX mappings of the sample corresponding to 530 \({}^{\circ}\)C (quenched), and 530 \({}^{\circ}\)C with 20 min of dwell time (normal procedure). Figure 4c shows the Sn and Ge distribution across the depth of these samples, exhibiting higher accumulation of Ge on the surface of the layer for the interrupted sample (Figure 4c, left hand side). The sample with dwell time shows intermixed Ge and Sn throughout the depth of the absorber (Figure 4c, right hand side). This result confirms the previously discussed observation from Raman and EDX and illustrates the formation and decay of an initial front-surface Ge gradient during this process. We note that the Sn content in the layer stays constant throughout all the samples.