## 4 Discussion
For higher substrate temperatures above 220 \({}^{\circ}\)C there seems to be a destruction of the important pn-junction at the CIGS/buffer interface resulting in deteriorated solar cell parameters. In addition, the substrate temperature during In\({}_{x}\)S\({}_{y}\) sputtering has an influence on the chemical composition of the In\({}_{x}\)S\({}_{y}\) buffer and CIGS/buffer interface. In this work, we observed an increasing sulfur/indium ratio starting from a value of 3:2 (In\({}_{2}\)S\({}_{3}\)) grown at RT with increasing substrate temperature during sputtering, similar to the trend in our previous experiments, but with a different starting ratio of around 1:1 [17], and similar to results obtained by [PERSON] et al. [31]. The sodium accumulation in the In\({}_{x}\)S\({}_{y}\) buffer and at the CIGS/buffer interface and the copper-depleted CIGS surface seem to be the most likely candidates to increase the \(V_{\text{OC}}\) of the 220 \({}^{\circ}\)C cell compared to the RT cell. Sodium addition to CIGS in general increases the \(V_{\text{OC}}\), e.g., in case a sodium-free substrate is used and sodium is intentionally incorporated with a precursor [32] or post-deposition process [33]. Sodium accumulation in the buffer and at the CIGS/buffer interface was also found after post-annealing processes for CBD Zn(O,S)-buffered devices [34]. A sodium accumulation on the nano scale was observed by atom probe tomography measurements at the CIGS/In\({}_{\text{x}}\)S\({}_{\text{y}}\) interface for samples annealed at temperatures above 225 \({}^{\circ}\)C compared to the RT samples without sodium accumulation [12]. The observed copper depletion at the CIGS surface could result in an ordered vacancy compound with a slightly higher bandgap due to a lowered valence band maximum. This feature could promote an improved band alignment in combination with the In\({}_{\text{x}}\)S\({}_{\text{y}}\) buffer and could be also a reason for improved \(V_{\text{OC}}\) values as reported for evaporated In\({}_{\text{x}}\)S\({}_{\text{y}}\):Na buffer on Cu(In,Ga)(S,Se)\({}_{2}\)[27]. The formation of such a Cu-poor CuIn\({}_{\text{5}}\)S\({}_{\text{8}}\) composition at the CIGS/sputtered In\({}_{\text{x}}\)S\({}_{\text{y}}\) interface was observed at relatively high temperatures around 340 \({}^{\circ}\)C [31]. In general, the integral copper content of the CIGS absorber can significantly influence the efficiency and the dominating recombination mechanism of In\({}_{\text{x}}\)S\({}_{\text{y}}\)-buffered devices [35]. The phenomena of copper diffusion at the CIGS/In\({}_{\text{2}}\)S\({}_{\text{3}}\) interface, when investigated, was mostly isolated from alkali elements in the past. [PERSON] et al. clearly showed with high kinetic energy X-ray photoelectron spectroscopy that above 200 \({}^{\circ}\)C, the copper diffusion from CIGS into In\({}_{\text{2}}\)S\({}_{\text{3}}\) is enhanced [36]. Recent calculations with density functional theory also took sodium into account and revealed an interplay between sodium and copper diffusion at the CIGS/In\({}_{\text{2}}\)S\({}_{\text{3}}\) interface, and the formation of Na- and Cu-containing phases is very likely due to strong negative driving force energies (energy releasing processes). The even lower driving force energy value for sodium implies that sodium diffusion is more favored than copper diffusion and would therefore be actuated earlier [37]. This difference could be one explanation for our observed results from the depth profiles: First, there is a sodium diffusion up to 200 \({}^{\circ}\)C with only small changes in copper. For higher temperatures \(>\)220 \({}^{\circ}\)C the copper diffusion is no longer hindered by the sodium and copper diffusion takes place. Following this argumentation, the important parameter for an increased efficiency is the sodium accumulation at the CIGS/buffer interface and a moderate copper depletion at the CIGS surface. An enhanced copper enrichment of the In\({}_{\text{2}}\)S\({}_{\text{3}}\) layer would be detrimental.