## 3 Results
### Analysis of In\({}_{x}\)S\({}_{y}\) Layers and CIGS/Buffer Interface
#### 3.2.2 Chemical Composition of In\({}_{x}\)S\({}_{y}\)/CIGS Interface Region  
Depth profiles were performed by ToF-SIMS in order to get an idea what influences the efficiency and \(V_{\rm OC}\) and to reveal changes in the chemical composition of the In\({}_{x}\)S\({}_{y}\) buffer and CIGS/buffer interface due to the different substrate temperatures during In\({}_{x}\)S\({}_{y}\) sputtering. The quantified ToF-SIMS depth profiles shown in Figure 4 highlight changes in the chemical composition of the In\({}_{x}\)S\({}_{y}\) buffer layer deposited on CIGS. The chemical composition of In\({}_{x}\)S\({}_{y}\) grown at RT is stoichiometric, i.e., the layer has a homogeneous In\({}_{2}\)S\({}_{3}\) composition over the whole 60 nm thickness; whereas for substrate temperatures higher than 200 \({}^{\circ}\)C, the In\({}_{x}\)S\({}_{y}\) composition becomes more sulfur-rich and indium-deficient, and the layers tend to be more inhomogenous. ToF-SIMS depth profiles obtained on samples sputtered at 160 and 180 \({}^{\circ}\)C (omitted here for clarity) reflect the same trend as the RT sample. Since pure argon without any addition of oxygen was used as sputtering gas, the oxygen concentration in all layers is expectedly very low, i.e., the S/(S+O) ratio is close to 1, indicating that no  
Figure 3: (**a**) GIXRD patterns measured with an incident angle of 0.5\({}^{\circ}\) on approximately 500 nm thick In\({}_{x}\)S\({}_{y}\) layers on suprasil quartz glass; (**b**) Transmittance measurement of about 500 nm thick In\({}_{x}\)S\({}_{y}\) layers on suprasil quartz glass; (**c**) Corresponding sqrt(\(\alpha\)\(E\))-\(E\)-plot (**indirect** bandgap energy for extraction of \(E_{\rm g}\) value was used). oxygen is incorporated during the sputtering process even at the higher substrate temperatures of 220 and 240 \({}^{\circ}\)C. Small amounts of oxygen are only detected at the In\({}_{\rm x}\)S\({}_{\rm y}\) surface. In addition to changes in the composition of the In\({}_{\rm x}\)S\({}_{\rm y}\) buffer, there is a pronounced copper depletion of the CIGS surface, which is more obvious for layers deposited at higher substrate temperatures of 220 and 240 \({}^{\circ}\)C compared to RT, 160, 180, and 200 \({}^{\circ}\)C. In a more detailed consideration, the copper depletion at the CIGS surface was estimated to rise from around 10 nm at RT, up to approximately 70 nm at 240 \({}^{\circ}\)C.  
ToF-SIMS depth profiles were also performed on complete CIGS solar cells from ZnO:Al front contact down to the soda-lime glass substrate to reveal differences in chemical composition between CIGS solar cells with efficiencies of 7.1% and 14.6% with In\({}_{\rm x}\)S\({}_{\rm y}\) buffer layers sputtered at RT and at 220 \({}^{\circ}\)C, respectively (Figure 5). The depth profile for the sample fabricated at higher substrate temperature exhibits a strong increase in the sodium signal inside the In\({}_{\rm x}\)S\({}_{\rm y}\) buffer layer and at the CIGS/buffer interface (note the logarithmic scale here). The sodium is originally diffusing from the soda-lime glass substrate through the molybdenum back contact into the CIGS layer at elevated deposition temperatures during the CIGS co-evaporation process but was not intentionally incorporated into the CIGS absorber or In\({}_{\rm x}\)S\({}_{\rm y}\) buffer layer. Figure 5: ToF-SIMS depth profiles performed with oxygen sputtering source on complete CIGS solar cells with stacking sequence ZnO:Al/i-ZnO/In\({}_{\rm x}\)S\({}_{\rm y}\)/CIGS/Mo/glass. (**a**) Solar cell with 7.1% efficiency and In\({}_{\rm x}\)S\({}_{\rm y}\) buffer deposited at RT; (**b**) Solar cell with 14.6% efficiency and In\({}_{\rm x}\)S\({}_{\rm y}\) sputtered at 220 \({}^{\circ}\)C substrate temperature. The enhanced sodium signal is highlighted with a dashed red circle. The observed sodium accumulation combined with a moderate copper depletion at the CIGS/In\({}_{x}\)S\({}_{y}\) interface region could be the most significant effect for an efficiency improvement of In\({}_{2}\)S\({}_{3}\) buffered CIGS-based solar cells.