## 3 Results
### Analysis of In\({}_{x}\)S\({}_{y}\) Layers and CIGS/Buffer Interface  
#### 3.2.1 Crystal Structure and Bandgap Energy of Sputtered In\({}_{x}\)S\({}_{y}\)  
Figure 3a depicts the GIXRD patterns obtained on 500 nm thick In\({}_{x}\)S\({}_{y}\) layers on glass substrates. Both layers deposited at RT and 220 \({}^{\circ}\)C are crystalline and exhibit the tetragonal \(\beta\)-In\({}_{2}\)S\({}_{3}\) structure [PDF no. 96-400-0814]. It should be noted that the reflexes of the cubic high-temperature structure [PDF no. 96-151-8188] overlap with most of the tetragonal reflexes, and [PERSON] et al. reported recently the formation of cubic In\({}_{2}\)S\({}_{3}\) by sputtering [12]. Therefore, a coexistence of this cubic \(\alpha\)-phase could not be totally excluded in our case and probably this phase could occur in case there are locally very high temperatures above 717 K [13]. Even the sputtering power (density) [14] and thickness of the In\({}_{x}\)S\({}_{y}\) layers [15] could influence the structural properties of the deposited In\({}_{x}\)S\({}_{y}\) films. The high-temperature modification \(\gamma\)-In\({}_{2}\)S\({}_{3}\) which typically forms for temperatures above 1049 K, as reported by [PERSON] et al. [13] is very unlikely to occur during our sputtering conditions.  
Figure 3b illustrates the difference in transmittance of In\({}_{x}\)S\({}_{y}\) films deposited at RT and 220 \({}^{\circ}\)C under otherwise identical conditions on suprasil quartz glass substrates. The evaluated thicknesses from these transmittance measurements are 490 and 480 nm for RT and 220 \({}^{\circ}\)C, respectively. Thus, there is only a slight reduction in the sputter deposition rate with temperature. The main finding is that the optical absorption edge slightly shifts to higher energies for the sample with the In\({}_{x}\)S\({}_{y}\) buffer deposited at 220 \({}^{\circ}\)C. According to a model with an indirect bandgap energy (\(E_{\rm g}\)) for In\({}_{2}\)S\({}_{3}\) the values of \(E_{\rm g}\) = 2.02 eV (RT) and \(E_{\rm g}\) = 2.08 eV (220 \({}^{\circ}\)C), respectively, were extracted by extrapolation of the linear region in the sqrt(\(\alpha\)_E_)-_E_-plot, as illustrated in Figure 3c. In case a model with a direct bandgap \(E_{\rm g}\) is used for the evaluation (not shown here), higher \(E_{\rm g}\) values would result but with the same trend--a slight increase in \(E_{\rm g}\) with elevated temperature. However, the corresponding \((\alpha\)_E_)\({}^{2}\)-_E_-plots show no distinct linear shape as expected for a semiconductor with a direct bandgap. Generally, an increase in  
Figure 2: Solar cell parameters of CIGS thin-film solar cells with 60 nm thick In\({}_{x}\)S\({}_{y}\) buffer layers sputtered at different substrate temperatures. For each substrate temperature 20 solar cells were measured: (**a**) Power conversion efficiency \(\eta\); (**b**) Open circuit voltage \(V_{\rm OC}\); (**c**) Fill factor FF; (**d**) Short-circuit current density \(J_{\rm SC}\).  
bandgap energy might be expected as a result of the incorporation of oxygen in In\({}_{2}\)S\({}_{3}\)[16]. In our case, this oxygen could probably stem from oxygen/water residual from the sputtering system, which could be more easily incorporated in the growing In\({}_{x}\)S\({}_{y}\) film deposited at higher substrate temperatures. Nevertheless, we could not clearly detect a higher amount of oxygen inside the In\({}_{x}\)S\({}_{y}\) layer grown at higher temperatures as reported in Section 3.2.2. A more likely explanation could be changes in chemical composition, i.e., in the sulfur/indium ratio as also shown Section 3.2.2.  
As a conclusion, only slight differences in structural and optical material properties between both In\({}_{2}\)S\({}_{3}\) layers sputtered at RT and 220 \({}^{\circ}\)C were observed, which is an indication that pronounced changes in solar cell performance could be more affected by the interface formation between CIGS absorber and In\({}_{x}\)S\({}_{y}\) buffer layer.