## 4 Discussion  
We have demonstrated that we could achieve suitable film quality for In\({}_{x}\)S\({}_{y}\) buffer layers with the very fast deposition method of rf-magnetron sputtering applied to CIGS thin-film solar cells to realize efficiencies above 15%. For our sputtered buffer layers, we applied high sputtering deposition rates around 0.8 nm/s; compared to slower deposition processes such as ALD, CBD, ILGAR, and thermal evaporation. Nevertheless, there is still an efficiency gap compared to the CdS-buffered reference cells, which was also observed in our previous works with sputtered In\({}_{x}\)S\({}_{y}\) buffers [10; 17] and reported by other groups for CIGS cells with sputtered In\({}_{x}\)S\({}_{y}\) buffers [12; 18; 19], evaporated In\({}_{x}\)S\({}_{y}\) buffers [20; 21], and our own cells with In\({}_{x}\)S\({}_{y}\) layers grown by metal-organic chemical vapor deposition [22]. For both 500 nm thick In\({}_{x}\)S\({}_{y}\) layers deposited at RT and 220 \({}^{\circ}\)C substrate temperature on glass substrates, we obtained crystalline structures without any huge difference between both layers. We indicated the tetragonal \(\beta\)-phase for In\({}_{2}\)S\({}_{3}\). This result is in accordance with structural investigations on rf-sputtered In\({}_{x}\)S\({}_{y}\) layers reported by other groups [14; 15; 23], but also for structural investigations on evaporated In\({}_{x}\)S\({}_{y}\) layers [20; 24]. In contrast, high-resolution transmission electron microscopy analyses revealed nanocrystalline structures for our oxygen-containing CBD-In\({}_{x}\)(O,S)\({}_{y}\)[25] and sputtered In\({}_{x}\)(O,S)\({}_{y}\) buffer layers deposited from an In\({}_{2}\)(O\({}_{0.25}\)S\({}_{0.75}\))\({}_{3}\) target [26] at the CIGS/buffer interface in the complete device. The extracted bandgap energy (indirect) for our In\({}_{x}\)S\({}_{y}\) is in the range of 2.0-2.1 eV, thus not perfectly ideal for an application as a buffer in CIGS thin-film solar cells. With the addition of dopants like chlorine [8], oxygen [16], or sodium [1], the bandgap energy can be increased to suitable values to reduce parasitic absorption of the buffer and obtain high \(J_{\text{SC}}\). In addition, a beneficial band alignment with chalcopyrite-based absorber is necessary to obtain high \(V_{\text{OC}}\)[27] and close the efficiency gap to the CdS-buffered reference cells. In this sense, the surface termination of the CIGS absorber could play an important role. The parasitic absorption can be also reduced by using thinner In\({}_{x}\)S\({}_{y}\) buffers with thicknesses below 20 nm. In this case the subsequent high-resistive layer becomes more important in terms of band alignment. It should be noted that the power density during sputtering could influence the bandgap value of In\({}_{x}\)S\({}_{y}\) layers as well [14]. Our extracted bandgap energy increases slightly around 0.06 eV with the substrate temperature during In\({}_{x}\)S\({}_{y}\) sputtering. This trend with increasing temperature was also observed by other groups for sputtered In\({}_{x}\)S\({}_{y}\) layers [17; 18; 28]. In contrast, [PERSON] et al. reported an opposite trend as found for their In\({}_{x}\)S\({}_{y}\) deposited by pulsed direct magnetron sputtering [23]. From theoretical density calculations there is only a very small difference between direct and indirect transitions in \(\beta\)-In\({}_{2}\)S\({}_{3}\)[29; 30]. The substrate temperature during In\({}_{x}\)S\({}_{y}\) sputtering has a major influence on the performance of the device. Solar cells with In\({}_{x}\)S\({}_{y}\) sputtered at RT result in poor efficiencies whereas a maximum seems to be around 200 \({}^{\circ}\)C. For higher temperatures \(>\)240 \({}^{\circ}\)C, there is a decrease in efficiency and \(V_{\text{OC}}\) again. The strong dependency of solar cell efficiency on substrate temperature during sputtering and sample pre-conditioning was also observed in previous experiments at ZSW [10; 17] and by other groups [31].