# Coupling and Trapping of Light in Thin-Film Solar Cells Using Modulated Interface Textures
## Results
### Good Coupling and Scattering Using Double Textured ZnO:Al  
A double texture is needed to generate a suitable texture for maximum current output. According to the previously discussed test series 1 and 2, this texture should exhibit large features with significant depth to facilitate good light-trapping at the back reflector and smaller very sharp features to enable a good light-coupling at the same time. Note, that high aspect ratios cause an area increase of the contact interface, which results in stronger interface recombination [67], and sharp features may deteriorate the quality of the absorber layer [68]. A tradeoff between good optical and good electrical device performance must be found. One possibility to generate a double structure with sputter deposited ZnO:Al is the use of a double etching step with two acids. It was already demonstrated that etching in HF in comparison to etching in HCl shows more points of attack on the same base material and leads to sharper surface features. Using a double etching step in first HCl to define the primary texture with larger features and second HF to define the secondary texture with smaller sharper features shows the possibility to generate a double-textured light-trapping/coupling concept. The ZnO:Al film properties, that are controlled by the deposition conditions, influence the texturing outcome of both etching steps. Therefore, lateral and vertical dimensions of the etch features had been adjusted by the deposition and by the two etching processes [69]. The ranges of the deposition parameters are given in Table 1.  
The sputtered ZnO:Al front contacts were optimized for tandem solar cell application using a high rate sputtering process on float glass. Figure 7 shows two SEM surface images of the optimized ZnO:Al single texture after the first etching step in HCl (120 s, left) and the optimized double texture after the second etching step in HF (20 s). For optimization, both deposition parameters and etching parameters were adjusted in test series 3. During the second etching step, small craters have formed on top of the primary HCl-etched texture. There are reports in the literature that further demonstrate the double texture type of surface [64; 70]. The textures shown here are realized with high rate sputtering processes onto industrial float glass. The presented single- and double-textured ZnO:Al films were optimized regarding their optical performance in tandem cells. The size and density of small craters in the secondary texture varied between the different ZnO:Al depositions. The deposition and etching conditions had to be adjusted to yield a trade-off between good HCl etching conditions ensuring a suitable crater size for light-trapping and good HF etching conditions ensuring the formation of suitable smaller craters for light-coupling. Figure 8 shows the external quantum efficiency (EQE) and the cell reflectance values of the single-textured sample (ST, blue line) and the double textured (DT, red) sample in Figure 7. The short-circuit current densities calculated using the EQE of the top and bottom cells are given in the graph. By applying the double texture an increased short-circuit current density for both top and bottom cell is observed. In this case the increase in top cell current is more pronounced with 0.8 mA/cm\({}^{2}\) as compared to an increase of 0.3 mA/cm\({}^{2}\) in the bottom cell. The cell reflectance measurement clearly shows less cell reflection and hence better light coupling with the double-textured front contact in the visible range. As the second etching step applied for the double texture reduces the front contact film thickness, less NIR absorption occurs in the front contact ZnO:Al layer increasing the bottom cell current. In this particular case, the enhancement of the bottom cell current is attributed to lower parasitic absorption in the front contact rather than better light-trapping.