# Coupling and Trapping of Light in Thin-Film Solar Cells Using Modulated Interface Textures
## Results  
### Solar Cells on Single Textured ZnO:Al with Different Feature Size  
All ZnO:Al samples were deposited within the chosen deposition regime in Table 1 and exhibit carrier concentrations in the range of 2.8-3.3 \(\times\) 10\({}^{20}\) cm\({}^{-3}\) and carrier mobilities in the range of 25-45 cm\({}^{2}\)V\({}^{-1}\)s\({}^{-1}\). The sheet resistance of all films in test series 1 and 2 range between 4.8 and 21.5 \(\Omega\). The sheet resistance is low enough for good electrical performance of thin-film silicon tandem solar cells. The spread in electrical properties and slightly different film thicknesses might cause minor effects of parasitic free carrier absorption in the long wavelength range [5]. In Figures 2 and 3 the short-circuit current densities (calculated using EQE measurement) of the a-Si:H top cell (a), \(\upmu\)c-Si:H bottom cell (b), and the root-mean square roughness (Rq) of the ZnO:Al front contact texture (c) are displayed as a function of the average feature diameter (dave) on the ZnO:Al surface for series 1 and 2, respectively. The ZnO:Al films of series 1 were prepared at various deposition conditions and have been etched either in HF only (shaded area) or in HCl by single step etching. The HF etched ZnO:Al films show a significantly smaller and less spread average feature diameter than samples etched in HCl. The different surface textures cause variations in optical solar cell performance. All HCl etched samples show almost the same short-circuit current density in the top cell whereas the short-circuit current density in the bottom cell increases with increasing average feature diameter. It is noted that for all HCl etched ZnO:Al samples the roughness increases with increasing feature diameter as well. The HF etched samples, on the other hand, show the smallest average feature diameter, however, due to the steeper crater angles some of these samples exhibited higher roughness than the HCl etched samples with slightly larger feature diameter. The roughness in conjunction with the small feature diameter leads to an increase in top cell short-circuit current density. The bottom cell current, however, does not reach the maximum value of the HCl etched sample. While small features around 400 nm with high roughness exceeding 90 nm boost the short-circuit current density in both top and bottom cell, larger features (ca. 850 nm) with roughness above 90 nm only improve the bottom cell short-circuit current density. The second test series is used to extend the investigated parameter regime of lateral feature diameter and roughness further (see Figure 3). The surface textures were created on the same ZnO:Al base material. Using an initial HF etching step--50 s for all samples--numerous points of attack were generated on the ZnO:Al surface. A subsequent dip in HCl then widened the craters [46]. With this method, a variety of different average feature diameters can be fabricated. Figure 2: Short-circuit current density calculated from external quantum efficiency measurements of (**a**) top cell Jsc, top, (**b**) bottom cell Jsc, bottom, and (**c**) rms-roughness Rq as a function of the average feature diameter of textured ZnO:Al surface for test series 1. The sample ‘B’ is further discussed in Section 3.2. In comparison with test series 1, the roughness values of test series 2 are significantly lower. Especially the textures with small feature sizes (200 nm-400 nm) exhibit roughness values below 60 nm. The roughness values of samples with larger feature sizes, however, saturated at about 70 nm. The solar cells were deposited in a different deposition system, so the quantitative current density values are not directly comparable to series 1. The short-circuit current density of the top cells does not show any significant variation with the surface texture. The maximum in bottom cell current density was observed at feature sizes of about 700 nm, which is slightly smaller than in test series 1.