# Coupling and Trapping of Light in Thin-Film Solar Cells Using Modulated Interface Textures
## 2 Materials and Methods
### Texture Etched ZnO:Al  
All ZnO:Al films were textured in a wet-chemical etching step. Three test series were evaluated as illustrated in Figure 1b: test series 1 and 2 consist of single textures with various feature sizes in a wide range, while series 3 was prepared for optimization of single and double textures. The texturing step was performed either in 0.5 w/w% HCl, or 1 w/w% HF, or a combination of double etching processes to yield a broad variety of surface textures [46; 47]. Samples in test series 1 were textured in a single HCl (29 s-120 s) or a single HF (180 s) step. Samples in test series 2 were textured in a double etch step with an initial HF etch step (50 s) and a subsequent HCl dip with various durations (0-32 s). The ZnO:Al front contact layers in test series 1 and 2 both feature a simple, single surface texture level. The samples in Section 3.3 (test series 3) on the other hand were textured either in a single HCl etch step or an HCl etch step (29-150 s) followed by an HF dip etch (20-30 s). Details regarding the etching processes with HF and HCl can be found elsewhere [45; 48; 49]. The two-step etched samples in test series 3 exhibit surface textures with two superimposed texture levels--large and small craters, the so-called double texture.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Layer**} & **Power** & **Process** & **Heater** & **Argon** & **Oxygen** & **Nitrogen** & \multirow{2}{*}{**Vcarrier**} & **No.** \\  & **Output** & **Pressure** & **Temperature** & **Flow** & **Flow** & **Flow** & **Vcarrier** & **of Passes** \\ \hline  & [kW] & [Pa] & [\({}^{\circ}\)C] & [sccm] & [sccm] & [sccm] & [mm/s] & \\ SiO\({}_{x}\)N\({}_{y}\) & 4 & 0.3 & 25 & 200 & 18 & 100 & 3 & 2 \\ ZnO:Al & 10 & 0.3–1 & 430–500 & 200 & - & - & 8 & 7–10 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Deposition parameters of the SiO\({}_{x}\)N\({}_{y}\) and the ZnO:Al layer. vcarrier is the velocity of the carrier while passing the target during dynamic deposition.  
After etching the sample surfaces were characterized by atomic force microscopy (AFM) and scanning electron microscope (SEM) measurements. In the case of AFM measurements, the 3D topography data were analyzed. A watershed algorithm was applied, to detect the surface features' boundaries and statistically evaluate the average feature diameter. The light-scattering properties of the rough ZnO:Al and silicon/back contact interfaces were analyzed using angular resolved scattering (ARS) measurements of the reflected light to estimate the surface texture evolution during the silicon deposition. This method uses a laser (wavelength 550 nm) to illuminate the surface and detects the distribution of the light scattered into different angles. Though far field measurements do not provide access to the light enhancement in solar cells [17], far field angular intensity distribution (AID) ia a good indicator for the surface topography, because information regarding the roughness, angle distribution or lateral feature diameter influence the measured results [20; 50; 51].