# 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
Based on the deviations in solar cell performance between test series 1 and 2 it can be concluded that not only the feature size but also the roughness of the texture and therefore the depth of the features significantly influence the optical performance [57; 12]. In order to interpret the function of different texture features, it is necessary to understand the two basic effects of enhancing the optical performance of a solar cell: light-coupling and light-trapping. Improved light-coupling causes the light to enter the solar cell efficiently by decreasing reflection at the interface between ZnO:Al and silicon. Here, the reduced reflection is achieved by an effective index grading at the rough ZnO:Al/silicon interface. The second optical effect, light-trapping, is caused to a large extend by light scattering at the rough reflector. The texture of the back reflector in superstrate technology is mainly influenced by two factors: the front contact texture and the evolution of the growing absorber layer's surface. This modification is governed by a flattening of the rough interface and by the growth inherent texture of the absorber layers [58; 59; 60; 21]. Studies have shown that the growth of silicon is non-conformal [58; 21; 61]. The original front contact texture is overgrown by a cauliflower-like structure in which the dimensions scale with the deposited film thickness. Keeping this in mind the trends in short-circuit current density that was measured in relation to the average feature diameter and the roughness of the textured ZnO:Al surface can be attributed to the afore mentioned optical effects light-coupling and light-trapping. Features of around 400 nm in diameter with a significant roughness (HF etching, series 1) represent a good effective medium and induce better light-coupling into the solar cell. Figure 4 shows cell reflection measurements on HF and HCl etched samples from series 1. The cell reflection is visibly reduced in the short wavelength region using the HF etched samples as compared to the HCl etched samples. The effect will also affect the long wavelength range, but it is hidden behind the light-trapping effect. Therefore, the current of top  
Figure 3: 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 samples ‘A’, ‘C’, and ‘D’ are further discussed in Section 3.2.  
and bottom cells are both increased. The strong interference fringes in the near-infrared region are caused by the low absorption coefficient of the silicon (below 100 cm\({}^{-1}\) at 1000 nm). The amplitude of the interference is reduced for effectively scattering interfaces. Figure 5 shows the external quantum efficiency (EQE) of the top and bottom cells of an HF and HCl etched ZnO:Al sample from test series 1. In addition, we provide optical absorption spectra for a typical ZnO:Al front contact and top and bottom cell absorber calculated from absorption coefficients. Note that for the bottom cell we calculated the absorption for twice the absorber layer thickness given the light is absorbed a second time after reflection at the rear side. The strong absorption enhancement in near infrared spectrum in the solar cells proves significant light-trapping for both types of texture. The HF etched ZnO:Al leads to an improved light-coupling due to a refractive index grading (blue line). The increase in the visible range of the a-Si:H cell especially indicates that more light enters the solar cell. This part of the spectrum is generally absorbed very efficiently and does not rely on any pathway elongation. Thus it is mainly governed by the light-coupling. Figure 4: Cell reflection measurements of various HF (blue lines) and HCl (red lines) etched samples from series 1.