# 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
Figure 5: External quantum efficiency (EQE) of an HF etched sample with improved light-coupling (blue line) and HCl etched sample with improved light-trapping (red line). The calculated top and bottom cell short-circuit current densities are indicated. The dotted lines represent absorption of typical layers calculated from absorption coefficients given in literature [52] for 1000 nm ZnO:Al (short dotted), 320 nm a-Si:H, and 3120 nm ucSi:H, assuming a single path absorption for top cell and path length enhancement of 2 utilizing the back reflector for the bottom cell absorber. The calculations neglect any light trapping, reflection or parasitic absorption losses in other layers. The reduction in primary cell reflection would also improve the bottom cell quantum efficiency in the NIR spectral range. However, the low NIR absorption in silicon and reflection at the rear reflector result in an additional contribution to reflected light and reflection does not provide sufficient information. The enhanced EQE in the NIR proves increased absorption from the light-trapping effect induced by the large features etched with HCL. Thus, the two effects light-coupling and -trapping can be distinguished quite clearly here. The maximum value in the short-circuit current density of the bottom cell can be attributed to a better light-trapping effect. In test series 1, the front contact features are larger (ca. 850 nm lateral) and show a significant depth (rms roughness around 120 nm). Therefore, these structures cannot be smoothened completely by the non-conformal silicon growth and the back reflector still shows a significant texture. In Figure 5 the EQE of the tandem cell with the highest bottom cell current density in test series 1 is displayed (red line). The increase in current in the bottom cell occurs especially in the NIR portion of the light, which strongly relies on a good light-trapping mechanism. The top cell current, on the other hand, shows a strong reduction in the short-circuit current density in comparison to the HF etched surface (blue line) which shows good light coupling properties. In addition to these effects there is a slight shift from the top cell to the bottom cell short-circuit current density in the transition region around 650 nm when changing from HF to HCl etched which also contributes to the differences in the top and bottom cell current densities. This could be due to a minor difference in cell thickness and/or the different scattering behavior at the front contact. Since the presented test series involved variously deposited and etched samples there might be additional effects involved influencing the absolute optical performance of each front contact within the tandem cell devices. The individual carrier density and film thickness after texture etching and the different back reflector textures may lead to variations in parasitic absorption. However, this mainly affects the bottom cells due to the excellent transparency of all ZnO:Al films in the visible spectral range. Furthermore the crystallinity of the uc-Si:H bottom cell was affected by different front contact textures although the amorphous top cell will alleviate the effects of the front contact texture on the bottom cell [62]. In the bottom cells of test series 2, the crystalline volume fraction as determined by Raman scattering measurements deviates between 39.6-53.6%. Therefore the bottom cells can exhibit different absorption coefficients although the samples were processed within the same silicon deposition. The reduction of reflection losses are attributed to the textured ZnO:Al/silicon interface. In addition, the HF etching of glass might lead to rough features or modify the glass morphology [63]. This could reduce reflection at the glass/air interface. However, we have never observed any change in glass haze or reflection, which is consistent with the low concentration of the hydrofluoric acid and the short etching duration. Thus, we completely neglected this effect.