## 4 Discussion  
In traditional thin-film solar cells, the degree of surface roughness (i.e., feature height) is small compared to the absorber layer thickness. As a result, the deposition of the absorber layer onto such a surface is approximately the same as the deposition onto a flat substrate. However, when the aspect ratio of the nanostructures on the nanostructured substrate is high (i.e., \(>\)1) and the absorber layer thickness is small compared to the nanostructure height (i.e., the \"folded junction\" configuration of a thin-film solar cell on a nanostructured substrate) the material quality of the absorber layer is a critical issue in the device performance. In the devices considered above with small nanostructures, the aspect ratio of the smaller nanostructures is low (approximately 1) and the height of the smaller nanostructures is small compared to the absorber layer thickness. As a result, the material quality (as indicated by the open circuit voltage of the solar cells) is high for the region of low surface roughness (i.e., primarily smaller nanostructures). However, in the regions with increasing numbers of higher aspect ratio structures for a given area and with height comparable to the absorber layer thickness, the open circuit voltage becomes progressively lower with the increase in areal density of higher aspect ratio structures, i.e., the material quality of the absorber layer becomes progressively worse. Furthermore, the consistent fill factor of all devices suggests that no pinholes exist in the devices, i.e., no shorting occurred. As a result, increased carrier recombination caused by reduced material quality is the primary issue, with carrier recombination increasing with increased areal density of nanostructures, thereby offsetting any enhancements in carrier generation via improved photon management. This reduced material quality is primarily due to the poor conformal coverage of the a-Si layer around the larger nanostructures [8]; such effects are more acute for microcrystalline Si solar cells (see, e.g., [9]). Note that our nanostructures have an added complication of being nonuniform in distribution and orientation. Although such nonuniformities can contribute to the  
Figure 4: FIB SEM image of a solar cell similar to device 15 in Figure 3; note that the vertical scale bar is corrected for the viewing angle, i.e., 476 nm (cs) indicates that the actual height of the nanostructure after correcting for the viewing angle is 476 nm. Each high aspect ratio nanostructure (or group of nanostructures) corresponds to a large dome-like feature in the top-view SEM images of the devices (e.g., device 30 has a high number of such high aspect ratio features), and the regions with the smaller nanostructures of lower aspect ratio correspond to the flatter regions in the top SEM images of the devices (e.g., device 3 predominantly has such low aspect ratio features). The scale bar in between the two top-view SEM images on the right is the scale bar for both images. absorber material degradation via inhomogeneity in the absorber layer deposition, the primary cause of the material degradation is the lack of conformal deposition of the absorber material that is further exacerbated with increased aspect ratio of the nanostructures that are substantially oriented perpendicular to the substrate. A convenient figure of merit to determine whether material quality issues come into play is the relative thickness (absorber layer thickness/nanostructure height) \(t_{relative}\) divided by the aspect ratio of the relevant nanostructures on the nanostructured substrate \(AR\), i.e., \(t_{relative}/AR\). If the figure of merit is less than 1 (\(t_{relative}/AR<1\)), then one must consider the effects of deposition onto the nanostructured substrate on the material quality of the absorber layer. The primary effects on the material quality are structural defects caused by poor conformal coverage of the absorber layer.