## 1 Introduction  
Although texturing of the transparent electrode of thin-film solar cells (particularly amorphous silicon solar cells) has long been used to enhance light absorption (and thus the conversion efficiency) via light trapping, such texturing has involved low roughness (i.e., structures with heights less than 100 nm) and thus is limited regarding the degree of light absorption enhancement that is possible in thin-film solar cells (see, e.g., Reference [1]). With the recent developments in nanotechnology, a new approach called photon (light) management [2; 3; 4; 5; 6; 7] in thin-film solar cells has been developed to further enhance the light absorption in thin-film solar cells; the ultimate goal is to enable even thinner films to be used while still providing higher conversion efficiency. Of the wide variety of approaches to perform photon management in thin-film solar cells, one particularly attractive approach is the use of nanostructured substrates with an array of nanostructures of high aspect ratio that have greater height from the substrate; such a nanostructured morphology manifests itself in the subsequently deposited thin-film solar cell, thereby allowing improved optical absorption [2; 3; 4; 5; 6; 7; 8]. The higher aspect ratio structures that can be produced are particularly advantageous for enhancing the light absorption in thin-film solar cells with very thin absorber layers, e.g., amorphous silicon (a-Si) solar cells deposited onto a nanostructured substrate comprised of an array of nanopillars (see e.g., [5]). Because of the \"folded\" morphology of the subsequently deposited thin-film solar cell on the nanostructured substrate, typically based on a p-n or p-i-n junction semiconductor device, we refer to such a device as a \"folded junction\" solar cell [2; 3; 4].  
Despite the great progress in the development of photon management in thin-film solar cells using nanostructured substrates, most of the studies on the use of nanostructured substrates focus on the improvement in optical absorption in folded junction solar cells and do not consider the effect of reduced material quality of thin-film solar cells deposited on such nanostructured substrates. Understanding the relationship between material quality and nanostructured morphology is crucial for practical implementation of nanostructured substrates in thin-film solar cells.  
Here, we report the observation of a correlation between the reduction of the open circuit voltage (related to the degradation of the material quality) of a-Si thin-film solar cells and the areal number density of high aspect ratio structures of the front contact (i.e., transparent conducting electrode of the solar cells in the superstrate configuration) of each solar cell. As discussed above, the high aspect ratio nanostructures enable enhanced light absorption in the folded junction solar cells via improved photon management; as a result, a higher areal density of such nanostructures corresponds to an overall increase in the light absorbed by the folded junction solar cell. However, the enhanced light absorption does not necessarily correspond to enhanced conversion efficiency, particularly if the material quality of the absorber layer is degraded [9]. Thus, we report a trade-off between photon management efficacy and material quality in a-Si folded junction solar cells. The results highlight the importance of considering the material quality of the absorber layer of thin-film solar cells deposited onto nanostructured substrates.