## 3 Results and Discussion  
### Morphological Characterizations  
As an example, Figure 3a shows the top-view SEM micrograph, acquired after the complete removal of the embedding nanoporous AAO template, of a dense array of vertically-aligned Cu NWs prepared via ECD at a reduction potential of \(-\)0.2 V applied versus the Ag/AgCl reference electrode. The inset presents a corresponding tilted SEM image of the Cu NW \"forest\". As can be seen, the obtained Cu NWs retain their predominant vertical orientation with respect to the substrate after AAO host removal. Moreover, a homogeneous array of Cu NWs was obtained, whereas it was experimentally observed that their lengths varied between 250-350 nm (depending the duration of the ECD process) and their average diameter was about 90 nm. The latter value, coupled with an observed center-to-center distance of about 150 nm, resembles a densely-packed Cu NW array obtained while employing a supported AAO template with a superficial porosity (P) of \(\sim\)32.5%, as estimated with Equation (4) [36]:  
\[\mathrm{P}\left[\%\right]=\frac{\pi}{2\sqrt{3}}\cdot\frac{\phi^{2}}{\mathrm{D }^{2}}\cdot 100 \tag{4}\]  
where \(\phi\) is the average NW diameter and D is the mean distance between the centers of two adjacent NWs, with both geometrical parameters directly evaluated through SEM analysis. Notably, the estimated porosity value was slightly lower compared with other studies [23; 24; 25; 27] due to the specifically selected parameters engaged in the anodization process, ultimately yielding Cu NWs of slightly reduced density. This further facilitates adequate penetration of the CdTe through the NW array. Figure 3b-d presents cross-sectional SEM micrographs of the Cu NWs supported on the Au-coated Si substrate and subsequently covered by CdTe thin films obtained by VTE (Figure 3b), RF-MS (Figure 3c), and ECD (Figure 3d). As can be seen, the evaporated CdTe film (Figure 3b) is homogeneous with an obvious intrinsic columnar nature due to the selected parameters employed in the VTE process, which favored growth along (111) crystalline plane of the \"zinc blende\"-type structure of CdTe, as reported elsewhere [37] and as confirmed later by XRD analysis. However, the anisotropic behavior of the CdTe evaporation process, with a high associated deposition rate (\(\sim\)700 nm/min), determined the formation of a \(\sim\)2.1 um-thick CdTe thin film in a relatively short time of \(\sim\)3 min, as directly measured by SEM. Consequently, as can be observed in Figure 3b, the penetration depth of the CdTe through the Cu NWs was observed to be extremely low, rendering the method rather unsuitable to properly fill the gaps in the Cu NW array. In contrast, CdTe deposition via the RF-MS procedure (Figure 3c) led to slightly better results in terms of CdTe protrusion through the Cu NWs, although the multiple voids detected still render it unacceptable. The improved penetration can be explained by the isotropic nature of the sputtering process, which is typically coupled with a lower deposition rate (\(\sim\)70 nm/min), as the thickness of the CdTe was \(\sim\)5.5 \(\upmu\)m obtained in \(\sim\)90 min, as observed in Figure 3c. However, it can be assumed that in both CdTe deposition scenarios (i.e., VTE and RF-MS), superior results could be achieved through better control of the Cu NW aspect ratio and/or by further reducing the density of the Cu NWs forming the array. This would incur the cost of diminishing the active surface at the collecting electrode/inorganic absorber layer interface within a \"substrate\"-type photovoltaic structure based on semiconducting A\({}^{\Pi}\)BVI heterojunctions. Notably, the ECD process (Figure 3d) revealed promising results, a consistent CdTe thin film being electrochemically synthesized through the Cu NWs array, as the top-view SEM image depicted in the inset of Figure 3d demonstrates.