## 3 Results and Discussion
### Morphological Characterizations
The result can be attributed to the \"bottom-up\" nature of the ECD process, wherein CdTe film growth occurs through electrochemical reduction mechanisms close to the working electrode surface, as previously explained. In addition, the measured thickness of the electrochemically-synthesized CdTe film was \(\sim\)250 nm, obtained in \(\sim\)33 min, points to a low growth rate of CdTe (\(\sim\)7 nm/min) achieved under optimized conditions with the Te-diluted electrolyte (ECD potential and process temperature). The latter conditions favor the growth of a compact CdTe able to penetrate the Cu NW array effectively, thus facilitating the achievement of an excellent interface between the Cu NW-based collecting electrode and the subsequent CdTe absorber layer in a \"substrate\"-type solar cell configuration. As previously mentioned, the protrusion capability of the CdTe through the NWs is affected by both the NWs' density and their average aspect ratio, as well as being notably affected by the intrinsic morphological and structural properties of the subsequently-deposited CdTe thin films, which inherently lead to different topography features. In this context, complementary information in addition to SEM images (i.e., surface roughness and overall topography) was obtained through AFM analysis performed on control samples (fabricated in the same conditions), allowing investigation of individual CdTe thin films  
Figure 3: (**a**) Top-view scanning electron microscope (SEM) image of a dense array of Cu NWs supported on the Au-coated Si substrate. The inset shows corresponding tilted view representation of the vertically-aligned Cu NW array. SEM observations were performed after the complete removal of the supported AAO template. (**b**–**d**) Cross-section SEM micrographs of the CdTe thin film as-deposited on top of the Cu NW array by VTE (**b**), by RF-MS (**c**), and by ECD (**d**). The inset in (**d**) represents the corresponding top surface view of the CdTe-embedded Cu NW array. The scale bar of the insets is 250 nm. prepared by all three methods (i.e., VTE, RF-MS, and ECD), which was expected to feature the same intrinsic properties as the CdTe deposits onto the Cu NW \"forest\". Figure 4 presents AFM images acquired in tapping mode (top view: Figure 4a,c,e; corresponding 3D representation: Figure 4b,d,f) of the CdTe thin films deposited in the same conditions as previously described by VTE (Figure 4a,b) and RF-MS (Figure 4c,d) onto glass substrates and by ECD (Figure 4e,f) onto an Au-coated Si substrate. The scanned area was established as \(5\times 5\) um\({}^{2}\) for all samples. The images were further post-processed using the Gwyddion 2.52 software package [38], with the corresponding statistical parameters (i.e., RMS, Ssk, and K coefficients) are collected in Table 1. As can be seen, all samples present good uniformity of the CdTe thin films and a granular surface aspect that changes with the employed deposition method. Notably, the Z-range is dramatically reduced for the sputtered CdTe film (Figure 4c,d), suggesting a more flattened and smoothened CdTe film surface relative to the other two deposition methods (i.e., VTE and ECD). This behavior can most probably be attributed to the isotropic nature of the sputtering process, as well as to increased surface mobility of the species arriving to the substrate, allowing very conformal and compact thin films to be obtained. This observation is further confirmed by the calculated RMS values (see Table 1), which show a value of 2.40 nm for the sputtered CdTe film, almost two times lower compared to the RMS values of the evaporated (3.54 nm) and electrodeposited (4.93 nm) CdTe thin films, respectively. Furthermore, the surface evenness of the prepared CdTe thin films can be analyzed in terms of the statistical coefficients (Skewness, Ssk and Excess Kurtosis, K) shown in Table 1.