## 2 Materials and Methods  
### Sample Preparation  
Analytical grade reagents (Merck KGaA, Darmstadt, Germany) were used throughout this work as provided. Ultra-pure deionized water (DIW) supplied from a Direct-Q 3 UV (Merck KGaA, Darmstadt, Germany) water purification system was used for preparing the aqueous electrolytic solutions and for rinsing the samples during the fabrication process flow. A gaseous nitrogen (N\({}_{2}\)) cylinder (5.0 purity from Linde Gaz S.R.L., Ilfov, Romania) was employed for blow-drying operations. Dense arrays of vertically-aligned Cu NWs were first prepared through electrochemical synthesis within supported nanoporous AAO templates, then in a second step were covered by CdTe thin films synthesized using three deposition methods (Figure 1). Moreover, where appropriate, additional control samples of CdTe thin films deposited on either Au-coated silicon (Si) or glass substrates were prepared under the same conditions as described below in order to allow subsequent morphological, structural, and optical studies of the obtained CdTe layers, assuming that the evaluated properties were comparable to the CdTe films within the prepared CdTe/Cu nanostructured interfaces. The latter assumption is especially valid for micrometer-thick films (as in this work), in which case the underlying substrate structure and surface roughness effects can be neglected, as they predominately manifest near the substrate-film interface. In the first stage, the substrate was prepared by in-situ direct-current (DC) sputtering of a multi-layer sequence consisting of Ti/Au/Ti/Al (5/50/5/1000 nm) onto a freshly cleaned p-doped <100> Si substrate (525 um thickness and 5 m\(\Omega\)-cm resistivity) (SIEGERT WAFER GmbH, Aachen, Germany) using commercially-available targets (Kurt J. Lesker Limited, Saint Leonards-on-sea, United Kingdom) and a Plassys MP500S magnetron as sputtering equipment (PLASSYS Bestek, Marolles-en-Hurepoix, France) (Figure 1a). The processing plasma parameters have been detailed elsewhere [27]. In this configuration, the thin Au film is the actual electrode, the aluminum (Al) layer is introduced to develop the supported AAO template through anodization, and the ultra-thin titanium (Ti) film is used to enhance the adhesion between successive metallic layers. The as-prepared substrate was afterwards subjected to an anodization process in order to entirely convert the Al film into a supported nanoporous AAO template with parallel quasi-cylindrical nanochannels reaching the Au conducting underlayer. The electrochemical process was carried out in a thermo-regulated electrolytic cell, using a two-electrode configuration by applying 60 V from a Keithley 2460 High Current SourceMeter (Source Measure Unit, Tektronix Inc., Beaverton, OR, USA) in 0.3 M oxalic acid solution kept at 2 \({}^{\circ}\)C, followed by a chemical widening process of the nanopores in 0.5 M H\({}_{2}\)SO\({}_{4}\) at 40 \({}^{\circ}\)C for 85 min (Figure 1b). The current curve obtained for entirely anodizing the Al layer (1 \(\upmu\)m thick) is presented in Figure 2a. As explained elsewhere [23; 24; 27], the measured current density drops at the beginning of the process due to the formation of a compact barrier-type Al\({}_{2}\)O\({}_{3}\) on the initial Al surface, leading to a plateau when a competitive equilibrium between field-enhanced Al\({}_{2}\)O\({}_{3}\) dissolution and Al\({}_{2}\)O\({}_{3}\) formation is established before sharply increasing at the end of the process as the nanopores progressively approach the Au electrode. Subsequently, an optimized electrochemical deposition protocol was employed to grow Cu within the AAO nanopores, by using a three-electrode potentiostatic setup using a Voltalab PGZ-100 Potentiostat (Radiometer Analytical Inc., Hegnau, Switzerland). In brief, \(-\)0.2 V was applied versus an Ag/AgCl reference electrode (KCl-saturated, E = 0.197 V against Normal Hydrogen Electrode, NHE) installed in a Cu commercially-available electroplating bath kept at room temperature.