**2.3. Cardboard**

**Figure 5** gathers the 3D profilometry scans of the different paper substrates. As expected, given the fibers' width, the WFP1 and 4 have the surface with the highest root mean square (RMS), exceeding 12 µm. On the opposite side, the TP has the smoothest surface with *RMS* ~4 µm, while COP and RP have similar and slightly higher values of RMS (~5 µm). The COP paper is optimized for printing and therefore has a lower porosity and higher hydrophobicity (water contact angle of 101°) when compared to WFP (water contact angle of <10°), which is hydrophilic in nature, given the dimension of its fibers and high porosity. The high concentration of small size fibers, compactness, and smooth surface of the vegetable papers, PP and TP, also lead to high hydrophobicity (water contact angle of 124° and 95°,

The water content and its affinity to the paper substrate also plays an important role as a plasticizer or softening agent, thus influencing paper properties such as flexibility, elasticity, strength, and rigidity, which should be adjusted not only to the fabrication process, but also to the ink impregnation and overall printing quality. A low moisture content give rise to a hard and brittle paper, while a water content too high leads to creasing, delayed ink drying,

**Figure 5.** 3D profilometer on a 0.5 × 0.5 mm area of the six paper samples: shown in Figure 4: (a) CPP, (b) RP, (c) WFP1,

The analysis of XRD diffraction (**Figure 6a**) highlights the referred differences between COP

(respectively at 2*θ* = 14.7°, 16.8°, and 22.7°) associated to semicrystalline cellulose type I (also referred to as native cellulose), the XRD of COP reveals intense peaks between 28° and 50°

typically present in paper manufacturing, either from pigments that are commonly used in

10, 110, and 200 crystallographic planes

) [41, 81]. Calcium carbonate and clay are

respectively).

44 Nanostructured Solar Cells

and poor finish [80].

(d) WFP4, (e) PP, and (f) TP.

and the other paper substrates. Besides the common 1¯

related to the presence of calcium carbonate (CaCO<sup>3</sup>

papermaking or the water mineral content.

The liquid packaging cardboard characterization by SEM and 3D profilometry (**Figure 7**) reveals a continuous and crack-free aluminum foil, shaped to the cardboard roughness (*RMS* ~6 µm). As for TG-DSC analysis, published elsewhere [48], there are two endothermic peaks. The first peak, detected at 99.3°C with a weight loss of 6.6%, is related to the release of free water, and the second peak, at 353°C with weight loss of weight loss of 57.7%, is linked to

**Figure 7.** Surface characterization analysis of LPC. (a) SEM. Adapted from Araújo et al. [73], with permission from IOP Publishing. (b) Total and diffuse reflectivity in the visible region. The inset shows the 3D profilometer on a 3 × 2 mm area. Adapted from Vicente et al. [48], with permission from the Royal Society of Chemistry. (c) XRD diffractogram.

the cardboard and LDPE decomposition. It should also be noted that although the weight loss is negligible up to 250°C, the LDPE, responsible for the adhesion of the aluminum foil to the cardboard, starts to degrade at 200°C; thus, the substrate should be considered thermally stable up to 200°C in order to assure that the release of organic species will not influence the device fabrication.
