**2.2. Standard paper**

Attenuated total reflectance-Fourier-transformed infrared (FTIR-ATR) spectroscopy provides information regarding the chemical species, chemical bonding state of cellulose, and to infer on the water content. Measurements were recorded using an attenuated total reflectance (ATR) sampling accessory (Smart iTR) with the following conditions: incident angle of 45°; 4000–650 cm−1 range; 4 cm−1 resolution; 32 scans; 20°C. The FTIR-ATR spectrum (**Figure 2b**) of BC and NCC resembles that typically obtained for plant-based cellulose and depicts the characteristic cellulose absorption peaks, i.e., hydrogen-bonded OH stretching at 3400 cm−1, C−H and C−O stretching vibrations, 2900 and 1060 cm−1, respectively. The O–H bending vibration assigned to the absorbed water is also observed with the presence of a

The BC membranes have a smooth structure of long entangled filaments of extruded cellulose fibrils with an average width of 20–100 nm (**Figure 3a**) and a surface *RMS* roughness in the order of 60 nm [68, 69, 78]. As for NCC, one can observe a smooth and compact morphology

**Figure 2.** (a) XRD diffractogram of bacterial cellulose (BC) and cotton-based nanocrystalline cellulose (NCC) dried

**Figure 3.** SEM micrograph of the surface morphology of (a) bacterial cellulose and (b) nanocrystalline cellulose membranes (NCC) produced by evaporation and the corresponding cross section (inset). Adapted from Gaspar et al.

(**Figure 3b**) and also a needle-like structure in the cross-section inset.

membranes; (b) FTIR spectrum of BC and NCC dried membranes; (c) total transmittance.

peak at 1640 cm-1 [40].

42 Nanostructured Solar Cells

[40], with permission from IOP Publishing.

The SEM micrographs in **Figure 4** show the morphology of the paper samples. COP has a high-density structure of intertwined cellulose fibers, with different shapes and sizes (some wider than 20 µm) and also calcium carbonate agglomerates. RP presents a similar structure and fibril size dispersion, without the presence of calcium carbonate agglomerates. As for the morphology of *Whatman* filter papers (WFP1 and 4), one can observe wide fibers (>20 µm) embedded in a matrix of small ones (<5 µm), whose concentration can be correlated to the intended filter permeability (**Figure 4c** and **d**). Both COP and RP have low porosity and high hydrophobicity (water-contact angle around 100°) when compared to WFP1 and 4 samples, which have a water-contact angle of <10°. The samples with the smoothest, more uniform, and compact surfaces are from parchment (PP) and tracing paper (TP), where the high concentration of small size fibers completely fills the gaps and surface between the larger fibers. The SEM for the PP samples (**Figure 4e**) shows a similar matrix structure of large and small fibers, with dimensions that can exceed 40 µm in width, embedded in a matrix of small ones (<5 µm) but the smoothest and most homogeneous surface is the TP substrate (**Figure 4f**).

**Figure 4.** SEM micrographs of the six paper samples: (a) Commercial office paper (CPP), (b) raw paper (RP), (c) *Whatman* filter paper sample 1 (WFP1), (d) *Whatman* filter paper sample 4 (WFP4), (e) parchment paper (PP), and (f) tracing paper (TP).

**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°, respectively).

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

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, and poor finish [80].

The analysis of XRD diffraction (**Figure 6a**) highlights the referred differences between COP and the other paper substrates. Besides the common 1¯ 10, 110, and 200 crystallographic planes (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° related to the presence of calcium carbonate (CaCO<sup>3</sup> ) [41, 81]. Calcium carbonate and clay are typically present in paper manufacturing, either from pigments that are commonly used in papermaking or the water mineral content.

**Figure 6.** (a) XRD diffractogram of the different papers analyzed in Figures 4 and 5. All paper substrates present the 11¯0, (110), and (002) diffraction peaks of cellulose I (native cellulose). COP also shows intense peaks associated with the presence of CaCO<sup>3</sup> ; (b) FTIR spectrum; (c) total transmittance as a function of wavelength.

In the FTIR-ATR spectra (**Figure 6b**), all samples reveal the bands associated to cellulose, such as OH, C−H, and C−O stretching bands, at 3400, 2900, and 1060 cm−1, respectively. It is also observed a peak at 1640 cm−1 for the O–H bending vibration which corresponds to the absorbed water [40]. The 3600–3100 cm−1 broad band in the region provides information regarding the hydrogen bonds. The peaks from amorphous cellulose are sharper and have lower intensity, which can be correlated with the scission of the intra- and intermolecular hydrogen bonds [75]. Tracing paper is the sample with the most distinctive and low intensity OH stretch band.

Transmittance can also be an important characteristic of paper devices, for example, in OLEDs; the large optical haze is attractive for organic solar cells or in paper touchscreens to eliminate glare under direct light [44]. The total transmittance spectra of the different paper substrates are shown in **Figure 6c**. Tracing and parchment paper are the most translucid, although there is always a fraction of light that is transmitted regardless of the paper substrate. Scattered light is the main component of transmitted light, which translates in a high haze factor.

Information regarding thermal stability can be obtained by thermogravimetry and differential scanning calorimetry (TG-DSC) analysis (~7.5 mg of sample mass, heated at atmospheric pressure, from 25°C to 550°C with a heating rate of 5°C min−1). Results show an initial weight loss (<10%), at temperatures close to 100°C, which corresponds to the desorption of free water from the cellulose fibers. Thermal degradation of cellulose, by depolymerization and the formation and evaporation of levoglucosan, or oxidative decomposition, occurs for temperatures above 300°C and is followed by a weight loss above 70%.
