**3. Characteristics of electronic devices on paper**

Although plasma-based processes, such as physical vapor deposition (PVD) and PECVD, may be relatively complicated and expensive in comparison to what is common in the graphics and printing industries (roll-to-roll), these techniques give rise to higher efficiency devices. Therefore, it is important to develop a better understanding of how these plasma processes can be applied and are influenced by paper substrates, so that a comparison can be established for the same devices produced with organic materials and printing techniques. As an example, in the more recent demonstrations of transistors on paper, there is still typically a trade-off between the transistor performance and its printability. While high-performance conventional silicon-based transistors have been attached to a silicon-coated paper substrate [34], screen printed silicon nanoparticle-based transistors on paper showed rather poor characteristics [82]. Laboratory scale techniques were also used when fabricating conventional organic field-effect transistors (OFETs) operating at high [22] or low voltages [83] on paper substrates, while simple electrochemical organic transistors operating at low voltages have been produced on polyethylene-coated paper [84] and on photo paper [85].

In this section, we provide examples of field effect transistors (FETs) fabricated on the studied substrates described in the previous section and highlight the impact cellulose has on the device properties (**Figure 8**). The transistor is one of the essential active components in electronics and its fabrication complexity makes it an ideal element to test the viability of paper electronics, and since it requires energy to operate, it is also an opportunity for solar power. In the specific case of FETs, paper is not only the support but also an active part of the device the gate dielectric—according to the electric double layer (EDL) formation due to the mobile protons within the cellulose matrix [39, 41, 86]. This cellulose role is similar to what happens in electric double-layer capacitors.

**Figure 8.** *I*DS–*V*GS transfer characteristics obtained at *V*DS = 15 V for GIZO field effect transistors using bacterial cellulose (BC) and nanocrystalline cellulose (NCC) as gate dielectric.

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

**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.

Although plasma-based processes, such as physical vapor deposition (PVD) and PECVD, may be relatively complicated and expensive in comparison to what is common in the graphics and printing industries (roll-to-roll), these techniques give rise to higher efficiency devices. Therefore, it is important to develop a better understanding of how these plasma processes can be applied and are influenced by paper substrates, so that a comparison can be established for the same devices produced with organic materials and printing techniques. As an example, in the more recent demonstrations of transistors on paper, there is still typically a trade-off between the transistor performance and its printability. While high-performance conventional silicon-based transistors have been attached to a silicon-coated paper substrate [34], screen printed silicon nanoparticle-based transistors on paper showed rather poor characteristics [82]. Laboratory scale techniques were also used when fabricating conventional organic field-effect transistors (OFETs) operating at high [22] or low voltages [83] on paper substrates, while simple electrochemical organic transistors operating at low voltages have

In this section, we provide examples of field effect transistors (FETs) fabricated on the studied substrates described in the previous section and highlight the impact cellulose has on the device properties (**Figure 8**). The transistor is one of the essential active components in electronics and its fabrication complexity makes it an ideal element to test the viability of paper electronics, and since it requires energy to operate, it is also an opportunity for solar power. In the specific case of FETs, paper is not only the support but also an active part of the device the gate dielectric—according to the electric double layer (EDL) formation due to the mobile

**3. Characteristics of electronic devices on paper**

been produced on polyethylene-coated paper [84] and on photo paper [85].

device fabrication.

46 Nanostructured Solar Cells

The transistor fabrication comprises, on one side of the paper, a GIZO (Ga<sup>2</sup> O3 -In<sup>2</sup> O3 -ZnO) thin film, to create the channel region, followed by the Al e-beam evaporation of the source/drain electrodes (S/D). On the opposite side of the paper substrate, a conductive IZO (In<sup>2</sup> O3 -ZnO) thin film is deposited by RF sputtering to work as gate electrode. To ensure the proper functioning, the devices were annealed in air for 30 min at 150°C.

The electronic performance of the BC-based FETs is similar to the FETs reported in the literature using commercial paper as dielectric, reaching a *I* ON/*I*OFF modulation ratio above 10<sup>4</sup> , whereas NCC-based FETs show a saturation mobility above 7 cm<sup>2</sup> V−1 s−1, and a *I* ON/*I*OFF modulation ratio higher than 105 [40].

The creation of the EDL depends on the mobile-free ions present in the paper matrix and fundamentally on the sorbed water within the paper. To infer the role of water content in the paper, due to humidity variations, and relate it with the performance of FETs, Fourier transform infrared (FTIR) spectroscopy was performed under normal atmosphere and in vacuum, for devices fabricated on tracing paper (TP).

In the FTIR-ATR spectra, the bands related to the adsorbed water are mainly located around 700 cm−1, which corresponds to the out-of-plane vibrations of OH groups or to the rotational vibrations of H<sup>2</sup> O molecules, whereas OH bending of adsorbed water is ascribed to the band observed at 1635 cm−1. In order to assess the major differences among the samples, the spectra were normalized to the intensity of the 2900 cm−1 band, since it is not susceptible to variations in crystallinity or water content [69] (**Figure 9a**). The measurements taken for different vacuum times reveal that there is an abrupt decrease in intensity for the band at 1635 cm-1. This observation is related to the structure compactness, whereas the more porous the structure, the more it facilitates adsorption and desorption of water.

**Figure 9.** Correlation between electrical properties and water content variation for tracing paper (TP). (a) ATR-FTIR spectra of the water content variation (bands 3600–3000 cm−1 and 1635 cm−1); (b) capacitance (C-f), and tan *δ* (tan *δ*-f) variation with frequency at atmospheric pressure and after 15 min of vacuum pumping; (c) high frequency region of the Cole-Cole plot; (d) transfer characteristics (*V*DS = 15 V) of the paper gated GIZO FETs under atmospheric pressure and vacuum. Adapted from Gaspar et al. [74] Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

Water and mobile-free ions are intrinsically linked to EDL formation, when using paper as dielectric. To understand the role of ions and water, spectroscopic impedance can provide information of the paper electrical properties, namely, the capacitance variation with frequency (C-f) and bulk resistance (**Figure 9b**).

The C-f plot was determined between 10 mHz and 1 MHz using an AC excitation voltage of 500 mV [41]. In this frequency range, there is an increase in the capacitance for low frequencies, which is a typical behavior for the electrode polarization as a result of the interaction between the charged electrode surface with the free charges in the paper. For 10 mHz, the capacitance of TP is 1.8 µF cm−2, and when submitted to vacuum, the capacitance decreases more than three orders of magnitude. This difference is explained by the sorbed water which is the main source of protons (H<sup>+</sup> ) and hydroxyls groups (OH- ) responsible for the behavior at lower frequencies. Drawing a parallel with FTIR-ATR results, it is possible to recognize the high susceptibility of the TP to the water variation content.

The loss tangent (tan *δ*) allows one to infer the relaxation frequency, by separating the contributions of the bulk material itself from the EDL regime. For high frequencies, the tan *δ* is low since there is no sufficient time for large dipole creation; whereas for low frequencies, the electric field action drives the ions to form the EDL, which in turn reduces tan *δ* [40]. The bulk resistance can also be determined from the impedance data through the Cole-Cole plot (**Figure 9c**), where the resistance of bulk material in parallel with the geometric capacitance describe a semicircle at high frequencies; the estimated resistivity for TP is 4.9 × 107 Ω cm. A possible explanation for this value relies in the amount of sorbed water, wherein a higher concentration can maximize the ability of free ions to migrate thought the paper matrix [41].

Regarding the other electrical properties of FETs, the saturation mobility on TP (*µ*SAT = 2.3 cm<sup>2</sup> V−1 s−1) is significantly lower compared to those fabricated on light paper (7 cm<sup>2</sup> V−1 s−1 for NCC). Thus, the saturation mobility is intimately related to the density of the matrix and fraction of small fibers. The paper morphology also influences similarly the on/off current ratio (TP, *I* on/*I* off = 2.5 × 10<sup>4</sup> ) and minimizes the gate leakage current (*I* GS).

As for device properties, the FETs characteristics are directly affected by the decrease of water content within the paper bulk (**Figure 9d**). The susceptibility of TP to water loss, due to humidity changes, leads to no current modulation after 15 min of vacuum since the desorbed water does not allow the EDL formation. The compactness and smoothness of small fibers can give paper a higher ability to bind and retain water and ions, allowing the formation of EDLs at the interface of paper, thus enabling the semiconductor to operate under low humidity for a prolonged period [74].
