**4. Electrochromic devices on paper**

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 fre-

**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

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

Drawing a parallel with FTIR-ATR results, it is possible to recognize the high susceptibility of

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

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>

) responsible for the behavior at lower frequencies.

V−1 s−1 for NCC).

quency (C-f) and bulk resistance (**Figure 9b**).

the TP to the water variation content.

) and hydroxyls groups (OH-

of protons (H<sup>+</sup>

permission.

48 Nanostructured Solar Cells

matrix [41].

The interest in electrochromic materials has grown from the mid-1980s, especially due to the application of these materials in smart windows for energy efficiency and indoor control [87, 88]. Nevertheless, new applications in the field of smart labels and displays have triggered the development of easier and cost-effective deposition processes that simultaneously allow patterning of the active area [89]. Inkjet printing is a good alternative for this purpose since it is processed at low temperatures, is a cost-effective technique, easily scalable for mass production, produces low waste, and can be adapted to different substrates and patterns. Nevertheless, this fabrication method can have some drawbacks as the quality of the deposited film is highly dependent on the ink characteristics, like viscosity and surface tension, combined with the printer specifications and surface properties of the paper substrate [90, 91]. So far, inkjet printed electrochromic films have been applied mostly to organic materials [92], but inorganic materials can also be easily adapted [93, 94].

An electrochemical device comprises two electrodes separated by an electrolyte, and paper can be a viable substrate to produce electrochromic displays as the next proof-of-concept shows. The electrodes are functionalized, by inkjet printing, with the electrochromic material, WO<sup>3</sup> ·H<sup>2</sup> O, and at least one of the supports for the electrodes has to be fully transparent (window layer). In the case of the device shown here, liquid packaging cardboard (LPC) was used as the paper substrate and the transparent support is an ITO-coated PET substrate (for the sake of device operation and proper encapsulation), but nanocrystalline cellulose can be a viable alternative, given its high transparency and low sheet resistance of the TCOs deposited on its surface, as demonstrated by the good quality FETs fabricated on this substrate. The devices are assembled in a sandwich structure, where the ITO-coated PET/WO<sup>3</sup> and LPC/ WO<sup>3</sup> are the working and counter electrodes, respectively, with the composite solid polymer electrolyte (fabricated according to Santos et al. [95]) in the middle and double-sided tape for encapsulation and spacing between electrodes. Potentiostat measurements characterize the electrochemical properties and colorimetric data studies the variations in color between different working stages.

Tungsten trioxide (WO<sup>3</sup> ) is a well-known inorganic electrochromic compound with a very high transmittance modulation, multiple oxidation states, and good cycle stability. The general reaction that describes its electrochromic behavior can be written as follows:

$$WO\_3 \text{(transpart/white)} \, \*y \, M^\* \, \*y \, e^- \Leftrightarrow \, M\_yWO\_3 \text{(blue}\,\text{)}\tag{2}$$

The coloration occurs when the tungsten metallic center is reduced, by the application of an external potential, and a small cation (M<sup>+</sup> = H<sup>+</sup> , Li<sup>+</sup> , K<sup>+</sup> ,…) from the electrolyte intercalates into the material to compensate the charge insertion. The tungsten bronze (*My WO*<sup>3</sup> ) is then formed, showing the blue color in the electrode. Since this is a reversible process, when a potential with inversed polarity is applied, the reaction occurs in the opposite direction and the material returns to its initial state with a transparent/white color [89]. The full mechanism is still under discussion but the most accepted theory is explained by small polaron transitions for the amorphous material and *Drude*-like free electron scattering for crystalline. The major difference between these two mechanisms is the electron localization or delocalization:

$$\mathcal{W}^{\vee}\mathcal{O}\_3 + y\,\mathcal{M}^+ + y\,e^- \Leftrightarrow \mathcal{M}\_y\mathcal{W}^{\vee}\_x\mathcal{W}^{\vee\vee}\_{1-x}\mathcal{O}\_3\tag{3}$$

The *x* represents the number of *W* sites. However, at higher values of *y* the reaction gets irreversible and the tungsten bronze turns red or golden [96].

#### **4.1. Liquid packaging cardboard (LPC) electrochromic device assessment**

The crystallographic structure of the synthesized WO<sup>3</sup> , analyzed by XRD (**Figure 10**), depicts a series of diffraction peaks that can be indexed to the reference pattern from the International Centre for Diffraction Data (ICDD) of the orthorhombic hydrated (or tungstic acid) WO<sup>3</sup> (*ortho-*WO<sup>3</sup> ·H<sup>2</sup> O) with ICDD No. 01–084–0886. Tungstic acid is already known to have a good electrochromic behavior, which is dependent on both the hydration level and crystalline structure [97, 98]. In this work, the terminal W=O and W–H<sup>2</sup> O bonds can increase the conduc-

**Figure 10.** (a) XRD diffractogram of the WO<sup>3</sup> ·H<sup>2</sup> O deposited film and the corresponding reference ICDD pattern. (b) SEM images of the WO<sup>3</sup> inkjet printed pattern detail on ITO-coated PET substrate.

tivity and diffusion of Li<sup>+</sup> ions, whereas the crystalline structure should improve the stability of the film [99].

Tungsten trioxide (WO<sup>3</sup>

50 Nanostructured Solar Cells

*W O*<sup>3</sup>

delocalization:

(*ortho-*WO<sup>3</sup>

·H<sup>2</sup>

**Figure 10.** (a) XRD diffractogram of the WO<sup>3</sup>

images of the WO<sup>3</sup>

external potential, and a small cation (M<sup>+</sup>

*WVI O*<sup>3</sup>

) is a well-known inorganic electrochromic compound with a very

+ *y e*<sup>−</sup> ⇔ *My W O*<sup>3</sup>

*<sup>V</sup> W*1−*<sup>x</sup>*

(blue ) (2)

*WO*<sup>3</sup>

) is then

,…) from the electrolyte intercalates

*IV O*<sup>3</sup> (3)

, analyzed by XRD (**Figure 10**), depicts

O bonds can increase the conduc-

O deposited film and the corresponding reference ICDD pattern. (b) SEM

high transmittance modulation, multiple oxidation states, and good cycle stability. The gen-

The coloration occurs when the tungsten metallic center is reduced, by the application of an

formed, showing the blue color in the electrode. Since this is a reversible process, when a potential with inversed polarity is applied, the reaction occurs in the opposite direction and the material returns to its initial state with a transparent/white color [89]. The full mechanism is still under discussion but the most accepted theory is explained by small polaron transitions for the amorphous material and *Drude*-like free electron scattering for crystalline. The major difference between these two mechanisms is the electron localization or

+ *y e*<sup>−</sup> ⇔ *My Wx*

The *x* represents the number of *W* sites. However, at higher values of *y* the reaction gets irre-

a series of diffraction peaks that can be indexed to the reference pattern from the International Centre for Diffraction Data (ICDD) of the orthorhombic hydrated (or tungstic acid) WO<sup>3</sup>

electrochromic behavior, which is dependent on both the hydration level and crystalline

O) with ICDD No. 01–084–0886. Tungstic acid is already known to have a good

 = H<sup>+</sup> , Li<sup>+</sup> , K<sup>+</sup>

eral reaction that describes its electrochromic behavior can be written as follows:

into the material to compensate the charge insertion. The tungsten bronze (*My*

+ *y M*<sup>+</sup>

**4.1. Liquid packaging cardboard (LPC) electrochromic device assessment**

·H<sup>2</sup>

inkjet printed pattern detail on ITO-coated PET substrate.

versible and the tungsten bronze turns red or golden [96].

The crystallographic structure of the synthesized WO<sup>3</sup>

structure [97, 98]. In this work, the terminal W=O and W–H<sup>2</sup>

(transparent/ white ) +*y M*<sup>+</sup>

The ink produced has to meet the demands of the inkjet printer used. For that, the viscosity and surface tension were between 1.5–2 cP and 30–40 dyne cm-1, respectively, while the filtration assured that the nozzles would not clog during printing. As expected, the printed pattern does not show a continuous film, but the eye resolution cannot detect these irregularities [94]. The patterns were defined as a square with 1 × 1 cm for the LPC and 0.5 cm diameter circle for the ITO-coated PET substrate. This design facilitates the alignment of both electrochromic layers upon encapsulation.

Usually, the coloration of electrochromic materials is studied by UV-Vis spectroscopy; however, due to the opacity of the LPC substrate, the different colors of the device were represented by the CIE 1931 *Yxy* diagram as recommended by the *Commission Internationale de L'Eclairage*. In the CIE diagram, the quantities *x* and *y* of Eq. (3) are represented by the two Cartesian coordinates represented and are obtained by analyzing the pictures in *ImageJ* software. The *x–y* coordinates for each colored and bleached state are represented inside the diagram depicted in **Figure 11a**. It can be observed that there is a notable color difference between the bleached and colored states, especially at −0.6 V, where the reaction becomes irreversible. As the potential increases, the *y* factor in Eq. (3) also increases and the Li<sup>+</sup> ions become trapped in the crystalline structure of the WO<sup>3</sup> .

The electrochemistry of the reaction studied by cyclic voltammetry (**Figure 11b**), under the same potential range (0.3 to −0.3 V and 0.6 to −0.6 V), shows that coloration occurs when the voltage and current decrease until a minimum negative value while bleaching takes place when voltage and current reach a maximum and then stabilize. In this work the capacity of the films is almost ineligible but it can be explained by the small area of the film and consequently the low amount of WO<sup>3</sup> material. The low current and voltage required to govern the on/off stages of the electrochromic device can be easily met by the current state-of-the-art organic [55] and inorganic [48, 49] solar cells fabricated on paper.

**Figure 11.** (a) *Y*xy chromaticity diagram for the bleached (0.3 V) and colored samples at both −0.3 and −0.6 V. (b) Cyclic voltammograms of the final devices with LPC/WO<sup>3</sup> and PET ITO/WO<sup>3</sup> electrodes in the potential ranges of 0.3 to −0.3 V and 0.6 to −0.6 V, at a scan rate of 10 and 50 mV s−1, respectively. The inset illustrates the schematic representation of an electrochromic device.
