Paper Electronics Utilizing Screen Printing and Vapor Phase Polymerization

The rise of paper electronics has been accelerated due to the public push for sustainability. Electronic waste can potentially be avoided if certain materials in electronic components can be substituted for greener alternatives such as paper. Within this report, it is demonstrated that conductive polymers poly(3,4‐ethylenedoxythiophene) (PEDOT), polypyrrole, and polythiophene, can be synthesized by screen printing combined with vapor phase polymerization on paper substrates and further incorporated into functional electronic components. High patterning resolution (100 µm) is achieved for all conductive polymers, with PEDOT showing impressive sheet resistance values. PEDOT is incorporated as conductive circuitry and as the active material in all‐printed electrochromic displays. The conductive polymer circuits allow for functional light emitting diodes, while the electrochromic displays are comparable to commercial displays utilizing PEDOT on plastic substrates.


Introduction
Green electronics is a growing trend within the scientific community. [1,2] While the term is not yet fully defined, green electronics typically encompasses various strategies to produce electronics with a lower environmental impact compared to current technologies. The need to satisfy consumer desires for more DOI: 10.1002/adsu.202300058 electronics while being considerate with electronic waste is a challenging conundrum. Traditional electronic (based on silicon, metals, and plastics) waste in 2021 was recorded at 3 billion tons with the consumer markets only continuing to rise, implying that innovative technologies regarding more sustainable electronics must be developed. [3] The replacement of traditional components and printed circuit boards with printed and flexible electronics is one avenue for potentially reducing electronic waste due to the possibility of recycling. However, due to the ease of production and costs, flexible and printed electronics generally use plastic substrates. Since most plastics are based on fossil sources and may contribute to the environmental issue of microplastics, there is a need to transition to more sustainable alternatives.
Paper electronics is another field forming within the scientific literature where plastic is replaced with bio-based paper substrates. [4,5] Paper is not only part of the natural carbon dioxide cycle, but it is also a material that can be easily recycled. By using paper as an integral part of electronic components, their recycling could become more efficient and widespread in the future. For these advantages, utilizing paper in electronics has produced a variety of example applications ranging from energy storage devices [6] to sensors [7] and other optoelectronic components. [8] Paper and other porous materials also present opportunities where traditional electronics are difficult to apply, such as soft robotics, implantable bioelectronics and wearable electronics. [9] Unfortunately, due to paper's inherent hygroscopicity and porosity, and therefore susceptibility to humidity and water, it is not an easy material to use for commercial electronics. [10] The field of printed electronics generally incorporates organic materials and inks such as conductive polymers. The most common conductive polymer reported in the scientific literature is poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). [11,12] PEDOT:PSS is often used as the active component in organic electrochromic devices and organic electrochemical transistors. [13] While a useful organic material, PE-DOT:PSS is made up of two polymers; the conductive polymer (PEDOT) and a solubilizing, insulating polymer (PSS). Therefore, the conductivity can be increased by only using PEDOT and not PSS. Many previous reports have demonstrated thin, high conductivity PEDOT films using a synthesis technique coined vapor phase polymerization (VPP). [14,15] VPP involves the deposition of an oxidant solution (traditionally by spin coating but more recently by printing technologies [16][17][18] ) before the monomer is introduced in vapor form to polymerize where the oxidant has been deposited. Usually, a washing step is included to remove the excess oxidant and unreacted monomer. In this way, patterning the oxidant species by printing technologies allows the VPP-PEDOT to be manufactured in patterned form which has proved difficult with other synthesis techniques. Recently, we have demonstrated the combination of screen printing and VPP for producing thin, highly conducting PEDOT patterns. [16,17] Within these reports we have employed the oxidant species iron (III) trifluoromethanesulfonate (FeOtf), which has been proven to provide thin, high conductive PEDOT. [19] The advantages of VPP over a conventional PEDOT:PSS ink were shown by enhancing printing resolution, conductivity and performances in organic electrochemical transistors and electrochromic displays. These electronic devices were printed on plastic substrates. However, a recent development in substrate formation and printing expertise has presented the conventional vertical electrochromic display structure printed on transparent nanocellulose substrates moving toward more sustainable electrochromic devices. [20] The substrates were created in a slow casting method, and the resulting substrates became brittle and therefore difficult to handle.
Within this report, we have brought the combination of VPP and screen printing into the field of paper electronics using commercial opaque paper and transparent paper as two examples. We have sourced a paper substrate, called Cristal paper, that is transparent, manufactured in a roll-to-roll machine and easy to handle. We show that using this transparent paper and an opaque paper combined with screen printing and VPP can produce more sustainable electrochromic displays. The "green" substrates were compared to the standard PET (polyethylene terephalate) plastic substrates and the VPP-PEDOT was compared to standard PE-DOT:PSS screen printing ink.

Results and Discussion
Within the field of electronics, and printed electronics specifically, transitioning to paper and other recyclable materials is required in order to achieve the global sustainability goals. One of the most widely used additive manufacturing technologies in printed electronics is screen printing. [21] While paper substrates for printed electronics exist, these substrates are typically precoated with fillers. Pure paper substrates are more difficult to apply to screen printing technologies due to the nature of cellulose fibers. The fibrous and porous structure of paper can introduce contraction and stretchability issues when exposed to varying humidity and temperature in addition to wettability and absorption problems with functional inks. To highlight the contraction issues, print tests are presented in Figure S1 (Supporting Information). This experiment involves sequential printing and curing of two layers of ink to observe if the alignment throughout the design is successful. A percentage of contraction between the two printed layers is then possible to calculate. The values reported in Figure S1 (Supporting Information) are significant (a contraction of 1.35%) and the photographs show visually that this contraction would lead to a print failure of the small features in multilayered electronic components. This effect can be circumvented by the use of heat treatments immediately before printing, but contraction issues can still be encountered (dependent on inks, curing type, etc.).
Previously, we have reported the combination of screen printing and VPP in the creation of conductive polymer patterns. [16,17] In this work, we build on this technique and replace the plastic substrates employed in all previous studies for more sustainable paper substrates. A schematic on the VPP process and how it is combined with screen printing and paper substrates can be found in Figure S2, Supporting Information).
In this work we have chosen two types of paper substrates to evaluate, as shown in Figure 1, and compared them to standard PET substrates ( Figure 1A). The Cristal paper is produced from regenerated cellulose bleached softwood pulp and is semitransparent ( Figure 1B). While this substrate is less transparent compared to the PET substrate, the RISE logo placed below the paper can still be clearly observed. The second paper substrate ( Figure 1C) is an opaque paper made from cellulose fibers with a recoatable latex coating (referred to as Spantex paper), which is suited for printed electronics due to its smooth surface.
Following screen printing of the oxidant ink, exposure to EDOT monomer and a washing step, PEDOT:Otf is formed on the control PET ( Figure 1D), transparent Cristal paper ( Figure 1E) and opaque Spantex paper ( Figure 1F). While these images of PEDOT:Otf highlight the transparency of the substrates and the PEDOT:Otf, high resolution (100 μm) structures can be obtained by employing screens with finer features. Figure S3 (Supporting Information) shows the printability of the oxidant in higher resolution patterning where all three substrates show impressive line edge and ink leveling. The resolution is consistent with our previous reports of combining screen printing and VPP. [16,17] It could be argued that the Cristal paper has slightly better edge definition than the other substrates and this could be related to the greater hydrophobicity of the substrate. The flexibility of the PEDOT:Otf was also shown to be excellent by observing the sheet resistance before and after the PEDOT:Otf on Cristal paper was bent around several cylinders with different diameters. No observable increase in the sheet resistance was discerned after the samples were bent over a 12 mm diameter cylinder, as shown in Table S1 (Supporting Information). The water contact angle measurements of the substrates in Figure S4 (Supporting Information) shows that the Cristal paper has the highest contact angle, which explains why the water-based oxidant does not spread as significantly as compared to the other substrates, and thereby results in better line definition. This is then translated into the resolution and line definition of the PEDOT:Otf shown in Figure 2. Interestingly, the PEDOT:Otf on the PET control substrate, while still acceptable, shows the poorest appearance of the three samples, which, from the contact angle data, may be expected. The scanning electron microscopy insets of the three samples in Figure 2 show the non-porous nature of PET and the Cristal paper, while the porous nature of the Spantex paper is preserved.
It should also be noted that the VPP process on paper is not limited to PEDOT synthesis. Previously, we have shown that the combination of screen printing and VPP has been successful with other conductive polymers such as polypyrrole or polythiophene. [17] As expected, these other conductive polymers   can also be synthesized on paper substrates. Figure S5 (Supporting Information) shows both polypyrrole and polythiophene on Spantex and Cristal paper. It can be seen that both conductive polymers were successfully synthesized, and the resolution is excellent. The polymerization of different conductive polymers can be taken further through screen printing technology to print consecutive layers that are aligned allowing all three conductive polymers in patterned form on a single substrate as seen in Figure 3. The synthesis and properties of polypyrrole and polythiophene via VPP and screen printing on PET substrates can be found in a previous report. [17] In the field of printed and organic electronics, PEDOT:PSS is often incorporated in devices such as electrochromic displays, [22] transistors, [11] and sensors, either as conductor or for its electrochemical switchability. Therefore, comparing the PEDOT:Otf fabricated via VPP within this work to commercial PEDOT:PSS on paper substrates is a good benchmark in order to assess the material combinations. Comparisons between these two materials on PET can be found in previous reports. [16] Figure 4 shows a comparative study between PEDOT:Otf and PEDOT:PSS on Cristal paper. Cristal paper was chosen so that spectroelectrochemical measurements were possible through the samples. Both PE-DOT:PSS and PEDOT:Otf appear to possess good resolution and line edges on the Cristal paper in Figure 4A,B, respectively. The spectra comparisons in Figure 4C,D show the electrochromic behavior of the conductive polymers. In agreement with previous results, PEDOT:Otf and PEDOT:PSS show a similar behavior both at 0 and −0.9 V. [17,23] Upon the application of −0.9 V, the neutral band of the dedoped PEDOT at ≈600 nm becomes more profound, which translates that the conductive polymers on the Cristal paper can behave as electrochromic active layers in all-printed electrochromic displays.
Comparative PEDOT:PSS prints on PET, Cristal and Spantex paper can be found in the supporting information ( Figure S6, Supporting Information). On all three substrates PEDOT:PSS shows a motley appearance due to the necessary solubilizing agents within the PEDOT:PSS ink. Unfortunately, this leads to electrical disconnects in small feature prints, while PEDOT:Otf shows better uniformity and the electrical connection remains. It should be noted that the PEDOT:Otf materials created via screen printing and VPP in this report, due to the increased viscosity necessary for screen printing, reduce the overall electrical conductivity compared to the lab scale optimized PEDOT:Otf process utilizing spin-coating deposition. [19] An important parameter of PEDOT materials for printed electronics is their electrical properties, in particular their sheet resistance. The sheet resistance was measured on the Cristal paper to ensure a suitable value for printed electronics was achieved. Table 1 shows the values obtained using a four-point probe method on square shaped 1 × 1 cm 2 structures. The values 198 (PEDOT:Otf) and 381 (PEDOT:PSS) Ω □ −1 are acceptable for use within the field of printed electronics, since they are relatively close to the sheet resistance of PEDOT:Otf on the PET control sample (160 Ω □ −1 ).
To further compare PEDOT:Otf and PEDOT:PSS, various printed electronic components are demonstrated. One basic   investigation is to evaluate whether PEDOT can be used as the conducting material in printed circuitry, to enable the functionality of light emitting diodes (LEDs). Figure 5 shows the printed PEDOT:Otf with LEDs mounted and adhered with anisotropic adhesive to highlight the possibilities with this material deposited on paper. While the PEDOT:Otf was successful in allowing the LED to function, the commercial PEDOT:PSS was not. The success of using PEDOT:Otf as circuitry within the field of printed electronics will potentially allow for other, more expensive, haz-ardous or limited in resource, materials to be replaced. PEDOT also has the added advantage of being biocompatible and has been incorporated in bioelectronic in vitro applications, which may allow printed electronic components to function within the human body. [24] Figure 5 demonstrates that the printed PEDOT:Otf patterns are showing sufficient electronic conductivity for electronics components, which is further emphasized by incorporating the PEDOT:Otf in all-printed electrochromic displays.
All-printed electrochromic devices on transparent plastic substrates are a mature technology that has entered the consumer market, where they are often employed as visual indicators in low-cost sensor systems. [25,26] Recently, we have published a report showcasing such all-printed electrochromic displays on transparent nanocellulose substrates. [20] While the nanocellulose substrates were almost fully transparent, they were brittle and the use of nanocellulose films as substrates is still in its infancy, which leads to slow substrate manufacturing and high costs. However, the more established transparent Cristal paper is manufactured by the roll and is more cost efficient while still maintaining good transparency. For these reasons, Cristal paper makes an excellent carrier candidate for the electrochromic display technology.
Substituting Cristal paper for the traditional PET substrates is not a trivial experiment. As stated previously in this article, we must take into account the contraction of the substrate in addition to the overall roughness. The differences in the surface free energy of the substrate (measured by liquid drop contact angle) can also influence the success and manufacturing yield of the electrochromic displays.
By evaluating different sets of printing parameters, we were successful in manufacturing all-printed electrochromic displays using PEDOT:Otf and PEDOT:PSS as the electrochromic material on the transparent Cristal paper substrate, see Figure 6A,B, respectively. Flexible 7-segment displays, which could be used as an informative display, in addition to insets showing simple square shaped devices for characterization purposes are shown. Figure S7 (Supporting Information) shows a graphical schematic of the layers screen printed to create the functional electrochromic display. The color contrast value (ΔE*) of the PEDOT:Otf-based electrochromic displays printed on the Cristal paper was calculated as 21.6 and the L*a*b* color coordinates can be seen in Figure S7 (Supporting Information), hence, the color contrast is comparable to the PEDOT:PSSbased devices (ΔE* = 20.6). However, the PEDOT:Otf-based electrochromic display exhibits superior retention time compared to the PEDOT:PSS-based device, with a stable color contrast for at least 30 min, while the color contrast of the PEDOT:PSS-based device dropped to below 80% of its original value. The electrochemical switching behavior of a square shaped electrochromic display can also be found in Figure 6D.
To investigate the stability of PEDOT:Otf-based electrochromic displays, the color contrast and the electrochemical switching behavior were both measured after 60 days of storage in ambient environment (20°C, 45 %RH). The L*, a*, and b* color coordinates, as well as the overall ΔE* color contrast, were almost identical with the data obtained immediately after the screen printing process ( Figure S7, Supporting Information). The data, based on the color contrast measurements of two electrochromic displays after 60 days of storage, are shown in Table S2 (Supporting Information). The electrochemical switching behavior of the same displays are shown in Figure S8 (Supporting Information). The current levels after storage are similar to those obtained immediately after manufacturing ( Figure 6D), which indicates that no degradation of the PEDOT:Otf film has occurred. However, the switching behavior differs in one aspect; the switching time has improved upon storage. In Figure 6D, the transition at 4.5 s indicates a fully switched display segment, while the same transition occurs after only 1.5 s in the devices measured after storage. This was unexpected but may be explained by that the electrolyte layers in the displays, which are non-encapsulated, have absorbed more water upon two months of storage.

Conclusion
Within this report we have shown the combination of screen printing and vapor phase polymerization on paper substrates. Two types of papers were investigated and showed that the oxidant has good printability on both, which can result in high resolution (100 μm) patterns of either poly(3,4-ethylenedioxythiophene) (PEDOT), polypyrrole or www.advancedsciencenews.com www.advsustainsys.com polythiophene. Patterns of PEDOT:Otf were shown to have sufficiently high electronic conductivity to be used as conductors to enable the functionality of LEDs as well as the color changing electrodes in electrochromic devices. Using the transparent Cristal paper, conventional electrochromic displays were fabricated using screen printing technology, and the performances of the resulting devices were comparable, or even improved, compared to those relying on commercial PEDOT:PSS ink. The proof-of-concept displays presented in this report could pave the way for more environmentally friendly, all-printed electronic components, including also the carrying substrate.
Spantex and Cristal (both trademark from Ahlstrom) were kindly provided by Ahlstrom from the mills in Billingsfors, Sweden and Rottersac, France, respectively.
Methods-Contraction Measurements of Paper Substrates: The paper substrates (Cristal and Spantex) were stored in ambient conditions (20°C and 50% RH). A red UV lacquer was printed on the substrates (using an expansion test screen mesh) and cured by UV and heat (120°C) through a belt oven at 1 m min −1 . The time in the oven was 2 min and the UV dose was ≈800 mJ cm −1 . Immediately after, a black lacquer was printed. The contraction of the paper substrates was measured in the microscope by measuring the mismatch of the printed layers, see Figure S1 (Supporting Information).
Methods-VPP Procedure: The oxidant inks were created similar to those reported previously. Fe(Otf) (9 wt%) was mixed in water and followed by the addition and of PEG-PPG-PEG (15 wt%) and PEG-ran-PPG (30 wt%). The ink was mixed in a DAC 600.1-CM 50 SpeedMixer at 2000 rpm for 8 min.
Fabrication of VPP-PEDOT, VPP-polypyrrole and VPP-polythiophene on paper was achieved by using a semi-automatic EKRA screen printer to deposit the oxidant ink. The screen meshes used were 120-34 (120 threads per cm with a thread diameter of 34 μm). The oxidant ink was deposited onto various paper substrates (in addition to the control PET substrate) and then heated on a 70°C hotplate for 2 min to allow the imprinted screen mesh to spread and become a uniform pattern before being placed in the VPP chamber.
A separate VPP chamber was used for the respective conductive polymer. The VPP chambers were composed of sealed boxes that could accommodate an A4 sheet with space for a small reservoir of monomer. The polymerization times were 30 min unless otherwise stated. After the polymerization the samples were placed on a 70°C hotplate for 2 min to complete the polymerization, followed by a washing step with 2-propanol and drying with compressed air.
The conventional electrochromic displays fabricated on the transparent Cristal paper were similar to those created previously, using several aligned screen printed layers. The VPP-PEDOT was created with the procedure described above, albeit with the polymerization time of 3 h. After the washing and drying of the VPP-PEDOT, the dielectric layer was deposited and cured by UV light followed by the electrolyte which is also cured by UV-light. Next the carbon was deposited and cured at 80°C and finally the silver was deposited and also cured at 80°C. The lower curing temperature was used to minimize the expansion and contraction of the paper.

Methods-Scanning Electron Microscopy:
Scanning electron microscopy (SEM) was carried out with a Zeiss Sigma 500 Gemini. The printed and polymerized structures on paper were each mounted on a carousel sample holder and grounded using copper tape. Measurements were performed using the in-lense detector and an electron gun EHT of 2.5-3.5 kV.
Methods-Spectroelectrochemical and Color Contrast Measurements: The spectroelectrochemical measurements were performed by using a Fiber Optic spectrometer AvaSpec and a Biologic-200 potentiostat. The conductive polymer/paper samples were utilized as a working electrode in an electrochemical cell inside a quartz cuvette, also comprising a Ag/AgCl reference electrode, a Pt wire counter electrode and a PBS (Phosphate Buffer Saline) solution (0.01 m) as the electrolyte.
The color contrast (ΔE*) values were obtained by measuring the L*a*b* color coordinates using a spectrophotometer (Mercury, purchased from Datacolor). The maximum color contrast was obtained by measuring the color coordinates of the electrochromic displays switched to their fully reduced and fully oxidized states (±3 V). In addition to this, the retention time of the electrochromic displays was also determined by monitoring the color contrast decay versus time upon leaving a fully reduced display segment in open circuit mode.
Methods-Electrical Measurements: 4-Point probe measurements were conducted on the samples using a Signatone 4-point resistance probe and a Keithley 2400. The switching behavior of the electrochromic displays was obtained by using a parameter analyzer (HP/Agilent 4155B) to record the current versus time upon continuous voltage supply (3 V applied on the counter electrode).
Methods-Bending Measurements: A bending apparatus was used where a mechanical lever can reproducibly bend samples over cylinders. The cylinders can be exchanged in order to test different diameters. Cylinders of 32-, 25-, and 12-mm diameters were used with PEDOT:Otf on Cristal paper. Sheet resistance values were recorded before and after bending with an average of three measurements.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.

Data Availability Statement
The data that support the findings of this study are available from the corresponding author upon reasonable request.