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Wu et al. Soft Sci 2024;4:29 https://dx.doi.org/10.20517/ss.2024.21 Page 11 of 22

Direct printing methods
The direct printing methods for fabricating active layer patterns in flexible ECDs can be classified into four
main categories: screen printing, inkjet printing, bespoke dispenser printing, and brush printing. The screen
printing method allows for high-yield and large-scale manufacturing with high resolution (≈ 70 μm). The
screen-printed ink dispersing the electrochromic materials in solvents and surfactants with a suitable
viscosity (500-5,000 cP) and surface tension (25-40 dyn/cm) can be uniformly and closely deposited on
various substrates with patterns . Recent studies reported that the patterned electrochromic electrodes
[96]
have been screen-printed on PEDOT:PSS paper, ITO-PET substrates, and PEDOT-PET substrates [96-98] .
Brooke et al. also developed a screen printing technology combined with vapor phase polymerization to
deposit various patterned polymer [poly(dimethylpropylene-dioxythiophene) (PProDOT-Me ), PPy] films
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on the PEDOT-PET substrate. This work demonstrates all screen-printed flexible ECDs with various
patterns, highlighting their potential for multicolor displays [Figure 7A] . Besides, inkjet printing is the
[99]
most commonly used technique because of its advantages of cost-efficiency and ingredient-saving,
simplified processing, and fabricating of high-resolution patterns . This technology has been widely used
[31]
in rigid and flexible electronics and can print large-scale products [72,100,101] . The crucial point for high-quality
inkjet printing is to ensure the ink has satisfactory rheological properties such as optimum viscosity (1-
10 cP), surface tension (28-35 dyn/cm), and appropriate contact angle on the electrode. Besides, the
particles with nano-size (< 200 nm) in the ink should be compatible with the solvent system, forming
uniform and stable particle dispersion, which ensures smooth and consistent ink droplets through
nozzles . Our group has put continuous effort into inkjet printing various electrochromic ink on rigid,
[102]
flexible, and stretchable substrates, achieving versatile inkjet-printed electrochromic devices [30,52,103-107] . For
example, the inkjet-printed WO -PEDOT:PSS layer on a flexible Ag grid-PET electrode with various
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patterns [Figure 7B] , the inkjet-printed WO electrode on a stretchable PEDOT:PSS-AgNWs-PDMS
[103]
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electrode with “NTU” patterns . Besides, Chen et al. have precisely inkjet-printed the multicolor patterned
[52]
electrochromic film at target positions of the ITO-PEN substrate. The multicolor phenomenon exhibiting
the coloration from red to blue is realized by regulating the ratio of Fe/Ru-Metallo-supramolecular
polymers (MEPE) solution as a series of inkjet inks. It was further fabricated as a flexible ECD, switching
from bleaching to multicolor states at different voltages [Figure 7C] . Besides, the inkjet printing technique
[22]
can be extendedly used for both the electrochromic layer and electrode with a simplified process, and the
individual printed PEDOT:PSS pixels can be realized with a printed connected Au electrode on cellulose di-
acetate foil. By attaching the gelatin electrolyte, the assembled biodegradable ECD prototype can present
quantities of display information, including battery level, weather conditions, and time [Figure 7D] .
[79]
Moreover, a newly water-based additive-free ink has been optimized for inkjet printing patterned
ammonium-functionalized viologen derivatives film with high resolutions, exhibiting excellent photo- and
electrochromic performances [Figure 7E] . The bespoke dispenser printing is another reported direct
[63]
printing method for active layer patterning. This method can allow higher viscosity ink (10-1,000 cP) and
larger particle size. Moreover, the ink should exhibit thixotropic behavior to accommodate the printer trait,
reducing viscosity under shear during dispensing and staying stable at rest . For instance, a WO -based
[108]
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electrochromic film on the ITO-PET flexible substrate was fabricated by the modified electrostatic force-
assisted dispensing printing with an adjustable pattern width from 693 to 256 μm. By using the dmFc as the
soluble anodic species, two WO flexible electrodes with different patterns have been fabricated as the
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symmetric electrochromic device, alternately showing dual images with applied bias under bending
conditions [Figure 7F] . For the brush printing method, a higher viscosity ink (100-10,000 cP) is required
[109]
to ensure it adheres to the brush and substrate without dripping . Currently, the brush printing method
[110]
applied in flexible ECDs is only reported on one paper, where the patterned PEDOT film is painted on the
flexible conductive tape with a brush . The above direct printing methods provide more efficient and facile
[36]
ways of fabricating the active layer patterns. To ensure high-quality printing, factors of ink formulation,
printing parameters, and post-printing treatments should be carefully considered and optimized.