Page 62 - Read Online

Page 8 of 22 Wu et al. Soft Sci 2024;4:29 https://dx.doi.org/10.20517/ss.2024.21

easily bendable flexible chalcogenoviologens-based display composed of two ITO-PET electrodes with
different etched patterns where the information of “¥ 9.9 !”/“ON SALE !” and “□”/“Δ” can alternatively
[85]
exhibit under corresponding applied voltages [Figure 4B] . Moreover, our group fabricated a patterned
digital array electrode by wet-etching the ultrathin flexible ITO glass, realizing dynamic switching of
multiple number patterns and significantly improving the information capacity of WO -based ECDs
3
[83]
[Figure 4C] . Apart from the etching methods on commercial flexible electrodes, there are other advanced
approaches for mounting conductive patterns on flexible substrates. For example, the conductive
PEDOT:PSS ink was directly screen-printed and inkjet-printed on flexible substrates (such as pre-engraved
PET and paper substrates) as patterned electrodes. They can provide favorable electrical connections for the
ECDs and are successfully applied as promising applications (electrochromic biosensors and paper-based
ECDs) [60,91] . Zhao et al. prepared a transparent Ni grid-PET electrode with multiple designed patterns
through lithography and electrodeposition processes . The prepared Ni electrode has a high optical
[29]
transmittance, electrical conductivity, and mechanical stability, which is advantageous for efficient electron
interaction inside the flexible ECDs under various mechanical conditions and demonstrates its suitability
for high-performance flexible ECD with fast switching speed and good bending capacity. Besides, our group
utilizes the template-assisted filtration method and transfers the desired AgNWs patterns onto the PDMS
[54]
elastomer substrate . This stretchable electrode was successfully assembled into the first stretchable and
wearable patterned WO -based ECD, demonstrating its display function under 50% strain [Figure 4D].
3
Similar template-assisted technology with spray coating method has also been adopted to fabricate
[32]
patterned electrodes , such as AgNWs/PEDOT:PSS-PET electrodes [Figure 4E] . Overall, the strategies
[26]
for designing electrode patterns have shown advanced progress on flexible and stretchable ECDs. The
streamlined and feasible preparation techniques would always be preferentially considered for future
displays. More importantly, the high information capacity and good display quality with high resolution will
be promising targets for the electrode pattern design strategies.

Active layer pattern design
The strategies of designing active layer patterns in flexible and stretchable ECDs have also been most widely
adopted over the last decade. These strategies are mainly classified into template-assisted and direct printing
methods. With these strategies, only the active patterned electrochromic layer will process the
electrochemical redox reaction and exhibit its optical properties variation upon electricity. In the following
part, we will comprehensively explain how these methods achieve active layer patterning and attain the
corresponding display functionalities of the fabricated electrochromic devices.

Template-assisted methods
As for the template-assisted methods, the common fabrication process is to deposit the active layer on the
electrode with the mounted mask. So far, the most developed deposition methods are spray-coating, spin-
coating, magnetron sputtering, electron beam thermal deposition, electrodeposition, and electrochemical
polymerization. The deposition conditions and adhesion between the template and electrode determine the
selection of the template. For example, a stable metal sheet was used as the shadow mask for thermally
depositing a bowknot-like patterned WO layer on a Cr/Ag-PET electrode under severe conditions (low
3
vacuum). The prepared patterned electrochromic electrode was further assembled into the device,
exhibiting dynamic multicolor under different potentials [Figure 5A] . Chen et al. have precisely fabricated
[92]
the two-dimensional (2D) PEDOT micropatterns with a high fidelity of 97.3% compared to the patterned
agarose template through the localized electrochemical polymerization method [Figure 5B] . It was
[78]
realized by strong adhesion between the ITO electrode and agarose with electrolyte. The ionic transport
kinetics of the electrochromic layer was boosted owing to the 2D structure of PEDOT which resulted in
faster switching speed and electrochromic performances. This micropatterned PEDOT was further designed
with the logo of Xiamen University and illustrated its application as a flexible ECD. Besides, a 2D ordered