Electrochromics for deformable display application
Date of Issue2016-12-28
School of Materials Science and Engineering
Advanced Materials Research Centre
Deformable electronics with the ability to conform to non-planar and curvature surfaces while keeping the functionality is under vigorous development recently to cater for the ever-growing and demanding need of people for mechanically compliant consumer electronics that could achieve human-machine interactions in a deformable way. Prompted by such needs, deformable electronics like wearable bands to capture signals for monitoring human health; stretchable optical projector or camera to realize virtual reality display and foldable smartphones that allows the ultra-portability and durability are under development in the market place as the next generation electronics to realize unprecedented functionalities unreachable for the conventional rigid electronics. As a very important component for the realization of human-machine interaction, a display system is greatly favored to feedback and transport information. Electrochromic display represents a non-emissive display genre with the advantage of high contrast, low power consumption and facile device assembly. The idea of adopting electrochromism for deformable display and information communication is still at its cradle and progressed in a limited fashion. In this study, the research is focused on feasible approaches and techniques for the realization of deformable electrochromic devices, in particular stretchable, wearable and foldable electrochromic devices. To realize these, the research started from the design of deformable electrodes. In order to meet the requirements for low charge transfer resistance with rapid switching and large optical modulation, electrodes need to be endowed with high conductivity. The electrochromic devices subject to deformation are expected to suffer performance degradation as small as possible, which makes requisite the mechanical stability of the electrode against stretching or folding. Ag nanowires with the metallic low resistance and the reticulate network shape of high mechanical compliance are potential with an excellent preservation of the network resistance due to inter-sliding mechanism to accommodate strain. The network structure with open voids that obviates strong absorption could also lead to optical transparency of the percolating network. These beneficial properties well leveraged could be integrated with a stretchable or foldable substrate to function as the deformable electrode. An embedded Ag nanowires network structure in PDMS was engineered to obtain the stretchable electrode. The electrode maintains an excellent conductivity at both the relaxed and 50% strained state. A nanopaper transfer technique was discovered and discussed in the thesis; the versatile transfer technique renders possible the transfer of various functional nanomaterials of different dimensions. Transparent conductive Ag nanowires percolating electrode was demonstrated with a high quality through this technique. The robust electrode was demonstrated as an excellent substrate to practice folding for the electrochromic devices. The display function is realized through the integration of electrochromes on the mentioned deformable electrodes. Unlike rigid conventional electrochromic devices, the morphology and thickness of the electrochrome need to be carefully designed in order to accommodate the deformation. Electrochemical deposition from the precursor offers an excellent solution to manually control the physical property of the electrochromic layer to fulfill the deformation function. Discrete WO3 islands and thin WO3 coatings were deposited for stretchable and foldable electrochromic applications respectively. The performance at relaxed and strained state as well as folded state was analyzed systematically to illuminate approaches for the buildup of durable deformable devices. A fully foldable solid state transparent electrochromic device based on small molecule slime and SWCNT protected Ag nanowire electrode was demonstrated as a potential technique for highly deformable electrochromics. In conclusion, the thesis together with the herein encompassed techniques and approaches is believed to be able to provide some insightful ideas on the emerging topic of deformable electrochromics and meanwhile it serves as a good guidance for the realization of durable deformable electrochromic devices for display applications.