Layer-by-layer assembly of polymeric complexes for electrochromic applications
Date of Issue2017-03-09
School of Materials Science and Engineering
Layer-by-Layer (LbL) self-assembly has been proven to be a very facile and versatile method for thin film fabrication with precise control of the film components and structures. This technique possesses great advantage on integrating materials with different functionalities into a film with designed structures. These features enable LbL self-assembly technique to be a suitable platform for electrochemical applications, which usually require integration of multiple functional components and regulation of interface properties. Conventional polymer LbL films adopt individual polymers as building blocks. However, these films suffer from slow deposition process with only limited amount of materials could be deposited onto the substrate within a deposition cycle. Moreover, LbL films fabricated from individual polymers usually demonstrate compact structure, which also hinders their application in fabricating thin films of which non-compact configuration is preferred. Due to the basic requirements of LbL technique that the adjacent layer should directly interact with each other, this method possesses inherent drawbacks where multi-interfaces are not favourable. Further exploration and modification of LbL technique is desired for better utilization of this prospected technique on electrochemical applications. In this thesis, we focus on developing polymeric complexes as building blocks for LbL self-assembly, in order to surmount the drawbacks and broaden the application field of LbL technique. Polymeric complexes (PCs) are supramolecular assemblies built up by nonstoichiometric combination of two components. Compared with free polymers, PCs have several advantages including rich varieties of compositions, relatively large dimensions and diverse structures. Thus introducing PCs into LbL enriches the diversity of LbL films, especially for fast deposition of LbL films with special structures and functionalities. However, there are limited reports on using PCs as building blocks for LbL deposition for electrochemical device applications. Aiming at exploiting the potential and merits of adopting PCs in LbL for electrochemical related studies and applications, this thesis is devoted to develop suitable PCs as building blocks to fabricate LbL films for three typical components in an electrochromic device: the electrochromic active layer, the solid polymer electrolyte (SPE) layer as well as the transparent conducting film (TCF). Based on the basic mechanism and requirements of these three functional layers, different types of PCs were developed to achieve superior performance. Polyelectrolyte-polyelectrolyte complexes (PECs) were chosen as building blocks for electrochromic active layer, contributing to a fast growth LbL film with porous structure due to the relative large dimensions and charge accumulation of PECs. The resultant electrochromic active films presented larger colouration modulation as well as fast kinetics compared to conventional LbL films. Polymer-alphaCD inclusion complexes via host-guest interaction were designed and adopted as building blocks for LbL SPE layer fabrication. Taking merits of the steric effect of polymer-alphaCD complexes, the resultant LbL films possessed lower glass transition temperature and higher ionic conductivity compared to LbL films made from uncomplexed polymers. Conductive PCs were selected to form capping layer on conducting Ag nanowire network. The LbL overcoated films lowered surface roughness of the metallic network without sacrificing the conductivity of the whole layer. The existence of LbL coating was demonstrated to protect Ag nanowire network from oxidation corrosion during the redox reaction. In conclusion, adopting PCs as building blocks was demonstrated to be a feasible and effective method to fabricate LbL films for electrochemical device applications, surmounting the drawbacks of conventional LbL technique and delivering superior performance.