High-contrast electrochromic thin films via layer-by-layer assembly of starlike and sulfonated polyaniline

In this Article, we report the layer-by-layer (LbL) assembly and electrochromic properties of polyaniline-tethered cubic polyhedral oligomeric silsesquioxane (POSS-PANI)/sulfonated polyaniline (SPANI) multilayer thin films. The interaction betweenPOSS-PANI and SPANI is characterized using X-ray photoelectron spectroscopy (XPS), ultraviolet-visible-near-infrared spectroscopy, and four-point probe conductivity. We show that the inclusion of SPANI during LbL assembly effectively dopes the underlying POSS-PANI layer and extends the conjugation length, as evidenced by probing the surface layers with XPS. We also demonstrate that the POSS-PANI/SPANI multilayer films have more electroactive units, lower band gap energies, and higher electrical conductivity values compared to those of POSS-PANI/poly(2-acrylamido-methane-2- propanesulfonic acid) (PAMPS) and spin-coated SPANI films. Under constant applied potentials, a 50 bilayer film of POSS-PANI/SPANI, (POSS-PANI/SPANI) ₅₀, shows significant enhancement in optical contrast compared with (POSS-PANI/PAMPS)₅₀. Furthermore, the switching kinetics of (POSS-PANI/ SPANI)₅₀ are much faster than that of the spin-coated SPANI. The improvement in electrochromic contrast under dynamic switching conditions is attributed to the presence of a larger number of electrochromic units in POSS-PANI/SPANI, the loose packing structure of POSS-PANI brought by its starlike molecular architecture, and the unique morphology created by the LbL assembly.