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 between POSS-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.