Nanoparticles of poly(3-hexylthiophene): Toward a solvent-independent performance of electrochromic films

Nanoparticles of poly(3-hexylthiophene), P3HT(NP), uniquely enable the preparation of stable dispersions in environmentally-friendly media and thus offer a sustainable liquid phase fabrication of electrochromic device structures. In this work, we assess the electrochromic performance of P3HT(NP) films spray-coated from either tetrahydrofuran (THF)-water or chloroform (CHCl3)-ethanol dispersions on ITO substrates. The nanoparticle films exhibit consistent and reproducible high optical contrast values of around 50 %, t90-switching speeds of about 0.45 s and a cycling stability of approximately 200 cycles for a 20 % performance retention, independent of the solvent being used. Conversely, non-nanostructured P3HT films spray-coated from THF or CHCl3 reveal a strong solvent dependent variability in their electrochromic behavior presenting low optical contrast, high switching speeds and fast degradation rates in the case of CHCl3. The solvent independent electrochromic characteristics of P3HT nanoparticle films is related to a consistent availability of accessible electroactive sites provided by a homogeneous porous P3HT network structure formed on the underlying substrate, as probed by SEM and profilometric studies. Our findings reveal that the use of nanoparticles of P3HT and its environmentally benign liquid phase processing, a concept which is extendable to other electrochromic polymers, opens a sustainable pathway toward the large-area fabrication of electrochromic device structures with favorable and consistent performance parameters.