Enhanced electrochromic performance of flexible WFexOyCz films by O2 gas addition using low temperature plasma polymerization

• Electrochromic WFexOyCz films synthesized by low temperature plasma polymerization.
• O2 gas addition enhances electrochromic performance of WFexOyCz films.
• Organic-inorganic hybrid WFexOyCz films synthesized.
• Flexible and electrochromic WFexOyCz films produced.

An investigation was conducted into the prominent lithium electrochromic performance of flexible organo-tungsten-iron oxide (WFexOyCz) films deposited onto flexible (60 Ω/□ polyethylene terephthalate/indium tin oxide) substrates. The simple one-step low temperature (∼23 °C) plasma polymerization method was used to inject the precursors tungsten hexacarbonyl [W(CO)6] and biscyclopentadienyl iron [Fe(C5H5)2] into the plasma chamber, whilst being mixed with O2 gases at various gas flow rates. The plasma-polymerized WFexOyCz films at a certain addition of oxygen gases achieved outstanding lithium electrochemical reversibility of Li+ ion intercalation and de-intercalation proven by a potential sweep switching between −1 V and 1 V at a scan rate of 50 mV/s in a 1 M LiClO4-propylene carbonate electrolyte. High values for optical transmittance modulation (ΔT) of up to 83.2%, optical density (ΔOD) of up to 1.08 and color efficiency (η) of up to 62.3 cm2/C at a wavelength of 854.9 nm were obtained for the amorphous WFexOyCz films synthesized by the addition of oxygen gases at a flow rate of 17 sccm. The results verified that the proper addition of oxygen gas provides a suitable routine to enhance the electrochromic properties of plasma-polymerized WFexOyCz films.