In recent years research in conjugated electrochromic polymers has made synthetic strides to complete the color palette of colored-to-colorless materials, as well as developed the ability to predictably mix solutions of these polymers for fine-tuning of coloration and precise color-matching. More recently, we have taken our family of polymers and begun investigating the effects of solution-processing technique and device architecture on their performance.
Here we report on a quantitative analysis to differentiate the effects of processing methods on the electrochromic properties of a material. We demonstrate the application of this analysis towards spin-coating, along with two roll-to-roll compatible processes – airbrush spraying and blade coating – for three electrochromic polymers with different repeat-unit motifs. Our results indicate a consistent trend showing the coating process affording the highest density of available redox sites led to the highest contrast films, with spun-coated and blade-coated films offering higher contrast and airbrush-sprayed films offering the lowest contrast. Surprisingly however, in spite of large differences in the surface morphologies of films obtained from these different coating methods, as well as in the electrochemical properties, the differences in the electrochromic performance are small across the different processing techniques. Furthermore, we show that coloration efficiency is not correlated with the achievable contrast of a material.
Using this understanding of how the coating technique affects film performance, we coated high-contrast films and incorporated them into electrochromic devices to examine how the choice of counter electrode material impacts their performance. While not widely studied, the choice of counter electrode is critical for the electrochromic polymer film to achieve its highest film performance. We found that using a counter electrode material that sufficiently charge-balances the electrochromic polymer film has large implications not only on the contrast, but also on the stabilities, operating voltages, and switching speeds of the device. From these studies, solid-state devices were fabricated on the benchtop achieving high contrasts with operational voltages of +/- 1V, and switching stability in excess of 10 000 cycles.