Exploring unbalanced electrode configurations for electrochromic devices

Abstract

Electrochromic cells consisting of a black-to-transmissive propylenedioxythiophene/benzothiadiazole copolymer active layer and a minimally colored dioxypyrrole-based charge storage layer were constructed, and the effect of the redox capacity ratio between the two layers was evaluated. This ratio was tuned between large excesses of either the active layer or the charge storage layer (an unbalanced configuration), to cells where both films had the same redox capacity (a balanced configuration), and the effects on cell voltage, optical contrast, switching time and switching stability were assessed. The inclusion of an auxiliary reference electrode allowed us to monitor the redox behavior of both the active layer and the charge storage layer during cell operation. With this setup, we show that the redox behavior of each layer is highly dependent on the redox capacity ratio between the two films. We also demonstrate that, in principle, any electrochromic material that has a colorless state can serve as an optically inactive charge storage material if it is present in sufficient excess in an electrochromic cell. Additionally, by monitoring the redox behavior of each individual layer during cell operation, we show that performance loss in electrochromic cells during long-term stability testing is not necessarily due to an irreversible electrochemical or structural degradation of the active material or the charge storage layer. Rather we observe a drift in the oxidation state of both layers, suggesting that the losses are reversible, and could be mitigated through proper electrochemical control of the cells. The main benefit of an unbalanced configuration is the substantial lowering of the operating voltage without compromising switching time or the contrast as long as transparency of the charge storage material is maintained.