Tetrathiafulvalene: effective organic anodic materials for WO(3)-based electrochromic devices

Finding a new, effective anodic species is a challenge for achieving simpler low-voltage tungsten trioxide (WO(3))-based electrochromic devices (ECDs). In this work, we utilize tetrathiafulvalene (TTF) and demonstrate its reversible redox behaviors as an electrolyte-soluble anodic species. The concentration of TTF in the electrolyte is varied to optimize device performance. When the TTF concentration is low (0.01 M), a smaller maximum transmittance difference (ΔT(max) ∼ 34.2%) and coloration efficiency (η ∼ 59.6 cm(2) C(−1)) are measured. Although a better performance of ΔT(max) ∼ 93.7% and η ∼ 74.5 cm(2) C(−1) is achieved at 0.05 M TTF, the colored state could no longer return to its original form. We conclude that 0.03 M of TTF is the appropriate concentration for high-performance WO(3) ECDs with high optical contrast and reversible EC behaviors. The irreversible EC transition at high concentrations of TTF is attributed to the agglomeration of TTF molecules.