Ionic additive in an ionogel for a large area long lived high contrast electrochromic device

Abstract

Heptyl viologen (HV) based electrochromic devices (ECD) suffer from poor write-erase efficiencies and become permanently colored after a few redox switches, thus limiting their use as electrochromic windows or mirrors. This formidable issue is addressed by incorporating an ionic additive: a disodium salt of ethylenediamine tetra-acetic acid (EDTA), in the ionogel electrolyte. EDTA, via electrostatic interactions, binds to the HV+• radical cation and prevents its' further reduction to the undesirable pale colored neutral HV0, by wedging in-between the moieties and inhibiting their undesirable stacking. A large area HV/EDTA in gel/Prussian blue (PB) ECDs of ~8 cm × 6 cm dimensions outperforms the HV/gel/PB ECD which is evidenced in the enhanced transmission modulation (ΔTmax: 73.1% at 606 nm), coloration efficiency (η: 346.2 cm2 C−1) and superior chromaticity coordinates (colored: L*,a,b: 40, 10, −65) compared to lower magnitudes of 69.2%, 270.3 cm2 C−1 and (L*,a,b: 50.3, 8, −48.5) respectively. When evaluated as a mirror, HV/EDTA in gel/PB ECD shows a reflectance modulation of ~65% which does not alter significantly, even at −10 °C or at +60 °C, ratifying its thermal robustness. The HV/EDTA in gel/PB ECD shows color and bleach times of 16 and 35 s, retains ~93% of its’ initial contrast after 104 cycles and after aging for 2 years, continues to deliver a transmission modulation of 68%. This study successfully demonstrates a cost effective, easily implementable, scaled-up HV/EDTA in gel/PB based long-lived durable ECD for energy efficient smart window and rearview mirror applications.