Rational design of the electrolyte systems for the photochromic device

Abstract

A photochromic (PC) device, a simplified version of the photoelectrochromic (PEC) device without the transparent conducting oxide (TCO) layer, can modulate the intensity of transmitted light by automatically changing its transmittance. To effectively regulate the transmittance of sunlight, the PC device should possess a high optical contrast and reasonable transition time. However, these properties cannot be achieved simultaneously due to the competitive coloring and bleaching reactions. In this study, the effects of the concentration ratio of redox couples (tetramethylthiourea/tetramethylformaminium disulfide (TMTU/TMFDS2+)) and the concentration of lithium source on the performance of the PC device were systematically investigated. The results confirmed that the intrinsic limitation can be overcome by the rational design of the electrolyte system. The PC device containing an optimized TMTU/TMFDS2+ (0.05 M/0.1 M) redox couple and 1 M LiClO4 provided excellent coloration depth (~51% transmission change at 670 nm) and fast bleaching speed (80% recovery in under 1.5 h). In addition, long-term durability (>100 h) tests of the PC device revealed the high stability of the investigated electrolyte system. The in-depth discussion in combination with the experimental results provided in this work can pave the way for the future rational design of PC devices.