Application of di-ureasil ormolytes based on lithium tetrafluoroborate in solid-state electrochromic displays

Abstract

In this study the sol–gel synthetic strategy was used to prepare organic–inorganic hybrid electrolyte systems composed of a di-urea cross-linked poly(ethylene oxide) (PEO)/siloxane (di-ureasil) matrix with a wide concentration range of lithium tetrafluoroborate (LiBF₄). The assembly and preliminary characterization of a prototype solid-state electrochromic device based on a four-layer sandwich structure incorporating these hybrid electrolytes doped with controlled quantities of LiBF₄ are reported. The composition of hybrid xerogels was indicated using the notation d-U(900)_nLiBF₄ and d-U(600)_nLiBF₄. In this representation the average molecular weight of the host di-ureasil framework is identified as d-U(900) or d-U(600) and the subscript n expresses the salt content in terms of the number of ether oxygen atoms per Li⁺ cation. The most conducting electrolyte of these hybrid systems is the d-U(900)₃₅LiBF₄ composition (1.70 × 10⁻⁴ S cm⁻¹ at about 95 °C). The lowest decomposition temperature was observed at a composition of n = 2.5 in both the d-U(900)_nLiBF₄ and the d-U(600)_nLiBF₄ electrolyte systems (250 °C and 243 °C, respectively). Electrochromic devices assembled with d-U(900)_nLiBF₄ and d-U(600)_nLiBF₄ electrolytes with compositions of 20 ≤ n ≤ 35 presented good color contrast, a maximum optical density of 0.23 and an encouraging performance in the coloring/bleaching process. The average transmittance in the visible region of the spectrum was above 74% for all the bleached samples analyzed.