Low-voltage electrochromic device for photovoltaic-powered smart windows

Abstract

We report the properties of an all-solid-state electrochromic (EC) device that can be switched over a useful range of optical transmissions with voltages below 1 V. This switching voltage is smaller than required by other solid-state EC devices reported to date. We attribute the lower-than-normal switching voltage to the use of a thermally evaporated MgF2 thin film as the lithium ion conducting layer. Electrochemical impedance spectroscopy studies show that high lithium ion conductivity and low interfacial barriers for lithium exchange with the adjacent electrochromic and ion storage layers make MgF2 a good choice for the ion conductor in EC devices. This reduction in switching voltage is a first step toward powering an EC device by an integrated semitransparent single-junction photovoltaic (PV) cell. In a side-by-side bench test, where the EC device is connected to a semitransparent a-SiC:H PV cell having on open circuit voltage of 0.87 V, a relative transmission change in the EC device of 40% is achieved in less than 60 s.